tag:theconversation.com,2011:/ca/topics/string-theory-156/articlesString Theory – The Conversation2024-03-03T19:19:44Ztag:theconversation.com,2011:article/2243722024-03-03T19:19:44Z2024-03-03T19:19:44ZGravity experiments on the kitchen table: why a tiny, tiny measurement may be a big leap forward for physics<figure><img src="https://images.theconversation.com/files/579074/original/file-20240301-30-ecsdm7.jpg?ixlib=rb-1.1.0&rect=3%2C16%2C2131%2C1536&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/string-theory-physical-processes-quantum-entanglement-733672807">Shutterstock</a></span></figcaption></figure><p>Just over a week ago, European physicists <a href="https://www.science.org/doi/10.1126/sciadv.adk2949">announced</a> they had measured the strength of gravity on the smallest scale ever. </p>
<p>In a clever tabletop experiment, researchers at Leiden University in the Netherlands, the University of Southampton in the UK, and the Institute for Photonics and Nanotechnologies in Italy measured a force of around 30 attonewtons on a particle with just under half a milligram of mass. An attonewton is a billionth of a billionth of a newton, the standard unit of force.</p>
<p>The researchers <a href="https://www.eurekalert.org/news-releases/1035222">say</a> the work could “unlock more secrets about the universe’s very fabric” and may be an important step toward the next big revolution in physics. </p>
<p>But why is that? It’s not just the result: it’s the method, and what it says about a path forward for a branch of science critics say may be trapped in a loop of <a href="https://www.prospectmagazine.co.uk/ideas/technology/38913/is-particle-physics-at-a-dead-end">rising costs and diminishing returns</a>.</p>
<h2>Gravity</h2>
<p>From a physicist’s point of view, gravity is an extremely weak force. This might seem like an odd thing to say. It doesn’t feel weak when you’re trying to get out of bed in the morning!</p>
<p>Still, compared with the other forces that we know about – such as the electromagnetic force that is responsible for binding atoms together and for generating light, and the strong nuclear force that binds the cores of atoms – gravity exerts a relatively weak attraction between objects. </p>
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<p>And on smaller scales, the effects of gravity get weaker and weaker.</p>
<p>It’s easy to see the effects of gravity for objects the size of a star or planet, but it is much harder to detect gravitational effects for small, light objects.</p>
<h2>The need to test gravity</h2>
<p>Despite the difficulty, physicists really want to test gravity at small scales. This is because it could help resolve a century-old mystery in current physics.</p>
<p>Physics is dominated by two extremely successful theories. </p>
<p>The first is general relativity, which describes gravity and spacetime at large scales. The second is quantum mechanics, which is a theory of particles and fields – the basic building blocks of matter – at small scales. </p>
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<a href="https://theconversation.com/approaching-zero-super-chilled-mirrors-edge-towards-the-borders-of-gravity-and-quantum-physics-162785">Approaching zero: super-chilled mirrors edge towards the borders of gravity and quantum physics</a>
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<p>These two theories are in some ways contradictory, and physicists don’t understand what happens in situations where both should apply. One goal of modern physics is to combine general relativity and quantum mechanics into a theory of “quantum gravity”. </p>
<p>One example of a situation where quantum gravity is needed is to fully understand black holes. These are predicted by general relativity – and we have observed huge ones in space – but tiny black holes may also arise at the quantum scale. </p>
<p>At present, however, we don’t know how to bring general relativity and quantum mechanics together to give an account of how gravity, and thus black holes, work in the quantum realm.</p>
<h2>New theories and new data</h2>
<p>A number of approaches to a potential theory of quantum gravity have been developed, including <a href="https://theconversation.com/explainer-string-theory-2983">string theory</a>, <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5567241/">loop quantum gravity</a> and <a href="https://link.springer.com/article/10.1007/s41114-019-0023-1">causal set theory</a>.</p>
<p>However, these approaches are entirely theoretical. We currently don’t have any way to test them via experiments.</p>
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<p>To empirically test these theories, we’d need a way to measure gravity at very small scales where quantum effects dominate.</p>
<p>Until recently, performing such tests was out of reach. It seemed we would need very large pieces of equipment: even bigger than the world’s largest particle accelerator, the Large Hadron Collider, which sends high-energy particles zooming around a 27-kilometre loop before smashing them together. </p>
<h2>Tabletop experiments</h2>
<p>This is why the recent small-scale measurement of gravity is so important.</p>
<p>The experiment conducted jointly between the Netherlands and the UK is a “tabletop” experiment. It didn’t require massive machinery.</p>
<p>The experiment works by floating a particle in a magnetic field and then swinging a weight past it to see how it “wiggles” in response.</p>
<p>This is analogous to the way one planet “wiggles” when it swings past another.</p>
<p>By levitating the particle with magnets, it can be isolated from many of the influences that make detecting weak gravitational influences so hard.</p>
<p>The beauty of tabletop experiments like this is they don’t cost billions of dollars, which removes one of the main barriers to conducting small-scale gravity experiments, and potentially to making progress in physics. (The latest proposal for a bigger successor to the Large Hadron Collider would <a href="https://www.nature.com/articles/d41586-024-00353-9">cost US$17 billion</a>.)</p>
<h2>Work to do</h2>
<p>Tabletop experiments are very promising, but there is still work to do.</p>
<p>The recent experiment comes close to the quantum domain, but doesn’t quite get there. The masses and forces involved will need to be even smaller, to find out how gravity acts at this scale. </p>
<p>We also need to be prepared for the possibility that it may not be possible to push tabletop experiments this far.</p>
<p>There may yet be some technological limitation that prevents us from conducting experiments of gravity at quantum scales, pushing us back toward building bigger colliders.</p>
<h2>Back to the theories</h2>
<p>It’s also worth noting some of the theories of quantum gravity that might be tested using tabletop experiments are very radical.</p>
<p>Some theories, such as loop quantum gravity, suggest <a href="https://theconversation.com/time-might-not-exist-according-to-physicists-and-philosophers-but-thats-okay-181268">space and time may disappear</a> at very small scales or high energies. If that’s right, it may not be possible to carry out experiments at these scales.</p>
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<p>After all, experiments as we know them are the kind of thing that happen at a particular place, across a particular interval of time. If theories like this are correct, we may need to rethink the very nature of experimentation so we can make sense of it in situations where space and time are absent.</p>
<p>On the other hand, the very fact we can perform straightforward experiments involving gravity at small scales may suggest that space and time are present after all.</p>
<p>Which will prove true? The best way to find out is to keep going with tabletop experiments, and to push them as far as they can go.</p><img src="https://counter.theconversation.com/content/224372/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Sam Baron receives funding from the Australian Research Council.</span></em></p>A new measurement of gravity at small scales hints at an alternative to billion-dollar experiments for the future of physics.Sam Baron, Associate Professor, Philosophy of Science, The University of MelbourneLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2030102023-05-15T15:43:17Z2023-05-15T15:43:17ZTheory of everything: how progress in physics depends on asking the right questions<figure><img src="https://images.theconversation.com/files/524343/original/file-20230504-29-15yttd.jpg?ixlib=rb-1.1.0&rect=152%2C197%2C5838%2C5793&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Calabi-Yau manifold: a proposed structure of extra dimensions of space in string theory. </span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/calabiyau-manifold-structure-extra-dimensions-space-1228700050">vchal/Shutterstock</a></span></figcaption></figure><iframe src="https://embed.acast.com/638f4b009a65b10011b94c5e/64353c62de066f001110361d" frameborder="0" width="100%" height="190px"></iframe>
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<p>When I began my undergraduate physics degree (around 20 years ago), “What is the <a href="https://theconversation.com/great-mysteries-of-physics-do-we-really-need-a-theory-of-everything-203534">theory of everything</a>?” was a question that I heard often. It was used as a label for how theoretical physicists were trying to develop a deeper understanding of the elementary building blocks of our universe and the forces that govern their dynamics.</p>
<p>But is it a good question? Is it helpful in guiding scientists towards the discoveries that will advance our understanding to the next level? After all, good science relies on asking good questions. Or is it just <a href="https://bigthink.com/starts-with-a-bang/theories-of-everything/">“wishful thinking”</a>?</p>
<p>Arguably, the question “What is the theory of everything?” reminds us that good science doesn’t have to start with the best questions. Let me explain what I mean.</p>
<p>Suppose we play a game. I have a deck of cards, and each card is printed with the name and a photograph of a different animal. I choose a card, and your job is to ask questions to find out which animal I have chosen. Of course, to ask a discerning question, you first need to know something about animals.</p>
<p>The first time you play, you may not be familiar with which animals are in the deck, and your first question is “Does it live in the sea?”. My answer is “No,” and the game continues. Then it is your turn to pick a card. You look carefully through the deck to make your choice, and you realise that it only contains land animals. “Does it live in the sea?” seemed like a good question to start with, but it was not.</p>
<p>We take turns, and the more we play, the quicker we seem to figure out which card has been chosen. Why? We have become better at asking good questions.</p>
<p>The role that questions play in scientific research is similar. We start from some level of understanding, and we ask questions based on that level of understanding to try to improve it. As our understanding builds, we refine our questions and get more insightful answers.</p>
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<img alt="" src="https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><em>This is article is accompanied by a podcast series called <a href="https://podfollow.com/great-mysteries-of-physics">Great Mysteries of Physics</a> which uncovers the greatest mysteries facing physicists today – and discusses the radical proposals for solving them.</em></p>
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<p>This is how progress is made. The same is true of asking “What is the theory of everything?”: the goodness of a scientific question is not immutable.</p>
<h2>Why a ‘theory of everything’?</h2>
<p>The <a href="https://home.cern/tags/standard-model#:%7E:text=The%20Standard%20Model%20of%20particle,of%20scientists%20around%20the%20world.">Standard Model of Particle Physics</a>, one <a href="https://theconversation.com/the-standard-model-of-particle-physics-may-be-broken-an-expert-explains-182081">of the pillars of modern science</a>, is a success of reductionism - the idea that things can be explained by breaking them down into smaller parts.</p>
<p>The model, which is written in a mathematical language called <a href="https://www.damtp.cam.ac.uk/user/tong/whatisqft.html">quantum field theory</a>, describes how elementary particles move around and interact with one another. It explains the nature of three out of four of the known fundamental forces: electromagnetism, and the weak and strong forces that govern processes on subatomic scales. It does not include gravity, the fourth force.</p>
<p>The model accounts for <a href="https://theconversation.com/great-mysteries-of-physics-4-does-objective-reality-exist-202550">quantum mechanics</a>, which describes the probabilistic nature of the dynamics of subatomic particles, and Einstein’s special theory of relativity, which describes what happens when relative speeds are close to the speed of light – no small achievement.</p>
<p>The assumption in asking “What is the theory of everything?” is that the Standard Model will one day be found to be embedded within a larger structure (with more elemental ingredients) that provides us with a unified explanation of the fundamental forces including gravity. Gravity, in fact, is this question’s ultimate focus. </p>
<p>But the question “What is the theory of everything?” gives very little guidance as to what such a theory of everything might look like. We need some better questions.</p>
<p>Now, there are good reasons to expect that such a unified explanation of the fundamental forces might exist: the Standard Model includes the celebrated Higgs mechanism, from which the <a href="https://theconversation.com/higgs-boson-ten-years-after-its-discovery-why-this-particle-could-unlock-new-physics-beyond-the-standard-model-186076">Higgs boson</a> arises. It explains why fundamental particles known as the W and Z bosons, which transmit the weak force, acquire a mass. It also explains why the photon, which transmits the electromagnetic force, does not.</p>
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<img alt="CMS experiment at Cern." src="https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=393&fit=crop&dpr=1 600w, https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=393&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=393&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=494&fit=crop&dpr=1 754w, https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=494&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=494&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">CMS experiment at Cern.</span>
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<p>As a result, electromagnetism and the weak force, which is involved in the nuclear fusion that powers stars, behave differently at low energies: the electromagnetic force acts over very large distances, whereas the weak force acts only over very short distances. The Higgs mechanism also explains why, at higher energies, these two forces start to behave as a single “electroweak” force. This is called electroweak unification.</p>
<p>Now, if electromagnetism and the weak force combine in this way, why not all the forces in the Standard Model? Unifying these two with the strong force, the force that holds the ingredients of atomic nuclei together, is the aim of grand unified theories. Theoretical ideas such as <a href="https://home.cern/science/physics/supersymmetry">supersymmetry</a>, which postulates a symmetry between force carriers and matter particles, suggest that <a href="https://bigthink.com/starts-with-a-bang/theories-of-everything/">the strength of these three forces could get tantalisingly close at high enough energies</a>.</p>
<p>And if the electromagnetic, weak and strong forces turn out to be unified, why not gravity, too?</p>
<p>Gravity is described by <a href="https://theconversation.com/how-einsteins-general-theory-of-relativity-killed-off-common-sense-physics-50042">Einstein’s General Theory of Relativity</a>, which applies on large scales or at low energies. But if we want a consistent quantum theory of gravity that applies on the smallest scales, quantum field theory isn’t enough. We need mathematical frameworks that can consistently incorporate both general relativity and quantum mechanics.</p>
<p>The “everything” in a “theory of everything” refers to all the known forces of nature: electromagnetism, the weak force, the strong force, and gravity (and new, <a href="https://theconversation.com/new-physics-latest-results-from-cern-further-boost-tantalising-evidence-170133">hypothetical forces</a>, too) and the particles that they act between. The “theory” refers to the existence of some common mathematical framework that describes all of the “everything”.</p>
<p>One such common mathematical framework is <a href="https://theconversation.com/stephen-hawking-had-pinned-his-hopes-on-m-theory-to-fully-explain-the-universe-heres-what-it-is-93440">string theory</a>, which supposes that the most fundamental building blocks of the universe are tiny strings that vibrate in extra spatial dimensions beyond the three of our everyday experience. </p>
<h2>Better questions</h2>
<p>Questions are the guide to scientific inquiry. The question “What is the theory of everything?” only speculates at a destination, but it gives very little direction.</p>
<p>Frameworks such as supersymmetry and string theory were not developed to answer the question “What is the theory of everything?” directly. They were motivated by better questions about what a theory of all the fundamental forces needs to explain and what it might look like, questions like: Why is there a huge discrepancy between the energy scales of the Standard Model and quantum gravity? Why do quantum mechanics and general relativity seem to be incompatible?</p>
<p>But the “whys” that theoretical physicists ask develop as our understanding develops, and the questions that we are now posing are getting us even closer than ever to an understanding of all the known forces of nature. </p>
<p>These new “whys” hint at <a href="https://doi.org/10.48550/arXiv.2006.06872">remarkable connections between very different areas of physics and mathematics</a>: Why does the physics of holograms seem to help us to understand gravity? Why does this seem to be connected to the properties of large collections of random numbers? Why do the rules of quantum information seem to explain the physics of black holes?</p>
<p>But this is not a case of “out with the old and in with the new”. Instead, these new questions have been reached by building on what has been learnt from developing and studying possible “Theories of Everything”, like string theory.</p>
<p>And these new questions are good questions. The exciting thing is that they still may not be the best questions, and having them to guide us doesn’t necessarily mean that we know where we will end up. That is what scientific discovery is all about.</p>
<p><em>You can listen to Great Mysteries of Physics via any of the apps listed above, our <a href="https://feeds.acast.com/public/shows/638f4b009a65b10011b94c5e">RSS feed</a>, or find out how else to listen here. You can also read a <a href="https://cdn.theconversation.com/static_files/files/2634/MoP__Ep6_-_Theory_of_Everything_TRANSCRIPT.docx.pdf?1681292977">transcript of the episode here</a>.</em></p><img src="https://counter.theconversation.com/content/203010/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Peter Millington is a Senior Research Fellow in the Particle Theory Group at the University of Manchester, UK, where he holds a UK Research and Innovation Future Leaders Fellowship and a Royal Society International Exchanges Grant. Peter Millington is a Member of the Institute of Physics, UK and serves on the Institute of Physics High Energy Particle Physics Group Committee.</span></em></p>Good questions guide good science, but that doesn’t mean we know where we’ll end up.Peter Millington, Senior Research Fellow and UKRI Future Leaders Fellow in the Particle Theory Group, Department of Physics and Astronomy, University of ManchesterLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2017292023-03-22T13:05:59Z2023-03-22T13:05:59ZThe multiverse: how we’re tackling the challenges facing the theory<figure><img src="https://images.theconversation.com/files/516902/original/file-20230322-1063-gxs4l1.jpg?ixlib=rb-1.1.0&rect=44%2C0%2C6000%2C3368&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
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<p>The idea of a multiverse consisting of “parallel universes” is a popular science fiction trope, recently explored in the Oscar-winning movie <a href="https://www.imdb.com/title/tt6710474/">Everything Everywhere All At Once</a>. However, it is within the realm of scientific possibility. </p>
<p>It is important to state from the start that the existence (or not) of the multiverse <a href="https://theconversation.com/the-theory-of-parallel-universes-is-not-just-maths-it-is-science-that-can-be-tested-46497">is a consequence</a> of our present understanding of the fundamental laws of physics – it didn’t come from the minds of whimsical physicists reading too many sci-fi books. </p>
<p>There are different versions of the multiverse. The first and perhaps most popular version comes from quantum mechanics, which governs the world of atoms and particles. It suggests a particle <a href="https://theconversation.com/four-common-misconceptions-about-quantum-physics-192062">can be in many possible states</a> simultaneously – until we measure the system and it picks one. According to <a href="https://plato.stanford.edu/entries/qm-manyworlds/">one interpretation</a>, all quantum possibilities that we didn’t measure are realised in other universes.</p>
<h2>Eternal inflation</h2>
<p>The second version, the cosmological multiverse, arises as a consequence of <a href="https://www.space.com/42261-how-did-inflation-happen-anyway.html">cosmic inflation</a>. In order to explain the fact that the universe today looks roughly similar everywhere, the physicist Alan Guth proposed in 1981 that the early universe underwent a period of accelerated expansion. During this period of inflation, space was stretched such that the distance between any two points were pushed apart faster than the speed of light.</p>
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<p><em>This is article is accompanied by a podcast series called <a href="https://podfollow.com/great-mysteries-of-physics">Great Mysteries of Physics</a> which uncovers the greatest mysteries facing physicists today – and discusses the radical proposals for solving them.</em></p>
<hr>
<p>The theory of inflation also predicted the existence of the <a href="https://www.esa.int/Science_Exploration/Space_Science/Planck/History_of_cosmic_structure_formation">primordial seeds</a> which grew into cosmological structures such as stars and galaxies. This was triumphantly detected in 2003 by observations of tiny temperature fluctuations in the cosmic microwave background, which is the light left over from the Big Bang. It was subsequently measured with exquisite precision by the space experiments <a href="https://map.gsfc.nasa.gov/#:%7E:text=The%20Wilkinson%20Microwave%20Anisotropy%20Probe,new%20Standard%20Model%20of%20Cosmology.">WMAP</a> and <a href="https://www.esa.int/Enabling_Support/Operations/Planck">Planck</a>. </p>
<p>Due to this remarkable success, cosmic inflation is <a href="https://www.forbes.com/sites/startswithabang/2019/05/11/ask-ethan-how-well-has-cosmic-inflation-been-verified/">now considered the de facto theory</a> of the early universe by most cosmologists.</p>
<p>But there was a (perhaps unintended) consequence of cosmic inflation. During inflation, space is stretched and smoothed over very large scales – usually much larger than the observable universe. Nevertheless, cosmic inflation must end at some point, else our universe wouldn’t have been able to evolve to what it is today.</p>
<p>But physicists soon realised that if inflation really is true, some regions of space-time would continue to inflate even as inflation ended in the others. The regions that continue to inflate can be considered a separate, inflating universe. This process continues indefinitely, with inflating universes producing even more inflating universes, creating a multiverse of universes.</p>
<p>This phenomenon is dubbed “eternal inflation”. First described by physicists Paul Steinhardt and Alex Vilenkin in 1983, eternal inflation remained a curious artefact of cosmic inflation until the early 21st century, when it was <a href="https://member.ipmu.jp/yuji.tachikawa/stringsmirrors/2007/linde.pdf">combined with an idea</a> from string theory to produce a controversial yet compelling explanation of why our physical laws are what they are today.</p>
<p>String theory is not yet proven, but it is presently our best hope for a theory of everything – uniting quantum mechanics and gravity. However, physically realistic string theories <a href="https://theconversation.com/stephen-hawking-had-pinned-his-hopes-on-m-theory-to-fully-explain-the-universe-heres-what-it-is-93440">must possess ten or more dimensions</a> (rather than our normal three spatial dimensions plus time). Thus, to describe our present universe, six or more of these dimensions must be “compactified” – curled up in a such way that we can’t see them.</p>
<figure class="align-center ">
<img alt="Yellow hosepipe with water coming out on green grass." src="https://images.theconversation.com/files/516903/original/file-20230322-764-nf6b9f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/516903/original/file-20230322-764-nf6b9f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/516903/original/file-20230322-764-nf6b9f.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/516903/original/file-20230322-764-nf6b9f.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/516903/original/file-20230322-764-nf6b9f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/516903/original/file-20230322-764-nf6b9f.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/516903/original/file-20230322-764-nf6b9f.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A hosepipe is 3D but from a distance it can look like a one-dimensional line – with dimensions seemingly compactified.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/manchester-uk-july-2018-yellow-hosepipe-1132110485">RootsShoots/Shutterstock</a></span>
</figcaption>
</figure>
<p>The mathematical procedure for this is known. The problem (some might say the feature) of this process is that there are at least 10<sup>500</sup> ways to do this compactification – and this mind-bogglingly huge set of possibilities is called the “string landscape”. Each compactification will yield a different set of physical laws, potentially corresponding to a different universe. This begs two crucial questions: where are we in the string landscape, and why?</p>
<p>Eternal inflation provides an elegant answer to the first question: each inflating universe of the multiverse realises a different point in the string landscape, so all possible physical laws can exist somewhere in the multiverse. But why is our universe so great at producing intelligent life like us? Well, some universes should, statistically speaking, <a href="https://theconversation.com/great-mysteries-of-physics-2-is-the-universe-fine-tuned-for-life-201720">be like ours</a> – and we live in the universe in which our physical laws are the ones we observe. </p>
<p>However, this view is highly controversial – many argue it is not a scientific argument and it has spurred an intensive inquiry. </p>
<h2>Testability</h2>
<p>The obvious challenge with the multiverse is its observability. Suppose it does exist, is it then possible to observe the other universes, even in principle? For the quantum multiverse, the answer is no – different universes don’t communicate. But in the inflationary multiverse, the answer is “yes, if we are lucky”. </p>
<p>Since the different universes occupy the same physical space, neighbouring universes could in principle collide with each other, possibly leaving relics and imprints in our observable universe. A research collaboration led by Hiranya Peiris of University College London and Matthew Johnson of the <a href="https://perimeterinstitute.ca/who-we-are">Perimeter Institute</a> showed that such collisions <a href="https://journals.aps.org/prd/abstract/10.1103/PhysRevD.85.103502">should indeed leave imprints</a> on the cosmic microwave background (light left over from the Big Bang) that can be searched for – although so far, these signatures have not been found.</p>
<p>The next challenge is theoretical. Some theorists have suggested that most of the universes in the string landscape are actually mathematically inconsistent – unable to exist in the way our universe does. They instead <a href="https://www.scientificamerican.com/article/string-theory-may-create-far-fewer-universes-than-thought/">exist in a swampland</a> of solutions – and in particular, solutions of string theory which permit cosmic inflation seem to be difficult to find.</p>
<p>There is deep disagreement among string theorists and cosmologists on whether string theory can describe inflation, even in principle. This conundrum is both vexing and exciting – it suggests that one of the two ideas is wrong, either of which will lead to a revolution in theoretical physics.</p>
<p>Finally, the very premise of cosmic inflation is now being challenged. The raison d'etre of cosmic inflation is that, regardless of how the early universe looked, inflation would dynamically drive the cosmos to the smooth universe we see today. However, it has never been rigorously investigated whether cosmic inflation can actually begin in the first place. </p>
<p>This is because the equations describing the beginning of the process are too complicated to solve analytically. But this question is now being rigorously tested by several research groups around the world, including my own at King’s College London, where the power of modern high performance computing is brought to bear on solving these formerly intractable equations. So watch this space.</p><img src="https://counter.theconversation.com/content/201729/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Eugene Lim receives funding from STFC, FQXI and Leverhulme Trust.</span></em></p>From string theory to observations, the multiverse theory is far from safe.Eugene Lim, Professor in Theoretical Physics, King's College LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2022022023-03-22T12:06:32Z2023-03-22T12:06:32ZGreat Mysteries of Physics 3: is there a multiverse?<figure><img src="https://images.theconversation.com/files/516427/original/file-20230320-1591-qs31qo.jpg?ixlib=rb-1.1.0&rect=50%2C39%2C1301%2C714&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The light left over from the Big Bang, seen by the Planck satellite.</span> <span class="attribution"><span class="source">ESA/ LFI & HFI Consortia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><iframe src="https://embed.acast.com/638f4b009a65b10011b94c5e/6419b6379099ce0011fbbec6" frameborder="0" width="100%" height="190px"></iframe>
<p><iframe id="tc-infographic-807" class="tc-infographic" height="100px" src="https://cdn.theconversation.com/infographics/807/1668471fb1e76a459995c87bd439c36b04b754ac/site/index.html" width="100%" style="border: none" frameborder="0"></iframe></p>
<p>Interest in the multiverse theory, suggesting that our universe is just one of many, spiked following the release of the movie <a href="https://www.imdb.com/title/tt6710474/">Everything Everywhere All At Once</a>. The film follows Evelyn Wang on her journey to connect with versions of herself in parallel universes to ultimately stop the destruction of the multiverse.</p>
<p>The multiverse idea has long been an inspiration for science fiction writers. But does it have any basis in science? And if so, is it a concept we could ever test experimentally? </p>
<p>That’s the topic of the third episode of our podcast Great Mysteries of Physics – hosted by me, Miriam Frankel, science editor at The Conversation, and supported by FQxI, the Foundational Questions Institute.</p>
<p>“One way to think of a multiverse is just to say: ‘Well, the universe might be really, really big – much bigger than our observable universe – and so there could be other regions of the universe that are far beyond our horizon that have different things happening in them’,” explains Katie Mack, Hawking chair in cosmology and science communication at the Perimeter Institute for Theoretical Physics in Canada. “And I think that idea is totally well accepted in cosmology.”</p>
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<figcaption><span class="caption">Everything Everywhere All At Once’s rock scene.</span></figcaption>
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<p>The idea that there could be other parts of the cosmos with different physical laws, processes and histories is hard to ignore. And the concept is consistent with the theories of quantum mechanics, which governs the micro-world of atoms and particles, string theory (an attempt at a theory of everything) – and also with cosmic inflation, which says the infant universe blew up hugely in size during a brief growth spurt, and then continued to grow at a less frantic pace. These theories each give rise to their own version of the multiverse theory.</p>
<p>For Andrew Pontzen, a professor of cosmology at University College London in the UK, quantum mechanics is the best reason to believe in the multiverse. According to quantum mechanics, particles can be in a mix of different possible states, such as locations, which is known as a “superposition”. But when we measure them, the superposition breaks and each particle randomly “picks” one state.</p>
<p>So what happens to the other possible outcomes? “There’s a brilliant way of understanding this which is to imagine that, actually, the reality we experience is just one kind of facet of a much more complicated multiverse, where pretty much anything that can happen does happen and we just experience one version of events,” explains Pontzen. “Although it sounds crazy, it’s sort of the least crazy option for understanding how quantum mechanics can be right.”</p>
<p>Not all physicists are fans of the multiverse, though. Many argue that if it’s impossible to ever observe other universes, the multiverse can’t be a scientific theory. “I think it’s fine for entertainment,” says Sabine Hossenfelder, a research fellow at the Frankfurt Institute of Advanced Studies, who describes the multiverse as “ascientific”. “You sometimes hear people talk about some kind of mathematical evidence. [But] that’s just not a thing – evidence is something that you actually observe.”</p>
<p>There is currently no observational support for the multiverse theory. However, Mack doesn’t think that necessarily means it is unscientific. “I don’t think that hypothesising the existence of something unobservable is inherently unscientific,” she argues. “The wave function [in quantum mechanics] is unobservable. We have ways to infer the existence of the wave function because the maths all works perfectly. That we never directly observe it is a little beside the point, because it’s such a basic part of the science.”</p>
<p>Pontzen, though, is optimistic that we may one day be able to see signs of a collision with another universe in the cosmic microwave background, which is the light left over from the Big Bang. He is also working on a laboratory experiment trying to shed light on how a baby universe could actually physically be born from a multiverse. </p>
<p><em>You can listen to Great Mysteries of Physics via any of the apps listed above, our <a href="https://feeds.acast.com/public/shows/638f4b009a65b10011b94c5e">RSS feed</a>, or find out how else to listen here. You can also read a <a href="https://cdn.theconversation.com/static_files/files/2631/MoP__Ep3_-_Multiverse_TRANSCRIPT.docx_%281%29.pdf?1681213830">transcript of the episode here</a>.</em></p><img src="https://counter.theconversation.com/content/202202/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Pontzen has received funding from UK Research and Innovation Quantum Technologies for Fundamental Physics. Katie Mack and Sabine Hossenfelder have nothing to disclose.</span></em></p>Some physicists believe we could one day find evidence of other universes.Miriam Frankel, Podcast host, The ConversationLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1410862020-07-30T07:59:28Z2020-07-30T07:59:28ZIf our reality is a video game, does that solve the problem of evil?<figure><img src="https://images.theconversation.com/files/346114/original/file-20200707-194418-1334xzr.jpg?ixlib=rb-1.1.0&rect=477%2C98%2C4528%2C3721&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/artificial-intelligence-concept-virtual-human-avatar-1068287444">Shutterstock/kmls</a></span></figcaption></figure><p>Pandemics and natural disasters cause pain and suffering to millions worldwide and can challenge the very foundations of human belief systems. They can be particularly challenging for those who believe in an all-knowing and righteous God. The <a href="https://theconversation.com/is-god-good-in-the-shadow-of-mass-disaster-great-minds-have-argued-the-toss-137078">Lisbon earthquake</a> of 1755, for example, shook the previously unquestioned faith of many and led Voltaire to question whether this really could be the best of all possible worlds. </p>
<p>When the <a href="https://www.historyextra.com/period/20th-century/spanish-flu-the-virus-that-changed-the-world/">Spanish flu</a> struck in 1918, some chose to see it as divine punishment for the sins of mankind and looked to prayer, rather than science, for salvation. Notoriously, the Bishop of Zamora resisted calls from the Spanish authorities to close his churches and instead insisted on holding additional masses and processions.</p>
<p>From a theological standpoint, natural disasters and pandemics inevitably raise the profile of the long-standing and much-debated “problem of evil”. Here is philosopher <a href="https://en.wikipedia.org/wiki/Galen_Strawson">Galen Strawson’s</a> take on <a href="https://www.nybooks.com/articles/2012/12/06/what-can-be-proved-about-god/">the problem</a>:</p>
<blockquote>
<p>We can, for example, know with certainty that the Christian God does not exist as standardly defined: a being who is omniscient, omnipotent and wholly benevolent. The proof lies in the world, which is full of extraordinary suffering…belief in such a God, however rare, is profoundly immoral. It shows contempt for the reality of human suffering, or indeed any intense suffering.</p>
</blockquote>
<p>But suppose the person who was directly responsible for creating the world wasn’t God but some far lesser, far more fallible being. Someone more akin to an ordinary human engineer or scientist – or even a movie director or video-game designer. Let us further suppose that the diseases and disasters that can be found in the world are all the result of design choices, freely made by this non-divine designer of worlds. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/346304/original/file-20200708-3978-1bgx34v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/346304/original/file-20200708-3978-1bgx34v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=362&fit=crop&dpr=1 600w, https://images.theconversation.com/files/346304/original/file-20200708-3978-1bgx34v.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=362&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/346304/original/file-20200708-3978-1bgx34v.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=362&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/346304/original/file-20200708-3978-1bgx34v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=454&fit=crop&dpr=1 754w, https://images.theconversation.com/files/346304/original/file-20200708-3978-1bgx34v.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=454&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/346304/original/file-20200708-3978-1bgx34v.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=454&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The Creation of Adam - a reproduction from a section of Michelangelo’s fresco on the Sistine Chapel ceiling.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/creation-adam-digital-sketch-reproduction-section-1608382417">Shutterstock/FreedaMichaux</a></span>
</figcaption>
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<p>This may seem fantastically far fetched. But in the realm of physics just these kinds of scenarios are being played out as scientists work on the complex mathematics behind lab-created “pocket universes” and tech leaders, such as Elon Musk, explore the potential of <a href="https://www.wired.com/story/heres-how-elon-musk-plans-to-stitch-a-computer-into-your-brain/">brain-machine interfaces</a>.</p>
<p>It’s also important to appreciate that if this <em>were</em> the case then for many theists God could no longer be blamed for much of the suffering that exists in our world and the problem of evil would be very largely solved.</p>
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<img alt="" src="https://images.theconversation.com/files/288776/original/file-20190820-170910-8bv1s7.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/288776/original/file-20190820-170910-8bv1s7.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/288776/original/file-20190820-170910-8bv1s7.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/288776/original/file-20190820-170910-8bv1s7.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/288776/original/file-20190820-170910-8bv1s7.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/288776/original/file-20190820-170910-8bv1s7.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/288776/original/file-20190820-170910-8bv1s7.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><strong><em>This article is part of Conversation Insights</em></strong>
<br><em>The Insights team generates <a href="https://theconversation.com/uk/topics/insights-series-71218">long-form journalism</a> derived from interdisciplinary research. The team is working with academics from different backgrounds who have been engaged in projects aimed at tackling societal and scientific challenges.</em> </p>
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<p>Why? Because for theists human beings are creatures of a very special sort: thanks to our God-given free will we have the ability to <em>choose</em> whether we act well or badly. And, generally speaking, God does not interfere with these choices or their consequences. If a free agent acts appallingly (committing murder, rape or genocide) the resulting “moral evil” is to be greatly regretted, but God should not be blamed. The fault lies entirely with the person who freely chose to act in this way. </p>
<h2>Morality and natural evils</h2>
<p>Morality and free will are deeply intertwined. If someone does something very wrong, they aren’t morally at fault if they only acted in that way because they were hypnotised or brainwashed. Similarly, if someone performs a good act (giving food to a starving child, say) but only did so because a gun was pointed at their heads, they are not morally praiseworthy. </p>
<p>Most religious believers hold that humans have the capacity to make free choices. They also believe that anyone who chooses to do the right things can expect to be rewarded by God, whereas those who act wrongly can expect to be punished. For this to be possible God has to not only provide us with free will, he also has to allow us to carry out those actions we freely choose to perform – the bad ones included. </p>
<p>This “free will solution” to the problem of evil has been a mainstay of theology since it was elaborated by <a href="http://sites.nd.edu/ujournal/files/2014/07/Peterson_05-06.pdf">St Augustine</a> more than 1,500 years ago. From the theological perspective, the so-called “natural evils” pose a far more intractable problem. These include all the vast amounts of suffering caused by diseases, earthquakes and floods along with the agonies suffered by animals. As normally construed, these sources of suffering are <em>not</em> moral evils, since they are not the result of freely chosen human actions. </p>
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<img alt="" src="https://images.theconversation.com/files/344076/original/file-20200625-33569-1tbdbps.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/344076/original/file-20200625-33569-1tbdbps.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/344076/original/file-20200625-33569-1tbdbps.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/344076/original/file-20200625-33569-1tbdbps.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/344076/original/file-20200625-33569-1tbdbps.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/344076/original/file-20200625-33569-1tbdbps.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/344076/original/file-20200625-33569-1tbdbps.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Apocalyptic vision of giant tsunami waves crashing small coastal town.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/apocalyptic-dramatic-background-giant-tsunami-waves-154310390">Shutterstock/IgorZh</a></span>
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<p>Hence the problem posed by such evils for anyone who believes that God created our world. Couldn’t a creator that is truly all-powerful, all-knowing and good have made a much better job of it? In fact, wouldn’t it have been quite easy for God to ensure that the world contains far fewer natural evils? A few tweaks to human DNA would provide <a href="https://www.medicalnewstoday.com/articles/325178">immunity to cancer</a>. A slightly different tweak would provide immunity to viruses. When designing the animals an all-powerful God would not need to rely on the incredibly slow and imperfect method of evolution by natural selection – a process which inevitably results in vast amounts of <a href="https://imagejournal.org/article/darwin-and-the-problem-of-time/">pain and suffering</a>.</p>
<p>On the other hand, if the maker of our world was not all-powerful, or all-knowing, or as good as it’s possible to be, then it’s not surprising to find ourselves living in the sort of world we do. </p>
<h2>Alternate realities and bubbles</h2>
<p>As for why we should take seriously the idea that there can be makers of worlds who are less than divine, there is no shortage of relevant scenarios to be found in science, science fiction and philosophy.</p>
<p>Among the obstacles that <a href="https://home.cern/about">Cern</a> had to overcome when constructing the Large Hadron Collider (the very large and powerful machine which discovered the <a href="https://theconversation.com/could-the-higgs-nobel-be-the-end-of-particle-physics-18978">Higgs boson</a> in 2012) was persuading a worried public that running the collider would not create a <a href="https://home.cern/resources/faqs/will-cern-generate-black-hole">mini-black hole</a> that would escape the confines of the lab and go on to consume the entire planet. Although there was no real danger of this happening, such worries were by no means entirely groundless. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/346310/original/file-20200708-47-hnx79q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/346310/original/file-20200708-47-hnx79q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=286&fit=crop&dpr=1 600w, https://images.theconversation.com/files/346310/original/file-20200708-47-hnx79q.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=286&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/346310/original/file-20200708-47-hnx79q.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=286&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/346310/original/file-20200708-47-hnx79q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=359&fit=crop&dpr=1 754w, https://images.theconversation.com/files/346310/original/file-20200708-47-hnx79q.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=359&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/346310/original/file-20200708-47-hnx79q.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=359&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The Higgs boson was detected in 2012 in the experiments, conducted with the Large Hadron Collider at CERN.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/cern-european-organization-nuclear-research-where-1287557641">Shutterstock/DVISIONS</a></span>
</figcaption>
</figure>
<p>As long ago as the 1980s and 1990s, <a href="https://ocw.mit.edu/courses/physics/8-286-the-early-universe-fall-2013/video-lectures/lecture-1-inflationary-cosmology-is-our-universe-part-of-a-multiverse/">Alan Guth</a> and <a href="https://slate.com/culture/2004/05/the-creation-of-the-universe.html">Andrei Linde</a> (respected physicists and pioneers of the now widely accepted <a href="http://www.ctc.cam.ac.uk/outreach/origins/inflation_zero.php">inflationary cosmology</a>) raised the possibility that scientists might soon be able to create “bubble” or “pocket” universes in a laboratory. Initially sub-microscopic, the newly created bubble universe rapidly expands and soon constitutes a full-scale cosmos in its own right. These new universes create their own space and time as they grow, so they take up no room at all in our world and pose no threat to us.</p>
<p>The energy driving the expansion of the envisaged pocket universes derives from the same <a href="http://nautil.us/issue/48/chaos/the-inflated-debate-over-cosmic-inflation">inflationary field</a> that cosmologists believe was responsible for an explosive expansion in our own universe that took place shortly after the big bang. During this brief period the scale of the universe’s expansion was enormous, it got trillions of times bigger in little more than an instant. But since the negative energy perfectly cancels the positive energy of the matter being created, no energy conservation laws are infringed. As Guth is fond of remarking, the universe is the ultimate free lunch.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-how-does-an-experiment-at-the-large-hadron-collider-work-42846">Explainer: how does an experiment at the Large Hadron Collider work?</a>
</strong>
</em>
</p>
<hr>
<p>Various methods for creating universes in labs have since been proposed, including compressing a few grams of ordinary matter into a <a href="https://www.nytimes.com/1987/04/14/science/physicist-aims-to-create-a-universe-literally.html">very small volume</a> to create small black holes and deploying <a href="https://arxiv.org/pdf/gr-qc/0602084.pdf">stable magnetic monopoles</a> to create exotic spacetime structures. Precisely controlling the physical laws that govern the worlds created by these methods will not be easy. But physicists have <a href="http://articles.adsabs.harvard.edu//full/1995QJRAS..36..193H/0000193.000.html">not ruled out the possibility</a> of fine tuning their basic physical constants to render them more capable of sustaining the complex structures needed for life.</p>
<p>Even if creating such universes requires knowledge and technology that we do not currently possess, a scientifically more advanced civilisation could easily possess what is required. Hence Linde’s playful quip: “Does this mean that our universe was created, not by a divine design, but by a physicist hacker?”</p>
<h2>The simulation argument</h2>
<p>This is one potential route to creating an entire world. But there are other possibilities, too. Perhaps in reality humans are all characters living inside something akin to a vast multi-player online video game, running on a <a href="https://www.youtube.com/watch?v=tlTKTTt47WE">super-powerful computer</a>. </p>
<p>By the 1980s and 90s science fiction writers such as <a href="https://www.theguardian.com/books/2010/oct/23/surface-detail-iain-banks-review">Iain M Banks</a> <a href="https://en.wikipedia.org/wiki/Eternity_(novel)">Greg Bear</a> and <a href="https://en.wikipedia.org/wiki/Diaspora_(novel)">Greg Egan</a> had started to explore the fictional possibilities of wholly computer-generated virtual realities in impressive depth and detail. The inhabitants of these worlds might seem to have ordinary physical bodies and brains, but like everything else in these worlds, their bodies and brains were virtual rather than physical, existing only as data flowing through a computer’s innards. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/wOi9i1vqJ60?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>The 1982 Disney production <a href="https://www.imdb.com/title/tt0084827/">TRON</a> was an early movie depiction of this sort of wholly computer-generated virtual world. The human protagonists are converted into data (or “digitised”) by a specially adapted laser beam, which allows them to embark on adventures in a digital virtual reality. The movie’s ground-breaking computer-generated imagery may be unremarkable by contemporary standards, but they are vastly more sophisticated than those found in the early video game <a href="https://www.youtube.com/watch?v=e4VRgY3tkh0">PONG</a>, one of the main <a href="https://www.nytimes.com/1982/07/04/movies/special-effects-are-revolutionizing-film.html?pagewanted=2">inspirations for the movie</a>.</p>
<p>In 2003 the philosopher Nick Bostrom published his much-discussed “simulation argument”, the upshot of which is that not only are TRON-style virtual worlds perfectly possible, there is a significant probability that <a href="https://www.simulation-argument.com">we are living in one</a>. Bostrom’s initially surprising conclusion is based on some by no means implausible assumptions regarding the computational capacity that future computers are likely to possess (<a href="https://www.nature.com/articles/35023282">astonishingly vast</a>, it turns out). </p>
<p>If we do exist inside a computer simulation, then since we are all conscious (at least while we’re awake) it must be possible for a computer to generate the kinds of experiences we are enjoying right now. If consciousness required a biological brain, Bostrom’s simulation scenario wouldn’t get off the ground. But science fiction writers were not the only people to be impressed by the arrival of computers. </p>
<p>In the 1970s and 80s increasing numbers of philosophers came round to the view that conscious mentality is not essentially biological in character. Slogans such as, “mind is related to brain as software is related to hardware” seemed very plausible, not only to philosophers but to psychologists and neuroscientists too. If mentality is essentially a matter of information flow (as the <a href="https://plato.stanford.edu/entries/computational-mind/#Plu">computer analogy</a> suggested) then anything could possess a mind provided it processes information in the right sorts of ways. And computers seemed at least as well suited to this task as a biological brain.</p>
<p>Less radical forms of virtual worlds are also possible and the <a href="https://www.youtube.com/watch?v=B70Hapf_okE">Matrix movies</a> provide a well-known example. In this scenario most humans find themselves living somewhere that seems similar to contemporary Earth. In reality, their entire environment is, in effect, a communal mass hallucination – a wholly virtual world produced by a powerful computer hooked into people’s brains via a neural interface. But it doesn’t seem like that: the virtual world seems just as real as our world. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/gCZBY7a8kqE?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>Smaller scale variants of this scenario are also possible. Instead of an entire planetary population being simultaneously plugged into the same virtual world, just a few people are. Perhaps you are a 22nd-century schoolchild, enjoying a virtual lesson supplied via a tiny but highly sophisticated neural interface, spending a bit of time learning what it was like to be an early 21st-century person leading a perfectly ordinary life. In an hour or so your lesson will finish and your version of the 21st century will come to an end. </p>
<h2>A video game? Seriously?</h2>
<p>A Matrix-style brain-computer interface is capable of controlling every aspect of a subject’s sensory consciousness down to the smallest detail. If it weren’t, it wouldn’t be able to supply a completely lifelike total virtual reality experience, involving vision, hearing, smell, taste and touch. Society does not possess anything close to this kind of technology at present. But there is every reason to believe it is possible, in principle, and rapid advances are already being made. </p>
<p>The Pentagon’s Defense Advanced Research Projects Agency (Darpa) made <a href="https://www.wired.com/2015/03/woman-controls-fighter-jet-sim-using-mind/">headlines</a> in 2017 when one of its neural interfaces allowed a paralysed woman to control a jet plane in a flight simulator. More recently, Elon Musk’s Neuralink start-up announced that it had designed a <a href="https://www.documentcloud.org/documents/6204648-Neuralink-White-Paper.html">neurosurgical robot</a> capable of inserting 192 electrodes a minute into a rat’s brain without triggering bleeding and experiments involving humans are expected to begin soon.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/elon-musk-says-were-probably-living-in-a-computer-simulation-heres-the-science-60821">Elon Musk says we're probably living in a computer simulation – here's the science</a>
</strong>
</em>
</p>
<hr>
<p>The science and technology needed to undertake this kind of world-making will be more advanced than anything we possess at present, but not by enormous or inconceivable margins. These are technologies we might reasonably expect to develop within a century or so – perhaps sooner.</p>
<p>In any event, the capabilities of these world-makers evidently fall far short of the capabilities of the omniscient, omnipotent and wholly benevolent God of traditional theism. Given the world’s many and varied imperfections, if there is a creator at all, doesn’t it seem more reasonable to suppose that it is of the non-divine variety? Someone more akin to the physicist hacker envisaged by Linde, or the virtual-reality programmers envisaged by Bostrom?</p>
<p>Adopting this hypothesis does not mean the theistic God is entirely redundant – far from it. Theists can still be confident that God is the ultimate creative force in the cosmos. Maybe it was God who brought the primordial cosmos into existence and furnished it with natural laws that allowed its less-than-divine inhabitants to develop the capability of acting as world-makers in their own right, with all the <a href="https://reducing-suffering.org/lab-universes-creating-infinite-suffering/">moral responsibilities</a> this brings. Although there is (at present) no way for us to find out what this divinely created world was like, we can be certain of one thing: being far better designed, it contains far fewer natural evils than can be found in this world, and so far less death and suffering.</p>
<p>But would a benevolent God allow less-than-divine people to create their own worlds? There is at least one compelling reason to think they would. As recent history has shown (think of the suffering resulting from the actions of Hitler, Stalin or Mao) God grants people a great deal of leeway when it comes to making choices that have horrendous consequences for untold millions of innocent men, women and children.</p>
<p>The problem of evil has bedevilled monotheistic religions ever since their inception, and the idea of extending the free-will solution to encompass natural evil has always been available. But until very recently, the idea that anything other than a being possessing supernatural powers could create a world such as ours was almost impossible to take seriously. This is no longer the case. </p>
<hr>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/313478/original/file-20200204-41481-1n8vco4.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/313478/original/file-20200204-41481-1n8vco4.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=112&fit=crop&dpr=1 600w, https://images.theconversation.com/files/313478/original/file-20200204-41481-1n8vco4.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=112&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/313478/original/file-20200204-41481-1n8vco4.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=112&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/313478/original/file-20200204-41481-1n8vco4.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=140&fit=crop&dpr=1 754w, https://images.theconversation.com/files/313478/original/file-20200204-41481-1n8vco4.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=140&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/313478/original/file-20200204-41481-1n8vco4.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=140&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption"></span>
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<p><em>For you: more from our <a href="https://theconversation.com/uk/topics/insights-series-71218?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=InsightsUK">Insights series</a>:</em></p>
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<li><p><em><a href="https://theconversation.com/the-world-needs-pharmaceuticals-from-china-and-india-to-beat-coronavirus-138388?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=InsightsUK">The world needs pharmaceuticals from China and India to beat coronavirus
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<p class="fine-print"><em><span>Barry Dainton does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>What if the being responsible for creating our world wasn’t God, but some far lesser, far more fallible being like a scientist or video game designer?Barry Dainton, Professor of Philosophy, University of LiverpoolLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1368302020-04-21T03:30:50Z2020-04-21T03:30:50ZA new kind of physics? Stephen Wolfram has a radical plan to build the universe from dots and lines<figure><img src="https://images.theconversation.com/files/329283/original/file-20200420-152607-1c2b25g.png?ixlib=rb-1.1.0&rect=922%2C0%2C3047%2C1928&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://wolframphysics.org/visual-gallery/">Wolfram Physics Project</a></span></figcaption></figure><p>Stephen Wolfram is a cult figure in programming and mathematics. He is the brains behind <a href="https://www.wolframalpha.com">Wolfram Alpha</a>, a website that tries to answer questions by using algorithms to sift through a massive database of information. He is also responsible for <a href="https://www.wolfram.com/mathematica/">Mathematica</a>, a computer
system used by scientists the world over.</p>
<p>Last week, Wolfram launched a new venture: the <a href="https://wolframphysics.org">Wolfram Physics Project</a>, an ambitious attempt to develop a new physics of our universe. The new physics, he <a href="https://writings.stephenwolfram.com/2020/04/finally-we-may-have-a-path-to-the-fundamental-theory-of-physics-and-its-beautiful/">declares</a>, is computational. The guiding idea is that everything can be boiled down to the application of simple rules to fundamental building blocks.</p>
<h2>What’s the point of the ‘new physics’?</h2>
<p>Why do we need such a theory? After all, we already have two extraordinarily successful physical theories. These are general relativity – a theory of gravity and the large-scale structure of the universe – and quantum mechanics – a theory of the basic constituents of matter, sub-atomic particles, and their interactions. Haven’t we got physics licked?</p>
<p>Not quite. While we have an excellent theory of how gravity works for large objects, such as stars and planets and even people, we don’t understand gravity at extremely high energies or for extremely small things. </p>
<p>General relativity “breaks down” when we try to extend it into the miniature realm where quantum mechanics rules. This has led to a quest for the holy grail of physics: a theory of quantum gravity, which would combine what we know from general relativity with what we know from quantum mechanics to produce an entirely new physical theory.</p>
<p>The current best approach we have to quantum gravity is <a href="https://theconversation.com/explainer-string-theory-2983">string theory</a>. This theory has been a work in progress for 50 years or so, and while it has achieved some success there is a growing dissatisfaction with it as an approach. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-string-theory-2983">Explainer: String theory</a>
</strong>
</em>
</p>
<hr>
<h2>How is Wolfram’s approach different?</h2>
<p>Wolfram is attempting to provide an alternative to string theory. He does so via a branch of mathematics called graph theory, which studies groups of points or nodes connected by lines or edges. </p>
<p>Think of a social networking platform. Start with one person: Betty. Next, add a simple rule: every person adds three friends. Apply the rule to Betty: now she has three friends. Apply the rule again to every person (including the one you started with, namely: Betty). Keep applying the rule and, pretty soon, the network of friends forms a complex graph.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/329330/original/file-20200421-126552-184ej9w.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/329330/original/file-20200421-126552-184ej9w.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=310&fit=crop&dpr=1 600w, https://images.theconversation.com/files/329330/original/file-20200421-126552-184ej9w.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=310&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/329330/original/file-20200421-126552-184ej9w.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=310&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/329330/original/file-20200421-126552-184ej9w.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=389&fit=crop&dpr=1 754w, https://images.theconversation.com/files/329330/original/file-20200421-126552-184ej9w.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=389&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/329330/original/file-20200421-126552-184ej9w.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=389&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">In Wolfram’s theory, applying a simple rule multiple times creates a complex network of points and connections.</span>
<span class="attribution"><span class="source">Samuel Baron</span></span>
</figcaption>
</figure>
<p>Wolfram’s proposal is that the universe can be modelled in much the same way. The goal of physics, he suggests, is to work out the rules that the universal graph obeys. </p>
<p>Key to his suggestion is that a suitably complicated graph looks like a geometry. For instance, imagine a cube and a graph that resembles it. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/329332/original/file-20200421-126563-12dps5e.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/329332/original/file-20200421-126563-12dps5e.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=259&fit=crop&dpr=1 600w, https://images.theconversation.com/files/329332/original/file-20200421-126563-12dps5e.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=259&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/329332/original/file-20200421-126563-12dps5e.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=259&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/329332/original/file-20200421-126563-12dps5e.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=326&fit=crop&dpr=1 754w, https://images.theconversation.com/files/329332/original/file-20200421-126563-12dps5e.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=326&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/329332/original/file-20200421-126563-12dps5e.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=326&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">In the same way that a collection of points and lines can approximate a solid cube, Wolfram argues that space itself may be a mesh that knits together a series of nodes.</span>
<span class="attribution"><span class="source">Samuel Baron</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>Wolfram argues that extremely complex graphs resemble surfaces and volumes: add enough nodes and connect them with enough lines and you form a kind of mesh. He maintains that space itself can be thought of as a mesh that knits together a series of nodes in this fashion.</p>
<h2>What does this have to do with physics?</h2>
<p>How can complicated meshes of nodes help with the project of reconciling general relativity and quantum mechanics? Well, quantum theory deals with discrete objects with discrete properties. General relativity, on the other hand, treats the universe as a continuum and gravity as a continuous force. </p>
<p>If we can build a theory that can do what general relativity does but that starts from discrete structures like graphs, then the prospects for reconciling general relativity and quantum mechanics start to look more promising. If we can build a geometry that resembles the one given to us by general relativity using a discrete structure, then the prospects look even better.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/329284/original/file-20200420-51952-4a40vm.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/329284/original/file-20200420-51952-4a40vm.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/329284/original/file-20200420-51952-4a40vm.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=260&fit=crop&dpr=1 600w, https://images.theconversation.com/files/329284/original/file-20200420-51952-4a40vm.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=260&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/329284/original/file-20200420-51952-4a40vm.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=260&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/329284/original/file-20200420-51952-4a40vm.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=326&fit=crop&dpr=1 754w, https://images.theconversation.com/files/329284/original/file-20200420-51952-4a40vm.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=326&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/329284/original/file-20200420-51952-4a40vm.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=326&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Stephen Wolfram believes that space itself may be a complex mesh of points connected together by means of a simple rule that is iterated many times.</span>
<span class="attribution"><a class="source" href="https://wolframphysics.org/visual-gallery/">Wolfram Physics Project</a></span>
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<h2>So is it time to get excited?</h2>
<p>While Wolfram’s project is promising, it does contain more than a hint of hubris. Wolfram is going up against the Einsteins and Hawkings of the world, and he’s doing it without a life spent publishing in physics journals. (He did publish several physics papers as a teenage prodigy, but that was 40 years ago, as well as a book <a href="https://writings.stephenwolfram.com/2017/05/a-new-kind-of-science-a-15-year-view/">A New Kind of Science</a>, which is the spiritual predecessor of the Wolfram Physics Project.)</p>
<p>Moreover, his approach is not wholly original. It is similar to two existing approaches to quantum gravity: causal set theory and loop quantum gravity, neither of which get much of a mention in Wolfram’s grand designs.</p>
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Read more:
<a href="https://theconversation.com/einstein-to-weinstein-the-lone-genius-is-an-exception-to-the-rule-14998">Einstein to Weinstein: the lone genius is an exception to the rule</a>
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<p>Nonetheless, the project is notable for three reasons. First, Wolfram has a broad audience and he will do a lot to popularise the approach that he advocates. Proponents of loop quantum gravity in particular lament the predominance of string theory within the physics community. Wolfram may help to underwrite a paradigm shift in physics. </p>
<p>Second, Wolfram provides a very careful <a href="https://wolframphysics.org/technical-introduction/">overview</a> of the project from the basic principles of graph theory up to general relativity. This will make it easier for individuals to get up to speed with the general approach and potentially make contributions of their own. </p>
<p>Third, the project is “open source”, inviting contributions from citizen scientists. If nothing else, this gives us all something to do at the moment – in between baking sourdough and playing Animal Crossing, that is.</p><img src="https://counter.theconversation.com/content/136830/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Samuel Baron receives funding from the Australian Research Council (Discovery Early Career Researcher Award: DE180100414).</span></em></p>Mathematical maverick Stephen Wolfram’s latest ambitious project calls on members of the public to help him find the rules that control the world.Sam Baron, Associate professor, Australian Catholic UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1014982018-08-15T10:17:18Z2018-08-15T10:17:18ZWhat is nothing? Martin Rees Q&A<figure><img src="https://images.theconversation.com/files/231777/original/file-20180813-2918-1k7lp6o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Galaxy history revealed by the Hubble Space Telescope.</span> <span class="attribution"><span class="source">NASA</span></span></figcaption></figure><p><em>Philosophers have debated the nature of “nothing” <a href="https://en.wikipedia.org/wiki/The_Void_(philosophy)">for thousands of years</a>, but what has modern science got to say about it? In an interview with The Conversation, Martin Rees, Astronomer Royal and Emeritus Professor of Cosmology and Astrophysics at the University of Cambridge, explains that when physicists talk about nothing, they mean empty space (vacuum). This may sound straightforward, but experiments show that empty space isn’t really empty – there’s a mysterious energy latent in it which can tell us something about the fate of the universe.</em></p>
<p><em>Rees was interviewed for The Conversation’s Anthill podcast <a href="https://theconversation.com/anthill-28-on-nothing-101622">on Nothing</a>. This Q&A is based on an edited transcript of that interview.</em></p>
<p><strong>Q: Is empty space really the same as nothing?</strong></p>
<p>A: Empty space seems to be nothing to us. By analogy, water may seem to be nothing to a fish – it’s what’s left when you take away all the other things floating in the sea. Likewise, empty space is conjectured to be quite complicated. </p>
<p>We know that the universe is very empty. The average density of space is about one atom in every ten cubic metres – far more rarefied than any vacuum we can achieve on Earth. But even if you take all the matter away, space has a kind of elasticity which (as was recently confirmed) allows <a href="https://theconversation.com/explainer-what-are-gravitational-waves-53239">gravitational waves</a> – ripples in space itself – to propagate through it. Moreover, we’ve learned that there is an exotic kind of energy in empty space itself.</p>
<p><strong>Q: We first learned about this vacuum energy in the 20th century with the rise of quantum mechanics, which governs the tiny world of atoms and particles. It suggests that empty space is made up of a field of fluctuating background energy – giving rise to waves and virtual particles that pop into and out of existence. They can even create a tiny force. But what about empty space on large scales?</strong></p>
<p>A: The fact that empty space exerts a large-scale force was discovered 20 years ago. Astronomers found that the expansion of the universe was accelerating. This was a surprise. The expansion had been known for more than 50 years, but everyone expected that it would be slowing down because of the gravitational pull that galaxies and other structures exert on each other. It was therefore a big surprise to find that this deceleration due to gravity was overwhelmed by something “pushing” the expansion. There is, as it were, energy latent in empty space itself, which causes a sort of repulsion which outweighs the attraction of gravity on these large scales. This phenomenon – dubbed <a href="https://theconversation.com/the-experiments-trying-to-crack-physics-biggest-question-what-is-dark-energy-52917">dark energy</a> – is the most dramatic manifestation of the fact that empty space is not featureless and irrelevant. Indeed it determines our universe’s long term fate.</p>
<p><strong>Q: But is there a limit to what we can know? At a scale of a trillion trillion times smaller than an atom, quantum fluctuations in spacetime can give rise to not just virtual particles, but to virtual black holes. This is a range that we cannot observe, and where we have to combine theories of gravity with quantum mechanics to probe what happens theoretically – something that’s notoriously difficult to do.</strong> </p>
<p>A: There are several theories that aim to understand this, the most famous being <a href="https://theconversation.com/stephen-hawking-had-pinned-his-hopes-on-m-theory-to-fully-explain-the-universe-heres-what-it-is-93440">string theory</a>. But none of these theories have yet engaged with the real world – so they are still untested speculation. But I think nearly everyone accepts that space itself could have a complicated structure on this tiny, tiny scale where gravitational and quantum effects meet. </p>
<p>We know that our universe has three dimensions in space: you can go left and right, backwards and forwards, up and down. Time is like a fourth dimension. But it’s a strong suspicion that if you were to magnify a little point in space so that you were probing this tiny, tiny scale … you would find that it is a tightly wound origami in about five extra dimensions that we don’t see. It’s rather as when you look at a hosepipe from a long way away, you think it is just a line. But when you look closer, you see that one dimension was in fact three dimensions. String theory involves complex mathematics – so do the rival theories. But that’s the kind of theory we’re going to need if we are to understand at the deepest level the nearest to nothingness that we can imagine: namely empty space.</p>
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Read more:
<a href="https://theconversation.com/aliens-very-strange-universes-and-brexit-martin-rees-qanda-75277">Aliens, very strange universes and Brexit – Martin Rees Q&A</a>
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<p><strong>Q: Within our current understanding, how can we explain our entire universe expanding from nothing? Could it really just start off from a bit of fluctuating vacuum energy?</strong></p>
<p>A: Some mysterious transition or fluctuation could have suddenly triggered a part of space to expand – at least that’s what some theorists think. The fluctuations intrinsic to quantum theory would be able to shake the entire universe if it were squeezed to a sufficiently tiny scale. That would happen at a time of about 10<sup>-44</sup> seconds – what’s called the <a href="https://www.universetoday.com/79418/planck-time/">Planck time</a>. That’s a scale when time and space are intertwined so that the idea of a clock ticking away makes no sense. We can extrapolate our universe with high confidence back to a nanosecond, and with some confidence right back much closer to the Planck time. But thereafter, all bets are off because … physics on this scale has to be superseded by some grand, more complicated theory.</p>
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<img alt="" src="https://images.theconversation.com/files/231778/original/file-20180813-2897-a9vf2f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/231778/original/file-20180813-2897-a9vf2f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=311&fit=crop&dpr=1 600w, https://images.theconversation.com/files/231778/original/file-20180813-2897-a9vf2f.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=311&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/231778/original/file-20180813-2897-a9vf2f.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=311&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/231778/original/file-20180813-2897-a9vf2f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=390&fit=crop&dpr=1 754w, https://images.theconversation.com/files/231778/original/file-20180813-2897-a9vf2f.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=390&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/231778/original/file-20180813-2897-a9vf2f.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=390&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><strong>Q: If it is possible that a fluctuation of some random part of empty space gave rise to the universe, why couldn’t exactly the same thing happen in another part of empty space – giving birth to parallel universes in an infinite multiverse?</strong></p>
<p>A: The idea that our Big Bang <a href="https://theconversation.com/the-theory-of-parallel-universes-is-not-just-maths-it-is-science-that-can-be-tested-46497">is not the only one</a> and that what we see with our telescopes is a tiny fraction of physical reality is popular among many physicists. And there are many versions of a cyclic universe. It was only 50 years ago that strong evidence for a Big Bang first emerged. But there have ever since been speculations about whether this is just an episode in a cyclic universe. And there’s been growing traction for the concept that there’s far more to physical reality than the volume of space and time that we can probe – even with the most powerful telescopes.</p>
<p>So we’ve no idea whether there was one Big Bang or many – there are scenarios which predict many Big Bangs and some which predict one. I think we should explore them all.</p>
<p><strong>Q: How will the universe end?</strong></p>
<p>A: The most straightforward long range forecast predicts that the universe goes on expanding at an accelerating rate, becomes ever emptier and ever colder. The particles in it may decay, making the dilution proceed indefinitely. We would end up with, in a sense, a huge volume of space, but it would be even emptier than space is now. That is one scenario, but <a href="https://theconversation.com/the-fate-of-the-universe-heat-death-big-rip-or-cosmic-consciousness-46157">there are others</a> that involve the “direction” of dark energy reversing from repulsion to attraction, so that there will be a collapse to a so-called “<a href="https://www.universetoday.com/37018/big-crunch/">Big Crunch</a>”, when the density heads towards infinity again. </p>
<p>There’s also an idea, due to physicist <a href="https://www.maths.ox.ac.uk/people/roger.penrose">Roger Penrose</a>, that the universe goes on expanding, becoming ever more dilute, but somehow – when it’s got nothing in it apart from the photons, particles of light – things can be “re-scaled”, so that after this huge dilution, space becomes in a sense the generator of some new Big Bang. So that’s a rather exotic version of the old cyclic universe – but please don’t ask me to explain Penrose’s ideas.</p>
<p><strong>Q: How confident are you that science can ultimately crack what nothing is? Even if we could prove that our universe started from some strange fluctuation of a vacuum field, don’t we have to ask where that vacuum field came from?</strong></p>
<p>A: Sciences try to answer questions, but every time we answer them, new ones come into focus – we’ll never have a complete picture. When I was starting research in the late 1960s, it was controversial whether there had been a Big Bang at all. Now that’s no longer controversial, and we can say with about 2% precision what the universe was like all the way back from the present 13.8 billion years to a nanosecond. That is huge progress. So it’s not absurdly optimistic to believe that in the next 50 years, the challenging issues about what happens at the quantum or <a href="http://www.ctc.cam.ac.uk/outreach/origins/inflation_zero.php">“inflationary” eras</a> will be understood. </p>
<p>But of course this raises another question: how much of science is going to be accessible to the human brain? It could turn out, for instance, that the mathematics of string theory is in some sense a correct description of reality, but that we will never be able to understand it well enough to check it against any genuine observation. Then we may have to await the emergence of some kind of post-humans to get a fuller understanding. </p>
<p>But everyone who ponders these mysteries should realise that the physicist’s empty space – vacuum – is not the same as the philosopher’s “nothing”.</p><img src="https://counter.theconversation.com/content/101498/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Martin Rees does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>From a mysterious energy of empty space to parallel universes, cosmology’s view of ‘nothing’ is anything but boring.Martin Rees, Emeritus Professor of Cosmology and Astrophysics, University of CambridgeLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/935082018-03-16T13:23:05Z2018-03-16T13:23:05ZI was a student of Stephen Hawking’s – here’s what he taught me<figure><img src="https://images.theconversation.com/files/210778/original/file-20180316-104663-kdnb6m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Hawking at the University of Cambridge.</span> <span class="attribution"><span class="source">Lwp Kommunikáció/Flickr</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Like many students of my generation, Stephen Hawking had already had enormous influence on me long before we ever met. When I was hesitating about my A-level choices, it was his book <a href="https://en.wikipedia.org/wiki/A_Brief_History_of_Time">A Brief History of Time</a> that convinced me to continue with physical sciences. In 1994, Hawking and mathematical physicist <a href="https://www.maths.ox.ac.uk/people/roger.penrose">Roger Penrose</a> gave a series of inspiring lectures about cosmology in Cambridge. As a direct result, I chose courses on black holes and relativity for my fourth year of study at the University of Cambridge. </p>
<p>I first saw Hawking when I was an undergraduate. At that time he was living in an apartment building just behind my student house. He was already so famous that friends would come to my room just to watch him leaving and entering his apartment. But as an undergraduate I never tried to talk with him, feeling much too junior and intimidated. </p>
<p>After I finished my fourth year, I was invited in to talk to Hawking, who was already using a speech synthesizer, about options for my PhD. I was quite nervous when I first met him, but he jumped straight into physics and soon we were discussing black holes. I became a student at the time of the “<a href="http://discovermagazine.com/2008/dec/13-the-man-who-led-the-second-superstring-revolution">Second String Theory Revolution</a>” in theoretical physics. Hawking had not worked actively in string theory, but he was very keen to understand the new ideas. </p>
<p>Following that meeting, he sent me off to read all the papers that <a href="https://www.sns.ias.edu/witten">Edward Witten</a>, a famous string theorist, had written that year. My task was to come back and summarise them for him – the student teaching the master. It’s difficult to describe how hard this task actually was: Hawking expected me to jump straight to the frontier of string theory as a starting graduate student. He also chose the title for my PhD thesis: “Problems in M-theory”, which I worked on from 1995 to 1998.</p>
<p>I can only hope that my explanations of string theory were helpful. Hawking went back and forth on his views on M-theory, but eventually ended up thinking that it may be <a href="https://theconversation.com/stephen-hawking-had-pinned-his-hopes-on-m-theory-to-fully-explain-the-universe-heres-what-it-is-93440">our best bet</a> for a theory of everything.</p>
<h2>No hand-holding</h2>
<p>PhD students were enormously important to Hawking. In the early phase of his illness, his students helped take care of him. By the time I became his student he needed round-the-clock nursing. At this point, his students were no longer involved in his physical care, but remained essential to his research. Theoretical physics begins with ideas and concepts, but these then evolve into explicit detailed calculations. Hawking had a remarkable ability to do complex calculations in his head, but he still relied on collaborators to develop and complete his research projects.</p>
<p>Theoretical physicists typically give early PhD students “safe” research projects, and guide them through the calculations required. As the students develop, the projects become more ambitious and risky and students are expected to work independently. However, PhD students working with Hawking did not have the luxury of this gentle introduction – he needed us to work on his own high-risk, high-gain projects. </p>
<p>Hawking’s communication via his speech synthesizer was necessarily concise and he simply could not provide detailed guidance about calculations, making it extremely challenging to work with him. But it was also stimulating, forcing students to be creative and independent. He did give praise when he thought it was due. He once sent me away with a very hard problem – finding exact rotating black hole solutions of Einstein’s equations with a cosmological constant – and was stunned when I came back a few days later with the solution. I can’t even remember exactly what he said but I will never forget his enormous smile. </p>
<p>Hawking was a determined and stubborn person. On many occasions he got through serious medical issues <a href="https://theconversation.com/stephen-hawking-martin-rees-looks-back-on-colleagues-spectacular-success-against-all-odds-93379">with sheer determination</a>. This same determination could make him very difficult to work. But it could also push research projects forward: Hawking would refuse to give up on seemingly unsolvable problems. </p>
<p>In fact, never giving up is the main thing Hawking has taught me – to keep attacking problems from different directions, to reach for the hardest problems and find a way to solve them. It’s immensely important as a scientist, but also in other aspects of life.</p>
<h2>Pithy one-liners</h2>
<p>Hawking was devoted to his family. His eyes would light up when one of his children came to visit or when he proudly showed us pictures of his first grandchild. In many respects, Hawking treated his PhD students and collaborators as a second family. However busy he was, he always made time for us, often making dignitaries wait outside his office while he talked physics with a student. He would eat lunch with us several times per week, and funded a weekly lunch for the wider group to bring everyone together. </p>
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<span class="caption">Hawking ahead of a lecture, Asturias.</span>
<span class="attribution"><span class="source">Eloy Alonso/EPA</span></span>
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<p>There were many occasions when physics discussions merged seamlessly into social activities: going to the pub, eating dinner at one of his favourite Cambridge restaurants, and so on. Hawking had a wonderful sense of humour. He turned his communication difficulties into an advantage, composing pithy one-liners. For instance when <a href="http://www.nbcnews.com/id/5452537/ns/technology_and_science-space/t/hawking-changes-his-mind-black-holes/">changing his mind</a> about what happens to information in a black hole, he announced it in the pub by turning the volume up on his synthesizer, saying simply: “I’m coming out.” He would discuss anything and everything in a social setting: politics, movies, other branches of science, music. </p>
<p>As we worked in closely related fields, we saw each other regularly even after I finished my PhD. In 2017, I attended a conference in Cambridge celebrating his 75th birthday. The list of participants illustrates Hawking’s influence on academia and beyond. Many of his former students and collaborators have gone on to become leaders in research in cosmology, gravitational waves, black holes and string theory. Others have had huge impact outside academia, such as <a href="http://www.nathanmyhrvold.com/">Nathan Myhrvold</a> at Microsoft. </p>
<p>There is currently pressure on academics <a href="https://theconversation.com/academics-admit-feeling-pressure-to-embellish-possible-impact-of-research-56059">to demonstrate the immediate impact</a> of their research on society. It is perhaps worth reflecting that impact is not easily measurable on short time scales. Hawking’s was truly blue-sky research – and yet it has fascinated millions, attracting many into scientific careers. His academic legacy is not just the remarkable science he produced, but the generations of minds he shaped. </p>
<p>There’s no doubt Hawking’s death is a huge loss to physics. But personally, what I will miss most is his humour and the general feeling of inspiration I got from being around him.</p><img src="https://counter.theconversation.com/content/93508/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Marika Taylor receives funding from EPSRC and STFC. </span></em></p>Hawking wasn’t able to give his students a gentle introduction, but he did provide a lot of inspiration and support.Marika Taylor, Professor in Theoretical Physics, University of SouthamptonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/934402018-03-15T15:36:07Z2018-03-15T15:36:07ZStephen Hawking had pinned his hopes on ‘M-theory’ to fully explain the universe – here’s what it is<figure><img src="https://images.theconversation.com/files/210604/original/file-20180315-104671-1v6k6ba.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Stephen Hawking.</span> <span class="attribution"><span class="source">Lwp Kommunikáció/Flickr</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Rumour has it that Albert Einstein spent his last few hours on Earth <a href="http://www.bbc.co.uk/sn/tvradio/programmes/horizon/einstein_symphony_prog_summary.shtml">scribbling something</a> on a piece of paper in a last attempt to formulate a theory of everything. Some 60 years later, another legendary figure in theoretical physics, Stephen Hawking, may have <a href="https://theconversation.com/stephen-hawking-martin-rees-looks-back-on-colleagues-spectacular-success-against-all-odds-93379">passed away</a> with similar thoughts. We know Hawking thought something called “M-theory” is <a href="https://arstechnica.com/science/2012/07/steven-hawking-on-time-travel-m-theory-and-extra-terrestrial-life/">our best bet</a> for a complete theory of the universe. But what is it?</p>
<p>Since the formulation of Einstein’s <a href="https://theconversation.com/how-einsteins-general-theory-of-relativity-killed-off-common-sense-physics-50042">theory of general relativity</a> in 1915, every theoretical physicist has been dreaming of reconciling our understanding of the infinitely small world of atoms and particles with that of the infinitely large scale of the cosmos. While the latter is effectively described by Einstein’s equations, the former is predicted with extraordinary accuracy by the so-called <a href="https://theconversation.com/explainer-standard-model-of-particle-physics-2539">Standard Model</a> of fundamental interactions.</p>
<p>Our current understanding is that the interaction between physical objects is described by four <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/funfor.html">fundamental forces</a>. Two of them – gravity and electromagnetism – are relevant for us on a macroscopic level, we deal with them in our everyday life. The other two, dubbed strong and weak interactions, act on a very small scale and become relevant only when dealing with subatomic processes.</p>
<p>The standard model of fundamental interactions provides a unified framework for three of these forces, but gravity cannot be consistently included in this picture. Despite its accurate description of large scale phenomena such as a planet’s orbit or galaxy dynamics, general relativity breaks down at very short distances. According to the standard model, all forces are mediated by specific particles. For gravity, a particle called the graviton does the job. But when trying to calculate how these gravitons interact, nonsensical infinities appear.</p>
<p>A consistent theory of gravity should be valid at any scale and should take into account the quantum nature of fundamental particles. This would accommodate gravity in a unified framework with the other three fundamental interactions, thus providing the celebrated theory of everything. Of course, since Einstein’s death in 1955, a lot of progress has been made and nowadays our best candidate goes under the name of M-theory.</p>
<h2>String revolution</h2>
<p>To understand the basic idea of M-theory, one has to go back to the 1970s when scientists realised that, rather than describing the universe based on point like particles, you could describe it in terms of tiny oscillating strings (tubes of energy). This new way of thinking about the fundamental constituents of nature turned out to solve many theoretical problems. Above all, a particular oscillation of the string could be interpreted as a graviton. And unlike the standard theory of gravity, string theory can describe its interactions mathematically without getting strange infinities. Thus, gravity was finally included in a unified framework. </p>
<p>After this exciting discovery, theoretical physicists devoted a lot of effort to understanding the consequences of this seminal idea. However, as often happens with scientific research, the history of string theory is characterised by ups and downs. At first, people were puzzled because it predicted the existence of a particle which travels faster than the speed of light, dubbed a “tachyon”. This prediction was in contrast with all the experimental observations and cast serious doubt on string theory. </p>
<p>Nevertheless, this issue was solved in the early 1980s by the introduction of something called “supersymmetry” in string theory. This predicts that every particle has a superpartner and, by an extraordinary coincidence, the same condition actually eliminates the tachyon. This first success is commonly known as “<a href="http://theory.caltech.edu/people/jhs/strings/str133.html">the first string revolution</a>”. </p>
<p>Another striking feature is that string theory requires the existence of ten spacetime dimensions. Currently, we only know of four: depth, height, width and time. Although this might seem a major obstacle, several solutions have been proposed and nowadays it is considered as a notable feature, rather than a problem. </p>
<p>For example, we could somehow be forced to live in a four dimensional world without any access to the extra dimensions. Or the extra dimensions could be “compactified” on such a small scale we wouldn’t notice them. However, different compactifications would lead to different values of the physical constants and, therefore, different physics laws. A possible solution is that our universe is just one of many in <a href="https://theconversation.com/the-theory-of-parallel-universes-is-not-just-maths-it-is-science-that-can-be-tested-46497">an infinite “multiverse”</a>, governed by different physics laws. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=424&fit=crop&dpr=1 600w, https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=424&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=424&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=533&fit=crop&dpr=1 754w, https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=533&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=533&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Are there other universes?</span>
<span class="attribution"><span class="source">Pixabay.</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>This may seem odd, but a lot of theoretical physicists are coming around to this idea. If you are not convinced you may try to read the novel <a href="http://www.geom.uiuc.edu/%7Ebanchoff/Flatland/">Flatland: a romance of many dimensions</a> by Edwin Abbott, in which the characters are forced to live in two space dimensions and are unable to realise there is a third one.</p>
<h2>M-theory</h2>
<p>But there was one remaining pressing issue that was bothering string theorists at the time. A thorough classification showed the existence of five different consistent string theories, and it was unclear why nature would pick one out of five.</p>
<p>This is when M-theory entered the game. During the <a href="http://discovermagazine.com/2008/dec/13-the-man-who-led-the-second-superstring-revolution">second string revolution</a>, in 1995, physicists proposed that the five consistent string theories are actually only different faces of a unique theory which lives in eleven spacetime dimensions and is known as M-theory. It includes each of the string theories in different physical contexts, <a href="https://www.quantamagazine.org/why-is-m-theory-the-leading-candidate-for-theory-of-everything-20171218/">but is still valid for all of them</a>. This extremely fascinating picture has led most theoretical physicists to believe in M-theory as the theory of everything – it is also more mathematically consistent than other candidate theories.</p>
<p>Nevertheless, so far M-theory has struggled in producing predictions that can be tested by experiments. Supersymmetry is <a href="https://theconversation.com/large-hadron-collider-sees-tantalising-hints-of-a-new-particle-that-could-revolutionise-physics-52457">currently being tested</a> at the Large Hadron Collider. If scientists do find evidence of superpartners, that would ultimately strengthen M-theory. But it still remains a challenge for current theoretical physicists to produce testable predictions and for experimental physicists to set up experiments to test them. </p>
<p>Most great physicists and cosmologists are driven by a passion to find that beautiful, simple description of the world that can explain everything. And although we are not quite there yet, we wouldn’t have a chance without the sharp, creative minds of people like Hawking.</p><img src="https://counter.theconversation.com/content/93440/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Lorenzo Bianchi receives funding from the European Union’s Horizon 2020 research and innovation
programme under the Marie Sklodowska-Curie grant agreement No 749909.</span></em></p>Stephen Hawking thought a form of string theory could be our best bet for a ‘theory of everything’.Lorenzo Bianchi, Marie Curie Fellow in Theoretical Physics, Queen Mary University of LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/496462015-11-30T00:22:31Z2015-11-30T00:22:31ZEinstein’s folly: how the search for a unified theory stumped him to his dying day<figure><img src="https://images.theconversation.com/files/103578/original/image-20151130-11614-8zpwab.jpg?ixlib=rb-1.1.0&rect=0%2C304%2C4517%2C2934&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Albert Einstein wrestled with unifying gravity with electromagnetism and quantum mechanics until his dying days.</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/Category:Portraits_of_Albert_Einstein#/media/File:Albert_Einstein_1947.jpg">Oren Jack Turner/Wikimedia Commons</a></span></figcaption></figure><p>This month marks exactly <a href="https://theconversation.com/au/topics/general-relativity-centenary">100 years</a> since Albert Einstein submitted the first paper fully describing the <a href="https://theconversation.com/au/topics/general-relativity">general theory of relativity</a>. It was both breathtaking and revolutionary. </p>
<p>Simply stated, gravity is a geometric property of <a href="https://einstein.stanford.edu/SPACETIME/spacetime2.html#fourth_dimension">spacetime</a> that is allowed to be curved. It was like looking at Newton’s world through the bottom of a glass.</p>
<p>General relativity is based on Einstein’s field equations, which describe the relation between the geometry of a four-dimensional description of spacetime, and the <a href="https://theconversation.com/without-einstein-it-would-have-taken-decades-longer-to-understand-gravity-50517">energy–momentum</a> contained in that spacetime. </p>
<p>Spacetime curvature is caused by <a href="https://theconversation.com/explainer-what-is-mass-49299">mass</a>; the more mass, the more spacetime is curved. This curvature can induce deflections or delays in the propagation of light. </p>
<p>Even close to home, our sun – not that massive as stars go – will alter the path of light near it. Newton’s theory predicts a deflection of light of 0.875 <a href="http://astronomy.swin.edu.au/cosmos/A/Arcsecond">seconds of arc</a> at the limb of the sun, whilst relativity predicted a deflection of 1.75 seconds of arc. Observations during <a href="http://www.powerhousemuseum.com/collection/database/?irn=355470">total solar eclipses of background star fields</a> confirmed Einsteins value. </p>
<p>Even had Einstein died shortly after his work on general relativity, he would still be regarded by many today as the greatest physicist who ever lived, and perhaps even the greatest scientist. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=340&fit=crop&dpr=1 600w, https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=340&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=340&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=427&fit=crop&dpr=1 754w, https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=427&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=427&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The first part of Einsteins defining GTR paper: Feldgleichungen der Gravitation (The Field Equations of Gravitation) Preussische Akademie der Wissenschaften, Sitzungsberichte, 1915.</span>
</figcaption>
</figure>
<h2>Towards a unified field theory</h2>
<p>However, whilst he continued to work on many problems up until his death in 1955, he is regularly described as failing in one particular area: the <a href="http://www.britannica.com/science/unified-field-theory">unified field theory</a>. </p>
<p>From the 1920s, Einstein tried to develop a <a href="http://www.aps.org/publications/apsnews/200512/history.cfm">unified theory</a> that melded general relativity and <a href="https://theconversation.com/let-there-be-light-celebrating-the-theory-of-electromagnetism-35723">electromagnetism</a>, representing the only two forces known to exist.</p>
<p>Such a theory would describe a single field in which all forces are mediated and the properties of all particles – which at the time were only electrons and protons, with the <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/particles/neutrondis.html">neutron</a> not discovered until 1932 – could be deduced.</p>
<p>Other players in the quest appeared. <a href="http://www-history.mcs.st-andrews.ac.uk/Biographies/Kaluza.html">Theodor Kaluza</a> showed that if spacetime had five dimensions, then four dimensions could reflect general relativity, and one could represent electromagnetism. In the burgeoning <a href="http://www.pbs.org/transistor/science/info/quantum.html">quantum world</a> of the mid-1920s, <a href="http://www-history.mcs.st-andrews.ac.uk/Biographies/Klein_Oskar.html">Oskar Klein</a> shrank Kaluza’s 5th dimension to be compact, in a sense offering a quantum mechanical interpretation. </p>
<p>Einstein drew upon other work if it could help his cause. He even looked at variations to the successful mathematical basis of general relativity. It is <a href="http://www.stmarys.ac.uk/news/news/ug-applied-physics/2014/09/physics-beyond-god-play-dice-einstein-mean/">widely reported</a> that he did not support quantum mechanics, but promoted it (suffered it?) being a derivative of an eventual unified theory. </p>
<h2>Strong developments</h2>
<p>In a way, his mathematical focus hindered his acceptance of ongoing, major discoveries in physics like quantum mechanics. The discovery of two new forces in addition to gravity and electromagnetism – the <a href="http://www.livescience.com/48575-strong-force.html">strong</a> and <a href="http://www.thestargarden.co.uk/Weak.html">weak</a> nuclear forces – also made his work of a unified field based only on two forces unattainable.</p>
<p>Protons and neutrons in atomic nuclei had to be held together by a strong attractive force. Mesons, the force carrying particles for the <a href="http://aether.lbl.gov/elements/stellar/strong/strong.html">strong nuclear force</a> were discovered experimentally in 1947. Enrico Fermi in 1933 tried to explain beta decay, which was a radioactive transmutation between protons and neutrons. It was related to a <a href="http://home.fnal.gov/%7Echeung/rtes/RTESWeb/LQCD_site/pages/weakforce.htm">weak nuclear force</a>. </p>
<p>Eventually Sheldon Glashow, Steven Weinberg, and Abdus Salam announced a unified theory of electromagnetism and the weak nuclear force in 1968. Their <a href="https://www.fnal.gov/pub/inquiring/matter/madeof/electroweakforce.html">electroweak theory</a> postulated the weak force carrier particles – W and Z bosons – which were then discovered in the 1980s. </p>
<p>We now know that all forces <em>apart from gravitation</em> are related mathematically, albeit with some differences in phenomena. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=750&fit=crop&dpr=1 600w, https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=750&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=750&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=943&fit=crop&dpr=1 754w, https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=943&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=943&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The rich galaxy cluster Cl 0024+17. Blue streaks near the centre are smeared images of very distant background galaxies. Their light is being bent and magnified by the intervening cluster, in an effect called gravitational lensing.</span>
</figcaption>
</figure>
<h2>Todays efforts at a unified field</h2>
<p>The major pathway to unification over the last three decades has been <a href="https://theconversation.com/explainer-string-theory-2983">string theory</a>. Two forms of string theory have ten and twenty one dimensions respectively. In a strange parallel, the miniaturisation or compactification of many dimensions in string theory is the modern day equivalent of the <a href="http://www.superstringtheory.com/experm/exper5.html">quantisation of a 5th dimension</a> by Klein. </p>
<p>Despite little predictive power, and critics attacking its relation to a <a href="http://www.scientificamerican.com/article/multiverse-the-case-for-parallel-universe/">multiverse</a>, no other areas towards unification theory appear as fruitful as string theory.</p>
<p>For thirty years a unified theory proved a worthy opponent of Einstein. He worked on it even on his penultimate day in Princeton Hospital. <a href="https://www.ias.edu/people/oppenheimer">J. Robert Oppenheimer</a> was later both unflattering,</p>
<blockquote>
<p>During all the end of his life, Einstein did no good. He turned his back on experiments […] to realise the unity of knowledge. </p>
</blockquote>
<p>…and <a href="http://www.hup.harvard.edu/catalog.php?isbn=9780674034525">envious</a>,</p>
<blockquote>
<p>Of course, I would have liked to be the young Einstein. This goes without saying.</p>
</blockquote>
<p>A consensus seems to exist: in later years, Einstein worked with mathematical blinkers, immune to relevant discoveries, and unable to change his method of investigation. </p>
<p>As James Joyce <a href="http://www.online-literature.com/james_joyce/ulysses/">wrote</a>:</p>
<blockquote>
<p>A man of genius makes no mistakes. His errors are volitional and are the portals of discovery.</p>
</blockquote>
<p>Failure and mistake are harsh words. They are often the precursors of discovery. The unified field was Einstein’s nemesis for a variety of reasons. Despite this, many envied his early genius and we should focus on this especially in this centenary year of the greatest physics revolution.</p><img src="https://counter.theconversation.com/content/49646/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Glen Mackie does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>After the triumph of general relativity, Albert Einstein spent the rest of his life chasing a unified theory, which eluded him right up until the end.Glen Mackie, Senior Lecturer in Astronomy & Astrophysics, Coordinator of Swinburne Astronomy Online, Swinburne University of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/500572015-11-24T17:08:28Z2015-11-24T17:08:28ZWill we have to rewrite Einstein’s theory of general relativity?<figure><img src="https://images.theconversation.com/files/102735/original/image-20151122-435-1eaferq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The fathers of modern physics, including Einstein, Millikan, Planck and others, in debate.</span> <span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Max_Planck#/media/File:Nernst,_Einstein,_Planck,_Millikan,_Laue_in_1931.jpg">Materialscientist/wikimedia</a></span></figcaption></figure><p>Einstein famously <a href="https://theconversation.com/how-einsteins-general-theory-of-relativity-killed-off-common-sense-physics-50042">laboured hard</a> to create the theory of general relativity, but it is less well known that he also helped to launch quantum mechanics, which <a href="http://physicsworld.com/cws/article/multimedia/2013/mar/04/why-did-einstein-say-god-doesnt-play-dice">he didn’t much care for</a>. These two views of the world are the very foundation stones of modern physics – without them we would not have things such as space travel, medical imaging, GPS systems or nuclear energy.</p>
<p>General relativity is unparalleled when it comes to describing the world on a large scale, such as planets and galaxies, while quantum mechanics perfectly describes physics on the smallest scale, such as the atom or even parts of the atom. Uniting the two into a consistent “theory of everything” is the single biggest challenge in physics today – and progress is slow.</p>
<h2>The birth of modern physics</h2>
<p>Our knowledge of the universe is based on a sequence of “natural laws”. With time many laws become morphed into new ones as a result of experimental evidence or changing conceptual prejudices. Einstein’s rejection of the concept of universal time was one of the most radical shifts in the history of physics. Its consequences have proved crucial to shaping some of the most profound developments in our understanding of nature. </p>
<p>By fusing the three dimensions of space (height, width and depth) with that of a time direction to construct a “spacetime structure”, a new symmetry of nature could be uncovered. When Einstein later added gravitation to his theories, it led to experimentally verifiable predictions as well as the prediction of gravitational waves and black holes, beyond the natural scope of Newton’s existing law of gravitation.</p>
<p>But Einstein didn’t just work on relativity. A <a href="http://physics.info/photoelectric/">big problem</a> at the time was the fact that Maxwell’s laws, describing electromagnetic phenomena, were unable to explain why faint ultraviolet light falling on metallic electrodes could induce sparks more easily than bright red light. Einstein suggested that this could be understood if the energy in the light wave wasn’t continuously distributed as a wave but rather as a shower of individual “light bullets” (photons – also known as “light quanta”), each with an energy proportional to the colour (frequency) of the light. Many scientists were sceptical of this groundbreaking thought, as so many experiments had already shown that light was a wave.</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/102884/original/image-20151123-18230-bbj53b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/102884/original/image-20151123-18230-bbj53b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=849&fit=crop&dpr=1 600w, https://images.theconversation.com/files/102884/original/image-20151123-18230-bbj53b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=849&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/102884/original/image-20151123-18230-bbj53b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=849&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/102884/original/image-20151123-18230-bbj53b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1066&fit=crop&dpr=1 754w, https://images.theconversation.com/files/102884/original/image-20151123-18230-bbj53b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1066&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/102884/original/image-20151123-18230-bbj53b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1066&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Millikan proved Einstein right.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Robert_Andrews_Millikan#/media/File:Millikan.jpg">Nobel foundation/wikipedia</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>One of them was Robert Millikan, who ironically eventually ended up <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/1923/millikan-bio.html">experimentally verifying</a> Einstein’s theory. Millikan also discovered that charged particles known as electrons have wave-like properties. Together with Einstein’s discovery, this pointed to a duality where both matter and light could be described as a particle or as a wave – an idea which led to the development of quantum mechanics by a number of scientists. </p>
<p>This theory has had wide applicability on the smallest of scales, where gravity can often be neglected as it is so weak compared to the other forces affecting particles. Not only has it led to a consistent description of matter and radiation observed in everyday life, it has also made predictions of new particles and processes that are now observed in high-energy accelerator experiments on Earth or cosmic events in space.</p>
<h2>The contenders</h2>
<p>To unify the description of matter and radiation quanta with gravitation it became natural to contemplate “gravitational quanta” that carry the force of gravitation. String theory has emerged as a candidate to do this. It states that matter is made up of vibrating extended structures, like tiny strings or membranes, rather than point-like particles. Each type of vibration of these structures corresponds to a particular state of matter. </p>
<p>One type of vibration also corresponds to a gravitational quantum. However, for the resulting quantum description to be consistent it becomes necessary to boost the dimension of spacetime by introducing additional space dimensions that are unobservable to the eye and current technology. To date, there has been <a href="http://discovermagazine.com/2005/aug/cover">no firm experimental confirmation</a> of string theory.</p>
<p>By contrast, in domains where gravitation appears irrelevant, quantum mechanics remains unchallenged, despite describing a very strange world. It states that particles can be in a number of different possible states at once. While the theory can predict a set of probabilities for the particle to be in a particular state, it cannot, in general, predict which probability will actually occur. </p>
<p>In such cases, one must take a large number of observations and then calculate average measurements. Furthermore, such averages depend on what properties are to be measured and when such measurement decisions are made. This peculiar world picture sits uncomfortably alongside Einstein’s world view of causal events and frozen histories in spacetime.</p>
<p>What’s more, according to quantum mechanics, one particle’s state can be correlated with another particle’s state, even if it is in a distant location. Einstein didn’t like this because it seemed to imply that correlations could occur over events that could not be connected by a beam of light, thereby breaking a rule that says nothing can travel faster than the speed of light. He felt that such “spooky action at a distance” was proof for the incompleteness of the theory, although <a href="https://theconversation.com/physicists-prove-quantum-spookiness-and-start-chasing-schrodingers-cat-48190">experimental evidence</a> since points to the contrary.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/102885/original/image-20151123-18257-fypsul.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/102885/original/image-20151123-18257-fypsul.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/102885/original/image-20151123-18257-fypsul.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/102885/original/image-20151123-18257-fypsul.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/102885/original/image-20151123-18257-fypsul.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/102885/original/image-20151123-18257-fypsul.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/102885/original/image-20151123-18257-fypsul.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Could the International Space Station be the key to probe the effects of gravity on quantum entanglement?</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Psychological_and_sociological_effects_of_spaceflight#/media/File:STS132_undocking_iss2.jpg">NASA/wikipedia</a></span>
</figcaption>
</figure>
<p>However, new experiments are underway to see whether gravitational interactions might influence such eerie action in unexpected ways. A research group in Vienna <a href="http://www.iop.org/news/13/apr/page_59834.html">proposes to use the International Space Station</a> to see how gravity might influence this action. A collection of entangled photon pairs will be created on Earth before one member of each pair is sent to the orbiting space station. There, a state known as polarisation will be recorded and compared with the state of its partner on Earth.</p>
<p>It is unclear whether quantum mechanics or general relativity will need either mathematical or conceptual modification in response to future experimental probing. But while the outcome is difficult to predict, Einstein’s influence has been and remains pivotal in this quest.</p><img src="https://counter.theconversation.com/content/50057/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Robin Tucker receives funding from EPSRC and STFC.</span></em></p>Physicists are working hard to unite Einstein’s theory of relativity with quantum mechanics. It’s no easy task.Robin Tucker, Professor in mathematical physics, Lancaster UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/459662015-09-28T05:32:25Z2015-09-28T05:32:25ZUnderstanding the hidden dimensions of modern physics through the arts<figure><img src="https://images.theconversation.com/files/95705/original/image-20150922-16698-1kqvv6k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Can the arts be a bridge to other worlds?</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/parksdh/9713995081/in/photolist-fNoLiP-D77HM-rUT5cx-pnTCYC-nmjt4Z-4tFJRm-4LsuM-hPBoYS-e3kE6B-96QyBj-jeXEcA-fky24x-vT4uBk-4tzBVc-nrZNnP-c8XrUy-9oTKUr-sQstBQ-5Bpk4Q-cC2gjf-aaoMeg-bqbJVx-4G2K3S-oyApa-nqhCRj-yNTFq-qqqTCV-nwXu22-ikpUps-xp4eDm-7uWEfq-eCxuRJ-7dpYrk-eiZAyY-cgyH7u-odVRjt-qrqX1C-dMta5V-cQczT5-owkJCB-u7LcTn-cQczWE-wMpZzc-wfghta-9EBncx-wqgCaK-qgC6m1-wN3ymr-bsKgrw-bsKibw">Daniel Parks</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span></figcaption></figure><p>Sometimes, the hardest job for a theoretical physicist is telling the story. The work in this field can be conducted entirely in the abstract, leaving outsiders (and the odd insider) bewildered, but there might be some assistance in the visualisation techniques developed by certain artists and writers. Cutting-edge theories are often motivated by aesthetics and simplicity, after all, and so the idea of a synergy between artists and scientists does not seem all that far-fetched. One clear example where the combination can work comes in the exploration and understanding of extra dimensions. </p>
<p>You may have heard that scientists often talk of these “other worlds”, but (hopefully), your everyday reality takes place in three dimensions of space, and one dimension of time. Physicists marry together these dimensions because of Albert Einstein’s special theory of relativity; it enables us to describe a point in (1+3) dimensions of (time+space) with four coordinates: (t, x, y, z). But from an abstract point of view, it makes a lot of sense to then ask: why just four? And in fact, many theories in physics can easily be formulated without being too specific about the number of dimensions. We can call it 1+D instead and open up the the possibility of more than three spatial dimensions: (t, x, y, z….). </p>
<p>But that’s where it gets hard, of course. Extra spatial dimensions are very hard to imagine, and even the scientists working with them (<a href="http://thequantummessenger.com">such as myself</a>) have a hard time visualising them. Now, this in itself is not proof that they do not exist. We also find it hard to imagine infinities, for instance, and <a href="http://www.physics.org/article-questions.asp?id=124">super-positions of quantum mechanical states</a>, but both these concepts are seen in nature. </p>
<h2>Hidden truths</h2>
<p>Physicists have of course <a href="http://thequantummessenger.com/index.php/2015/08/14/adding-dimensions-with-art-and-prose-1/#_ftn2">come up with tests</a> which allow for the existence of other dimensions, but the trouble is that this delivers results which imply we’re happily bumping along with the ones we’re all very familiar with.</p>
<p>But before concluding that this invalidates the whole discussion about more dimensions already, there are ways to get around this result. We already <a href="http://home.web.cern.ch/about/physics/extra-dimensions-gravitons-and-tiny-black-holes">knew</a> that the new dimensions would have to be very different from the ones we experience – otherwise we would be able to see them. In much the same way they may not show up in those tests which use <a href="http://thequantummessenger.com/index.php/2015/08/14/adding-dimensions-with-art-and-prose-1/#_ftnref2">force laws</a>. They may be very small, for instance, and folded away to make them invisible to us. Size and energy are inversely related in particle theories, so the smaller the dimensions are, the less likely it is that we will be able to probe them directly. </p>
<p>A popular example of how this works is by an ant on a piece of rope. From far away, the piece of rope seems one-dimensional, but only when you zoom in you can see that in the ant’s world the surface it sits on is really 2D. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/96034/original/image-20150924-17062-ou2c8h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/96034/original/image-20150924-17062-ou2c8h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/96034/original/image-20150924-17062-ou2c8h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=319&fit=crop&dpr=1 600w, https://images.theconversation.com/files/96034/original/image-20150924-17062-ou2c8h.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=319&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/96034/original/image-20150924-17062-ou2c8h.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=319&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/96034/original/image-20150924-17062-ou2c8h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=401&fit=crop&dpr=1 754w, https://images.theconversation.com/files/96034/original/image-20150924-17062-ou2c8h.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=401&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/96034/original/image-20150924-17062-ou2c8h.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=401&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Our ability to perceive even our own dimensions can be flawed.</span>
<span class="attribution"><a class="source" href="http://www.alexameade.com/">Alexa Meade</a></span>
</figcaption>
</figure>
<p>This limited ability to perceive dimensions even in our familiar world can be seen in the <a href="http://www.alexameade.com/">work of artist Alexa Meade</a>, who paints 3D installations and renders them 2D to our primitive eyes. And to start visualising extra dimensions instead, scientists may also take inspiration from the arts. </p>
<h2>Slicing</h2>
<p>A good starting point is to turn the question on its head: in <a href="https://www.youtube.com/watch?v=RxcMa4lay5Q">the 1884 novella Flatland</a> EA Abbott wrote about creatures living in fewer dimensions, instead of more. The creatures of his 2D world experienced 3D through cross sections of objects passing through. An illustration from the book appears below.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/93542/original/image-20150901-13419-qp08bc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/93542/original/image-20150901-13419-qp08bc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=220&fit=crop&dpr=1 600w, https://images.theconversation.com/files/93542/original/image-20150901-13419-qp08bc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=220&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/93542/original/image-20150901-13419-qp08bc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=220&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/93542/original/image-20150901-13419-qp08bc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=277&fit=crop&dpr=1 754w, https://images.theconversation.com/files/93542/original/image-20150901-13419-qp08bc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=277&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/93542/original/image-20150901-13419-qp08bc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=277&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Flatland, A Romance of Many Dimensions, EA Abbott</span></span>
</figcaption>
</figure>
<p>In exactly the same way we may use a computer to show what a cross section of a 4D image would look like in 3D, or in 2D. A 4D cube (a hypercube) may for instance be represented with this slicing method: </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/93540/original/image-20150901-13412-woiv6p.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/93540/original/image-20150901-13412-woiv6p.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=294&fit=crop&dpr=1 600w, https://images.theconversation.com/files/93540/original/image-20150901-13412-woiv6p.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=294&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/93540/original/image-20150901-13412-woiv6p.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=294&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/93540/original/image-20150901-13412-woiv6p.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=370&fit=crop&dpr=1 754w, https://images.theconversation.com/files/93540/original/image-20150901-13412-woiv6p.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=370&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/93540/original/image-20150901-13412-woiv6p.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=370&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Thomas Banchoff, Brown University</span></span>
</figcaption>
</figure>
<p>Interestingly, both are slices of the same object, but in the top set of images the slicing was started on a corner, and in the second one with a square. </p>
<p>EA Abbott wrote about slicing, but there may have been another way in which his flat creatures observed the 3D sphere. If the 3D sun had shone over it, a shadow would have been cast over the plane: this defines the linear perspective method. It has a foundation in ancient Greece, and modern artists still follow the <a href="https://www.khanacademy.org/humanities/renaissance-reformation/early-renaissance1/beginners-renaissance-florence/v/linear-perspective-brunelleschi-s-experiement">techniques developed by Renaissance architect Filippo Brunelleschi</a>, perhaps most famous for building the gigantic dome of on Florence cathedral. Jean-François Colonna has some <a href="http://www.lactamme.polytechnique.fr/">great examples of extra dimensional objects</a> created using the perspective method which have all the trappings of abstract art.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/93541/original/image-20150901-13392-ws979j.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/93541/original/image-20150901-13392-ws979j.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=334&fit=crop&dpr=1 600w, https://images.theconversation.com/files/93541/original/image-20150901-13392-ws979j.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=334&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/93541/original/image-20150901-13392-ws979j.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=334&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/93541/original/image-20150901-13392-ws979j.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=419&fit=crop&dpr=1 754w, https://images.theconversation.com/files/93541/original/image-20150901-13392-ws979j.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=419&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/93541/original/image-20150901-13392-ws979j.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=419&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">from http://www.drawspace.com/</span></span>
</figcaption>
</figure>
<h2>Light and shade</h2>
<p>In the same way that the light from our sun casts shadows on 2D surfaces, that is, in parallel lines, we may consider a hypothetical 4D sun which casts a shadow of a 4D object onto our 3D world. This is hard to visualise, but easy to program on a computer. A hypercube would then look like this: </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/93539/original/image-20150901-13425-q8lpfi.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/93539/original/image-20150901-13425-q8lpfi.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=649&fit=crop&dpr=1 600w, https://images.theconversation.com/files/93539/original/image-20150901-13425-q8lpfi.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=649&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/93539/original/image-20150901-13425-q8lpfi.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=649&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/93539/original/image-20150901-13425-q8lpfi.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=815&fit=crop&dpr=1 754w, https://images.theconversation.com/files/93539/original/image-20150901-13425-q8lpfi.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=815&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/93539/original/image-20150901-13425-q8lpfi.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=815&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">from Wolfram Mathworld</span></span>
</figcaption>
</figure>
<p>This is called a Schlegel diagram. Perhaps it is not immediately obvious how this is related to a shadow, but considering its contour lines may help.</p>
<p>If you can imagine adding one dimension, you can imagine adding several. Descriptions of string theory, for example, only make sense when formulated in as many as 11 dimensions. And though the result may grow in complexity with the number of dimensions, the techniques looked at here are not limited to 4D. </p>
<p>Difficulties with visualizing physical theories have never proved to be a valid basis for their rejection, but they have been an obstacle to understanding. The techniques developed for visualizing extra dimensions form a good example of how physicists may borrow and extrapolate techniques developed in the arts world, and how interdisciplinary collaborations may be beneficial to both fields.</p><img src="https://counter.theconversation.com/content/45966/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Djuna Croon receives funding from the University of Sussex. </span></em></p>Is a novella published 130 years ago our best bet for explaining the worlds of 4D and beyond?Djuna Croon, PhD Researcher, University of SussexLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/464972015-09-02T13:29:06Z2015-09-02T13:29:06ZThe theory of parallel universes is not just maths – it is science that can be tested<figure><img src="https://images.theconversation.com/files/93501/original/image-20150901-25717-1y3igv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Scientists are searching for collisions between different 'universe bubbles' in the cosmic microwave background.</span> <span class="attribution"><a class="source" href="https://pixabay.com/en/globe-earth-country-continents-73397/">Geralt</a></span></figcaption></figure><p>The existence of parallel universes may seem like something cooked up by science fiction writers, with little relevance to modern theoretical physics. But the idea that we live in a “<a href="http://www.astronomy.pomona.edu/Projects/moderncosmo/Sean%27s%20mutliverse.html">multiverse</a>” made up of an infinite number of parallel universes has long been considered a scientific possibility – although <a href="https://www.quantamagazine.org/20150317-sciences-path-from-myth-to-multiverse/">it is still a matter of vigorous debate</a> among physicists. The race is now on to find a way to test the theory, including searching the sky for signs of collisions with other universes.</p>
<p>It is important to keep in mind that the multiverse view is not actually a theory, it is rather a consequence of our current understanding of theoretical physics. This distinction is crucial. We have not waved our hands and said: “Let there be a multiverse”. Instead the idea that the universe is perhaps one of infinitely many is derived from current theories like <a href="https://theconversation.com/explainer-quantum-physics-570">quantum mechanics</a> and <a href="https://theconversation.com/explainer-string-theory-2983">string theory</a>.</p>
<h2>The many-worlds interpretation</h2>
<p>You may have heard the thought experiment of <a href="http://news.nationalgeographic.com/news/2013/08/130812-physics-schrodinger-erwin-google-doodle-cat-paradox-science/">Schrödinger’s cat</a>, a spooky animal who lives in a closed box. The act of opening the box allows us to follow one of the possible future histories of our cat, including one in which it is both dead and alive. The reason this seems so impossible is simply because our human intuition is not familiar with it. </p>
<p>But it is entirely possible according to the strange rules of quantum mechanics. The reason that this can happen is that the space of possibilities in quantum mechanics is huge. Mathematically, a quantum mechanical state is a sum (or superposition) of all possible states. In the case of the Schrödinger’s cat, the cat is the superposition of “dead” and “alive” states. </p>
<p>But how do we interpret this to make any practical sense at all? One popular way is to think of all these possibilities as book-keeping devices so that the only “objectively true” cat state is the one we observe. However, one can just as well choose to accept that all these possibilities are true, and that they exist in different universes of a multiverse.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/93451/original/image-20150831-25771-t0149q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/93451/original/image-20150831-25771-t0149q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=384&fit=crop&dpr=1 600w, https://images.theconversation.com/files/93451/original/image-20150831-25771-t0149q.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=384&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/93451/original/image-20150831-25771-t0149q.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=384&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/93451/original/image-20150831-25771-t0149q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=483&fit=crop&dpr=1 754w, https://images.theconversation.com/files/93451/original/image-20150831-25771-t0149q.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=483&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/93451/original/image-20150831-25771-t0149q.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=483&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Miaaaaultiverse.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/29233640@N07/8132455446">Robert Couse-Baker/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>The string landscape</h2>
<p><a href="https://theconversation.com/explainer-string-theory-2983">String theory</a> is one of our most, if not the most promising avenue to be able to <a href="https://www.quantamagazine.org/20140314-betting-on-the-future-of-quantum-gravity/">unify quantum mechanics and gravity</a>. This is notoriously hard because gravitational force is so difficult to describe on small scales like those of atoms and subatomic particles – which is the science of quantum mechanics. But string theory, which states that all fundamental particles are made up of one-dimensional strings, can describe all known forces of nature at once: gravity, electromagnetism and the nuclear forces.</p>
<p>However, for string theory to work mathematically, it requires at least ten physical dimensions. Since we can only observe four dimensions: height, width, depth (all spatial) and time (temporal), the extra dimensions of string theory must therefore be hidden somehow if it is to be correct. To be able to use the theory to explain the physical phenomena we see, these extra dimensions have to be “compactified” by being curled up in such a way that they are too small to be seen. Perhaps for each point in our large four dimensions, there exists six extra indistinguishable directions?</p>
<p>A problem, or some would say, a feature, of string theory is that there are many ways of doing this compactification –<a href="http://events.nytimes.com/2005/06/26/weekinreview/26read.html?ei=5070&en=305a968fba809092&ex=1121918400&pagewanted=print">10<sup>500</sup> possibilities is one number usually touted about</a>. Each of these compactifications will result in a universe with different physical laws – such as different masses of electrons and different constants of gravity. However there are also vigorous <a href="http://content.time.com/time/magazine/article/0,9171,1226142,00.html">objections to the methodology</a> of compactification, so the issue is not quite settled.</p>
<p>But given this, the obvious question is: which of these landscape of possibilities do we live in? String theory itself does not provide a mechanism to predict that, which makes it useless as we can’t test it. But fortunately, an idea from our study of early universe cosmology has turned this bug into a feature. </p>
<h2>The early universe</h2>
<p>During the very early universe, just after the Big Bang, the universe underwent a <a href="http://www.ctc.cam.ac.uk/outreach/origins/inflation_zero.php">period of accelerated expansion</a> called inflation. Inflation was invoked originally to explain why the current observational universe is <a href="http://www.physicsoftheuniverse.com/topics_bigbang_inflation.html">almost uniform in temperature</a>. However, the theory also predicted a spectrum of temperature fluctuations around this equilibrium which was later confirmed by several spacecraft such as <a href="http://aether.lbl.gov/www/projects/cobe/">Cosmic Background Explorer</a>, <a href="http://map.gsfc.nasa.gov/">Wilkinson Microwave Anisotropy Probe</a> and the <a href="http://www.esa.int/Our_Activities/Space_Science/Planck">PLANCK spacecraft</a>.</p>
<p>While the exact details of the theory are still being hotly debated, inflation is widely accepted by physicists. However, a consequence of this theory is that there must be other parts of the universe that are still accelerating. However, due to the quantum fluctuations of space-time, some parts of the universe never actually reach the end state of inflation. This means that the universe is, at least according to our current understanding, eternally inflating. Some parts can therefore end up becoming other universes, which could become other universes etc. This mechanism generates a infinite number of universes.</p>
<p>By combining this scenario with string theory, there is a possibility that each of these universes possesses a different compactification of the extra dimensions and hence has different physical laws. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/93458/original/image-20150831-25714-1r7rvpt.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/93458/original/image-20150831-25714-1r7rvpt.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=300&fit=crop&dpr=1 600w, https://images.theconversation.com/files/93458/original/image-20150831-25714-1r7rvpt.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=300&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/93458/original/image-20150831-25714-1r7rvpt.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=300&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/93458/original/image-20150831-25714-1r7rvpt.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=377&fit=crop&dpr=1 754w, https://images.theconversation.com/files/93458/original/image-20150831-25714-1r7rvpt.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=377&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/93458/original/image-20150831-25714-1r7rvpt.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=377&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The cosmic microwave background. Scoured for gravitational waves and signs of collisions with other universes.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:WMAP_2010_41GHz.png">NASA / WMAP Science Team/wikimedia</a></span>
</figcaption>
</figure>
<h2>Testing the theory</h2>
<p>The universes predicted by string theory and inflation live in the same physical space (unlike the many universes of quantum mechanics which live in a mathematical space), they can overlap or collide. Indeed, they inevitably must collide, <a href="http://discovermagazine.com/2009/oct/04-will-our-universe-collide-with-neighboring-one">leaving possible signatures</a> in the cosmic sky which we can try to search for.</p>
<p>The exact details of the signatures depends intimately on the models – ranging from <a href="http://fqxi.org/community/articles/display/155">cold or hot spots</a> in the cosmic microwave background to anomalous voids in the distribution of galaxies. Nevertheless, since collisions with other universes must occur in a particular direction, a general expectation is that any signatures will break the uniformity of our observable universe. </p>
<p>These signatures are actively being pursued by scientists. Some are looking for it directly through <a href="https://www.ucl.ac.uk/news/news-articles/1108/110802-first-test-of-multiverse">imprints in the cosmic microwave background</a>, the afterglow of the Big Bang. However, no such signatures are yet to be seen. Others are looking for indirect support such as gravitational waves, which are ripples in space-time as massive objects pass through. Such waves could directly prove the existence of inflation, which ultimately strengthens the support for the multiverse theory. </p>
<p>Whether we will ever be able to prove their existence is hard to predict. But given the massive implications of such a finding it should definitely be worth the search.</p>
<p><em>Read other articles from our cosmology series <a href="https://theconversation.com/uk/topics/cosmology-series">here</a>.</em></p><img src="https://counter.theconversation.com/content/46497/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Eugene Lim receives funding from UK STFC and FQXi.</span></em></p>The idea that our universe is just one in a ‘multiverse’ of parallel universes is increasingly gathering attention from cosmologists. But can we ever test the theory?Eugene Lim, Lecturer in theoretical particle physics & cosmology, King's College LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/434412015-06-24T10:24:39Z2015-06-24T10:24:39ZDon’t fear falling into a black hole – you may live on as a hologram<figure><img src="https://images.theconversation.com/files/86145/original/image-20150623-19415-1o5yk6p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Black holes don't deserve their bad reputation, says study.</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Black_hole_Cygnus_X-1.jpg">NASA/wikimedia</a></span></figcaption></figure><p>In the movie <a href="http://www.imdb.com/title/tt0816692/">Interstellar</a>, the main character Cooper escapes from a black hole in time to see his daughter Murph in her final days. Some have argued that the movie is so scientific that it <a href="http://www.bbc.co.uk/news/science-environment-33173197">should be taught in schools</a>. In reality, many scientists believe that <a href="http://www.dailymail.co.uk/sciencetech/article-2828836/Five-things-Interstellar-got-wrong-points-got-right-Space-experts-reveal-scientifically-accurate-film-actually-is.html">anything sent into a black hole would probably be destroyed</a>. But a new study suggests that this might not be the case after all. </p>
<p>The research says that, rather than being devoured, a person falling into a black hole would <a href="http://arxiv.org/abs/1506.04342">actually be absorbed into a hologram</a> – without even noticing. The paper challenges a rival theory stating that <a href="http://arxiv.org/abs/1207.3123">anybody falling into a black hole hits a “firewall”</a> and is immediately destroyed. </p>
<h2>Hawking’s black holes</h2>
<p>Forty years ago Stephen Hawking shocked the scientific establishment with his discovery that <a href="http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html">black holes aren’t really black</a>. Classical physics implies that anything falling through the horizon of a black hole can never escape. But Hawking showed that black holes continually emit radiation once quantum effects are taken into account. Unfortunately, for typical astrophysical black holes, the temperature of this radiation is far lower than that of the <a href="http://www.scientificamerican.com/article/what-is-the-cosmic-microw/">cosmic microwave background</a>, meaning detecting them is beyond current technology. </p>
<p>Hawking’s calculations are perplexing. If a black hole continually emits radiation, it will continually lose mass – eventually evaporating. Hawking realised that this implied a paradox: if a black hole can evaporate, the information about it will be lost forever. This means that even if we could measure the radiation from a black hole we could never figure out it was originally formed. This violates an important rule of quantum mechanics that states <a href="http://van.physics.illinois.edu/qa/listing.php?id=24045">information cannot be lost or created</a>.</p>
<p>Another way to look at this is that Hawking radiation poses a problem with determinism for black holes. Determinism implies that the state of the universe at any given time is uniquely determined from its state at any other time. This is how we can trace its evolution both astronomically and mathematically though quantum mechanics. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/86146/original/image-20150623-19371-18g4ko1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/86146/original/image-20150623-19371-18g4ko1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/86146/original/image-20150623-19371-18g4ko1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/86146/original/image-20150623-19371-18g4ko1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/86146/original/image-20150623-19371-18g4ko1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/86146/original/image-20150623-19371-18g4ko1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/86146/original/image-20150623-19371-18g4ko1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Lots of theories but only one way to find out for sure.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/gsfc/15084150039/in/photolist-oYWdhp-8GGV2G-bmeT2o-c8VEpo-tJbJf5-9KgqiH-9KgjhK-arsndK-85HPW2-njnKGG-7tUwtD-nACNem-njnKQN-nASwv1-njnF2D-9KgqhX-bjTBGb-2sEbya-asju2j-qbJxfD-eKKDT9-tnFPM-8mpnm4-961Htv-4c214s-bbeigR-bqBKGc-e7SYXx-m4S4yk-pk3ihS-5m4QNL-7HA85c-by9eag-4P72dh-85wsE-4pjcYr-dKokBK-bASA92-38gp2-cpHLM7-tNDXDU-9a81fE-7A6AQ-9a81fS-9a81fL-4Si5wm-4HmjVt-9a81fW-tNDZjC-uHm7jQ">NASA/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>This means that the loss of determinism would have to arise from <a href="http://plato.stanford.edu/entries/quantum-gravity/">reconciling quantum mechanics with Einstein’s theory of gravity</a> – a notoriously hard problem and ultimate goal for many physicists. Black hole physics provides a test for any potential quantum gravity theory. Whatever your theory is, it must explain what happens to the information recording a black hole’s history.</p>
<p>It took two decades for scientists to <a href="http://arxiv.org/abs/hep-th/9409089">come up with a solution</a>. They suggested that the information stored in a black hole is proportional to its surface area (in two dimensions) rather than its volume (in three dimensions). This could be explained by quantum gravity, where the three dimensions of space could be reconstructed from a two-dimensional world without gravity – much like a hologram. Shortly afterwards, string theory, the most studied theory of quantum gravity, <a href="http://www.aps.org/programs/honors/prizes/prizerecipient.cfm?first_nm=Juan&last_nm=Maldacena&year=2004">was shown to be holographic</a> in this way.</p>
<p>Using holography we can describe the evaporation of the black hole in the two-dimensional world without gravity, for which the usual rules of quantum mechanics apply. This process is deterministic, with small imperfections in the radiation encoding the history of the black hole. So holography tells us that information is not lost in black holes, but tracking down the flaw in Hawking’s original arguments has been surprisingly hard. </p>
<h2>Fuzzballs versus firewalls</h2>
<p>But exactly what the black holes described by quantum theory look like is harder to work out. In 2003, Samir Mathur <a href="http://researchnews.osu.edu/archive/fuzzball.htm">proposed that black holes are in fact fuzzballs</a>, in which there is no sharp horizon. Quantum fluctuations around the horizon region records the information about the hole’s history and thus Mathur’s proposal resolves the information loss paradox. However the idea has been criticised since it implies that somebody falling into a fuzzball has a very different experience to somebody falling into a black hole descried by Einstein’s theory of general relativity. </p>
<p>The general relativity description of black holes suggests that once you go past the event horizon, the surface of a black hole, you can go deeper and deeper. As you do, <a href="http://www.bbc.com/earth/story/20150525-a-black-hole-would-clone-you">space and time become warped</a> until they reach a point called the “singularity” at which point the laws of physics cease to exist. (Although in reality, you would be die pretty early on on this journey as you are <a href="http://casswww.ucsd.edu/archive/public/tutorial/GR.html">pulled apart by intense tidal forces</a>).</p>
<p>In Mathur’s universe, however, there is nothing beyond the fuzzy event horizon. Currently, <a href="http://arxiv.org/abs/1207.3123">a rival theory in quantum gravity</a> is that anybody falling into a black hole hits a “firewall” and is immediately destroyed. The firewall proposal has been criticised since (like fuzzballs) firewalls have drastically different behaviour at the horizon than general relativity black holes.</p>
<p>But Mathur argues that to an outside observer, somebody falling into a fuzzball looks almost the same as somebody falling into an Einstein black hole, even though those falling in have very different experiences. Others working on firewalls and fuzzballs may well feel that these arguments rely on properties of the example he used. Mathur used an explicit description of a very special kind of fuzzball to make his arguments. Such special fuzzballs are probably very different to the fuzzballs needed to describe realistic astrophysical black holes. </p>
<p>The debate about what actually happens when one falls into a black hole will probably continue for some time to come. The key question is to understand is not that the horizon region is reconstructed as a hologram – but exactly how this happens.</p><img src="https://counter.theconversation.com/content/43441/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Marika Taylor receives funding from the Science and Technologies Facilities Council (STFC); the Engineering and Physical Sciences Research Council (EPSRC) and the European Union under the Horizon 2020 programme. </span></em></p>Black holes may not be the ferocious killers they are made out to be, suggests study.Marika Taylor, Professor in theoretical physics, University of SouthamptonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/64182012-04-18T20:10:50Z2012-04-18T20:10:50ZThe origin of the universe: is there a role for God?<figure><img src="https://images.theconversation.com/files/9723/original/3x9vjtmd-1334722976.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The Big Bang theory and the existence of God are ideas often grappled with when thinking about how the universe was created.</span> <span class="attribution"><span class="source">DamienHR</span></span></figcaption></figure><p>Last week’s <a href="http://www.atheistconvention.org.au">Global Atheist Convention</a> and debates between prominent atheists and theologians in the <a href="http://www.youtube.com/watch?v=zqoqAxSO9VI">Australian media</a> has seen arguments about the existence of God getting a thorough airing. </p>
<p>In my view, one of the more interesting arguments – recently debated by evolutionary biologist and famous atheist Richard Dawkins and the archbishop of Sydney, Cardinal George Pell – was around <a href="http://www.abc.net.au/tv/qanda/txt/s3469101.htm">how the universe was created</a>. </p>
<p>This involved questions addressing what caused the <a href="http://en.wikipedia.org/wiki/Big_Bang">Big Bang</a> and whether that necessitated creating something from nothing, thus providing evidence for the existence of God. </p>
<p>As a science undergraduate, I came across two common answers to the question of what came before the Big Bang. The first was something like “as both space and time were created in the Big Bang, there was no time before the Big Bang; thus it doesn’t make sense to ask what came before it”. </p>
<p>The second was that, in simple terms, the whole universe is the result of a <a href="http://en.wikipedia.org/wiki/Quantum_fluctuation">quantum fluctuation</a> in some pre-universe vacuum. This latter view is clearly articulated by the Canadian-American theoretical physicist <a href="http://en.wikipedia.org/wiki/Lawrence_M._Krauss">Lawrence Krauss</a>, who has written a <a href="http://en.wikipedia.org/wiki/A_Universe_from_Nothing:_Why_There_is_Something_Rather_Than_Nothing">book</a> on the subject. </p>
<p>I have since discovered there are a plethora of mind-stretching (and highly speculative) hypotheses around the birth of our universe. They all involve the idea of a vast <a href="http://en.wikipedia.org/wiki/Multiverse">multiverse</a>, comprising a potentially infinite number of universes that are interrelated in various ways. So let’s jump right in and discuss some of these ideas.</p>
<h2>Eternal chaotic inflation</h2>
<p>This <a href="http://en.wikipedia.org/wiki/Eternal_inflation">hypothesis</a> was proposed by the Russian-American theoretical physicist <a href="http://en.wikipedia.org/wiki/Andrei_Linde">Andrei Linde</a>. </p>
<p>Eternal chaotic inflation is a variant of the <a href="http://en.wikipedia.org/wiki/Inflation_%28cosmology%29">inflation model of the universe</a>, which describes how the early universe was thought to have expanded extremely rapidly. </p>
<p>To visualise this hypothesis, imagine a chaotically fluctuating field. The peaks of that field then become points from which exponentially expanding patches of space grow to become “bubble universes”. </p>
<p>This multiverse has Big Bangs going off constantly in exponentially increasing numbers for all eternity. In this view, one of these Big Bangs resulted in the universe we currently observe, but our universe is just one within a huge multiverse.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/34zVzoZugG4?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
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<p></p>
<h2>Ekpyrotic Universe</h2>
<p>This <a href="http://en.wikipedia.org/wiki/Ekpyrotic_universe">hypothesis</a> gets its name from the Greek word Ekpyrotic (“conversion into fire”) and came from the work of physicists <a href="http://arxiv.org/abs/hep-th/0103239">Neil Turok and Paul Steinhardt</a>. </p>
<p>Apart from a different beginning, it’s very similar to the standard Big Bang theory in describing the evolution of structure in our universe. </p>
<p>To understand this hypothesis, we need to make a brief foray into <a href="https://theconversation.com/explainer-string-theory-2983">string theory</a> – a fascinating area of physics that has also been <a href="http://en.wikipedia.org/wiki/The_Trouble_with_Physics">somewhat controversial</a>. </p>
<p>String theory involves extra-spatial dimensions, which we can’t detect because they are “compactified” or curled up on very small scales. </p>
<p>It predicts particles as different types of vibrations on “strings” in these extra dimensions. There is a generalisation of string theory called <a href="http://en.wikipedia.org/wiki/Membrane_%28M-theory%29">M-theory</a>, which involves 11 dimensions and fundamental objects called “branes” (short for membranes). </p>
<p>M-theory predicts that branes exist in varying forms across all 11 dimensions. Our universe is seen as existing on or within a three-dimensional brane embedded in a higher-dimensional space.</p>
<p>To really get to grips with this hypothesis, we need to imagine two massive three-dimensional branes. We could think of them as some sort of proto-universes, embedded within a higher-dimensional space. </p>
<p>The Ekpyrotic Universe hypothesis then speculates that our universe was formed when two such branes collided. The two branes “stuck” together and released massive amounts of energy from the collision that was confined to stay with the three-dimensional branes. </p>
<p>This sudden burst of energy produced radiation and subsequently all the particles that make up our universe, in the same way as occurs within the standard Big Bang theory.</p>
<h2>Fecund universes</h2>
<p>This hypothesis, proposed by theoretical physicist <a href="http://en.wikipedia.org/wiki/Lee_Smolin">Lee Smolin</a>, magnifies the process of evolution to some of largest scales conceivable. </p>
<p>If correct, it would mean our universe was born from the heart of a <a href="https://theconversation.com/scary-monsters-and-supermassive-black-holes-4661">black hole</a> contained in another universe. </p>
<p>It would also imply that all the black holes in our universe are giving birth to new baby universes. The idea here is that the <a href="http://en.wikipedia.org/wiki/Black_hole#Singularity">singularity</a> at the heart of a black hole gives birth to a new space-time, resulting in a whole new universe separate to our own. </p>
<p>The fecund universes hypothesis (also known as <a href="http://evodevouniverse.com/wiki/Cosmological_natural_selection_(fecund_universes)">cosmological natural selection</a> further supposes that in each new universe the laws of physics will be slightly different from the mother universe that gave birth to it. </p>
<p>The fact that black holes would provide a mechanism for giving birth to new universes results in a natural selection pressure for universes with laws of physics that allow black hole formation. </p>
<p>This process of replication, mutation and selection pressure means that the multiverse is likely to be dominated by universes that allow black hole formation. </p>
<p>Our universe certainly <a href="http://www.sciencemag.org/content/300/5627/1898.abstract">falls into this category</a>.</p>
<figure>
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<h2>The selfish biocosm (SB)</h2>
<p>The <a href="http://www.biocosm.org">SB hypothesis</a> was developed by theorist and author <a href="http://en.wikipedia.org/wiki/James_N._Gardner">James N. Gardner</a> who, unlike the creators of the previously mentioned hypotheses, is not a physicist – though his hypothesis has been published in <a href="http://onlinelibrary.wiley.com/doi/10.1002/%28SICI%291099-0526%28200001/02%295:3%3C34::AID-CPLX7%3E3.0.CO;2-8/abstract">peer-reviewed journals</a>. </p>
<p>Gardner’s main idea is that life itself facilitates the feat of cosmic replication resulting in the birth of new baby universes. </p>
<p>This would occur after life had been evolving for billions of years, resulting in highly sophisticated intelligences that would infuse the entire universe. </p>
<p>These highly evolved intelligences would develop a level of understanding of the laws of physics that would allow them to create new baby universes. </p>
<p>Gardner further proposes these intelligences would choose to design new baby universes with laws of physics that would allow new intelligent forms of life to evolve. </p>
<p>This idea has some interesting implications. For one, it predicts the ability of the laws of physics in our universe to support life is not an accident, but results from an evolutionary chain of universes within a multiverse, with each generation becoming increasingly “bio-friendly”. </p>
<p>Gardner’s description of highly evolved intelligences – infused throughout their universe, with the ability to create new universes, potentially at will – could also fit very will with some definitions of God. </p>
<p>The SB hypothesis thus implies the future destiny of intelligence in our universe may be to become this type of “God”. Further, it implies this emerges from the laws of physics that have been “designed” by the “Gods” in our mother universe. </p>
<p>This puts a whole new slant on the debate between <a href="http://en.wikipedia.org/wiki/Darwinism">Darwinism</a> and <a href="http://en.wikipedia.org/wiki/Intelligent_design">Intelligent Design</a>, with the SB Hypothesis falling somewhere between the two. </p>
<p>Importantly, this obviously speculative hypothesis attempts to be consistent with current scientific knowledge and requires no intervention from an external God.</p>
<figure>
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</figure>
<p>Compared with these mind-stretching ideas about the origin of our universe and the potential structure of the multiverse, the debate between atheists and creationists about how, or if, the universe could be created out of nothing can perhaps be seen as simplistic. </p>
<p>The debate would become considerably more interesting if those on both sides more thoroughly familiarised themselves with these and other similar ideas. </p>
<p>While the possibility of any of these hypotheses being correct doesn’t provide new evidence for or against the existence of God, they do point to an ultimate reality that’s potentially far more interesting than we originally thought.</p><img src="https://counter.theconversation.com/content/6418/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ascelin Gordon receives funding from Australian Research Council.</span></em></p>Last week’s Global Atheist Convention and debates between prominent atheists and theologians in the Australian media has seen arguments about the existence of God getting a thorough airing. In my view…Ascelin Gordon, Research Fellow, Global Studies, Social Science & Planning, RMIT UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/25402011-09-11T20:11:41Z2011-09-11T20:11:41ZPeer Review: The Fallacy of Fine-Tuning<figure><img src="https://images.theconversation.com/files/3484/original/5507545821_3c67520e17_b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A universe composed differently could still support complex life.</span> <span class="attribution"><span class="source">Susan NYC</span></span></figcaption></figure><p><em>Welcome to Peer Review, a series in which we ask leading academics to review books written by people working in the same field.</em></p>
<p><em>Here Geraint Lewis, Professor of Astrophysics at the University of Sydney, reviews <a href="http://www.amazon.com/Fallacy-Fine-Tuning-Why-Universe-Designed/dp/1616144432/ref=sr_1_1?ie=UTF8&s=books&qid=1294782033&sr=1-1">The Fallacy of Fine-Tuning: Why the Universe is Not Designed for Us</a> by Victor J. Stenger, Professor Emeritus of Physics and Astronomy at the University of Hawaii.</em></p>
<p>We are a product of <a href="http://theconversation.com/explainer-theory-of-evolution-2276">evolution</a>, and are not surprised that our bodies seem to be well-suited to the environment. </p>
<p>Our leg bones are strong enough to allow for Earth’s gravitational pull – not too weak to shatter, not so massively over-engineered as to be wasteful.</p>
<p>But it could also be claimed we are special and the environment was formed and shaped for us. </p>
<p>This, as we know, is the basis of many religious ideas.</p>
<p>In recent years, such ideas have been expanded beyond Earth to look at the entire universe and our place within it. </p>
<p>The so-called <a href="http://www.discovery.org/a/91">Fine-Tuning Argument</a> – that the laws of physics have been specially-tuned, potentially by some Supreme Being, to allow human life to arise – is the focus of Victor J. Stenger’s book.</p>
<p>Stenger presents the mathematics underpinning cosmic evolution, the lifetime of stars, the quantum nature of atoms and so on. His central is that “fine-tuning” claims are fatally flawed.<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/3486/original/finetuning.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/3486/original/finetuning.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=900&fit=crop&dpr=1 600w, https://images.theconversation.com/files/3486/original/finetuning.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=900&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/3486/original/finetuning.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=900&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/3486/original/finetuning.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1131&fit=crop&dpr=1 754w, https://images.theconversation.com/files/3486/original/finetuning.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1131&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/3486/original/finetuning.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1131&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
</figcaption>
</figure> </p>
<p>He points out that some key areas of physics – such as the equality of the <a href="http://www.ndt-ed.org/EducationResources/HighSchool/Electricity/electriccharge.htm">charges on the electron and proton</a> – are set by conservation laws determined by symmetries in the universe, and so are not free to play with.</p>
<p>Some flaws in the theory, he argues, run deeper.</p>
<p>A key component of the fine-tuning argument is that there are many parameters governing our universe, and that changing any one of these would likely produce a sterile universe unlike our own. </p>
<p>But think of baking a cake. Arbitrarily doubling only the flour, or sugar or vanilla essence may end in a cooking disaster, but doubling all the ingredients results in a perfectly tasty cake.</p>
<p>The interrelationships between the laws of physics are somewhat more complicated, but the idea is the same. </p>
<p>A hypothetical universe in which gravity was stronger, the masses of the fundamental particles smaller and electomagnetic force weaker may well result in the following: a universe that appears a little different to our own, but is still capable of producing long-lived stars and heavy chemical elements, the basic requirements for complex life.</p>
<p>Stenger backs up such points with his own research, and provides access to a web-based program he wrote called <a href="http://www.colorado.edu/philosophy/vstenger/Cosmo/monkey.html">MonkeyGod</a>. </p>
<p>The program allows you to conjure up universes with differing underlying physics. And, as Stenger shows, randomly plucking universe parameters from thin air can still produce universes quite capable of harbouring life.</p>
<p>This book is a good read for those wanting to understand the fine-tuning issues in cosmology, and it’s clear Stenger really understands the science. </p>
<p>But while many of the discussions are robust, I felt that in places some elements of the fine-tuning argument were brushed aside with little real justification.</p>
<p>As a case in point, Stenger falls back on <a href="http://www.astronomy.pomona.edu/Projects/moderncosmo/Sean's%20mutliverse.html">multiverse theory</a> and the <a href="http://www.enotes.com/science-religion-encyclopedia/anthropic-principle">anthropic principle</a>, whereby we occupy but one of an almost infinite sea of different universes, each with a different law of physics. </p>
<p>In multiverse theory, most universes would be sterile (though we should not be surprised to find ourselves in a habitable universe). </p>
<p>While such a multiverse – the staple of <a href="http://theconversation.com/explainer-string-theory-2983">superstring</a> and <a href="http://www.damtp.cam.ac.uk/research/gr/public/qg_ss.html">brane</a> ideas of the cosmos – is often sold as science fact, it actually lies much closer to the world of science speculation (or, to many, fiction).</p>
<p>We are not out of the fine-tuning waters yet, but Stenger’s book is a good place to start getting to grips with the issues.</p>
<p><em>The Fallacy of Fine-Tuning: Why the Universe is Not Designed for Us by Victor J. Stenger (Prometheus Books) is available now.</em></p>
<p><em><strong>If you’re an academic and have a book you’d like reviewed, or if you’d like to review a book for us, <a href="mailto:paul.dalgarno@theconversation.edu.au">contact the science and technology editor</a>, subject: Peer Review.</strong></em></p><img src="https://counter.theconversation.com/content/2540/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Geraint Lewis does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Welcome to Peer Review, a series in which we ask leading academics to review books written by people working in the same field. Here Geraint Lewis, Professor of Astrophysics at the University of Sydney…Geraint Lewis, Professor of Astrophysics, University of SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/29832011-08-29T20:47:54Z2011-08-29T20:47:54ZExplainer: String theory<figure><img src="https://images.theconversation.com/files/3054/original/5882079207_f07920c392_b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">How long's a piece of string? You may want to sit down for a minute.</span> <span class="attribution"><span class="source">Gnu2000</span></span></figcaption></figure><p>String theory entered the public arena in 1988 when a BBC radio series Desperately Seeking Superstrings was broadcast. </p>
<p>Thanks to good marketing and its inherently curious name and features, it’s now part of popular discourse, mentioned in TV’s Big Bang Theory, Woody Allen stories, and countless science documentaries. </p>
<p>But what is string theory and why does it find itself shrouded in controversy? </p>
<h2>Life, the universe and the theory of everything</h2>
<p>Today we think of string theory in two ways. </p>
<p>It’s seen as a theory of everything – that is, a theory that aims to describe <a href="http://library.thinkquest.org/27930/forces.htm">all four forces of nature</a> within a single theoretical scheme.</p>
<p>These forces are:</p>
<ul>
<li>Electromagnetic force </li>
<li>Gravitational force</li>
<li>Weak nuclear force </li>
<li>Strong nuclear force.</li>
</ul>
<p>Electromagnetism and gravity are familiar to most people. The nuclear forces occur at a subatomic level, and are unobservable by the naked eye.</p>
<p>String theory is also used to describe <a href="http://plato.stanford.edu/entries/quantum-gravity/">quantum gravity</a>, a theory that combines Einstein’s <a href="http://www.youtube.com/watch?v=hbmf0bB38h0">theory of gravity</a> and the principles of <a href="http://www.thebigview.com/spacetime/quantumtheory.html">quantum theory</a>.</p>
<h2>Tangled beginnings</h2>
<p>But string theory began life more modestly, as a way to describe strongly interacting particles called <a href="http://www2.slac.stanford.edu/vvc/theory/hadrons.html">hadrons</a>. </p>
<p>Hadrons are now understood to be composed of <a href="http://www2.slac.stanford.edu/vvc/theory/quarks.html">quarks</a> connected by <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/particles/expar.html">gluons</a> but string theory viewed them as quarks connected by strings (tubes of energy). </p>
<p>Understood this way, string theory buckled under both new experimental evidence (leading to the crowning of <a href="http://www.frankwilczek.com/Wilczek_Easy_Pieces/298_QCD_Made_Simple.pdf">quantum chromodynamics</a> which describes the interactions of quarks and gluons) and also internal problems.</p>
<p>String theory involved too many particles, including a massless particle with so-called <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/particles/spinc.html">spin 2</a> – spin being the name used for the <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/amom.html">angular momentum</a> of particles.</p>
<p>As it happens, this is exactly the property possessed by the <a href="http://curious.astro.cornell.edu/question.php?number=535">graviton</a> – the carrier of gravitational force in the particle physics picture of the world. </p>
<h2>Beyond four dimensions</h2>
<p>This discovery meant that with a bit of skilful rebranding (and rescaling the energy of the strings to match the strength of gravitation), string theory shed its hadronic past and was reborn as a quantum theory of gravity. </p>
<p>All those other particles that were also problematic for the original string theory were able to capture the remaining non-gravitational forces too. This is how string theory took on its current role as describing all four forces together: a theory of everything.</p>
<p>But it could not shed many of its curious features. </p>
<p>One such feature was the necessity of many more space-time dimensions than are actually observed.</p>
<p>In a “bosonic” version of string theory (i.e. <em>without</em> matter or <a href="http://http://hyperphysics.phy-astr.gsu.edu/hbase/particles/spinc.html">fermions</a>, there would have to be 21 dimensions – 20 space dimensions and one time dimension.</p>
<p>In a theory <em>with</em> fermions, there would have to be nine spatial dimensions and one temporal, ten dimensions all together.</p>
<p>The problem is that we only perceive four dimensions: height, width, depth (all spatial) and time (temporal).</p>
<h2>Supersizing symmetry, downsizing dimensions </h2>
<p>The “super” in “superstring theory” refers to a symmetry, known as <a href="http://en.wikipedia.org/wiki/Supersymmetry">supersymmetry</a>, relating bosons and fermions. </p>
<p>There are five possible theories that involve matter in ten dimensions. This was previously seen as a problem since it was expected that a theory of everything should be unique.</p>
<p>The six unseen dimensions (ten minus the four dimensions of everyday life) are made too small to be observable, using a process known as <a href="http://www.brynmawr.edu/physics/MBSchulz/research/compactifications.html">compactification</a>. </p>
<p> </p>
<h2>Beautiful maths</h2>
<p>It is from this process that much of the extraordinarily beautiful (and <a href="http://www.math.upenn.edu/StringMath2011/">fiendishly difficult</a>) mathematics involved in string theory stems. </p>
<p>We have no trouble thinking of each event in the world as labeled by four numbers or coordinates (e.g., x,y,z,t). </p>
<p>A string-theoretic world adds another six coordinates, only they are crumpled up into a tiny space of radius comparable to the string length, so we don’t see them. </p>
<p>But, according to string theory, their effects can be seen indirectly by the way strings moving through spacetime will wrap around those crumpled, curled up directions. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/3202/original/Lunch_at_en.wikimedia.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/3202/original/Lunch_at_en.wikimedia.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/3202/original/Lunch_at_en.wikimedia.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/3202/original/Lunch_at_en.wikimedia.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/3202/original/Lunch_at_en.wikimedia.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/3202/original/Lunch_at_en.wikimedia.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/3202/original/Lunch_at_en.wikimedia.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A Calabi-Yau space, visible to strings, but not to us.</span>
<span class="attribution"><span class="source">Lunch</span></span>
</figcaption>
</figure>
<p>There are very many ways of hiding those six dimensions, yielding more possible stringy worlds (perhaps as many as <a href="http://www.edge.org/3rd_culture/susskind03/susskind_index.html">10<sup>500</sup></a>!). </p>
<h2>How long is a piece of string?</h2>
<p>This is why string theory is so controversial. It seemingly loses all predictive power since we have no way of isolating our world amongst this plenitude. </p>
<p>And what good is a scientific theory if it cannot make predictions?</p>
<p>One response is to say that these various theories are not really so different. In fact there are all sorts of exact relations known as <a href="http://web.physics.ucsb.edu/%7Estrings/superstrings/duality.htm">dualities</a> connecting them. </p>
<p>More recent developments based on these dualities include a new type of object with higher dimensions – so called <a href="http://www.slac.stanford.edu/slac/sass/talks/bernlochner.pdf">Dp-branes</a>. </p>
<p>These too can wrap around the compact dimensions to generate potentially observable effects. </p>
<p>Most importantly, they can also provide boundaries on which endpoints of strings sit.</p>
<p>Just to complicate things more, a new kind of theory has been discovered, this time in 11 dimensions: 11 dimensional supergravity - it is also <a href="http://www.youtube.com/watch?v=Iw07AyLu3n4">very beautiful mathematically</a>.</p>
<h2>Dial M for Multiverse</h2>
<p>String theorists are fond of saying that these six theories are aspects (special limits) of a deeper underlying theory, known as <a href="http://www.damtp.cam.ac.uk/research/gr/public/qg_ss.html">M-theory</a>. In this way, uniqueness is restored. </p>
<p>Or is it? </p>
<p>We still have the spectre of the 10<sup>500</sup> solutions or worlds. The great hope is that the number of solutions with features like our own world’s (with its four visible dimensions, particles of various types interacting with particular strengths, conscious observers, and so on) will be small enough to be able to extract testable predictions. </p>
<p>So far, though, the only real way of getting our world out of the theory involves the use of a <a href="http://lpsc.in2p3.fr/ams/aurelien/aurelien/CCDecMULTIV.pdf">multiverse</a> (a realistically interpreted ensemble of string theoretic worlds with differing physical properties) combined with the <a href="http://www.colorado.edu/philosophy/vstenger/Cosmo/ant_encyc.pdf">anthropic principle</a> (only some of these worlds have what it takes to support humans). </p>
<p>Needless to say, this does not entirely sit easy with critics of string theory!</p>
<p>But string theory has been making strides in other areas of physics. Notably in the physics of plasmas and of <a href="http://www.nature.com/news/2009/090719/full/news.2009.699.html">superconductors</a>. </p>
<p>Whether this success can be repeated within its proper realm (fundamental physics) remains to be seen.</p><img src="https://counter.theconversation.com/content/2983/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Dean Rickles receives funding from the ARC.</span></em></p>String theory entered the public arena in 1988 when a BBC radio series Desperately Seeking Superstrings was broadcast. Thanks to good marketing and its inherently curious name and features, it’s now part…Dean Rickles, Associate Professor, University of SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/4452011-05-08T21:01:16Z2011-05-08T21:01:16ZExplainer: the fifth dimension<figure><img src="https://images.theconversation.com/files/877/original/4291413264_137620e540_o.jpg?ixlib=rb-1.1.0&rect=67%2C168%2C850%2C625&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">We've got the time, if you've got the theory.</span> <span class="attribution"><span class="source">h.koppdelaney/Flickr</span></span></figcaption></figure><p>By now we’re used to the idea that the world has four dimensions: three spatial and one temporal. But what if there were a fifth dimension – what would that dimension look like, and how would it relate to time? </p>
<p>One of the central threads running through the philosophy of time concerns the idea that time flows. This doesn’t sound controversial: for most people, it flows in much the same way as a river. </p>
<p>But there are problems with this view.</p>
<p>If time flows, it’s surely reasonable to wonder about the rate it flows at. </p>
<p>But rates of flow are construed as ratios over time – a river flows at one metre per unit of time, say – so it would seem time should also flow at some rate over time: one second per second. </p>
<p>This doesn’t work, though: it’s like saying that, for every dollar you give me, I give you a dollar back: what could be gained in such a transaction? </p>
<p>We commonly think of time flowing into the future, away from the past, but time wouldn’t “go anywhere” if it flowed at one second per second.</p>
<h2>What to do?</h2>
<p>One option might be to construe the flow of time as a ratio of time over space, so that time flows, for example, at one second per metre. </p>
<p>But this too would be pretty odd: treating time as dependent on space in this fashion flies in the face of our intuitive understanding of its nature.</p>
<p>Another alternative might be to invoke a further temporal dimension – a fifth dimension – which can then be used as the yardstick for measuring temporal flow. </p>
<p>Call the ordinary temporal dimension “A” and this new temporal dimension “B”. In this view, time flows at one second of A per second of B. Thus, time now “goes somewhere” in that it charts a path through a higher dimension.</p>
<p>Appealing to a fifth dimension in this fashion is often seen as a strategy of last resort by philosophers of time: the idea is simply too wild to take seriously. </p>
<h2>The LHC </h2>
<p>It has been predicted that the <a href="https://theconversation.com/is-the-large-hadron-collider-a-time-machine-447">Large Hadron Collider</a> in Switzerland might generate particles that time-travel by taking shortcuts through a fifth dimension.</p>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/860/original/File_FifthDimension.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/860/original/File_FifthDimension.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/860/original/File_FifthDimension.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/860/original/File_FifthDimension.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/860/original/File_FifthDimension.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/860/original/File_FifthDimension.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/860/original/File_FifthDimension.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Sixties group The Fifth Dimension: way ahead of their time?</span>
<span class="attribution"><span class="source">Arnie Lee</span></span>
</figcaption>
</figure>
<h2>Spatial treatment</h2>
<p>Unfortunately, according to the physicists responsible for the relevant experimental predictions, the fifth dimension is spatial, not temporal. </p>
<p>Even if the fifth dimension <em>were</em> temporal, there’d still be a problem. This is because the experimental predictions conducted so far are produced against the backdrop of a particular physical theory: <a href="http://en.wikipedia.org/wiki/General_relativity">general relativity</a>.</p>
<p>General relativity undermines any basis we might have for believing that time flows at all, as it’s portrayed as a space-like dimension. This means that, very roughly, we should think time flows only if it’s coherent to think of space flowing, which seems implausible. </p>
<h2>On the plus side … </h2>
<p>Perhaps we have reason to take heart. Although general relativity plays a role in relevant experimental predictions, those predictions are actually coming from a particular theory of quantum gravity: <a href="http://www.nucleares.unam.mx/%7Ealberto/physics/string.html">string theory</a>.</p>
<p>This theory reconciles our best theories of the very big (general relativity) with the very small (<a href="http://www.thebigview.com/spacetime/quantumtheory.html">quantum theory</a>). </p>
<p>Some versions of string theory posit as many as 11 dimensions. </p>
<p>With so many, surely it’ll be possible to make some sense of the idea that time flows … won’t it?</p><img src="https://counter.theconversation.com/content/445/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Sam Baron does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>By now we’re used to the idea that the world has four dimensions: three spatial and one temporal. But what if there were a fifth dimension – what would that dimension look like, and how would it relate…Sam Baron, Postgraduate Research Student, University of SydneyLicensed as Creative Commons – attribution, no derivatives.