tag:theconversation.com,2011:/fr/topics/quantum-gravity-18150/articlesQuantum gravity – 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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<a href="https://theconversation.com/explainer-standard-model-of-particle-physics-2539">Explainer: Standard Model of Particle Physics</a>
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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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Read more:
<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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<a href="https://theconversation.com/explainer-string-theory-2983">Explainer: String theory</a>
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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/1812682022-04-14T05:55:33Z2022-04-14T05:55:33ZTime might not exist, according to physicists and philosophers – but that’s okay<figure><img src="https://images.theconversation.com/files/458074/original/file-20220414-12-s9hvgo.jpg?ixlib=rb-1.1.0&rect=17%2C2%2C1979%2C1119&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/composition-space-time-flight-spiral-roman-1221181900">Shutterstock</a></span></figcaption></figure><p>Does time exist? The answer to this question may seem obvious: of course it does! Just look at a calendar or a clock.</p>
<p>But developments in physics suggest the non-existence of time is an open possibility, and one that we should take seriously.</p>
<p>How can that be, and what would it mean? It’ll take a little while to explain, but don’t worry: even if time doesn’t exist, our lives will go on as usual.</p>
<h2>A crisis in physics</h2>
<p>Physics is in crisis. For the past century or so, we have explained the universe with two wildly successful physical theories: general relativity and quantum mechanics.</p>
<p>Quantum mechanics describes how things work in the incredibly tiny world of particles and particle interactions. <a href="https://theconversation.com/how-einsteins-general-theory-of-relativity-killed-off-common-sense-physics-50042">General relativity</a> describes the big picture of gravity and how objects move.</p>
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<a href="https://theconversation.com/how-einsteins-general-theory-of-relativity-killed-off-common-sense-physics-50042">How Einstein's general theory of relativity killed off common-sense physics</a>
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<p>Both theories work extremely well in their own right, but the two are thought to conflict with one another. Though the exact nature of the conflict is controversial, scientists generally agree both theories need to be replaced with a new, more general theory. </p>
<p>Physicists want to produce a theory of “quantum gravity” that <em>replaces</em> general relativity and quantum mechanics, while capturing the extraordinary success of both. Such a theory would explain how gravity’s big picture works at the miniature scale of particles.</p>
<h2>Time in quantum gravity</h2>
<p>It turns out that producing a theory of quantum gravity is extraordinarily difficult. </p>
<p>One attempt to overcome the conflict between the two theories is <a href="https://theconversation.com/explainer-string-theory-2983">string theory</a>. String theory replaces particles with strings vibrating in as many as 11 dimensions. </p>
<p>However, string theory faces a further difficulty. String theories provide a range of models that describe a universe broadly like our own, and they don’t really make any clear predictions that can be tested by experiments to figure out which model is the right one. </p>
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Read more:
<a href="https://theconversation.com/explainer-string-theory-2983">Explainer: String theory</a>
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<p>In the 1980s and 1990s, many physicists became dissatisfied with string theory and came up with a range of new mathematical approaches to quantum gravity. </p>
<p>One of the most prominent of these is <a href="https://www.space.com/loop-quantum-gravity-space-time-quantized">loop quantum gravity</a>, which proposes that the fabric of space and time is made of a network of extremely small discrete chunks, or “loops”.</p>
<p>One of the remarkable aspects of loop quantum gravity is that it appears to eliminate time entirely. </p>
<p>Loop quantum gravity is not alone in abolishing time: a number of other approaches also seem to remove time as a fundamental aspect of reality. </p>
<h2>Emergent time</h2>
<p>So we know we need a new physical theory to explain the universe, and that this theory might not feature time.</p>
<p>Suppose such a theory turns out to be correct. Would it follow that time <em>does not exist</em>? </p>
<p>It’s complicated, and it depends what we mean by <em>exist</em>.</p>
<p>Theories of physics don’t include any tables, chairs, or people, and yet we still accept that tables, chairs and people exist.</p>
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<img alt="A person walking beneath a large clock swinging from a rope." src="https://images.theconversation.com/files/458075/original/file-20220414-24-y990i8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/458075/original/file-20220414-24-y990i8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/458075/original/file-20220414-24-y990i8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/458075/original/file-20220414-24-y990i8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/458075/original/file-20220414-24-y990i8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/458075/original/file-20220414-24-y990i8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/458075/original/file-20220414-24-y990i8.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">If time isn’t a fundamental property of the universe, it may still ‘emerge’ from something more basic.</span>
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<p>Why? Because we assume that such things exist at a higher level than the level described by physics. </p>
<p>We say that tables, for example, “emerge” from an underlying physics of particles whizzing around the universe. </p>
<p>But while we have a pretty good sense of how a table might be made out of fundamental particles, we have no idea how time might be “made out of” something more fundamental.</p>
<p>So unless we can come up with a good account of how <a href="https://www.preposterousuniverse.com/blog/2013/10/18/is-time-real/">time emerges</a>, it is not clear we can simply assume time exists. </p>
<p>Time might not exist at any level.</p>
<h2>Time and agency</h2>
<p>Saying that time does not exist at any level is like saying that there are no tables at all.</p>
<p>Trying to get by in a world without tables might be tough, but managing in a world without time seems positively disastrous.</p>
<p>Our entire lives are built around time. We plan for the future, in light of what we know about the past. We hold people morally accountable for their past actions, with an eye to reprimanding them later on.</p>
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Read more:
<a href="https://theconversation.com/time-is-but-a-dream-or-is-it-928">Time is but a dream ... or is it?</a>
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<p>We believe ourselves to be <em>agents</em> (entities that can <em>take action</em>) in part because we can plan to act in a way that will bring about changes in the future. </p>
<p>But what’s the point of acting to bring about a change in the future when, in a very real sense, there is no future to act for?</p>
<p>What’s the point of punishing someone for a past action, when there is no past and so, apparently, no such action?</p>
<p>The discovery that time does not exist would seem to bring the entire world to a grinding halt. We would have no reason to get out of bed. </p>
<h2>Business as usual</h2>
<p>There is a way out of the mess. </p>
<p>While physics might eliminate time, it seems to leave <em>causation</em> intact: the sense in which one thing can bring about another. </p>
<p>Perhaps what physics is telling us, then, is that causation and not time is the basic feature of our universe. </p>
<p>If that’s right, then agency can still survive. For it is possible to reconstruct a sense of agency entirely in causal terms. </p>
<p>At least, that’s what Kristie Miller, Jonathan Tallant and I argue in <a href="https://global.oup.com/academic/product/out-of-time-9780192864888?facet_narrowbypubdate_facet=Next%203%20months&lang=en&cc=kw">our new book</a>.</p>
<p>We suggest the discovery that time does not exist may have no direct impact on our lives, even while it propels physics into a new era.</p><img src="https://counter.theconversation.com/content/181268/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>Cutting-edge theories of physics suggest time may not be real – but even if they’re right, life can still go on as usual.Sam Baron, Associate Professor, Philosophy of Science, Australian Catholic UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1627852021-06-17T20:05:52Z2021-06-17T20:05:52ZApproaching zero: super-chilled mirrors edge towards the borders of gravity and quantum physics<figure><img src="https://images.theconversation.com/files/406468/original/file-20210615-3629-ntfm3c.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C2746%2C1835&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Caltech / MIT / LIGO Lab</span>, <span class="license">Author provided</span></span></figcaption></figure><p>The <a href="https://www.ligo.caltech.edu">LIGO gravitational wave observatory</a> in the United States is so sensitive to vibrations it can detect the tiny ripples in space-time called gravitational waves. These waves are caused by colliding black holes and other stellar cataclysms in distant galaxies, and they cause movements in the observatory much smaller than a proton. </p>
<p>Now we have used this sensitivity to effectively chill a 10-kilogram mass down to less than one billionth of a degree above absolute zero.</p>
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Read more:
<a href="https://theconversation.com/explainer-why-you-can-hear-gravitational-waves-when-things-collide-in-the-universe-92356">Explainer: why you can hear gravitational waves when things collide in the universe</a>
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<p>Temperature is a measure of how much, and how fast, the atoms and molecules that surround us (and that we are made of) are moving. When objects cool down, their molecules move less. </p>
<p>“Absolute zero” is the point where atoms and molecules stop moving entirely. However, quantum mechanics says the complete absence of motion is not really possible (due to the <a href="https://theconversation.com/explainer-heisenbergs-uncertainty-principle-7512">uncertainty principle</a>). </p>
<p>Instead, in quantum mechanics the temperature of absolute zero corresponds to a “motional ground state”, which is the theoretical minimum amount of movement an object can have. The 10-kilogram mass in our experiment is about 10 trillion times heavier than the previous heaviest mass cooled to this kind of temperature, and it was cooled to nearly its motional ground state.</p>
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<span class="caption">One of the four Advanced LIGO 40-kg mirrors that are cooled near their quantum ground state.</span>
<span class="attribution"><span class="source">Danny Sellers / Caltech / MIT / LIGO Lab</span>, <span class="license">Author provided</span></span>
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<p>The work, <a href="https://science.sciencemag.org/cgi/doi/10.1126/science.abh2634">published today in Science</a>, is an important step in the ongoing quest to understand the gap between quantum mechanics — the strange science that rules the universe at very small scales — and the macroscopic world we see around us. </p>
<p>Plans are already under way to improve the experiment in more sensitive gravitational wave observatories of the future. The results may offer insight into the inconsistency between quantum mechanics and the theory of general relativity, which describes gravity and the behaviour of the universe at very large scales.</p>
<h2>How it works</h2>
<p>LIGO detects gravitational waves using lasers fired down long tunnels and bounced between two pairs of 40-kilogram mirrors, then combined to produce an interference pattern. Tiny changes in the distance between the mirrors show up as fluctuations in the laser intensity.</p>
<p>The motion of the four mirrors is controlled very precisely, to isolate them from any surrounding vibrations and even to compensate for the impact of the laser light bouncing off them. </p>
<p>This part may be hard to get your head around, but we can show mathematically that the <em>differences</em> in the motion of the four 40-kilogram mirrors is equivalent to the motion of a single 10-kilogram mirror. What this means is that the pattern of laser intensity changes we observe in this experiment is the same as what we would see from a single 10-kilogram mirror.</p>
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<a href="https://images.theconversation.com/files/406464/original/file-20210615-22-lyn95.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/406464/original/file-20210615-22-lyn95.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406464/original/file-20210615-22-lyn95.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406464/original/file-20210615-22-lyn95.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406464/original/file-20210615-22-lyn95.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406464/original/file-20210615-22-lyn95.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406464/original/file-20210615-22-lyn95.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406464/original/file-20210615-22-lyn95.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"></a>
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<span class="caption">One of the four Advanced LIGO 40-kg mirrors that are cooled near their quantum ground state.</span>
<span class="attribution"><span class="source">Matt Heintze / Caltech / MIT / LIGO Lab</span>, <span class="license">Author provided</span></span>
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<p>Although the temperature of the 10-kilogram mirror is defined by the motion of the atoms and molecules that make it up, we don’t measure the motion of the individual molecules. Instead, and largely because it’s how we measure gravitational waves, we measure the average motion of all the atoms (or the centre-of-mass motion). </p>
<p>There are at least as many ways the atoms can move as there are atoms, but we only measure one of those ways, and that particular dance move of all the atoms together is the only one we cooled. </p>
<p>The result is that while the four physical mirrors remain at room temperature and would be warm to the touch (if we let anyone touch them), the average motion of the 10-kilogram system is effectively at 0.77 nanokelvin, or less than one billionth of a degree above absolute zero.</p>
<h2>Squeezed light</h2>
<p>Our contribution to Advanced LIGO, as members of Australia’s <a href="https://www.ozgrav.org">OzGrav</a> gravitational wave research centre, was to design, install and test the “quantum squeezed light” system in the detector. This system creates and injects a specially engineered quantum field into the detector, making it more sensitive to the motion of the mirrors, and thus more sensitive to gravitational waves.</p>
<p>The squeezed light system uses a special kind of crystal to produce pairs of highly correlated or “entangled” photons, which reduce the amount of noise in the system. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/406911/original/file-20210617-19-zlv9gg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406911/original/file-20210617-19-zlv9gg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=257&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406911/original/file-20210617-19-zlv9gg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=257&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406911/original/file-20210617-19-zlv9gg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=257&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406911/original/file-20210617-19-zlv9gg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=323&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406911/original/file-20210617-19-zlv9gg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=323&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406911/original/file-20210617-19-zlv9gg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=323&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Australian National University scientists Nutsinee Kijbunchoo and Terry McRae build components for a quantum squeezed light source at LIGO Hanford Observatory in Washington, US.</span>
<span class="attribution"><span class="source">Nutsinee Kijbunchoo</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/were-going-to-get-a-better-detector-time-for-upgrades-in-the-search-for-gravitational-waves-100382">We're going to get a better detector: time for upgrades in the search for gravitational waves</a>
</strong>
</em>
</p>
<hr>
<h2>What does it all mean?</h2>
<p>Being able to observe one particular property of these mirrors approach a quantum ground state is a by-product of improving LIGO in the quest to do more and better gravitational wave astronomy, but it might also offer insights into the vexed question of quantum mechanics and gravity. </p>
<p>At very small scales, quantum mechanics allows many strange phenomena, such as objects being both waves and particles, or seemingly existing in two places at the same time. However, even though the macroscopic world we see is built from tiny objects that must obey quantum phenomena, we don’t see these quantum effects at larger scales. </p>
<p>One theory about why this happens is the idea of <em>decoherence</em>. This suggests that heat and vibrations from a quantum system’s surroundings disrupt its quantum state and make it behave like a familiar solid object.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/406465/original/file-20210615-15-a86of1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406465/original/file-20210615-15-a86of1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406465/original/file-20210615-15-a86of1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406465/original/file-20210615-15-a86of1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406465/original/file-20210615-15-a86of1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406465/original/file-20210615-15-a86of1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406465/original/file-20210615-15-a86of1.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">One of the four Advanced LIGO 40-kg mirrors that are cooled near their quantum ground state.</span>
<span class="attribution"><span class="source">Danny Sellers / Caltech / MIT / LIGO Lab</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>In order to measure gravitational waves, LIGO is designed to not be affected by heat or vibrations from its surroundings, but LIGO test masses are heavy enough for gravity to be a possible cause of decoherence. </p>
<p>Despite a century of searching, we have no way to reconcile gravity and quantum mechanics. Experiments like this, especially if they can get even closer to the ground state, might yield insight into this puzzle. </p>
<p>As we improve LIGO over the next few years, we can re-do this quantum mechanics experiment and maybe see what happens when we cross over from the classical world into the quantum world with human-sized objects.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-gravity-5256">Explainer: gravity</a>
</strong>
</em>
</p>
<hr>
<img src="https://counter.theconversation.com/content/162785/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Ernest McClelland receives funding from the Australian Research Council. </span></em></p><p class="fine-print"><em><span>Robert Ward has received funding from the Australian Research Council</span></em></p><p class="fine-print"><em><span>Terry McRae receives funding from the Australian Research Council. </span></em></p>The world’s biggest gravitational wave observatory is now probing the limits of quantum mechanics.David Ernest McClelland, Distinguised Professor and Director Centre for Gravitational Astrophysics, Australian National UniversityRobert Ward, Associate Investigator, OzGrav (ARC Centre of Excellence for Gravitational Wave Discovery), Research Fellow in Physics, Australian National UniversityTerry McRae, Research fellow, gravitational wave detection, Australian National UniversityLicensed 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>
<figcaption>
<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>
</figcaption>
</figure>
<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>
<hr>
<p>
<em>
<strong>
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>
</strong>
</em>
</p>
<hr>
<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/1150292019-04-14T18:23:27Z2019-04-14T18:23:27ZSeven common myths about quantum physics<figure><img src="https://images.theconversation.com/files/267900/original/file-20190406-115794-1f570ox.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C1678%2C1009&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Héloïse Chochois, "Embedded with Physicists” “Physics Reimagined” coll.</span>, <span class="license">Author provided</span></span></figcaption></figure><p>I have been popularising quantum physics, my area of research, for many years now. The general public finds the topic fascinating and covers of books and magazines often draw on its mystery. A number of misconceptions have arisen in this area of physics and my purpose here is to look at the facts to debunk seven of these myths.</p>
<p>Don’t worry, you don’t need to know much about quantum physics to read this article. I will mostly be explaining what quantum physics isn’t, rather than what it is…</p>
<h2>1. “Quantum physics is all about uncertainty”</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/268137/original/file-20190408-2914-uvil3m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/268137/original/file-20190408-2914-uvil3m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/268137/original/file-20190408-2914-uvil3m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=900&fit=crop&dpr=1 600w, https://images.theconversation.com/files/268137/original/file-20190408-2914-uvil3m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=900&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/268137/original/file-20190408-2914-uvil3m.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=900&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/268137/original/file-20190408-2914-uvil3m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1131&fit=crop&dpr=1 754w, https://images.theconversation.com/files/268137/original/file-20190408-2914-uvil3m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1131&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/268137/original/file-20190408-2914-uvil3m.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"></a>
<figcaption>
<span class="caption">Heisenberg’s ‘uncertainty principle’.</span>
<span class="attribution"><span class="source">Margaux Khalil and Janet Rafner</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>Wrong! Quantum physics is probably the most precise scientific discipline ever devised by humankind. It can predict certain properties with extreme accuracy, to 10 decimal places, which later experiments confirm exactly.</p>
<p>This myth originated partly in Werner Heisenberg’s <a href="https://en.wikipedia.org/wiki/Uncertainty_principle">“uncertainty principle”</a>. He showed that there is a limit to how accurately two quantities – for instance a particle’s speed and its position – can be measured simultaneously. When quantum physics is used to calculate other quantities, such as the energy, or the magnetic property of atoms, it is astounding in its precision.</p>
<h2>2. “Quantum physics can’t be visualised.”</h2>
<p>Quantum physics describes objects that are often “strange” and difficult to put into pictures: wave functions, superimposed states, probability amplitude, complex numbers to name but a few. People often say that they can only be understood with mathematical equations and symbols. And yet we physicists are always making representations of it when we teach and popularise it. We use graphs, drawings, metaphors, projections and many other devices. Which is just as well, because students and even veteran quantum physicists like us need a mental image of the objects being manipulated. The contentious part is the accuracy of these images, as it is difficult to represent a quantum object accurately.</p>
<p>Working together with designers, illustrators and video makers,
the <a href="http://www.physicsreimagined.com"><em>Physics Reimagined</em></a> research team seeks to “draw” quantum physics in all its forms: folding activities, graphic novels, sculptures, 3D animations, and on and on.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/268140/original/file-20190408-2924-15jlqpj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/268140/original/file-20190408-2924-15jlqpj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=424&fit=crop&dpr=1 600w, https://images.theconversation.com/files/268140/original/file-20190408-2924-15jlqpj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=424&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/268140/original/file-20190408-2924-15jlqpj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=424&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/268140/original/file-20190408-2924-15jlqpj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=533&fit=crop&dpr=1 754w, https://images.theconversation.com/files/268140/original/file-20190408-2924-15jlqpj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=533&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/268140/original/file-20190408-2924-15jlqpj.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">Design makes it possible to imagine what quantum particles could be.</span>
<span class="attribution"><span class="source">Paul Morin et al.</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>3. “Even scientists don’t really understand quantum physics”</h2>
<p>One of the leading lights in the field, <a href="https://en.wikipedia.org/wiki/Richard_Feynman">Richard Feynman</a> himself said: “I think I can safely say that nobody understands quantum mechanics.” But he then immediately added: “I am going to tell you what nature behaves like.” Niels Bohr, one of the founding fathers of the discipline, gives a good summary: “Those who are not shocked when they first come across quantum theory cannot possibly have understood it.”</p>
<p>Physicists do understand what they’re doing when they’re manipulating the quantum formalism. They just need to adapt their intuitions to this new field and its inherent paradoxes.</p>
<h2>4. “A few brilliant theorists came up with the entire concept of quantum physics”</h2>
<p>The entire history of quantum physics shows the exact opposite: at the very beginning, lab experiments threw up unexpected results, such as the photoelectric effect, black-body radiation, the light emission spectrum of atoms. Only later did brilliant theorists enter the scene, when Albert Einstein, Max Planck, Niels Bohr and others tried to provide explanations.</p>
<p>Further fundamental experiments followed, including electrons that bounced weirdly off nickel, silver atoms strangely deviated by a magnetic field, a perfectly conducting metal at low temperatures and so on. Theories and concepts then emerged once again: duality, spin or superconductivity were introduced. The highly productive “back and forth” exchanges between theory and practice are what physics is built on. Experiments generally come first, except in very few cases.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/268142/original/file-20190408-2921-oxhxq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/268142/original/file-20190408-2921-oxhxq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/268142/original/file-20190408-2921-oxhxq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/268142/original/file-20190408-2921-oxhxq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/268142/original/file-20190408-2921-oxhxq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=471&fit=crop&dpr=1 754w, https://images.theconversation.com/files/268142/original/file-20190408-2921-oxhxq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=471&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/268142/original/file-20190408-2921-oxhxq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=471&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The invention of superconductivity.</span>
<span class="attribution"><span class="source">Marine Joumard et al.</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>5. “Einstein was quantum physics’ worst enemy”</h2>
<p>Poor old Albert Einstein is often depicted as having been a virulent opponent of quantum physics, probably because of his famous quote, “God does not play dice with the universe.” Yet he wasn’t against it and what’s more, he created it! In 1905 Einstein wrote his foundational article, <a href="https://en.wikipedia.org/wiki/Annus_Mirabilis_papers">“On a Heuristic Viewpoint Concerning the Production and Transformation of Light”</a>, based on the work of <a href="https://en.wikipedia.org/wiki/Max_Planck">Max Planck</a>. In it, he proposed that light was made of small, individual and quantified bodies, called photons. This is what won him the Nobel Prize, in fact, not his work on the theory of relativity.</p>
<p>Einstein probably earned that reputation because of his discussions with <a href="https://en.wikipedia.org/wiki/Niels_Bohr">Niels Bohr</a>, especially on the idea of interpretation and quantum reality, as he didn’t accept the concept of nonlocality. Later, experiments on entanglement and violation of <a href="https://en.wikipedia.org/wiki/Bell%27s_theorem">Bell’s theorem</a> proved him wrong and showed the absence of hidden variables. Einstein fully appreciated the relevance of quantum physics, he just had a few problems with some of its implications, especially as regards locality.</p>
<h2>6. “Quantum physics has no practical use”</h2>
<p>Quantum physics is probably the most useful discipline in modern physics: once physicists understood how light, atoms and electrons worked, they were able to manipulate them. Lasers, MRI in hospitals, LEDs, flash memory, hard disks – and above all else, the transistor and electronics – all of these technologies were invented by quantum physicists.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/268143/original/file-20190408-2912-7crqjq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/268143/original/file-20190408-2912-7crqjq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=175&fit=crop&dpr=1 600w, https://images.theconversation.com/files/268143/original/file-20190408-2912-7crqjq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=175&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/268143/original/file-20190408-2912-7crqjq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=175&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/268143/original/file-20190408-2912-7crqjq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=220&fit=crop&dpr=1 754w, https://images.theconversation.com/files/268143/original/file-20190408-2912-7crqjq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=220&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/268143/original/file-20190408-2912-7crqjq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=220&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Lasers, maglev trains and MRI are just a few of the applications of quantum physics.</span>
<span class="attribution"><span class="source">Marine Joumard, Flammarion</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>7. “Quantum physics might explain certain alternative therapies and other mysteries”</h2>
<p>Many people who believe in paranormal phenomena and in certain “therapies” claim to be inspired by quantum physics. Indian-American Deepak Chopra is one of the most famous proponents of this approach. He has developed a kind of quantum mysticism in which a pseudo-New Age spirituality finds its credentials in scientific jargon such as “human quantum-body essence”, “localised field of energy and information with cybernetic feedback loops”, and “harmonisation of the quantum mechanical body”. He then purports to establish quantum relationships between mind, consciousness, matter and the universe. “Quantum therapies” also offer care protocols based on the body seen as “a vibration and energy field”, host to “vibrating states” and “bioresonances”.</p>
<p>This is dishonest on two counts. The first trick consists in using scientific terms to mystify quantum physics, when there is in fact no mystery. Lab experiments and daily living have shown its validity. On the other hand, none of the phenomena described by these therapies or beliefs have any scientific basis. Above all, words denote very precise meanings in quantum physics and they are entirely misused in these pseudo-sciences.</p>
<p>More cheating can be found when quantum properties are extrapolated to a human scale. To be absolutely clear, quantum properties such as superposition of states or quantisation don’t apply in the living world on a human scale. 2012 Nobel Prize–winner <a href="https://en.wikipedia.org/wiki/Serge_Haroche">Serge Haroche</a> proved this with his experiments. When an object interacts too much with its environment and becomes too large, it is no longer a quantum object.</p>
<p>However, I wouldn’t like to judge those who wish to test this approach, which belongs to the realm of belief, not science. Everyone can do as they wish, of course. I would only ask people to refrain from pretending it has any scientific basis in quantum physics. Any such claim is simply false.</p>
<p>That’s it! I hope that I managed to debunk quantum physics a little. In the end, it is just like any other scientific discipline…</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/268144/original/file-20190408-2905-3n5muy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/268144/original/file-20190408-2905-3n5muy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=408&fit=crop&dpr=1 600w, https://images.theconversation.com/files/268144/original/file-20190408-2905-3n5muy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=408&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/268144/original/file-20190408-2905-3n5muy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=408&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/268144/original/file-20190408-2905-3n5muy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=512&fit=crop&dpr=1 754w, https://images.theconversation.com/files/268144/original/file-20190408-2905-3n5muy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=512&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/268144/original/file-20190408-2905-3n5muy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=512&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">An equation that raises questions – but also provides answers.</span>
<span class="attribution"><span class="source">Chloé Passavant et al.</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure><img src="https://counter.theconversation.com/content/115029/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Julien Bobroff is the author of "Mon grand mécano quantique" mentioned in the article.</span></em></p>Quantum physics and its mysteries… And what if this supposedly incomprehensible science weren’t so difficult for non-scientists to understand?Julien Bobroff, Physicien, Professeur des Universités, Université Paris-SaclayLicensed 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.