tag:theconversation.com,2011:/ca/topics/speed-of-light-16770/articlesSpeed of light – The Conversation2023-03-20T12:45:51Ztag:theconversation.com,2011:article/1971892023-03-20T12:45:51Z2023-03-20T12:45:51ZWhy does time change when traveling close to the speed of light? A physicist explains<figure><img src="https://images.theconversation.com/files/514967/original/file-20230313-19-rrsoxs.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C2121%2C1412&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Time gets a little strange as you approach the speed of light.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/stars-light-motion-in-space-royalty-free-image/510216640">ikonacolor/iStock via Getty Images</a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&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><em><a href="https://theconversation.com/us/topics/curious-kids-us-74795">Curious Kids</a> is a series for children of all ages. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskidsus@theconversation.com">curiouskidsus@theconversation.com</a>.</em></p>
<hr>
<blockquote>
<p><strong>Why does time change when traveling close to the speed of light? – Timothy, age 11, Shoreview, Minnesota</strong></p>
</blockquote>
<hr>
<p>Imagine you’re in a car driving across the country watching the landscape. A tree in the distance gets closer to your car, passes right by you, then moves off again in the distance behind you.</p>
<p>Of course, you know that tree isn’t actually getting up and walking toward or away from you. It’s you in the car who’s moving toward the tree. The tree is moving only in comparison, or relative, to you – that’s what <a href="https://scholar.google.com/citations?user=QyArIUgAAAAJ&hl=en">we physicists</a> call <a href="https://www.amnh.org/exhibitions/einstein/time/its-all-relative">relativity</a>. If you had a friend standing by the tree, they would see you moving toward them at the same speed that you see them moving toward you.</p>
<p>In his 1632 book “<a href="https://www.loc.gov/item/12018406/">Dialogue Concerning the Two Chief World Systems</a>,” the astronomer Galileo Galilei first described the <a href="https://www.phys.unsw.edu.au/einsteinlight/jw/module1_Galileo_and_Newton.htm">principle of relativity</a> – the idea that the universe should behave the same way at all times, even if two people experience an event differently because one is moving in respect to the other.</p>
<p>If you are in a car and toss a ball up in the air, the physical laws acting on it, such as the force of gravity, should be the same as the ones acting on an observer watching from the side of the road. However, while you see the ball as moving up and back down, someone on the side of the road will see it moving toward or away from them as well as up and down.</p>
<h2>Special relativity and the speed of light</h2>
<p>Albert Einstein much later proposed the idea of what’s now known as <a href="https://futurism.com/special-relativity-simplified">special relativity</a> to explain some confusing observations that didn’t have an intuitive explanation at the time. Einstein used the work of many physicists and astronomers in the late 1800s to put together his theory in 1905, starting with two key ingredients: the principle of relativity and the strange observation that the speed of light is the same for every observer and nothing can move faster. Everyone measuring the speed of light will get the same result, no matter where they are or how fast they are moving. </p>
<p>Let’s say you’re in the car driving at 60 miles per hour and your friend is standing by the tree. When they throw a ball toward you at a speed of what they perceive to be 60 miles per hour, you might logically think that you would observe your friend and the tree moving toward you at 60 miles per hour and the ball moving toward you at 120 miles per hour. While that’s really close to the correct value, it’s actually slightly wrong. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/GguAN1_JouQ?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The experience of time is dependent on motion.</span></figcaption>
</figure>
<p>This discrepancy between what you might expect by adding the two numbers and the true answer grows as one or both of you move closer to the speed of light. If you were traveling in a rocket moving at 75% of the speed of light and your friend throws the ball at the same speed, you would not see the ball moving toward you at 150% of the speed of light. This is because nothing can move faster than light – the ball would still appear to be moving toward you at less than the speed of light. While this all may seem very strange, there is <a href="https://galileo.phys.virginia.edu/classes/252/adding_vels.html">lots of experimental evidence</a> to back up these observations.</p>
<h2>Time dilation and the twin paradox</h2>
<p>Speed is not the only factor that changes relative to who is making the observation. Another consequence of relativity is the concept of <a href="https://www.technologyreview.com/2019/12/07/65014/how-does-time-dilation-affect-aging-during-high-speed-space-travel/">time dilation</a>, whereby people measure different amounts of time passing depending on how fast they move relative to one another.</p>
<p>Each person experiences time normally relative to themselves. But the person moving faster experiences less time passing for them than the person moving slower. It’s only when they reconnect and compare their watches that they realize that one watch says less time has passed while the other says more.</p>
<p>This leads to one of the strangest results of relativity – the <a href="https://www.britannica.com/science/twin-paradox">twin paradox</a>, which says that if one of a pair of twins makes a trip into space on a high-speed rocket, they will return to Earth to find their twin has aged faster than they have. It’s important to note that time behaves “normally” as perceived by each twin (exactly as you are experiencing time now), even if their measurements disagree.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/h8GqaAp3cGs?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The twin paradox isn’t actually a paradox.</span></figcaption>
</figure>
<p>You might be wondering: If each twin sees themselves as stationary and the other as moving toward them, wouldn’t they each measure the other as aging faster? The answer is no, because they can’t both be older relative to the other twin. </p>
<p>The twin on the spaceship is not only moving at a particular speed where the frame of references stay the same but also accelerating compared with the twin on Earth. Unlike speeds that are relative to the observer, accelerations are absolute. If you step on a scale, the weight you are measuring is actually your acceleration due to gravity. This measurement stays the same regardless of the speed at which the Earth is moving through the solar system, or the solar system is moving through the galaxy or the galaxy through the universe. </p>
<p>Neither twin experiences any strangeness with their watches as one moves closer to the speed of light – they both experience time as normally as you or I do. It’s only when they meet up and compare their observations that they will see a difference – one that is perfectly defined by the mathematics of relativity.</p>
<hr>
<p><em>Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to <a href="mailto:curiouskidsus@theconversation.com">CuriousKidsUS@theconversation.com</a>. Please tell us your name, age and the city where you live.</em></p>
<p><em>And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.</em></p><img src="https://counter.theconversation.com/content/197189/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Michael Lam does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Your experience of time is relative because it depends on motion – more specifically, your speed and acceleration.Michael Lam, Assistant Professor of Physics and Astronomy, Rochester Institute of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1919272022-10-06T17:51:10Z2022-10-06T17:51:10ZWhat is quantum entanglement? A physicist explains the science of Einstein’s ‘spooky action at a distance’<figure><img src="https://images.theconversation.com/files/488150/original/file-20221004-12421-klkh40.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C5064%2C3294&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">When two particles are entangled, the state of one is tied to the state of the other. </span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/illustration/quantum-entanglement-conceptual-artwork-royalty-free-illustration/1333715460">Victor de Schwanberg/Science Photo Library via Getty Images</a></span></figcaption></figure><p>The <a href="https://theconversation.com/nobel-prize-physicists-share-prize-for-insights-into-the-spooky-world-of-quantum-mechanics-191884">2022 Nobel Prize in physics</a> recognized three scientists who made groundbreaking contributions in understanding one of the most mysterious of all natural phenomena: quantum entanglement.</p>
<p>In the simplest terms, quantum entanglement means that aspects of one particle of an entangled pair depend on aspects of the other particle, no matter how far apart they are or what lies between them. These particles could be, for example, electrons or photons, and an aspect could be the state it is in, such as whether it is “spinning” in one direction or another.</p>
<p>The strange part of quantum entanglement is that when you measure something about one particle in an entangled pair, you immediately know something about the other particle, even if they are millions of light years apart. This odd connection between the two particles is instantaneous, <a href="https://doi.org/10.1103/PhysRev.47.777">seemingly breaking a fundamental law of the universe</a>. Albert Einstein famously called the phenomenon “spooky action at a distance.”</p>
<p>Having spent the better part of <a href="https://scholar.google.com/citations?user=r8sBeycAAAAJ&hl=en&oi=ao">two decades conducting experiments rooted in quantum mechanics</a>, I have come to accept its strangeness. Thanks to ever more precise and reliable instruments and the work of this year’s Nobel winners, <a href="https://scholar.google.com/citations?user=-6d6dV4AAAAJ&hl=en&oi=sra">Alain Aspect</a>, <a href="https://scholar.google.com/citations?user=BDm2SGcAAAAJ&hl=en&oi=ao">John Clauser</a> and <a href="https://scholar.google.com/citations?user=cuqIY0oAAAAJ&hl=en&oi=ao">Anton Zeilinger</a>, physicists now integrate quantum phenomena into their knowledge of the world with an exceptional degree of certainty.</p>
<p>However, even until the 1970s, researchers were still divided over whether quantum entanglement was a real phenomenon. And for good reasons – who would dare contradict the great Einstein, who himself doubted it? It took the development of new experimental technology and bold researchers to finally put this mystery to rest.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A cat sitting in a box." src="https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.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"></a>
<figcaption>
<span class="caption">According to quantum mechanics, particles are simultaneously in two or more states until observed – an effect vividly captured by Schrödinger’s famous thought experiment of a cat that is both dead and alive simultaneously.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Cat_in_a_box_2.jpg#/media/File:Cat_in_a_box_2.jpg">Michael Holloway/Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>Existing in multiple states at once</h2>
<p>To truly understand the spookiness of quantum entanglement, it is important to first understand <a href="https://doi.org/10.1103/RevModPhys.71.S288">quantum superposition</a>. Quantum superposition is the idea that particles exist in multiple states at once. When a measurement is performed, it is as if the particle selects one of the states in the superposition.</p>
<p>For example, many particles have an attribute called spin that is measured either as “up” or “down” for a given orientation of the analyzer. But until you measure the spin of a particle, it simultaneously exists in a superposition of spin up and spin down.</p>
<p>There is a probability attached to each state, and it is possible to predict the average outcome from many measurements. The likelihood of a single measurement being up or down depends on these probabilities, <a href="https://theconversation.com/could-schrodingers-cat-exist-in-real-life-our-research-may-provide-the-answer-147752">but is itself unpredictable</a>.</p>
<p>Though very weird, the mathematics and a vast number of experiments have shown that quantum mechanics correctly describes physical reality.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A photo of Albert Einstein" src="https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=774&fit=crop&dpr=1 600w, https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=774&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=774&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=973&fit=crop&dpr=1 754w, https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=973&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=973&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Albert Einstein, Boris Podolsky and Nathan Rosen pointed out an apparent problem with quantum entanglement in 1935 that prompted Einstein to describe quantum entanglement as ‘spooky action at a distance.’</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Einstein-formal_portrait-35.jpg#/media/File:Einstein-formal_portrait-35.jpg">Sophie Dela/Wikimedia Commons</a></span>
</figcaption>
</figure>
<h2>Two entangled particles</h2>
<p>The <a href="https://doi.org/10.1103/PhysRev.48.696">spookiness of quantum entanglement</a> emerges from the reality of quantum superposition, and was clear to the founding fathers of quantum mechanics who developed the theory in the 1920s and 1930s.</p>
<p>To create entangled particles you essentially break a system into two, where the sum of the parts is known. For example, you can split a particle with spin of zero into two particles that necessarily will have opposite spins so that their sum is zero. </p>
<p>In 1935, Albert Einstein, Boris Podolsky and Nathan Rosen <a href="https://doi.org/10.1103/PhysRev.47.777">published a paper</a> that describes a thought experiment designed to illustrate a <a href="https://doi.org/10.1103/PhysRev.47.777">seeming absurdity of quantum entanglement</a> that challenged a foundational law of the universe.</p>
<p>A <a href="https://doi.org/10.1103/PhysRev.48.696">simplified version of this thought experiment</a>, attributed to David Bohm, considers the decay of a particle called the pi meson. When this particle decays, it produces an electron and a positron that have opposite spin and are moving away from each other. Therefore, if the electron spin is measured to be up, then the measured spin of the positron could only be down, and vice versa. This is true even if the particles are billions of miles apart.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Two blue circles with an arrow pointing up and an arrow pointing down." src="https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=405&fit=crop&dpr=1 600w, https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=405&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=405&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=509&fit=crop&dpr=1 754w, https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=509&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=509&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Entanglement can be created between a pair of particles with one measured as spin up and the other as spin down.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/spin-quantum-physics-and-computing-concept-royalty-free-image/1346594645?phrase=particle%20spin%20physics&adppopup=true">atdigit/iStock via Getty Images</a></span>
</figcaption>
</figure>
<p>This would be fine if the measurement of the electron spin were always up and the measured spin of the positron were always down. But because of quantum mechanics, the spin of each particle is both part up and part down until it is measured. Only when the measurement occurs does the quantum state of the spin “collapse” into either up or down – instantaneously collapsing the other particle into the opposite spin. This seems to suggest that the particles communicate with each other through some means that moves faster than the speed of light. But according to the laws of physics, nothing can travel faster than the speed of light. Surely the measured state of one particle cannot instantaneously determine the state of another particle at the far end of the universe?</p>
<p>Physicists, including Einstein, proposed a number of alternative interpretations of quantum entanglement in the 1930s. They theorized there was some unknown property – dubbed hidden variables – <a href="https://doi.org/10.1103/PhysRev.47.777">that determined the state of a particle before measurement</a>. But at the time, physicists did not have the technology nor a definition of a clear measurement that could test whether quantum theory needed to be modified to include hidden variables.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A photo of John Stuart Bell in front of a chalkboard." src="https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=609&fit=crop&dpr=1 600w, https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=609&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=609&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=766&fit=crop&dpr=1 754w, https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=766&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=766&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">John Bell, an Irish physicist, came up with the means to test the reality of whether quantum entanglement relied on hidden variables.</span>
<span class="attribution"><a class="source" href="https://cds.cern.ch/record/1823937">CERN</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Disproving a theory</h2>
<p>It took until the 1960s before there were any clues to an answer. John Bell, a brilliant Irish physicist who did not live to receive the Nobel Prize, devised a scheme to test whether the notion of hidden variables made sense.</p>
<p><a href="https://doi.org/10.1103/PhysicsPhysiqueFizika.1.195">Bell produced</a> an equation now known as Bell’s inequality that is always correct – and only correct – for hidden variable theories, and not always for quantum mechanics. Thus, if Bell’s equation was found not to be satisfied in a real-world experiment, local hidden variable theories can be ruled out as an explanation for quantum entanglement.</p>
<p>The experiments of the 2022 Nobel laureates, particularly those of <a href="https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.49.91">Alain Aspect</a>, were the first <a href="https://doi.org/10.1038/18296">tests of the Bell inequality</a>. The experiments used entangled photons, rather than pairs of an electron and a positron, as in many thought experiments. The results conclusively ruled out the existence of hidden variables, a mysterious attribute that would predetermine the states of entangled particles. Collectively, these and <a href="https://www.nature.com/articles/nature15759">many</a> <a href="https://doi.org/10.1038/35057215">follow-up</a> <a href="https://doi.org/10.1103/PhysRevD.14.2543">experiments</a> have vindicated quantum mechanics. Objects can be correlated over large distances in ways that physics before quantum mechanics can not explain.</p>
<p>Importantly, there is also no conflict with <a href="https://www.ams.org/journals/bull/1935-41-04/S0002-9904-1935-06046-X/S0002-9904-1935-06046-X.pdf">special relativity, which forbids faster-than-light communication</a>. The fact that measurements over vast distances are correlated does not imply that information is transmitted between the particles. Two parties far apart performing measurements on entangled particles <a href="https://www.forbes.com/sites/startswithabang/2020/01/02/no-we-still-cant-use-quantum-entanglement-to-communicate-faster-than-light/?sh=730ad18c4d5d">cannot use the phenomenon to pass along information</a> faster than the speed of light.</p>
<p>Today, physicists <a href="https://doi.org/0.1103/PhysRevLett.103.217402">continue to research quantum entanglement</a> and <a href="https://theconversation.com/a-quantum-computing-future-is-unlikely-due-to-random-hardware-errors-126503">investigate potential</a> <a href="https://theconversation.com/the-search-for-dark-matter-gets-a-speed-boost-from-quantum-technology-153604">practical applications</a>. Although quantum mechanics can predict the probability of a measurement with incredible accuracy, many researchers remain skeptical that it provides a complete description of reality. One thing is certain, though. Much remains to be said about the mysterious world of quantum mechanics.</p><img src="https://counter.theconversation.com/content/191927/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andreas Muller receives funding from the National Science Foundation. </span></em></p>A multitude of experiments have shown the mysterious phenomena of quantum mechanics to be how the universe functions. The scientists behind these experiments won the 2022 Nobel Prize in physics.Andreas Muller, Associate Professor of Physics, University of South FloridaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1830432022-05-23T19:57:43Z2022-05-23T19:57:43ZCurious Kids: what would happen if someone moved at twice the speed of light?<figure><img src="https://images.theconversation.com/files/463869/original/file-20220518-14-zdsmu6.jpeg?ixlib=rb-1.1.0&rect=17%2C85%2C2977%2C1908&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><blockquote>
<p><strong>I’m curious about what will happen if, hypothetically, someone moves with speed (that is) twice the speed of light? – Devanshi, age 13, Mumbai</strong></p>
</blockquote>
<p><a href="https://theconversation.com/au/topics/curious-kids-36782"><img src="https://images.theconversation.com/files/291898/original/file-20190911-190031-enlxbk.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=90&fit=crop&dpr=1" width="100%"></a></p>
<p>Hi Devanshi! Thanks for this great question.</p>
<p>As far as we know, it’s not possible for a person to move at twice the speed of light. In fact, it’s not possible for any <em>object</em> with the kind of mass you or I have to move faster than the speed of light. </p>
<p>However, for certain strange particles, travelling at twice the speed of light might be possible – and it might send those particles back in time.</p>
<h2>A universal speed limit</h2>
<p>One of our best physical theories at the moment is the <a href="https://theconversation.com/from-newton-to-einstein-the-origins-of-general-relativity-50013">theory of relativity</a>, developed by Albert Einstein. According to this theory, the speed of light operates as a universal speed limit on anything with mass. </p>
<p>Specifically, relativity tells us that nothing with mass can accelerate past the speed of light. </p>
<p>To accelerate an object with mass, we have to add energy. The faster we want the object to go, the more energy we’ll need.</p>
<p>The equations of relativity tell us that anything with mass – regardless of how much mass it has – would require an infinite amount of energy to be accelerated to the speed of light. </p>
<p>But all of the sources of energy we know of are finite: they are limited in some respect. </p>
<p>Indeed, it’s plausible the Universe only contains a finite amount of energy. That would mean there isn’t enough energy in the Universe to accelerate something with mass up to the speed of light.</p>
<p>Since you and I have mass, don’t expect to be travelling at twice the speed of light anytime soon.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/463866/original/file-20220518-16-dnqcm3.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Blue beams of light rushing past signify a fast moving object going through space" src="https://images.theconversation.com/files/463866/original/file-20220518-16-dnqcm3.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/463866/original/file-20220518-16-dnqcm3.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/463866/original/file-20220518-16-dnqcm3.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/463866/original/file-20220518-16-dnqcm3.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/463866/original/file-20220518-16-dnqcm3.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/463866/original/file-20220518-16-dnqcm3.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/463866/original/file-20220518-16-dnqcm3.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">According to Einstein, nothing bulky such as an object or human could accelerate faster than the speed of light.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
</figcaption>
</figure>
<h2>Tachyons</h2>
<p>This universal speed limit applies to anything with what we might call “ordinary mass”. </p>
<p>There are, however, hypothetical particles called <a href="https://www.space.com/tachyons-facts-about-particles#">tachyons</a> with a special kind of mass called “imaginary mass”. </p>
<p>There is no evidence tachyons exist. But according to relativity, their possible existence can’t be ruled out.</p>
<p>If they do exist, tachyons must always be travelling faster than the speed of light. Just as something with ordinary mass can’t be accelerated past the speed of light, tachyons can’t be slowed down to below the speed of light. </p>
<p>Some physicists believe that if tachyons exist, they would constantly be travelling backwards in time. This is why tachyons are associated with time travel in many science fiction books and movies. </p>
<p>There are ideas that we might someday harness tachyons to <a href="https://theconversation.com/curious-kids-is-time-travel-possible-for-humans-140703">build a time machine</a>. But for now this remains a distant dream, as we don’t have the ability to detect potential tachyons. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/curious-kids-is-time-travel-possible-for-humans-140703">Curious Kids: is time travel possible for humans?</a>
</strong>
</em>
</p>
<hr>
<h2>Shortcuts?</h2>
<p>It’s disappointing we can’t travel faster than the speed of light. The nearest star to us, other than the Sun, is 4.35 light years away. So, travelling at the speed of light, it would take more than four years to get there. </p>
<p>The <a href="https://theconversation.com/most-distant-star-to-date-spotted-but-how-much-further-back-in-time-could-we-see-180623#:%7E:text=The%20Hubble%20Space%20Telescope%20has,be%20able%20to%20see%20it.">farthest star</a> we’ve ever detected is 28 billion light years away. So you can pretty much give up on charting the entire Universe.</p>
<p>That said, relativity does allow for the existence of “<a href="https://theconversation.com/curious-kids-how-do-wormholes-work-90627">wormholes</a>”.</p>
<p>A wormhole is a shortcut between any two points in space. While a star might be 4.5 light years away in normal terms, it might only be a few hours away via a wormhole.</p>
<p>If there are any actual wormholes, they would let us travel great distances in a very short period of time – allowing us to get to the farthest reaches of the universe within a single lifetime. </p>
<p>Unfortunately, like tachyons, wormholes remain entirely hypothetical.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/463871/original/file-20220518-21-25ek7a.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Illustration showing a hypothetical wormhole open in space, bending spacetime around it." src="https://images.theconversation.com/files/463871/original/file-20220518-21-25ek7a.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/463871/original/file-20220518-21-25ek7a.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/463871/original/file-20220518-21-25ek7a.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/463871/original/file-20220518-21-25ek7a.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/463871/original/file-20220518-21-25ek7a.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/463871/original/file-20220518-21-25ek7a.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/463871/original/file-20220518-21-25ek7a.jpeg?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"></a>
<figcaption>
<span class="caption">You can think of a wormhole as a tunnel with two ends opening up to different points in spacetime.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
</figcaption>
</figure>
<h2>Strange possibilities</h2>
<p>Despite the fact we can’t genuinely travel faster than light, we can still try to imagine what it would be like to do so.</p>
<p>By thinking in this way, we are engaging in “<a href="https://www.reddit.com/r/bigbangtheory/comments/7e0m9j/lets_play_the_counterfactual_game/">counterfactual thinking</a>”. We are considering what things would, or might, be like if reality was different in some way.</p>
<p>There are many different possibilities we could consider, each with a different set of physical principles. </p>
<p>So we can’t say with any certainty what would happen if we were able to travel faster than light. At best, we can guess what <em>might</em> happen. Would we start to travel back in time, as some scientists think tachyons might do?</p>
<p>I’ll leave it to you and your imagination to come up with some ideas!</p><img src="https://counter.theconversation.com/content/183043/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>There’s a special type of particle called a ‘tachyon’ which would have to travel faster than the speed of light. But here’s the hitch – we can’t prove tachyons even exist.Sam Baron, Associate professor, Australian Catholic UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1708492021-11-15T13:11:18Z2021-11-15T13:11:18ZHave we made an object that could travel 1% the speed of light?<figure><img src="https://images.theconversation.com/files/429624/original/file-20211101-21-hweqmz.jpg?ixlib=rb-1.1.0&rect=389%2C463%2C8635%2C3891&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">It only takes light about eight minutes to go from the Sun to Earth.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/sunrise-in-space-royalty-free-image/174578300?adppopup=true"> loops7/E+ via Getty Images</a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&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><em><a href="https://theconversation.com/us/topics/curious-kids-us-74795">Curious Kids</a> is a series for children of all ages. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskidsus@theconversation.com">curiouskidsus@theconversation.com</a>.</em></p>
<hr>
<blockquote>
<p><strong>Have we made an object that could travel at at least 1% the speed of light? – Anadi, age 14, Jammu and Kashmir, India</strong></p>
</blockquote>
<hr>
<p><a href="https://www.universetoday.com/38040/speed-of-light-2/">Light is fast</a>. In fact, it is the fastest thing that exists, and a law of the universe is that nothing can move faster than light. Light travels at 186,000 miles per second (300,000 kilometers per second) and can go from the Earth to the Moon in just over a second. Light can streak from Los Angeles to New York in less than the blink of an eye.</p>
<p>While 1% of anything doesn’t sound like much, with light, that’s still really fast – close to 7 million miles per hour! At 1% the speed of light, it would take a little over a second to get from Los Angeles to New York. This is more than 10,000 times faster than a commercial jet.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/429627/original/file-20211101-25-c3f9c9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A spacecraft with the sun in the background." src="https://images.theconversation.com/files/429627/original/file-20211101-25-c3f9c9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429627/original/file-20211101-25-c3f9c9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=415&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429627/original/file-20211101-25-c3f9c9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=415&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429627/original/file-20211101-25-c3f9c9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=415&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429627/original/file-20211101-25-c3f9c9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=522&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429627/original/file-20211101-25-c3f9c9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=522&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429627/original/file-20211101-25-c3f9c9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=522&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 Parker Solar Probe, seen here in an artist’s rendition, is the fastest object ever made by humans and used the gravity of the Sun to get going 0.05% the speed of light.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Parker_Solar_Probe.jpg">NASA/Johns Hopkins APL/Steve Gribben</a></span>
</figcaption>
</figure>
<h2>The fastest things ever made</h2>
<p><a href="https://hypertextbook.com/facts/1999/MariaPereyra.shtml">Bullets</a> can go 2,600 mph (4,200 kmh), more than three times the speed of sound. The fastest aircraft is NASA’s <a href="https://www.wearethemighty.com/tech/the-8-fastest-man-made-objects-ever/">X3 jet plane</a>, with a top speed of 7,000 mph (11,200 kph). That sounds impressive, but it’s still only 0.001% the speed of light.</p>
<p>The fastest human-made objects are spacecraft. They use rockets to break free of the Earth’s gravity, which takes a speed of 25,000 mph (40,000 kmh). The spacecraft that is traveling the fastest is NASA’s <a href="https://www.cnet.com/home/energy-and-utilities/nasa-solar-probe-becomes-fastest-object-ever-built-as-it-touches-the-sun/">Parker Solar Probe</a>. After it launched from Earth in 2018, it skimmed the Sun’s scorching atmosphere and used the Sun’s gravity to reach 330,000 mph (535,000 kmh). That’s blindingly fast – yet only 0.05% of the speed of light.</p>
<h2>Why even 1% of light speed is hard</h2>
<p>What’s holding humanity back from reaching 1% of the speed of light? In a word, energy. Any object that’s moving has energy due to its motion. Physicists call this kinetic energy. To go faster, you need to increase kinetic energy. The problem is that it takes a lot of <a href="https://www.omnicalculator.com/physics/relativistic-ke">kinetic energy</a> to increase speed. To make something go twice as fast takes four times the energy. Making something go three times as fast requires nine times the energy, and so on. </p>
<p>For example, to get a teenager who weighs 110 pounds (50 kilograms) to 1% of the speed of light would cost 200 trillion Joules (a measurement of energy). That’s roughly the same amount of energy that 2 million people in the U.S. use in a day.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/429630/original/file-20211101-21-5bh2wr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A shiny golden-hued square with a small spacecraft attached in space with a planet in the background." src="https://images.theconversation.com/files/429630/original/file-20211101-21-5bh2wr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429630/original/file-20211101-21-5bh2wr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429630/original/file-20211101-21-5bh2wr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429630/original/file-20211101-21-5bh2wr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429630/original/file-20211101-21-5bh2wr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429630/original/file-20211101-21-5bh2wr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429630/original/file-20211101-21-5bh2wr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=603&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Solar sails, the thin shiny square seen in this artist’s rendition of the Japanese IKAROS spacecraft, could propel a spacecraft to 10% the speed of light.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:IKAROS_solar_sail.jpg#/media/File:IKAROS_solar_sail.jpg">Andrzej Mirecki via Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>How fast can we go?</h2>
<p>It’s possible to get something to 1% the speed of light, but it would just take an enormous amount of energy. Could humans make something go even faster?</p>
<p>Yes! But engineers need to figure out new ways to make things move in space. All rockets, even the sleek new rockets used by SpaceX and Blue Origins, <a href="https://www.sciencelearn.org.nz/resources/393-types-of-chemical-rocket-engines">burn rocket fuel</a> that isn’t very different from gasoline in a car. The problem is that burning fuel is very inefficient. </p>
<p>Other methods for pushing a spacecraft involve using <a href="https://www.nasa.gov/feature/glenn/2020/the-propulsion-we-re-supplying-it-s-electrifying">electric or magnetic forces</a>. <a href="https://www.nasa.gov/directorates/spacetech/niac/2012_Phase_II_fusion_driven_rocket/">Nuclear fusion</a>, the process that powers the Sun, is also much more efficient than chemical fuel. </p>
<p>Scientists are researching many other ways to go fast – even <a href="https://theconversation.com/warp-drives-physicists-give-chances-of-faster-than-light-space-travel-a-boost-157391">warp drives</a>, the faster-than-light travel popularized by Star Trek. </p>
<p>One promising way to get something moving very fast is to use a solar sail. These are large, thin sheets of plastic attached to a spacecraft and designed so that sunlight can push on them, like wind in a normal sail. A few spacecraft have used solar sails to show that they work, and scientists think that a solar sail could <a href="http://ffden-2.phys.uaf.edu/webproj/212_spring_2015/Robert_Miller/physics.html#:%7E:text=Solar%20sails%20have%20a%20maximum,the%20sail%20propelling%20it%20forward">propel spacecraft to 10% of the speed of light</a>. </p>
<p>One day, when humanity is not limited to a tiny fraction of the speed of light, we might <a href="https://tauzero.aero/">travel to the stars</a>.</p>
<hr>
<p><em>Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to <a href="mailto:curiouskidsus@theconversation.com">CuriousKidsUS@theconversation.com</a>. Please tell us your name, age and the city where you live.</em></p>
<p><em>And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.</em></p><img src="https://counter.theconversation.com/content/170849/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Chris Impey receives funding from the National Science Foundation and the Hearst Foundation.</span></em></p>The fastest things ever made by humans are spacecraft, and the fastest spacecraft reached 330,000 mph – only 0.05% the speed of light. But there are ways to go faster.Chris Impey, University Distinguished Professor of Astronomy, University of ArizonaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1573912021-04-23T17:38:33Z2021-04-23T17:38:33ZWarp drives: Physicists give chances of faster-than-light space travel a boost<figure><img src="https://images.theconversation.com/files/396639/original/file-20210422-24-1x8nzax.jpg?ixlib=rb-1.1.0&rect=37%2C19%2C1119%2C804&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Faster than light travel is the only way humans could ever get to other stars in a reasonable amount of time. </span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Wormhole_travel_as_envisioned_by_Les_Bossinas_for_NASA.jpg">Les Bossinas/NASA/Wikimedia Commons</a></span></figcaption></figure><p>The closest star to Earth is Proxima Centauri. It is about 4.25 light-years away, or about 25 trillion miles (40 trillion km). The fastest ever spacecraft, the now- in-space <a href="https://blogs.nasa.gov/parkersolarprobe/2018/10/29/parker-solar-probe-becomes-fastest-ever-spacecraft/">Parker Solar Probe</a> will reach a top speed of 450,000 mph. It would take just 20 seconds to go from Los Angeles to New York City at that speed, but it would take the solar probe about 6,633 years to reach Earth’s nearest neighboring solar system. </p>
<p>If humanity ever wants to travel easily between stars, people will need to go faster than light. But so far, faster-than-light travel is possible only in science fiction. </p>
<p>In Issac Asimov’s <a href="https://asimov.fandom.com/wiki/Foundation">Foundation series</a>, humanity can travel from planet to planet, star to star or across the universe using jump drives. As a kid, I read as many of those stories as I could get my hands on. I am now a theoretical physicist and study nanotechnology, but I am still fascinated by the ways humanity could one day travel in space. </p>
<p>Some characters – like the astronauts in the movies “Interstellar” and “Thor” – use <a href="https://www.space.com/20881-wormholes.html">wormholes to travel between solar systems</a> in seconds. Another approach – familiar to “Star Trek” fans – is warp drive technology. Warp drives are theoretically possible if still far-fetched technology. Two recent papers <a href="https://www.msn.com/en-au/news/techandscience/engineers-have-proposed-the-first-model-for-a-physically-possible-warp-drive/ar-BB1ed4KQ">made headlines</a> in March when <a href="https://doi.org/10.1088/1361-6382/abe692">researchers claimed</a> to <a href="https://doi.org/10.1088/1361-6382/abdf6e">have overcome</a> one of the many challenges that stand between the theory of warp drives and reality. </p>
<p>But how do these theoretical warp drives really work? And will humans be making the jump to warp speed anytime soon?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/396642/original/file-20210422-15-1fbhdiq.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A circle on a flat blue plane with the surface dipping down in front and rising up behind." src="https://images.theconversation.com/files/396642/original/file-20210422-15-1fbhdiq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/396642/original/file-20210422-15-1fbhdiq.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=278&fit=crop&dpr=1 600w, https://images.theconversation.com/files/396642/original/file-20210422-15-1fbhdiq.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=278&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/396642/original/file-20210422-15-1fbhdiq.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=278&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/396642/original/file-20210422-15-1fbhdiq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=349&fit=crop&dpr=1 754w, https://images.theconversation.com/files/396642/original/file-20210422-15-1fbhdiq.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=349&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/396642/original/file-20210422-15-1fbhdiq.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=349&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">This 2-dimensional representation shows the flat, unwarped bubble of spacetime in the center where a warp drive would sit surrounded by compressed spacetime to the right (downward curve) and expanded spacetime to the left (upward curve).</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Alcubierre.png#/media/File:Alcubierre.png">AllenMcC/Wikimedia Commons</a></span>
</figcaption>
</figure>
<h2>Compression and expansion</h2>
<p>Physicists’ current understanding of spacetime comes from Albert Einstein’s <a href="https://www.space.com/17661-theory-general-relativity.html">theory of General Relativity</a>. General Relativity states that space and time are fused and that nothing can travel faster than the speed of light. General relativity also describes how mass and energy warp spacetime – hefty objects like stars and black holes curve spacetime around them. This curvature is what you feel as gravity and why many spacefaring heroes worry about “getting stuck in” or “falling into” a gravity well. Early science fiction writers <a href="https://www.tor.com/2018/07/05/the-father-of-science-fiction-the-best-of-john-w-campbell/">John Campbell</a> and Asimov saw this warping as a way to skirt the speed limit. </p>
<p>What if a starship could compress space in front of it while expanding spacetime behind it? “Star Trek” took this idea and named it the warp drive. </p>
<p>In 1994, Miguel Alcubierre, a Mexican theoretical physicist, showed that compressing spacetime in front of the spaceship while expanding it behind was <a href="https://doi.org/10.1088/0264-9381/11/5/001">mathematically possible within the laws of General Relativity</a>. So, what does that mean? Imagine the distance between two points is 10 meters (33 feet). If you are standing at point A and can travel one meter per second, it would take 10 seconds to get to point B. However, let’s say you could somehow compress the space between you and point B so that the interval is now just one meter. Then, moving through spacetime at your maximum speed of one meter per second, you would be able to reach point B in about one second. In theory, this approach does not contradict the laws of relativity since you are not moving faster than light in the space around you. Alcubierre showed that the warp drive from “Star Trek” was in fact theoretically possible.</p>
<p>Proxima Centauri here we come, right? Unfortunately, Alcubierre’s method of compressing spacetime had one problem: it requires negative energy or negative mass.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/396645/original/file-20210422-16-1yaplky.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A 2–dimensional diagram showing how matter warps spacetime" src="https://images.theconversation.com/files/396645/original/file-20210422-16-1yaplky.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/396645/original/file-20210422-16-1yaplky.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=260&fit=crop&dpr=1 600w, https://images.theconversation.com/files/396645/original/file-20210422-16-1yaplky.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=260&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/396645/original/file-20210422-16-1yaplky.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=260&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/396645/original/file-20210422-16-1yaplky.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=327&fit=crop&dpr=1 754w, https://images.theconversation.com/files/396645/original/file-20210422-16-1yaplky.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=327&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/396645/original/file-20210422-16-1yaplky.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=327&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">This 2–dimensional representation shows how positive mass curves spacetime (left side, blue earth) and negative mass curves spacetime in an opposite direction (right side, red earth).</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:One-sided_spacetime_curvatures.png#/media/File:One-sided_spacetime_curvatures.png">Tokamac/Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>A negative energy problem</h2>
<p>Alcubierre’s warp drive would work by creating a bubble of flat spacetime around the spaceship and curving spacetime around that bubble to reduce distances. The warp drive would require either negative mass – a theorized type of matter – or a ring of negative energy density to work. Physicists have never observed negative mass, so that leaves negative energy as the only option. </p>
<p>To create negative energy, a warp drive would use a huge amount of mass to create an imbalance between particles and antiparticles. For example, if an electron and an antielectron appear near the warp drive, one of the particles would get trapped by the mass and this results in an imbalance. This imbalance results in negative energy density. Alcubierre’s warp drive would use this negative energy to create the spacetime bubble. </p>
<p>But for a warp drive to generate enough negative energy, you would need a lot of matter. Alcubierre estimated that a warp drive with a 100-meter bubble would <a href="https://doi.org/10.1088/0264-9381/11/5/001">require the mass of the entire visible universe</a>. </p>
<p>In 1999, physicist Chris Van Den Broeck showed that expanding the volume inside the bubble but keeping the surface area constant would <a href="https://doi.org/10.1088/0264-9381/16/12/314">reduce the energy requirements significantly</a>, to just about the mass of the sun. A significant improvement, but still far beyond all practical possibilities.</p>
<h2>A sci-fi future?</h2>
<p>Two recent papers – one by <a href="https://doi.org/10.1088/1361-6382/abdf6e">Alexey Bobrick and Gianni Martire</a> and another by <a href="https://doi.org/10.1088/1361-6382/abe692">Erik Lentz</a> – provide solutions that seem to bring warp drives closer to reality.</p>
<p>Bobrick and Martire realized that by modifying spacetime within the bubble in a certain way, they could remove the need to use negative energy. This solution, though, does not produce a warp drive that can go faster than light. </p>
<p>[<em>Over 100,000 readers rely on The Conversation’s newsletter to understand the world.</em> <a href="https://theconversation.com/us/newsletters/the-daily-3?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=100Ksignup">Sign up today</a>.]</p>
<p>Independently, Lentz also proposed a solution that does not require negative energy. He used a different geometric approach to solve the equations of General Relativity, and by doing so, he found that a warp drive wouldn’t need to use negative energy. Lentz’s solution would allow the bubble to travel faster than the speed of light.</p>
<p>It is essential to point out that these exciting developments are mathematical models. As a physicist, I won’t fully trust models until we have experimental proof. Yet, the science of warp drives is coming into view. As a science fiction fan, I welcome all this innovative thinking. In the <a href="https://www.youtube.com/watch?v=7-Q9CxKtZUA">words of Captain Picard</a>, things are only impossible until they are not.</p><img src="https://counter.theconversation.com/content/157391/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Mario Borunda does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>If humanity wants to travel between stars, people are going to need to travel faster than light. New research suggests that it might be possible to build warp drives and beat the galactic speed limit.Mario Borunda, Associate Professor of Physics, Oklahoma State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1533642021-02-01T13:09:54Z2021-02-01T13:09:54ZCould a human enter a black hole to study it?<figure><img src="https://images.theconversation.com/files/381286/original/file-20210129-19-1ly38eg.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A person falling into a black hole and being stretched while approaching the black hole's horizon.</span> <span class="attribution"><span class="source">Leo Rodriguez and Shanshan Rodriguez</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&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><em><a href="https://theconversation.com/us/topics/curious-kids-us-74795">Curious Kids</a> is a series for children of all ages. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskidsus@theconversation.com">CuriousKidsUS@theconversation.com</a>.</em></p>
<hr>
<blockquote>
<p><strong>Could a human enter a black hole to study it? – Pulkeet, age 12, Bahadurgarh, Haryana, India</strong></p>
</blockquote>
<hr>
<p>To solve the mysteries of black holes, a human should just venture into one. However, there is a rather complicated catch: A human can do this only if the respective black hole is supermassive and isolated, and if the person entering the black hole does not expect to report the findings to anyone in the entire universe. </p>
<p>We are <a href="https://www.grinnell.edu/user/rodriguezl">both</a> <a href="https://www.grinnell.edu/user/rodriguezs">physicists</a> who study black holes, albeit from a very safe distance. Black holes are <a href="https://doi.org/10.1093/mnras/stx1959">among the most abundant astrophysical objects in our universe</a>. These intriguing objects appear to be an essential ingredient in the <a href="https://link.springer.com/chapter/10.1007%2F978-3-642-39596-3_8">evolution of the universe</a>, from the Big Bang till present day. They probably had an <a href="https://doi.org/10.1093/mnras/stz2161">impact on the formation of human life in our own galaxy</a>. </p>
<h2>Two types of black holes</h2>
<p>The universe is littered with a <a href="https://www.washingtonpost.com/science/2019/11/29/scientists-find-monster-black-hole-so-big-they-didnt-think-it-was-possible/">vast zoo of different types of black holes</a>. </p>
<p>They can vary by size and be electrically charged, the same way electrons or protons are in atoms. Some black holes actually spin. There are two types of black holes that are relevant to our discussion. The first does not rotate, is electrically neutral – that is, not positively or negatively charged – and has the mass of our Sun. The second type is a supermassive black hole, with a mass of millions to even billions times greater than that of our Sun. </p>
<p>Besides the mass difference between these two types of black holes, what also differentiates them is the distance from their center to their “event horizon” – a measure called radial distance. The event horizon of a black hole is the point of no return. Anything that passes this point will be swallowed by the black hole and forever vanish from our known universe. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/379201/original/file-20210118-13-1j8ys1.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/379201/original/file-20210118-13-1j8ys1.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/379201/original/file-20210118-13-1j8ys1.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=598&fit=crop&dpr=1 600w, https://images.theconversation.com/files/379201/original/file-20210118-13-1j8ys1.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=598&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/379201/original/file-20210118-13-1j8ys1.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=598&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/379201/original/file-20210118-13-1j8ys1.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=752&fit=crop&dpr=1 754w, https://images.theconversation.com/files/379201/original/file-20210118-13-1j8ys1.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=752&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/379201/original/file-20210118-13-1j8ys1.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=752&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 distance from a black hole’s center of mass to where gravity’s pull is too strong to overcome is called the event horizon.</span>
<span class="attribution"><span class="source">Leo and Shanshan</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>At the event horizon, the black hole’s gravity is so powerful that no amount of mechanical force can overcome or counteract it. <a href="https://doi.org/10.1088/2053-2571/ab06bd">Even light</a>, the fastest-moving thing in our universe, cannot escape – hence the term “black hole.”</p>
<p>The radial size of the event horizon depends on the mass of the respective black hole and is key for a person to survive falling into one. For a black hole with a mass of our Sun (one solar mass), the event horizon will have a radius of just under 2 miles. </p>
<p>The supermassive black hole at the center of our Milky Way galaxy, by contrast, has a mass of roughly 4 million solar masses, and it has an event horizon with a radius of 7.3 million miles or 17 solar radii. </p>
<p>Thus, someone falling into a stellar-size black hole will get much, much closer to the black hole’s center before passing the event horizon, as opposed to falling into a supermassive black hole. </p>
<p>This implies, due to the closeness of the black hole’s center, that the black hole’s pull on a person will differ by a factor of 1,000 billion times between head and toe, depending on which is leading the free fall. In other words, if the person is falling feet first, as they approach the event horizon of a stellar mass black hole, the gravitational pull on their feet will be exponentially larger compared to the black hole’s tug on their head. </p>
<p>The person would experience spaghettification, and most likely not survive being stretched into a long, thin noodlelike shape.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/379434/original/file-20210119-21-1lb26xu.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/379434/original/file-20210119-21-1lb26xu.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/379434/original/file-20210119-21-1lb26xu.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=383&fit=crop&dpr=1 600w, https://images.theconversation.com/files/379434/original/file-20210119-21-1lb26xu.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=383&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/379434/original/file-20210119-21-1lb26xu.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=383&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/379434/original/file-20210119-21-1lb26xu.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=482&fit=crop&dpr=1 754w, https://images.theconversation.com/files/379434/original/file-20210119-21-1lb26xu.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=482&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/379434/original/file-20210119-21-1lb26xu.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=482&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">As the person approaches the event horizon of a a Sun-size black hole, the vast difference in gravitational pull between the inidvidual’s head and toes causes the person to stretch into a very long noodle, hence the term ‘spaghettification’.</span>
<span class="attribution"><span class="source">Leo and Shanshan Rodriguez</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Now, a person falling into a supermassive black hole would reach the event horizon much farther from the the central source of gravitational pull, which means that the difference in gravitational pull between head and toe is nearly zero. Thus, the person would pass through the event horizon unaffected, not be stretched into a long, thin noodle, survive and float painlessly past the black hole’s horizon.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/379435/original/file-20210119-24-1w07b7a.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/379435/original/file-20210119-24-1w07b7a.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/379435/original/file-20210119-24-1w07b7a.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=566&fit=crop&dpr=1 600w, https://images.theconversation.com/files/379435/original/file-20210119-24-1w07b7a.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=566&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/379435/original/file-20210119-24-1w07b7a.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=566&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/379435/original/file-20210119-24-1w07b7a.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=711&fit=crop&dpr=1 754w, https://images.theconversation.com/files/379435/original/file-20210119-24-1w07b7a.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=711&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/379435/original/file-20210119-24-1w07b7a.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=711&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A person falling into a supermassive black hole would likely survive.</span>
<span class="attribution"><span class="source">Leo and Shanshan Rodriguez</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Other considerations</h2>
<p>Most black holes that we observe in the universe are surrounded by very hot disks of material, mostly comprising gas and dust or other objects like stars and planets that got too close to the horizon and fell into the black hole. These disks are called accretion disks and are very hot and turbulent. They are most certainly not hospitable and would make traveling into the black hole extremely dangerous. </p>
<p>To enter one safely, you would need to find a supermassive black hole that is completely isolated and not feeding on surrounding material, gas and or even stars. </p>
<p>Now, if a person found an isolated supermassive black hole suitable for scientific study and decided to venture in, everything observed or measured of the black hole interior would be confined within the black hole’s event horizon.</p>
<p>Keeping in mind that nothing can escape the gravitational pull beyond the event horizon, the in-falling person would not be able to send any information about their findings back out beyond this horizon. Their journey and findings would be lost to the rest of the entire universe for all time. But they would enjoy the adventure, for as long as they survived … maybe ….</p>
<hr>
<p><em>Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to <a href="mailto:curiouskidsus@theconversation.com">CuriousKidsUS@theconversation.com</a>. Please tell us your name, age and the city where you live.</em></p>
<p><em>And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.</em></p><img src="https://counter.theconversation.com/content/153364/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>If you are a sci-fi junkie you’ve probably wondered what would happen if you were unlucky enough to fall into a black hole. How well you’d fare all depends on the type of black hole.Leo Rodriguez, Assistant Professor of Physics, Grinnell CollegeShanshan Rodriguez, Assistant Professor of Physics, Grinnell CollegeLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/972892018-06-07T10:53:35Z2018-06-07T10:53:35ZHow far away was that lightning?<figure><img src="https://images.theconversation.com/files/221632/original/file-20180604-175445-meat87.jpg?ixlib=rb-1.1.0&rect=587%2C71%2C3455%2C2583&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">One one thousand, two one thousand....</span> <span class="attribution"><a class="source" href="https://unsplash.com/photos/vP5Im4q8Z6g">Eric Ward/Unsplash</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>You probably do it. It might be ingrained from when you were a kid, and now it’s almost automatic. You see the flash of lightning – and you immediately start counting the seconds till it thunders.</p>
<p>But does counting really get you a good estimate for how far away the lightning is? Is this one of those old wives’ tales, or is it actually based on science? In this case, we have physics to thank for this quick and easy – and pretty accurate – calculation.</p>
<p>So what happens when a big storm rolls in?</p>
<p>The lightning you see is the <a href="https://www.nssl.noaa.gov/education/svrwx101/lightning/">discharge of electricity</a> that travels between clouds or to the ground. The thunder you hear is the rapid expansion of the air in response to the lightning’s intense heat.</p>
<p>If you’re really close to the lightning, you will see it and hear the thunder simultaneously. But when it’s far away, you see and hear the event at different times. That’s because <a href="https://morgridge.org/blue-sky/why-is-light-faster-than-sound/">light travels much faster than sound</a>. Think of sitting in the nosebleed seats at a baseball game. You see the batter hit the ball a second before you hear the crack of the bat. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.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"></a>
<figcaption>
<span class="caption">The visual part is instantaneous.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/noaaphotolib/27330291264">Pete Gregoire</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>When observing an event on Earth, you see things almost the instant they happen – the speed of light is so fast you can’t even detect the travel time. The speed of sound is much slower, which gives us time to do our calculation.</p>
<p>Let’s simplify the speed equation: <a href="https://en.wikipedia.org/wiki/Speed_of_sound">Sound travels a little over 700 miles per hour</a>, or 700 miles in 3,600 seconds. That means 7 miles traveled every 36 seconds. Make this even easier and round down to 7 miles every 35 seconds… or 1 mile every 5 seconds! Count to 5: If you hear thunder, the lightning occurred within 1 mile.</p>
<p>Now that you know how far away that lightning strike was, is it far enough to be <a href="https://www.weather.gov/safety/lightning-safety">a safe distance from the storm</a>? That’s actually a trick question. Thunder can be heard up to 25 miles away, and lightning strikes have been documented to occur as far as 25 miles from thunderstorms – known as a “<a href="https://www.nssl.noaa.gov/education/svrwx101/lightning/faq/">bolt from the blue</a>.” So if you can hear thunder, you’re close enough to be hit by lightning, and sheltering indoors or in an enclosed car is your safest bet.</p>
<p>And don’t count on the folk wisdom that lightning never strikes the same place twice to protect you. That one is just plain wrong. For example, lightning strikes the top of the <a href="https://www.livescience.com/13704-empire-state-building-lightning-strike.html">Empire State Building</a> an average of 23 times per year.</p><img src="https://counter.theconversation.com/content/97289/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Becky Bolinger receives funding from National Oceanic and Atmospheric Administration and the State of Colorado to monitor drought and climate conditions. </span></em></p>When you see a bolt of lightning, do you immediately start counting to see how far off a storm is? An atmospheric scientist parses the practice.Becky Bolinger, Assistant State Climatologist and Research Scientist in Atmospheric Science, Colorado State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/841292017-09-18T22:58:42Z2017-09-18T22:58:42ZSpeed plus control in new computer chip: slowing down light to sound<figure><img src="https://images.theconversation.com/files/186325/original/file-20170918-24064-3bm8hr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">You see it, then you hear it: light and sound travel at different speeds. </span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/lightening-over-sydney-skyline-night-522565483?src=zYm_8vqVO_Y0-3KxMmML4A-1-19">Reeva/shutterstock </a></span></figcaption></figure><p>Light travels fast – sometimes a little too fast when it comes to data processing. </p>
<p>Published today, our <a href="http://dx.doi.org/10.1038/s41467-017-00717-y">paper</a> describes a new memory chip design that allows us to temporarily slow down light to a manageable speed for better control of computer processing. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/aang9q1G2HQ?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Light energy slows down to become sound, which allows more control.</span></figcaption>
</figure>
<p>Light packets were successfully stored as high-pitch sound waves – about 1,000 times higher than ultrasound – in a wire on a microchip. Around 100-fold thinner than a human hair, the tiny wires were designed to guide light waves as well as high-frequency sound waves, known as hyper-sound. </p>
<p>It’s the first time this has been achieved.</p>
<p>The delay of the transferred information packet is caused by the large difference in speed of travel between light and sound. This is something we experience every time we try to determine how far a thunderstorm is away from us by counting the seconds between the lightning and the thunder.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/hold-it-right-there-how-and-why-to-stop-light-in-its-tracks-66210">Hold it right there: how (and why) to stop light in its tracks</a>
</strong>
</em>
</p>
<hr>
<h2>Why we use light in computing</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/186319/original/file-20170918-24051-1e00a5k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/186319/original/file-20170918-24051-1e00a5k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/186319/original/file-20170918-24051-1e00a5k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1352&fit=crop&dpr=1 600w, https://images.theconversation.com/files/186319/original/file-20170918-24051-1e00a5k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1352&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/186319/original/file-20170918-24051-1e00a5k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1352&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/186319/original/file-20170918-24051-1e00a5k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1699&fit=crop&dpr=1 754w, https://images.theconversation.com/files/186319/original/file-20170918-24051-1e00a5k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1699&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/186319/original/file-20170918-24051-1e00a5k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1699&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 new chip design, shown next to an Australian 50 cent piece.</span>
<span class="attribution"><span class="source">University of Sydney</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>Today even small laptops use multiple processors, such as dual or quad cores. This is even more evident in high-performance machines, supercomputers or large data centres. Dividing computation between several processors is a way to improve performance, known in computer language as parallel computing. </p>
<p>This parallelisation, however, raises new issues: the different cores have to talk to each other and perform in sync, like a big orchestra. Here electronics starts to reach its limits. The connections between the processors suffer from losses and produce heat. This is the main reason why your laptop gets hot. </p>
<p>At industrial scales, the heat is becoming almost unmanageable. Just last month there was an <a href="https://www.cnbc.com/2017/08/15/worlds-largest-data-center-to-be-built-in-arctic-circle.html">announcement</a> to build the world’s largest data centre inside the Arctic Circle, in order to deal with the heat problem of these centres.</p>
<p>Optical links between processors can help solve this problem: data encoded as light packets can provide large bandwidths, high speeds and do not produce heat.</p>
<h2>A blessing and a curse</h2>
<p>While the speed of light is of great advantage when sending data over the internet all around the globe, it is a real challenge to master on a small chip. </p>
<p>Light travels 300 metres in just a millionth of a second. To provide a connection between different processors, we need a way stop or delay the light at times when the receiving processor is still occupied. In other words, we need a buffer for light packets on a chip. </p>
<p>But buffering the optical data in common chip designs for electronic memory results in loss of speed and bandwidth. </p>
<p>Our new research shows all the characteristics of a light wave – that is, brightness, colour and phase – can be transferred to a hyper-sound wave, and by doing so can be buffered. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/186326/original/file-20170918-24099-x0h1vl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/186326/original/file-20170918-24099-x0h1vl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=212&fit=crop&dpr=1 600w, https://images.theconversation.com/files/186326/original/file-20170918-24099-x0h1vl.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=212&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/186326/original/file-20170918-24099-x0h1vl.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=212&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/186326/original/file-20170918-24099-x0h1vl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=266&fit=crop&dpr=1 754w, https://images.theconversation.com/files/186326/original/file-20170918-24099-x0h1vl.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=266&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/186326/original/file-20170918-24099-x0h1vl.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=266&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A light packet is converted to a sound wave as it moves through the chip, and then back to light as it exits.</span>
<span class="attribution"><span class="source">University of Sydney</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>One reason for the large data rates achieved using light lies in its ability to carry data simultaneously at different wavelengths, or colours. Using multiple colours is like opening additional lanes on a crowded highway. </p>
<p>What we experience as different colour in the case of light is a different pitch for a sound wave. We show that different colours can be stored as different pitched sound waves, and importantly can be unambiguously identified afterwards.</p>
<h2>Sound waves to store information</h2>
<p>The basic operation principles of our new design – which features a phenomenon known as delay line memory – are the following: </p>
<ul>
<li>a processor encodes the freshly-calculated data on light packets, and sends it towards the next processor </li>
<li>if this processor is still occupied, the light packet is transferred to a sound wave</li>
<li>the sound wave travels a hundred thousand times slower towards the processor, giving it the required time to finish the computation</li>
<li>the sound wave gets transferred back to a light packet, and can be further processed. </li>
</ul>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-how-scientists-invent-new-colours-80897">Explainer: how scientists invent new colours</a>
</strong>
</em>
</p>
<hr>
<p>This process resembles the operation of the first computers built at the beginning of the 20th century. Here information was temporarily stored in sound waves that propagated in mercury tubes while the processors were occupied.</p>
<p>So as computer chips are reaching their performance limits, the old idea of a delay line-based memory using sound waves is celebrating a comeback. This time it’s not in bulky mercury tubes, but tiny light wires on a microchip that are capable of processing much more data.</p><img src="https://counter.theconversation.com/content/84129/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Benjamin J. Eggleton receives funding from the Australian Research Council, the NSW Government and the US Air-force Office of Scientific Research.</span></em></p><p class="fine-print"><em><span>Birgit Stiller and Moritz Merklein do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The speed of light is of great advantage when sending data over the internet, but it is a real challenge to master on a small chip. A new design offers a solution: convert the energy to sound.Moritz Merklein, PhD candidate in Brillouin optomechanics, University of SydneyBenjamin J. Eggleton, Professor; ARC Laureate Fellow, Director, ARC Centre of Excellence for Ultrahigh bandwidth Devices for Optical Systems, University of SydneyBirgit Stiller, Research Fellow in Photonics, University of SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/494392015-11-26T19:12:33Z2015-11-26T19:12:33ZDon’t stop me now! Superluminal travel in Einstein’s universe<figure><img src="https://images.theconversation.com/files/103147/original/image-20151125-23821-59f1ak.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Hyperspace may one day be a reality.</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>The story of the drawn-out development of Albert Einstein’s revolutionary rewrite of the laws of gravity <a href="http://www.amazon.com/Subtle-Is-Lord-Science-Einstein/dp/0192806726">has been told many times</a>, but over the past 100 years it has given us extreme stars and black holes, expanding universes and gravitational mirages. Einstein also ensured you will never get lost, enabling the technology that helps your phone find your location with <a href="https://helix.northwestern.edu/article/satellites-smartphones-and-science-gps">pinpoint accuracy!</a> </p>
<p>Despite this scientific bounty, relativity appears to place strict limits on our exploration of Einstein’s universe, with any rocketship limited to travelling no faster than the speed of light. With the distance between stars measured in <a href="http://earthsky.org/astronomy-essentials/how-far-is-a-light-year">light years</a>, and the distance across galaxies being <a href="https://www.youtube.com/watch?v=dvwH8Qij0JY">hundreds of thousands of light years</a>, not to mention the complexities of <a href="http://newt.phys.unsw.edu.au/einsteinlight/jw/module4_time_dilation.htm">time dilation</a>, establishing and running a <a href="http://starwars.wikia.com/wiki/Galactic_Empire">galactic empire</a> is going to be a drawn out and messy affair. </p>
<h2>Bending time, bending space</h2>
<p>I’ve already <a href="https://theconversation.com/warp-drives-and-reality-new-hope-for-a-galactic-empire-5891">written</a> that all is not lost, as in 1994 physicist <a href="http://www.nucleares.unam.mx/%7Esoma/miembros/MAlcubierre.htm">Miguel Alcubierre</a> discovered something wonderful: that by bending <a href="https://en.wikipedia.org/wiki/Spacetime">space and time</a> just the right way will allow you to travel at any speed you want! While there are some <a href="http://www.universetoday.com/93882/warp-drives-may-come-with-a-killer-downside/">downsides</a>, with such a <a href="https://en.m.wikipedia.org/wiki/Warp_drive_(Star_Trek)">warp drive</a>, the speed of light <em>can</em> be broken. </p>
<p>However, a couple of questions spring to mind, not least how can this superluminal bubble of a warp drive be consistent with the rules of relativity. And if it is, why did it take until the 1990s for someone to notice this was the case. </p>
<p>After <a href="http://www.britannica.com/science/E-mc2-equation">E = mc²</a>, the fact that nothing can move faster than light is probably the most common fact known about Einstein’s special theory of relativity. So just what can superluminal motion actual mean? </p>
<p>Let’s begin with what Einstein was actually saying about racing a light beam. To Einstein, the race takes place “locally”, such as in a laboratory, where you start a particle with mass and a light beam off at the same time. In this case, the light beam always gets ahead.</p>
<p>But in his special theory, the details of space and time are the same everywhere. More technically, the union of the two – known as <a href="https://einstein.stanford.edu/SPACETIME/spacetime2.html">spacetime</a> – is flat, and we can compare the speed of a particle in the laboratory to a light ray somewhere off in the universe.</p>
<p>Things get messier in the general theory, as the presence of gravity ensures that the curvature of spacetime here is different to spacetime over there, and it is not possible to uniquely compare the speed of the particle in your laboratory to a light ray off in the distant universe. The only sensible comparison you can make is in your laboratory, and here the light ray still always wins.</p>
<p>The same is true in the curved spacetime of the warp drive. If your traveller in the warp bubble tries to race a particle and a light beam together, the light beam will always win.</p>
<p>An observer watching the bubble go by would calculate the light beam to be travelling faster than any light ray they create in their own laboratory. But this is not a problem, as it really makes no sense to compare velocities “there” with velocities “here”. </p>
<p>It is precisely this reason that cosmologists are happy to talk about galaxies receding from us faster than the speed of light due to the <a href="http://www.physics.uq.edu.au/download/tamarad/papers/SciAm_BigBang.pdf">expansion of the universe</a>.</p>
<h2>Metric mechanics</h2>
<p>Relativity had been around for almost 80 years before Alcubierre uncovered his solution. Why hadn’t people realised superluiminal travel was part of the theory?</p>
<p>The problem, of course, is the mathematically fiendish nature of Einstein’s equations. It is extremely difficult to calculate the curvature of spacetime and resultant action of gravity from any old distribution of mass and energy.</p>
<p>It can be mathematically simpler to define the properties of spacetime and then calculate the required distribution of mass and energy. And Alcubierre’s great insight was to realise a bubble could move at any speed as a rolling wave in spacetime. </p>
<p>However, such “metric mechanics” come with a downside: we may be able to find spacetimes that allow superluminal motion, but the required distribution of mass and energy may not be physically possible. </p>
<p>Those familiar with classical mechanics may remember that it is easier define a <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/gpot.html">gravitational potential</a> to determine forces, but these might require <a href="http://folk.uio.no/iliamu/Poisson.pdf">negative matter to physically exist</a>.</p>
<p>The same is true for the warp drive solution, requiring material with a negative energy density to bend and shape space-time appropriately. And while we have hints that such properties exist in the universe, we have no idea if we will be able to mine and forge it to fashion our spaceships. So we may never be able to build an Alcubierre warp drive.</p>
<p>But we should not allow this to demoralise us! Alcubierre’s insights should inspire us to continue to bend and stretch spacetime, to tease out the possibles still hidden within the mathematics. Most may be physically impossible to ever realise, but with sufficient imagination, and a stroke of luck, we may stumble across our pathway to the stars.</p><img src="https://counter.theconversation.com/content/49439/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Geraint Lewis receives funding from the Australian Research Council.</span></em></p>Many people think relativity puts a hard speed limit on the universe, but it actually opens up the possibility of faster-than-light travel - if we can overcome some significant practical hurdles.Geraint Lewis, Professor of Astrophysics, University of SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/411122015-05-08T09:53:28Z2015-05-08T09:53:28ZFaster-than-light travel: are we there yet?<figure><img src="https://images.theconversation.com/files/80723/original/image-20150506-10927-1o5e58k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">I can get you there fast!</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/craigyc/3976054279">Craig Cormack</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Long before the Empire struck back, before the United Federation of Planets federated, Isaac Asimov created <a href="http://knopfdoubleday.com/book/203444/foundation-foundation-and-empire-second-foundation/">Foundation</a>, the epic tale of the decline and fall of the Galactic Empire. Asimov’s Empire comprised 25 million planets, knit together by sleek spaceships hurtling through the galaxy.</p>
<p>And how did these spaceships cross the vast gulf between the stars? By jumping through hyperspace, of course, as Asimov himself explains in Foundation: </p>
<blockquote>
<p>Travel through ordinary space could proceed at no rate more rapid than that of ordinary light… and that would have meant years of travel between even the nearest of inhabited systems. Through hyper-space, that unimaginable region that was neither space nor time, matter nor energy, something nor nothing, one could traverse the length of the Galaxy in the interval between two neighboring instants of time. </p>
</blockquote>
<p>What the heck is Asimov talking about? Did he know something about a secret theory of faster-than-light travel? Hardly. Asimov was participating in a grand science fiction tradition: when confronted with an immovable obstacle to your story, make something up.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/80724/original/image-20150506-10950-vwyxry.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/80724/original/image-20150506-10950-vwyxry.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/80724/original/image-20150506-10950-vwyxry.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=407&fit=crop&dpr=1 600w, https://images.theconversation.com/files/80724/original/image-20150506-10950-vwyxry.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=407&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/80724/original/image-20150506-10950-vwyxry.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=407&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/80724/original/image-20150506-10950-vwyxry.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=511&fit=crop&dpr=1 754w, https://images.theconversation.com/files/80724/original/image-20150506-10950-vwyxry.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=511&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/80724/original/image-20150506-10950-vwyxry.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=511&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Nothing goes faster than light.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/thefatrobot/14996505415">Bastian Hoppe</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<h2>You can’t beat the speed of light</h2>
<p>The problem is that as far as we know, faster-than-light travel is impossible, making galactic empires, federations, confederacies and any other cross-galaxy civilizations impossible. But that’s so <em>inconvenient</em>. To evade the cosmic speed limit science fiction has created “warp-drives,” “hyperspace,” “subspace,” and other tricks that have become so ingrained, fans of science fiction don’t give them a second thought.</p>
<p>Everyone knows what the Enterprise is doing when it does this:</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/Kj178APgdno?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Warp drive, Mr Scott.</span></figcaption>
</figure>
<p>Or when the Millennium Falcon does this:</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/cSHYjrSLm4w?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">I’m going to make the jump to light speed!</span></figcaption>
</figure>
<p>Or when the Jupiter 2… actually the Robinson family tried to get to Alpha Centauri without any special effects:</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/80853/original/image-20150507-1212-atsn1h.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/80853/original/image-20150507-1212-atsn1h.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/80853/original/image-20150507-1212-atsn1h.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=464&fit=crop&dpr=1 600w, https://images.theconversation.com/files/80853/original/image-20150507-1212-atsn1h.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=464&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/80853/original/image-20150507-1212-atsn1h.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=464&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/80853/original/image-20150507-1212-atsn1h.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=583&fit=crop&dpr=1 754w, https://images.theconversation.com/files/80853/original/image-20150507-1212-atsn1h.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=583&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/80853/original/image-20150507-1212-atsn1h.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=583&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Good luck.</span>
<span class="attribution"><a class="source" href="http://reflectionsonfilmandtelevision.blogspot.com/2015/01/lost-in-space-50th-anniversary-blogging_7.html">Lost in Space 'The Derelict'</a></span>
</figcaption>
</figure>
<p>No wonder they got lost in space.</p>
<h2>Light sets the cosmic speed limit</h2>
<p>Why <em>can’t</em> we really exceed the speed of light? After all, people used to talk about a “sound barrier” up until the barrier was broken. But the speed of light is a much tougher barrier to crack. When scientists developed the theory of light back in the 19th century, it came with a special puzzle: their theory seemed to show that every observer should measure the same speed for light, about 186,000 miles per second. But that means if you try to chase a beam of light, no matter how fast you move, the light beam will still fly away from you at 186,000 miles per second. And what’s even more bizarre is that if you are moving at 99% of the speed of light, and your friend is standing still, both of you will see the light moving away at exactly the same speed.</p>
<p>Many scientists back then didn’t really believe this odd prediction, and the American physicist Albert Michelson (along with his collaborator Edward Morley) set out to measure how the speed of light would change due to the motion of the earth through space. But their famous <a href="http://www.juliantrubin.com/bigten/michelsonmorley.html">Michelson-Morley experiment</a> found no change at all. The speed of light seemed to be the same regardless of whether they measured it in the same direction the earth was moving, or in some other direction – a rare example of a non-discovery that turned out to be more important than a discovery!</p>
<h2>Enter Einstein and relativity</h2>
<p>Instead of trying to explain away this bizarreness, Albert Einstein embraced it. He built an entire theory, called <a href="http://www.fourmilab.ch/etexts/einstein/specrel/www/">special relativity</a>, around the idea that the speed of light is the same for everyone who measures it, no matter how fast they are moving in relation to the light. In order to accommodate this behavior for light, Einstein’s theory predicted that time and space would have to stretch or contract as someone traveled with increasing speed. And out of special relativity popped a cosmic speed limit: nothing could ever exceed the speed of light.</p>
<p>Relativity is a cornerstone of all of modern physics, and we have no reason to doubt it – no one has ever observed an object moving faster than light. There’s actually a minor clarification necessary here: Einstein’s speed limit is the speed of light <em>in a vacuum</em>. Light slows down when it moves through a material like water or glass, and then it’s perfectly possible to exceed this reduced speed of light – up to its speed in a vacuum, of course. Anything moving faster than light in water or glass produces the luminous equivalent of a sonic boom, called Čerenkov radiation. It’s what gives underwater nuclear reactors their attractive blue glow.</p>
<h2>But about that warp drive…</h2>
<p>Of all of the attempts to wiggle out of Einstein’s speed limit, probably the most plausible is theoretical physicist Miguel Alcubierre’s <a href="http://dx.doi.org/10.1088/0264-9381/11/5/001">“warp drive”</a>. Alcubierre’s proposal doesn’t violate the cosmic speed limit – it goes around it. Try filling a greasy frying pan with water and then put a drop of soap into the pan. The grease will fly away to the sides of the pan.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/80719/original/image-20150506-10950-1ysg2ty.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/80719/original/image-20150506-10950-1ysg2ty.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/80719/original/image-20150506-10950-1ysg2ty.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/80719/original/image-20150506-10950-1ysg2ty.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/80719/original/image-20150506-10950-1ysg2ty.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/80719/original/image-20150506-10950-1ysg2ty.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/80719/original/image-20150506-10950-1ysg2ty.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/80719/original/image-20150506-10950-1ysg2ty.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"></a>
<figcaption>
<span class="caption">A visualization of a warp field. The ship rests in a bubble of unaltered space, while what’s in front contracts and what’s behind stretches.</span>
<span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:Star_Trek_Warp_Field.png">Trekky0623</a></span>
</figcaption>
</figure>
<p>Alcubierre’s warp drive does the same thing with <em>space itself</em>. Alcubierre showed that by a suitable distribution of matter, you can shrink space in front of your spaceship and stretch it behind the spaceship, creating a small bubble around the ship that moves as fast as you like. Because space is contracting in front of the ship, the ship wouldn’t officially be moving faster than the speed of light. In fact, the ship would actually be at rest relative to the warp bubble, and the people inside the ship wouldn’t even feel any acceleration. Talk about a smooth ride!</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/pbKJ_onDy4E?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Everybody ready to say goodbye to our solar system? We’ll have to violate the weak energy condition….</span></figcaption>
</figure>
<p>There’s just one tiny problem…. Alcubierre’s space warp can only be generated by violating something called the “weak energy condition.” Scientists can’t prove that the weak energy condition is always true, but any violation would produce a lot of strange things, like negative energy densities, and possible <a href="http://www.cosmicyarns.com/2015/04/wormholes-galactic-subway-system_21.html">wormholes</a> or <a href="http://dx.doi.org/10.1103/PhysRevLett.61.1446">time machines</a>. Cool – sign me up for that! But we’ve never seen any actual violations of the weak energy condition. So the Alcubierre warp drive occupies a kind of physics twilight zone – not absolutely ruled out, but not very plausible, either.</p>
<p>So how will humanity ever reach the stars? The door marked “faster-than-light travel” has been slammed in our face and welded shut. We’ll have to sneak in some other way. Get to work!</p><img src="https://counter.theconversation.com/content/41112/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Robert Scherrer receives funding from the Department of Energy.</span></em></p>There’s a cosmic speed limit that unfortunately means you aren’t going to be firing up warp drive anytime soon.Robert Scherrer, Professor and Chair of Physics and Astronomy, Vanderbilt UniversityLicensed as Creative Commons – attribution, no derivatives.