tag:theconversation.com,2011:/us/topics/special-relativity-2599/articlesSpecial Relativity – The Conversation2023-11-13T13:33:49Ztag:theconversation.com,2011:article/2138362023-11-13T13:33:49Z2023-11-13T13:33:49ZIs time travel even possible? An astrophysicist explains the science behind the science fiction<figure><img src="https://images.theconversation.com/files/554607/original/file-20231018-19-hyrxxn.jpeg?ixlib=rb-1.1.0&rect=0%2C0%2C960%2C540&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">If traveling into the past is possible, one way to do it might be sending people through tunnels in space.</span> <span class="attribution"><a class="source" href="https://pixabay.com/photos/astronomy-desktop-space-galaxy-3217141/">by raggio5 via Pixabay</a></span></figcaption></figure><figure class="align-left ">
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<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>
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<p><strong>Will it ever be possible for time travel to occur? – Alana C., age 12, Queens, New York</strong></p>
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<p>Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time. </p>
<p>But is this just a fun idea for movies, or could it really happen?</p>
<p>The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the <a href="https://www.grc.nasa.gov/www/k-12/airplane/thermo.html">laws of thermodynamics</a>, it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time. </p>
<p>It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.</p>
<h2>Time is relative</h2>
<p>However, physicist Albert Einstein’s <a href="https://www.space.com/36273-theory-special-relativity.html">theory of special relativity</a> suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the <a href="https://www.space.com/15830-light-speed.html">speed of light</a> – 671 million miles per hour! – will experience time slower than a person on Earth. </p>
<p>People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is <a href="https://www.space.com/33411-astronaut-scott-kelly-relativity-twin-brother-ages.html">6 minutes and 5 milliseconds younger</a>.</p>
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<p>Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves <a href="https://www.space.com/20881-wormholes.html">wormholes</a>, or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past. </p>
<p>However, wormholes remain theoretical: Scientists have yet to spot one. It also looks like it would be <a href="https://galileospendulum.org/2015/01/26/why-wormholes-probably-dont-exist/">incredibly challenging</a> to send humans through a wormhole space tunnel.</p>
<h2>Paradoxes and failed dinner parties</h2>
<p>There are also paradoxes associated with time travel. The famous “<a href="https://www.discovermagazine.com/the-sciences/what-is-the-grandfather-paradox-of-time-travel">grandfather paradox</a>” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.</p>
<p>Famously, physicist Stephen Hawking tested the possibility of time travel by <a href="https://www.euronews.com/culture/2023/06/28/culture-re-view-the-day-stephen-hawking-threw-a-time-traveller-party">throwing a dinner party</a> where invitations noting the date, time and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up. </p>
<p>As he <a href="https://www.penguinrandomhouse.com/books/77014/black-holes-and-baby-universes-by-stephen-hawking/">pointed out</a>: “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”</p>
<h2>Telescopes are time machines</h2>
<p>Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel. As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago. </p>
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<figcaption><span class="caption">Telescopes are a kind of time machine – they let you peer into the past.</span></figcaption>
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<p>NASA’s newest space telescope, the <a href="https://theconversation.com/a-cosmic-time-machine-how-the-james-webb-space-telescope-lets-us-see-the-first-galaxies-in-the-universe-187015">James Webb Space Telescope</a>, is peering at galaxies that were formed at the very beginning of the Big Bang, about 13.7 billion years ago.</p>
<p>While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies and dreams.</p>
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<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/213836/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Adi Foord 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>Scientists are trying to figure out if time travel is even theoretically possible. If it is, it looks like it would take a whole lot more knowledge and resources than humans have now to do it.Adi Foord, Assistant Professor of Astronomy and Astrophysics, University of Maryland, Baltimore CountyLicensed as Creative Commons – attribution, no derivatives.tag: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">
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<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>
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<p><strong>Why does time change when traveling close to the speed of light? – Timothy, age 11, Shoreview, Minnesota</strong></p>
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<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>
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<figcaption><span class="caption">The experience of time is dependent on motion.</span></figcaption>
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<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>
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<figcaption><span class="caption">The twin paradox isn’t actually a paradox.</span></figcaption>
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<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>
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<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/1966492022-12-15T16:23:01Z2022-12-15T16:23:01ZHow the James Webb Space Telescope has revealed a surprisingly bright, complex and element-filled early universe – podcast<figure><img src="https://images.theconversation.com/files/501209/original/file-20221215-15338-7jlg2y.png?ixlib=rb-1.1.0&rect=5%2C191%2C1886%2C1613&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The James Webb Space Telescope is providing astronomers with images and data that reveal secrets from the earliest era of the universe.</span> <span class="attribution"><a class="source" href="https://webbtelescope.org/contents/media/images/2022/034/01G7DA5ADA2WDSK1JJPQ0PTG4A?news=true">NASA/STScI</a></span></figcaption></figure><p>If you want to know what happened in the earliest years of the universe, you are going to need a very big, very specialized telescope. Much to the joy of astronomers and space fans everywhere, the world has one – the <a href="https://theconversation.com/james-webb-space-telescope-an-astronomer-on-the-team-explains-how-to-send-a-giant-telescope-to-space-and-why-167516">James Webb Space Telescope</a>. </p>
<p>In this episode of “<a href="https://theconversation.com/uk/topics/the-conversation-weekly-98901">The Conversation Weekly</a>,” we talk to three experts about what astronomers have learned about the first galaxies in the universe and how just six months of data from James Webb is already changing astronomy. </p>
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<p>The James Webb Space Telescope successfully launched into space on Dec. 25, 2021. After about six months of travel, setup and calibration, the telescope began collecting data and NASA published the first <a href="https://theconversation.com/james-webb-space-telescope-an-astronomer-explains-the-stunning-newly-released-first-images-186800">stunning images</a>.</p>
<p>One of Webb’s nicknames is the “<a href="https://theconversation.com/is-the-james-webb-space-telescope-finding-the-furthest-oldest-youngest-or-first-galaxies-an-astronomer-explains-187915">first light telescope</a>.” This is because Webb was specifically designed to be able to see as far back as possible into the earliest days of the universe and detect some of the first visible light. </p>
<p>You can see these galaxies in the <a href="https://theconversation.com/james-webb-space-telescope-an-astronomer-explains-the-stunning-newly-released-first-images-186800">images NASA has released</a>. <a href="https://scholar.google.com/citations?user=AWluLnoAAAAJ&hl=en&oi=ao">Jonathan Trump</a>, an astronomer at the University of Connecticut, is on one of the teams working on some of the early James Webb data. He was watching the release of the first images live and noticed some things many nonastronomers might have missed. “In the background, behind these beautiful arcs and spirals and massive elliptical galaxies are these tiny, itty-bitty red smudges. That’s what I was most interested in, because those are some of the first galaxies in the universe.”</p>
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<span class="caption">This compound image shows some of the earliest galaxies ever seen, highlighted by the small boxes in the images on the left and right, and shown up close in the images in the center.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/feature/goddard/2022/nasa-s-webb-draws-back-curtain-on-universe-s-early-galaxies">NASA, ESA, CSA, Tommaso Treu (UCLA)</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
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<p>To see any of these galaxies from the earliest days of the universe would be exciting, but right off the bat, <a href="https://scholar.google.com/citations?user=oXVDWEcAAAAJ&hl=en&oi=ao">Jeyhan Kartaltepe</a>, an astronomer at the Rochester Institute of Technology, found something exciting when she started digging into the data. </p>
<p>“One of the things we’ve learned is that there are more of these galaxies than we expected to see.” In addition to working on identifying these early galaxies, Kartaltepe has been using Webb’s incredible resolution to study their structure and shape. “We expect there to be discs because discs form pretty naturally in the universe whenever you have something that’s rotating. But we’ve been seeing a lot of them, which has been a bit of a surprise.”</p>
<p>In addition to noting the shape of the galaxies in the early universe, astronomers like Trump are starting to be able to assess the <a href="https://arxiv.org/pdf/2207.12388.pdf">chemical composition of these galaxies</a>. He does this by looking at the spectrum of light James Webb is collecting. “We look at these distant galaxies and we look for particular patterns of emission lines. We often call them a chemical fingerprint because it really is like a particular fingerprint of particular elements in the gas in a galaxy.” </p>
<p>The universe started with just hydrogen and helium, but as stars formed and fused elements together, bigger, heavier elements started to emerge and fill in the periodic table as it is today. And just like Kartaltepe, Trump is finding evidence that things were happening faster in the early universe than astronomers expected. “I would’ve guessed that the universe would have struggled to make the periodic table and build up things. But that’s not what we found. Instead, the universe seems to have proceeded pretty rapidly.”</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/501210/original/file-20221215-20-lvlbo.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A photos showing thousands of galaxies in a night sky." src="https://images.theconversation.com/files/501210/original/file-20221215-20-lvlbo.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/501210/original/file-20221215-20-lvlbo.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=612&fit=crop&dpr=1 600w, https://images.theconversation.com/files/501210/original/file-20221215-20-lvlbo.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=612&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/501210/original/file-20221215-20-lvlbo.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=612&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/501210/original/file-20221215-20-lvlbo.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=769&fit=crop&dpr=1 754w, https://images.theconversation.com/files/501210/original/file-20221215-20-lvlbo.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=769&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/501210/original/file-20221215-20-lvlbo.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=769&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 photo shows Webb’s first deep-field image, a long exposure of a small part of the sky revealing thousands of galaxies, many of which are too faint for even Hubble to detect.</span>
<span class="attribution"><a class="source" href="https://webbtelescope.org/contents/media/images/2022/035/01G7DCWB7137MYJ05CSH1Q5Z1Z?news=true">NASA/STScI</a></span>
</figcaption>
</figure>
<p>The discoveries coming out of James Webb are already changing how astronomers think of the early universe and challenging much of the existing theory. But the truly exciting part is that we are just beginning to see what this telescope is capable of, as <a href="https://scholar.google.com/citations?user=npUHvbwAAAAJ&hl=en&oi=ao">Michael Brown</a>, an astronomer at Monash University, explains. </p>
<p>“I’ve been on science papers that have used literally just a couple of minutes of data,” Brown says. “The image quality is just so good that a couple of minutes can do amazing things.” But soon Webb will begin to do follow-up surveys, take deep-field images and stare at parts of the sky for days and even weeks. Over the coming months, years and decades, Webb is going to keep giving astronomers plenty to work on, and astronomers like Brown are excited. “There is just all this complexity there, and we are barely scratching the surface. This will be the stuff that people who are students now are going to devote their careers to. And it’s going to be marvelous.”</p>
<hr>
<p>This episode was produced by Katie Flood and Daniel Merino, with sound design by Eloise Stevens. It was written by Katie Flood and Daniel Merino. Mend Mariwany is the show’s executive producer. Our theme music is by Neeta Sarl. </p>
<p>You can find us on Twitter <a href="https://twitter.com/TC_Audio">@TC_Audio</a>, on Instagram at <a href="https://www.instagram.com/theconversationdotcom/">theconversationdotcom</a> or <a href="mailto:podcast@theconversation.com">via email</a>. You can also sign up to The Conversation’s <a href="https://theconversation.com/newsletter">free daily email here</a>. A transcript of this episode will be available soon. </p>
<p>Listen to “The Conversation Weekly” via any of the apps listed above, download it directly via our <a href="https://feeds.acast.com/public/shows/60087127b9687759d637bade">RSS feed</a>, or find out <a href="https://theconversation.com/how-to-listen-to-the-conversations-podcasts-154131">how else to listen here</a>.</p><img src="https://counter.theconversation.com/content/196649/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span> </span></em></p><p class="fine-print"><em><span>Jeyhan Kartaltepe receives funding from NASA and the National Science Foundation.</span></em></p><p class="fine-print"><em><span>Jonathan Trump receives funding from NASA and NSF. </span></em></p><p class="fine-print"><em><span>Michael J. I. Brown receives research funding from the Australian Research Council and Monash University.</span></em></p>It has been one year since the launch of the James Webb Space Telescope and six months since the first pictures were released. Astronomers are already learning unexpected things about the early universe.Daniel Merino, Associate Science Editor & Co-Host of The Conversation Weekly Podcast, The ConversationNehal El-Hadi, Science + Technology Editor & Co-Host of The Conversation Weekly Podcast, The ConversationLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1186412019-08-08T13:14:39Z2019-08-08T13:14:39ZNASCAR may be the fastest way to learn about physics<figure><img src="https://images.theconversation.com/files/286979/original/file-20190805-36399-kz4ouq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The laws of physics are on display at the Daytona International Speedway.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/february-26-2017-daytona-beach-florida-596617100">Action Sports Photography/Shutterstock.com</a></span></figcaption></figure><p>There’s just something thrilling about traveling at high speeds. Throughout history people have always pushed themselves to <a href="https://landspeedrecord.org/">go faster</a>, whether on foot, on horseback, on a boat or on a bicycle.</p>
<p>Nearly every weekend, today’s speed lovers can live vicariously by watching their favorite NASCAR drivers race around the track at death-defying speeds.</p>
<p>Maybe it’s the excitement in the crowd or maybe it’s the constant threat of danger that draws people to the sport. Or maybe its the feats of science and engineering that pull some spectators in. <a href="https://scholar.google.com/citations?user=w_InbNoAAAAJ&hl=en&oi=ao">As a physicist</a>, I love seeing all the physics principles on display during a NASCAR race. </p>
<h2>Speed</h2>
<p>NASCAR drivers travel at extremely high speeds, over 200 miles per hour. They accelerate so quickly that it takes them only around 3 to 3.5 seconds to go from zero to 60 mph. During this acceleration, the car must exert an average of 2,600 lbs of horizontal force against the track. This is comparable to the <a href="https://www.bio.fsu.edu/%7Egerick/bite_force/">bite force of a large American crocodile</a> or what it would take to lift a full-grown buffalo.</p>
<p>According to Einstein’s theory of special relativity, the faster you move through space, the slower your passage of time. So it’s fair to say that speed demon NASCAR drivers age a very tiny bit less than the rest of us. At the end of a 3.5 hour race, the drivers have aged about 0.5 nanoseconds less than the spectators who stayed still. If a driver raced nonstop at 200 mph for the next 50 years, he would age 70 microseconds less than the rest of us.</p>
<p>While NASCAR drivers are moving at incredibly fast speeds compared to the crowds in the stands, their speeds are small compared to what Einstein had in mind – like how fast light can travel, 670 million mph. The effect of relativity at the track is small, but it does exist.</p>
<h2>The track</h2>
<p>So how are drivers able to obtain these speeds?</p>
<p>As a car enters a turn, it naturally wants to continue in the direction it was originally going. To change direction to follow the curve of the oval-shaped track, a force must be applied.</p>
<p>The necessary force comes from the friction between the tires and the track. <a href="https://www.khanacademy.org/science/physics/forces-newtons-laws/inclined-planes-friction/a/what-is-friction">Friction</a> is the connection between the two that prevents them from sliding against one another. </p>
<p>So for drivers it’s a balancing act – they want to keep the pedal to the metal, but they can’t go so fast on a curve that their speed overpowers the maneuvering ability provided by friction. Go too quickly and the friction may not be enough to prevent the car from continuing in its original direction and sliding straight into the wall. Slow down too much and you fall behind the competition.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.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 banking of the track helps cars make those high-speed turns.</span>
<span class="attribution"><a class="source" href="https://unsplash.com/photos/Ur5VN_92g-k">Tim Trad/Unsplash</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>The way the track is designed can help out here. The turns are banked, meaning they are higher on the outside of the track and lower toward the center. Part of the force of the road pushing up on the car – what physicists call the <a href="https://www.khanacademy.org/science/physics/forces-newtons-laws/normal-contact-force/a/what-is-normal-force">normal force</a> – assists the frictional force of the tires and helps the car make it around the turn.</p>
<p><a href="https://www.nascar.com/gallery/talladega-superspeedway/#/0">Banking in the turns</a> at some of the fastest race tracks is comparable to the steepness of a playground slide. <a href="https://www.richmondraceway.com/About/About.aspx">Banking at Richmond International Raceway</a> allows cars to go approximately 1.3 times faster than they could without banking. Larger curves and higher banking, like those seen at Daytona and Talladega, allow the drivers to maintain a higher speed as they round those corners.</p>
<h2>Power</h2>
<p>Power is a measure of energy converted from one form to another in a set amount of time. In stock car racing, this conversion is from the chemical energy stored in gasoline to the kinetic energy of motion.</p>
<p>A NASCAR engine produces around <a href="https://www.nascar.com/news-media/2018/10/02/2019-rules-packages-announced-monster-energy-series/">750 horsepower</a> (560 kW), which exceeds a similar model street car that tops out around <a href="https://www.toyota.com/camry/features/interior/2550/2514/2532">300 horsepower</a>. During a race, the power conversion of a NASCAR engines is about 500 times the power usage of the <a href="https://www.eia.gov/tools/faqs/faq.php?id=97&t=3">typical American household</a> during the same period of time.</p>
<p>The cars’ power comes from burning gas as the engine rotates. The rotation of a NASCAR engine is 3.5 times faster than a standard street car and much more efficient, allowing it to combust more quickly and produce more power. </p>
<h2>Collisions</h2>
<p>With the high speed and power of stock cars come the risks of dangerous collisions. Some of the <a href="https://www.espn.com/racing/nascar/cup/news/story?id=5435268">hardest crashes in NASCAR</a> register around 80 G’s – that is, 80 times the acceleration of gravity that holds you to the planet. For perspective, amusement park rides top out around 6 G’s.</p>
<p>Safety elements try to extend the time, distance and area over which any collision takes place in an effort to lower these high forces. The principle is similar to the way gradually coming to a stop is less jarring than slamming on the brakes or the way a bed of nails spreads the weight of your body over a large area versus lying on a single nail. </p>
<p><a href="https://galvanizeit.org/project-gallery/nascar-safer-barrier">SAFER barriers</a> along the outside wall of the race track are made to crumple and dissipate a crash’s force over a large area. The front end of the car itself is also made to crumple, which extends the time of impact.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.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">Safety elements inside a NASCAR vehicle go way beyond the seatbelt you have in your car.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/long-pond-pa-june-04-juan-55276939">Action Sports Photography/Shutterstock.com</a></span>
</figcaption>
</figure>
<p>Carbon fiber seats in the car absorb more impact energy compared to aluminum seats. They stabilize the driver by wrapping around the rib cage and shoulders, and spread the impact force over a larger area.</p>
<p>A 5-point harness connects the driver to the car, once again spreading the area of impact. It also attaches the driver to the car, so he or she slows with the crumpling car rather than continuing forward at full speed until impact. </p>
<p>So next time you head to the track or tune in on TV, ponder some of the physics of NASCAR, as well as the contributions of scientists and engineers working behind the scenes to improve the speed, power and safety of the sport.</p>
<hr>
<p><em>This article has been updated to correct the force on the track during a car’s acceleration.</em></p>
<p>[ <em><a href="https://theconversation.com/us/newsletters?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=thanksforreading">Thanks for reading! We can send you The Conversation’s stories every day in an informative email. Sign up today.</a></em> ]</p><img src="https://counter.theconversation.com/content/118641/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Christine Helms 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>High speeds, the threat of dangerous crashes, the excitement of the crowd – and the laws of physics on full display. A physicist explains the science of NASCAR.Christine Helms, Assistant Professor of Physics, University of RichmondLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/939942018-07-02T10:40:57Z2018-07-02T10:40:57ZObserving the universe with a camera traveling near the speed of light<figure><img src="https://images.theconversation.com/files/225012/original/file-20180626-112641-fpyvp6.jpg?ixlib=rb-1.1.0&rect=51%2C67%2C1120%2C731&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">What could a 'relativistic camera' capture on the way to Alpha Centauri?</span> <span class="attribution"><a class="source" href="https://images.nasa.gov/details-GSFC_20171208_Archive_e000214.html">ESA/NASA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Astronomers strive to observe the universe via ever more advanced techniques. Whenever researchers invent a new method, unprecedented information is collected and people’s understanding of the cosmos deepens.</p>
<p>An ambitious program to blast cameras far beyond the solar system was announced in April 2016 by internet investor and science philanthropist Yuri Milner, late physicist Stephen Hawking and Facebook CEO Mark Zuckerberg. Called “<a href="https://breakthroughinitiatives.org/initiative/3">Breakthrough Starshot</a>,” the idea is to send a bunch of tiny nano-spacecraft to the sun’s closest stellar neighbor, the three-star Alpha Centauri system. Traveling at around 20 percent the speed of light – so as fast as 100 million miles per hour – the craft and their tiny cameras would aim for the smallest but closest star in the system, Proxima Centari, and its planet Proxima b, 4.26 light-years from Earth.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/xRFXV4Z6x8s?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Breakthrough Starshot aims to establish proof of concept for a ‘nanocraft’ driven by a light beam.</span></figcaption>
</figure>
<p>The Breakthrough Starshot team’s goal will rely on a number of as-yet unproven technologies. The plan is to use light sails to get these spacecraft further and faster than anything that’s come before – lasers on Earth will push the tiny ships via their super-thin and reflective sails. I have another idea that could piggyback on this technology as the project is gearing up: Researchers could get valuable data from these mobile observatories, even directly test Einstein’s theory of special relativity, long before they get anywhere close to Alpha Centauri.</p>
<h2>Technical challenges abound</h2>
<p>Achieving Breakthrough Starshot’s goal is by no means an easy task. The project relies on continuing technological development on three independent fronts.</p>
<p>First, researchers will need to dramatically decrease the size and weight of microelectronic components to make a camera. Each nanocraft is planned to be no more than <a href="http://breakthroughinitiatives.org/concept/3">a few grams in total</a> – and that will have to include not just the camera, but also other payloads including power supply and communication equipment.</p>
<p>Another challenge will be to build thin, ultra-light and highly reflective materials to serve as the “sail” for the camera. One possibility is to have <a href="https://en.wikipedia.org/wiki/Solar_sail">a single-layer graphene sail – just a molecule thick, only 0.345 nanometer</a>.</p>
<p>The Breakthrough Starshot team will benefit from the rising power and falling cost of laser beams. Lasers with <a href="http://breakthroughinitiatives.org/concept/3">100-Gigawatt power</a> are needed to accelerate the cameras from the ground. Just as wind fills a sailboat’s sails and pushes it forward, the photons from a high-energy laser beam can propel an ultralight reflective sail forward as they bounce back.</p>
<p>With the projected technology development rate, it will likely be at least two more decades before scientists can launch a camera traveling with a speed a significant fraction of the speed of light. </p>
<p>Even if such a camera could be built and accelerated, several more challenges must be overcome in order to fulfill the dream of reaching the Alpha Centauri system. Can researchers aim the cameras correctly so they reach the stellar system? Can the camera even survive the near 20-year journey without being damaged? And if it beats the odds and the trip goes well, will it be possible to transmit the data – say, images – back to Earth over such a huge distance?</p>
<h2>Introducing ‘relativistic astronomy’</h2>
<p>My collaborator Kunyang Li, a graduate student at Georgia Institute of Technology, and I <a href="https://doi.org/10.3847/1538-4357/aaa9b7">see potential in all these technologies</a> even before they’re perfected and ready to head out for Alpha Centauri. </p>
<p>When a camera travels in space at close to the speed of light – what could be called “relativistic speed” – Einstein’s special theory of relativity plays a role in how the images taken by the camera will be modified. Einstein’s theory states that in different “rest frames” observers have different measures of the lengths of space and time. That is, space and time are relative. How differently the two observers measure things depends on how fast they’re moving with respect to each other. If the relative speed is close to the speed of light, their observations can differ significantly. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=471&fit=crop&dpr=1 754w, https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=471&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=471&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Doppler effect explains how a source moving away from you will stretch the wavelengths of its light and look redder, while if it’s moving closer the wavelengths will shorten and look bluer.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Redshift_blueshift.svg">Aleš Tošovský</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Special relativity also affects many other things physicists measure – for example, the frequency and intensity of light and also the size of an object’s appearance. In the rest frame of the camera, the entire universe is moving at a good fraction of the speed of light in the opposite direction of the camera’s own motion. To an imaginary person on board, thanks to the different spacetimes experienced by him and everyone back on Earth, the light from a star or galaxy would appear bluer, brighter and more compact, and the angular separation between two objects would look smaller. </p>
<p>Our idea is to take advantage of these features of special relativity to observe familiar objects in the relativistic camera’s different spacetime rest frame. This can provide a new mode to study astronomy – what we’re calling “relativistic astronomy.”</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=462&fit=crop&dpr=1 600w, https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=462&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=462&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=581&fit=crop&dpr=1 754w, https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=581&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=581&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Observed image of nearby galaxy M51 on the left. On the right, how the image would look through a camera moving at half the speed of light: brighter, bluer and with the stars in the galaxy closer together.</span>
<span class="attribution"><a class="source" href="https://doi.org/10.3847/1538-4357/aaa9b7">Zhang & Li, 2018, The Astrophysical Journal, 854, 123</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>What could the camera capture?</h2>
<p>So, a relativistic camera would naturally serve as a <a href="https://www.quora.com/What-do-spectrographs-help-astronomers-determine-How-is-this-important">spectrograph</a>, allowing researchers to look at an intrinsically redder band of light. It would act as a lens, magnifying the amount of light it collects. And it would be a wide-field camera, letting astronomers observe more objects within the same field of view of the camera.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1060&fit=crop&dpr=1 600w, https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1060&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1060&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1332&fit=crop&dpr=1 754w, https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1332&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1332&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An example of redshift: On the right, absorption lines occur closer to the red end of the spectrum.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Redshift.svg">Georg Wiora</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Here’s one example of the kind of data we could gather using the relativistic camera. Due to the expansion of the universe, the light from the early universe is redder by the time it reaches Earth than when it started. Physicists call this effect redshifting: As the light travels, its wavelength stretches as it expands along with the universe. Red light has longer wavelengths than blue light. All this means that to see red-shifted light from the young universe, one must use the difficult-to-observe infrared wavelengths to collect it.</p>
<p>Enter the relativistic camera. To a camera moving at close to the speed of light, such redshifted light becomes bluer – that is, it’s now blueshifted. The effect of the camera’s motion counteracts the effect of the universe’s expansion. Now an astronomer could catch that light using the familiar visible light camera. The same Doppler boosting effect also allows the faint light from the early universe to be amplified, aiding detection. Observing the spectral features of distant objects can allow us to reveal the history of the early universe, especially <a href="https://www.symmetrymagazine.org/article/what-ended-the-dark-ages-of-the-universe">how the universe evolved after it became transparent</a> 380,000 years after the Big Bang.</p>
<p>Another exciting aspect of relativistic astronomy is that humankind can directly test the principles of special relativity using macroscopic measurements for the first time. Comparing the observations collected on the relativistic camera and those collected from ground, astronomers could precisely test the fundamental predictions of Einstein’s relativity regarding change of frequency, flux and light travel direction in different rest frames.</p>
<p>Compared with the ultimate goals of the Starshot project, observing the universe using relativistic cameras should be easier. Astronomers wouldn’t need to worry about aiming the camera, since it could get interesting results when sent in any direction. The data transmission problem is somewhat alleviated since the distances wouldn’t be as great. Same with the technical difficulty of protecting the camera.</p>
<p>We propose that trying out relativistic cameras for astronomical observations could be a forerunner of the full Starshot project. And humankind will have a new astronomical “observatory” to study the universe in an unprecedented way. History suggests that opening a new window like this will unveil many previously undetected treasures.</p><img src="https://counter.theconversation.com/content/93994/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Bing Zhang 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>An astronomer suggests an idea to piggyback on the ambitious Breakthrough Starshot project that aims to send nano spacecraft to Alpha Centauri at a major fraction of the speed of light.Bing Zhang, Professor of Astrophysics, University of Nevada, Las VegasLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/893532018-01-12T00:22:35Z2018-01-12T00:22:35ZQuantum speed limit may put brakes on quantum computers<figure><img src="https://images.theconversation.com/files/200986/original/file-20180105-26163-urueyr.jpg?ixlib=rb-1.1.0&rect=22%2C546%2C4970%2C4011&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">How fast can quantum computing get? Research shows there's a limit.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-vector/tachometer-speedometer-indicator-icon-performance-measurement-289456043">Vladvm/Shutterstock.com</a></span></figcaption></figure><p>Over the past five decades, standard computer processors have <a href="https://www.intel.com/content/www/us/en/silicon-innovations/moores-law-technology.html">gotten increasingly faster</a>. In recent years, however, the <a href="https://www.technologyreview.com/s/601441/moores-law-is-dead-now-what/">limits to that technology</a> have become clear: Chip components can only get so small, and be packed only so closely together, before they overlap or short-circuit. If companies are to continue building ever-faster computers, something will need to change. </p>
<p>One key hope for the future of increasingly fast computing is my own field, quantum physics. <a href="http://www.research.ibm.com/ibm-q/learn/what-is-quantum-computing/">Quantum computers</a> are expected to be much faster than anything the information age has developed so far. But my recent research has revealed that <a href="https://academicminute.org/2017/12/sebastian-deffner-university-of-maryland-baltimore-county-quantum-supremacy/">quantum computers will have limits of their own</a> – and has suggested ways to figure out what those limits are.</p>
<h2>The limits of understanding</h2>
<p>To physicists, we humans live in what is called the “<a href="https://www.wikihow.com/Understand-Classical-Physics">classical</a>” world. Most people just call it “the world,” and have come to understand physics intuitively: Throwing a ball sends it up and then back down in a predictable arc, for instance.</p>
<p>Even in more complex situations, people tend to have an unconscious understanding of how things work. Most people largely grasp that a car works by burning gasoline in <a href="https://auto.howstuffworks.com/engine1.htm">an internal combustion engine</a> (or <a href="https://auto.howstuffworks.com/electric-car.htm">extracting stored electricity from a battery</a>), to produce energy that is transferred through gears and axles to turn tires, which push against the road to move the car forward.</p>
<p>Under the laws of classical physics, there are theoretical limits to these processes. But they are unrealistically high: For instance, we know that a car can never go <a href="https://www.amnh.org/exhibitions/einstein/light/cosmic-speed-limit/">faster than the speed of light</a>. And no matter how much fuel is on the planet, or how much roadway or how strong the construction methods, <a href="https://www.space.com/36273-theory-special-relativity.html">no car will get close</a> to going even 10 percent of the speed of light.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/AInCqm5nCzw?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Explaining special relativity.</span></figcaption>
</figure>
<p>People never really encounter the actual physical limits of the world, but they exist, and with proper research, physicists can identify them. Until recently, though, scholars only had a rather vague idea that <a href="http://iopscience.iop.org/article/10.1088/1751-8121/aa86c6">quantum physics had limits too</a>, but didn’t know how to figure out how they might apply in the real world.</p>
<h2>Heisenberg’s uncertainty</h2>
<p>Physicists trace the history of quantum theory back to 1927, when German physicist Werner Heisenberg showed that the classical methods did not work <a href="https://doi.org/10.1007/BF01397280">for very small objects</a>, those roughly the size of individual atoms. When someone throws a ball, for instance, it’s easy to determine exactly where the ball is, and how fast it’s moving.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/200854/original/file-20180104-26145-1f5rms6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/200854/original/file-20180104-26145-1f5rms6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/200854/original/file-20180104-26145-1f5rms6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=379&fit=crop&dpr=1 600w, https://images.theconversation.com/files/200854/original/file-20180104-26145-1f5rms6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=379&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/200854/original/file-20180104-26145-1f5rms6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=379&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/200854/original/file-20180104-26145-1f5rms6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=476&fit=crop&dpr=1 754w, https://images.theconversation.com/files/200854/original/file-20180104-26145-1f5rms6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=476&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/200854/original/file-20180104-26145-1f5rms6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=476&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 radar gun can track a baseball as it moves from the pitcher’s hand to home plate.</span>
<span class="attribution"><a class="source" href="http://www.apimages.com/metadata/Index/AP-S-FL-USA-Radar-Guns-Baseball/fed349afba5f44cd853cb7f23a88db90/18/0">AP Photo/Charlie Riedel</a></span>
</figcaption>
</figure>
<p>But as Heisenberg showed, that’s not true for atoms and subatomic particles. Instead, an observer can see either where it is or how fast it’s moving – but not both at the exact same time. This is an uncomfortable realization: Even from the moment Heisenberg explained his idea, Albert Einstein (among others) <a href="https://en.wiktionary.org/wiki/God_does_not_play_dice_with_the_universe">was uneasy with it</a>. It is important to realize that this “quantum uncertainty” is not a shortcoming of measurement equipment or engineering, but rather how our brains work. We have evolved to be so used to how the “classical world” works that the actual physical mechanisms of the “quantum world” are simply beyond our ability to fully grasp. </p>
<h2>Entering the quantum world</h2>
<p>If an object in the quantum world travels from one location to another, researchers can’t measure exactly when it has left nor when it will arrive. The limits of physics impose a tiny delay on detecting it. So no matter how quickly the movement actually happens, it won’t be detected until slightly later. (The lengths of time here are incredibly tiny – quadrillionths of a second – but add up over trillions of computer calculations.)</p>
<p>That delay effectively slows down the potential speed of a quantum computation – it imposes what we call the “quantum speed limit.”</p>
<p>Over the last few years, research, to which <a href="https://quthermo.umbc.edu/group-members/sebastian-deffner/">my group</a> has <a href="https://quthermo.umbc.edu/publications/peer-reviewed-articles/">contributed significantly</a>, has shown how this quantum speed limit is determined under different conditions, such as using different types of materials in different magnetic and electric fields. For each of these situations, the quantum speed limit is a little higher or a little lower. </p>
<p>To everyone’s big surprise, we even found that sometimes unexpected factors can help speed things up, at times, in counterintuitive ways. </p>
<p>To understand this situation, it might be useful to imagine a particle moving through water: The particle displaces water molecules as it moves. And after the particle has moved on, the water molecules quickly flow back where they were, leaving no trace behind of the particle’s passage.</p>
<p>Now imagine that same particle traveling through honey. Honey has a higher viscosity than water – it’s thicker and flows more slowly – so the honey particles will take longer to move back after the particle moves on. But in the quantum world, the returning flow of honey can build up pressure that propels the quantum particle forward. This extra acceleration can make a quantum particle’s speed limit different from what an observer might otherwise expect.</p>
<h2>Designing quantum computers</h2>
<p>As researchers understand more about this quantum speed limit, it will affect how quantum computer processors are designed. Just as engineers figured out how to <a href="https://qz.com/852770/theres-a-limit-to-how-small-we-can-make-transistors-but-the-solution-is-photonic-chips/">shrink the size of transistors</a> and pack them more closely together on a classical computer chip, they’ll need some clever innovation to build the fastest possible quantum systems, operating as close as possible to the ultimate speed limit.</p>
<p>There’s a lot for researchers like me to explore. It’s not clear whether the quantum speed limit is so high it’s unattainable – like the car that will never even get close to the speed of light. And we don’t fully understand how unexpected elements in the environment – like the honey in the example – can <a href="https://doi.org/10.1103/PhysRevLett.114.233602">help to speed up</a> quantum processes. As technologies based on quantum physics become more common, we’ll need to find out more about where the limits of quantum physics are, and how to engineer systems that take the best advantage of what we know.</p><img src="https://counter.theconversation.com/content/89353/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Sebastian Deffner receives funding from NSF. </span></em></p>A future that continues to have increasingly fast computing depends on quantum physics – but research is showing that there are limits to how fast quantum computers can go.Sebastian Deffner, Assistant Professor of Physics, University of Maryland, Baltimore CountyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/699912016-12-19T23:28:00Z2016-12-19T23:28:00ZWhy don’t we teach Einstein’s theories in school?<figure><img src="https://images.theconversation.com/files/149155/original/image-20161208-18059-7syt30.jpg?ixlib=rb-1.1.0&rect=0%2C270%2C4517%2C2968&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Einstein's theories are still not taught in school.</span> <span class="attribution"><span class="source">Wikimedia</span></span></figcaption></figure><p>The discovery of <a href="https://theconversation.com/au/topics/gravitational-waves-9473">gravitational waves</a>, announced earlier this year, marked the ultimate test of Einstein’s <a href="https://theconversation.com/au/topics/general-relativity-161">general theory of relativity</a>. Einstein published his theory in the form of 10 abstract equations 101 years ago. The equations did away with Newton’s theory of gravity and replaced it with curved space and warped time.</p>
<p>Within weeks, <a href="http://www.physicsoftheuniverse.com/scientists_schwarzschild.html">Karl Schwarzschild</a> found a solution to Einstein’s equations. His conclusion was astonishing and almost unbelievable: it told us that time depends on altitude and that matter can create holes where <a href="https://theconversation.com/au/topics/black-holes-686">space and time come to an end</a>. </p>
<p>A few months later, Einstein himself found a solution to his own equations. This solution described waves in the curvature of spacetime that would ripple out at the speed of light whenever masses accelerated around each other.</p>
<p>For its first half-century, Einstein’s theory was controversial. Were the waves real or mere mathematical artefacts? Do gravitational waves deposit energy? Are the black holes hypothesised by astronomers the same black holes that Schwarszchild predicted, or are they some other very dense agglomerations of matter?</p>
<p>Over the past 40 years the evidence has mounted that gravitational waves actually exist and that black holes are the real thing. Thousands of physicists believed the theory well enough to devote years inventing technology for making the <a href="https://www.ligo.caltech.edu/">exquisitely sensitive detectors</a> required to prove the theory.</p>
<p>Yet when the <a href="https://theconversation.com/gravitational-waves-discovered-the-universe-has-spoken-54237">waves were finally discovered</a>, it still came as a shock. The shock was to suddenly know what for years had been a belief and a hope. </p>
<p>Suddenly we knew that the waves existed; we knew that our detectors could actually detect them and that the black holes out there are precisely the holes predicted by Schwarszchild. The discovery removed all remaining doubt that Einstein’s description of space, time and gravity is the best way we have of understanding the universe.</p>
<p>The first gravitational “sounds” detected revealed an unexpected number of very heavy black holes colliding throughout the universe. These discoveries are only explainable using Einsteinian thinking.</p>
<h2>Scientific masterpiece</h2>
<p>So 2016 is surely the year when Newtonian physics was consigned to the history books, to be replaced by Einsteinian physics.</p>
<p>When, then, does Newtonian physics – with its absolute time, fixed space and lack of gravitational waves – still dominates the school physics curriculum in most countries? Why aren’t Newton’s theories supplemented with Einstein’s more general ones to give students insight into our present best understanding of our universe?</p>
<p>Recently, 40 physicist-educators from around the world converged on the <a href="http://gravitycentre.com.au/">Gravity Discovery Centre</a> in Western Australia to explore how school science can be re-imagined in the era of gravitational wave astronomy. </p>
<p>All shared the vision that we owe it to our children to teach our best understanding of the nature of our universe, rather than the obsolete 19th-century science that still dominates our school curriculum.</p>
<p>We heard about three countries that are pioneering Einsteinian physics in the classroom: South Korea, Norway and Scotland. </p>
<p>Korean physicist and educator Hongbin Kim suggested that South Korea’s economic growth and innovative culture is closely linked to its massive emphasis on education. This has led to it performing much better than Australia in the <a href="http://www.theaustralian.com.au/national-affairs/education/students-slide-in-global-maths-and-science-rankings/news-story/bf2aa8be9b40805d6d9406c4392a3940">world rankings of maths and science education</a>.</p>
<p>One reason for Einstein’s absence from school science classes is that many people imagine that his theories require enormous mathematical skills. The educationalists at the conference emphatically rejected this viewpoint. </p>
<p>Others argued that Einsteinian physics should be taught for its sublime beauty. <a href="http://www.cpt.univ-mrs.fr/%7Erovelli/">Carlo Rovelli</a> stated:</p>
<blockquote>
<p>There are absolute masterpieces which move us intensely: Mozart’s Requiem, Homer’s Odyssey, the Sistine Chapel… Einstein’s jewel, the general theory of relativity, is a masterpiece of this order.</p>
</blockquote>
<p>Yet Einsteinian physics is more than a jewel to be admired. Einstein’s seminal works brought us photons, digital cameras, lasers, black holes, time warps, quantum entanglement and solar panels. His discoveries changed and defined the modern world. Time magazine declared him the <a href="http://content.time.com/time/magazine/0,9263,7601991231,00.html">Person of the 20th Century</a>.</p>
<p>Education researchers have overwhelming evidence that children are motivated and excited to learn Einsteinian physics, and equally that they are turned off by a stale and obsolete curriculum. </p>
<p><a href="https://goo.gl/tpoYyS">Jyoti Kaur</a>, from the Australian Einstein-First team at the University of Western Australia, showed that Einsteinian physics brought year 9 girls into parity with boys in their enthusiasm for learning Einstein’s physics and in their attitudes to physics.</p>
<h2>Entering a new era</h2>
<p>Gravitational wave detection has brought us a new way of observing the universe. Australians played a major role in the discovery and shared the <a href="https://breakthroughprize.org/News/32">Breakthrough Prize</a> for their achievement. </p>
<p>Now we can directly listen to extraordinary events and phenomena throughout the universe. We have already heard vast spacequakes created by huge merging black holes. In a few years as more detectors are built and improved, the universe will be online with daily signals that will help to reveal the entire history of matter in the universe, from the very first stars where the elements for life were created, to the ultimate death of matter itself in constantly growing black holes.</p>
<p>In Norway, Einstein’s general relativity is part of the upper secondary physics curriculum and many on-line learning resources are being developed. </p>
<p>In Scotland, the recent introduction of the so-called Curriculum for Excellence has seen Einsteinian physics firmly embedded in the senior physics national qualifications. Australia is lagging far behind and its slow decline in the rankings tell of lack of innovation in the curriculum.</p>
<p>So why isn’t relativity taught in Australia? One of the greatest impediments to re-imagining school science is the retraining of science teachers, whose training was devoid of Einsteinian physics. </p>
<p>Given that education drives innovation, governments and education departments should recognise that the failure to invest in updating the curriculum is a recipe for economic decline and loss of international competitiveness.</p><img src="https://counter.theconversation.com/content/69991/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Blair's research on the Einstein-First education project is a collaboration between University of Western Australia, Curtin University, Edith Cowan University, the Gravity Discovery Centre Foundation and the Graham (Polly) Farmer Foundation. The project is funded by the Australian Research Council. </span></em></p><p class="fine-print"><em><span>Ellen Karoline Henriksen, University of Oslo, Norway, receives funding from The Research Council of Norway and from The Olav Thon Foundation for the work mentioned in this article. </span></em></p><p class="fine-print"><em><span>Martin Hendry was an invited member of the SQA Physics Qualifications Design Team and Mathematics Excellence Group for the "Curriculum for Excellence" in Scotland, which has seen the introduction of significant elements of Einsteinian physics and Einsteinian thinking into the senior phase of the Scottish high school curriculum. From 2010 to 2012 he was partially funded by the UK Science and Technology Facilities Council as a Science in Society Fellow, leading an international programme of schools and public outreach on cosmology, astrophysics and relativity entitled "Exploring the Dark Side of the Universe".</span></em></p>Einstein’s theories of relativity underpin our understanding of the universe, yet they’re not taught in high school. How can we change that?David Blair, Director, Australian International Gravitational Research Centre, The University of Western AustraliaEllen Karoline Henriksen, Professor of physics education, University of OsloMartin Hendry, Professor of Gravitational Astrophysics and Cosmology, University of GlasgowLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/544932016-02-12T10:26:32Z2016-02-12T10:26:32ZWhat’s the point of theoretical physics?<figure><img src="https://images.theconversation.com/files/111194/original/image-20160211-29214-2zauf0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>You don’t have to be a scientist to get excited about breakthroughs in theoretical physics. Discoveries such as gravitational waves and <a href="https://theconversation.com/higgs-bosons-decay-confirms-physics-model-works-20882">the Higgs boson</a> can inspire wonder at the complex beauty of the universe no matter how little you really understand them. </p>
<p>But some people will always question why they should care about scientific advances that have no apparent impact on their daily life – and why we spend millions funding them. Sure, it’s amazing that we can study black holes thousands of light years away and that Einstein really was as much of a genius as we thought, but that won’t change the way most people live or work.</p>
<p>Yet the reality is that purely theoretical studies in physics can sometimes lead to <a href="https://theconversation.com/five-ways-particle-accelerators-have-changed-the-world-without-a-higgs-boson-in-sight-54187">amazing changes</a> in our society. In fact, several key pillars on which our modern society rests, from satellite communication to computers, were made possible by investigations that had no obvious application at the time. </p>
<h2>Quantum leap</h2>
<p>Around 100 years ago, <a href="https://theconversation.com/explainer-quantum-physics-570">quantum mechanics</a> was a purely theoretical topic, only developed to understand certain properties of atoms. Its founding fathers such as Werner Heisenberg and Erwin Schrödinger had no applications in mind at all. They were simply driven by the quest to understand what our world is made of. Quantum mechanics states that you cannot observe a system without changing it fundamentally by your observation, and initially its effects to society were of a philosophical and not a practical nature.</p>
<p>But today, quantum mechanics is the basis of our use of all semiconductors in computers and mobile phones. To build a modern semiconductor for use in a computer, you have to understand concepts such as the way electrons behave when atoms are held together in a solid material, something only described accurately by quantum mechanics. Without it, we would have been stuck using computers based on vacuum tubes.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111195/original/image-20160211-29175-1om523u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111195/original/image-20160211-29175-1om523u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111195/original/image-20160211-29175-1om523u.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111195/original/image-20160211-29175-1om523u.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111195/original/image-20160211-29175-1om523u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111195/original/image-20160211-29175-1om523u.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111195/original/image-20160211-29175-1om523u.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">
<figcaption>
<span class="caption">GPS: a relative success.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
</figcaption>
</figure>
<p>At a similar time as the key developments in quantum mechanics, Albert Einstein was attempting to better understand gravity, the dominating force of the universe. Rather than viewing gravity as a force between two bodies, he described it as a curving of space-time around each body, similar to how a rubber sheet will stretch if a heavy ball is placed on top of it. This was Einstein’s <a href="https://theconversation.com/how-einsteins-general-theory-of-relativity-killed-off-common-sense-physics-50042">general theory of relativity</a>. </p>
<p>Today the most common application of this theory is in GPS. To use signals from satellites to pinpoint your location you need to know the precise time the signal leaves the satellite and when it arrives on Earth. Einstein’s theory of general relativity means that the distance of a clock from the Earth’s centre of gravity affects how fast it ticks. And his theory of special relativity means that the speed a clock is moving at also affects its ticking speed.</p>
<p>Without knowing how to adjust the clocks to take account of these effects, we wouldn’t be able to accurately use the satellite signals to determine our position on the ground. Despite his amazing brain, Einstein probably could not have imagined this application a century ago. </p>
<h2>Scientific culture</h2>
<p>Aside from the potential, eventual applications of doing fundamental research, there are also direct financial benefits. Most of the student and post-docs working on big research projects like the Large Hadron Collider, will <a href="https://royalsociety.org/%7E/media/Royal_Society_Content/policy/publications/2010/4294970126.pdf">not stay in academia</a> but move into industry. During their time in fundamental physics, they are educated at the highest existing technical level and then take their expertise into working companies. This is like educating car mechanics in Formula One racing teams.</p>
<p>Despite these direct and indirect benefits, most theoretical physicists have a very different motive for their work. They simply want to improve humanity’s understanding of the universe. While this might not immediately impact everyone’s lives, I believe it is just as important a reason for pursuing fundamental research. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111199/original/image-20160211-29202-k3uaot.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111199/original/image-20160211-29202-k3uaot.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111199/original/image-20160211-29202-k3uaot.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111199/original/image-20160211-29202-k3uaot.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111199/original/image-20160211-29202-k3uaot.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111199/original/image-20160211-29202-k3uaot.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111199/original/image-20160211-29202-k3uaot.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">
<figcaption>
<span class="caption">Infinite inspiration.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
</figcaption>
</figure>
<p>This motivation may well have begun when humans first looked up at the night-sky in ancient times. They wanted to understand the world they lived and so spent time watching nature and creating theories about it, many of them involving gods or supernatural beings. Today we have made huge progress in our understanding of both stars and galaxies and, at the other end of the scale, of the tiny fundamental particles from which matter is built.</p>
<p>It somehow seems that every new level of understanding we achieve comes in tandem with new, more fundamental questions. It is never enough to know what we now know. We always want to continue looking behind newly arising curtains. In that respect, I consider fundamental physics a basic part of human culture.</p>
<p>Now we can wait curiously to find out what unforeseen spin-offs that discoveries such as the Higgs boson or gravitational waves might lead to in the long-term future. But we can also look forward to the new insights into the building-blocks of nature that they will bring us and the new questions they will raise.</p><img src="https://counter.theconversation.com/content/54493/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alexander Lenz receives funding from STFC. </span></em></p>There’s a good reason you should care about the discovery of gravitational waves, even if you don’t understand the science.Alexander Lenz, Deputy director, Institute for Particle Physics Phenomenology, Durham UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/545252016-02-12T06:00:43Z2016-02-12T06:00:43ZAustralia’s part in the global effort to discover gravitational waves<p>The historic <a href="https://theconversation.com/au/topics/gravitational-wave-discovery">discovery of gravitational waves</a> <a href="https://www.ligo.caltech.edu/news/ligo20160211">announced this week</a> involved the work of more than a thousand scientists working tirelessly in several different institutions, across many different countries and timezones.</p>
<p>Why is an entire village, albeit a diverse and disparate one, required to verify experimentally the last of Einstein’s major predictions in his theory of general relativity? And how does such a village function and coordinate in such a way that maximises scientific output?</p>
<h2>A tour of the village</h2>
<p><a href="http://www.ligo.org/">LIGO Scientific Collaboration</a> consists of <a href="https://theconversation.com/gravitational-waves-discovered-how-did-the-experiment-at-ligo-actually-work-54510">two individual experiments</a>, located at two sites in the United States, separated by 3,000 kilometres. At each site, a single, very high-power laser beam is split in two, and travels down two perpendicular four kilometre-long vacuum tunnels.</p>
<p>At the ends of these tunnels the laser hits large, 40-kilogram mirrors suspended by an intricate series of pendula to reduce shaking from external forces.</p>
<p>The laser light returns along the same tunnel, and recombines. Gravitational waves cause the actual length of each arm to change. The way the laser light recombines is used to determine this change.</p>
<p>In order to make a detection, the LIGO instruments needed to measure a change in arm length equal to 1,000th the diameter of a proton. Performing such a measurement is a remarkable technological feat that involves development across multiple scientific streams.</p>
<p>These fields include, but are not limited to; quantum physics and quantum metrology; high-powered optics; mechanical systems including thermal and vibrational control systems; general relativity and gravitation; theoretical astrophysics and traditional astronomy; large-scale computing … the list goes on. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/111279/original/image-20160212-29172-xz618n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/111279/original/image-20160212-29172-xz618n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111279/original/image-20160212-29172-xz618n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111279/original/image-20160212-29172-xz618n.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111279/original/image-20160212-29172-xz618n.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111279/original/image-20160212-29172-xz618n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111279/original/image-20160212-29172-xz618n.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111279/original/image-20160212-29172-xz618n.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">LIGO Laboratory operates two detector sites, one near Hanford in eastern Washington, and another near Livingston, Louisiana. This photo shows the Hanford detector site.</span>
<span class="attribution"><a class="source" href="https://www.ligo.caltech.edu/image/ligo20150731f">LIGO/Caltech</a></span>
</figcaption>
</figure>
<p>The multidisciplinary nature of this experiment is reflected in the structure of the LIGO Scientific Collaboration (LSC), which looks more like a corporate entity than a traditional scientific collaboration.</p>
<p>Among other things, there are many, many science working groups which fall within the scope of three main themes: instrument science, detector characterisation and data analysis.</p>
<p>Alongside the science working groups sit groups such as Education and Public Outreach, Diversity, and the Presentation and Publications Committee. </p>
<p>Each working group is a dynamic, scientific collaboration all unto themselves. Each has a chairperson, or multiple co-chairs, who report to the theme leaders who, in turn, report to the LSC spokesperson, executive committee and council.</p>
<h2>Who works in the village?</h2>
<p>So exactly how many scientists does it take to detect a gravitational wave? This particular effort took 1,006 scientists working tirelessly in 16 countries in 83 different institutions, located in 14 different timezones!</p>
<p>Research for the discovery was done all over North America, Brazil, throughout Europe, Russia, India, China and South-East Asia and Australia. </p>
<p>We, along with about 50 colleagues, work on this experiment in Australia. A majority of the leadership group work in institutes in the US, at places such as CalTech and MIT.</p>
<p>The result of this unfortunate circumstance is that full, collaboration-wide teleconferences typically take place between 2am and 4am in Australian time. Over the past two months, building to the announcement, this has affected our lives many times!</p>
<p>In general, science working groups hold weekly teleconferences. Many of us are part of working groups that only exist on two continents, making it possible to schedule meetings that also allow for a relatively normal existence. </p>
<p>Many of us also work in groups that have numerous members on three or more continents; very early, or very late teleconferences are not uncommon, but remind us of the scale of the collaboration and the international effect of our work.</p>
<h2>What do they do?</h2>
<p>As mentioned before, this is a precision measurement! Every aspect of the experiment is incredibly finely-tuned. For example, multiple groups and individuals around Australia work on the technology and design of the mirrors.</p>
<p>Monash University researcher Yuri Levin, while a PhD student of Kip Thorne’s at CalTech, developed the theoretical framework for computing thermal noise (which is now widely used within the collaboration). From this work it became clear that LIGO mirrors require exceptionally high-quality reflective coatings.</p>
<p>The coating noise Levin anticipated is now considered to be among the most serious sources of noise in the LIGO experiment.</p>
<p>Scientists at Adelaide University developed, installed and commissioned wavefront sensors for the LIGO mirrors that measure the mirrors’ change in shape due to the temperature of the high-powered laser, and corrects these distortions.</p>
<p>Researchers at CSIRO developed mirror coatings and polishing techniques for the initial phase of the LIGO experiment that lasted from 2002 to 2010. A team at the Australian National University developed tip-tilt mirror suspension systems that can be used to steer the laser light with remarkable accuracy.</p>
<p>A group at the University of Western Australia have built a mini-LIGO experiment that is used, among other things, to study an instability the high-powered laser can induce on the mirrors, causing them to wobble uncontrollably.</p>
<p>Each element and each component of the incredibly complex LIGO system undergoes incredible levels of development and scrutiny. </p>
<p>This is perhaps best exemplified in the data analysis sphere. In Australia, we have strong groups at Monash University, the universities of Melbourne and Western Australia, the Australian National University and Charles Sturt University.</p>
<p>We all work on developing and running computer software that can pick a tiny signal out of noisy data streams. Somewhat infamously, LIGO puts itself through a process called blind injections.</p>
<p>Blind injections are performed by a very small group of people. The team inject a fake signal into the data stream by artificially shaking the mirrors of the detector in such a way that makes it looks like a gravitational wave has passed through.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/UkrM9pRy43M?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>The unsuspecting data analysts play their usual games of analysing this data and, lo and behold, inevitably find the signal.</p>
<h2>An early result?</h2>
<p>The most famous of these blind injections occurred in September 2010. Very soon after the signal was automatically injected into the detectors, it was picked up with the initial data analysis algorithms.</p>
<p>The purported signal looked like it to came from the constellation Canis Major, and the event was subsequently called the “Big Dog”. The collaboration then went through a six-month process of vetting, checking, and re-checking the analysis, and even wrote up a full paper to be submitted to the journal.</p>
<p>An independent Detection Committee reviewed all of the results, and a collaboration-wide vote was held on whether to submit the paper for peer review – the result was an anonymous “yes”.</p>
<p>And then the envelope was opened: the signal was fake.</p>
<p>That exercise, while incredibly painful to many, shows just how seriously the LIGO Scientific Collaboration takes its science. That this latest detection was not a blind injection has been known by the entire collaboration for a long time – the experiment was only beginning to collect data, and the blind injection software had not yet been set up properly.</p>
<p>Less than one hour after the LIGO experiment wobbled from the gravitational wave on that fateful day on September 14, 2015, one of us (Lasky) and fellow Monash academic and LIGO researcher Eric Thrane, who sits on the fake injection committee, were sitting at our laptops at home when we both received an email titled “Very interesting event on ER8” (ER8 stands for Engineering Run 8, which was the name of the pre-science phase of the experiment).</p>
<p>A quick Skype conversation quickly ensued:</p>
<blockquote>
<p>Thrane: Have you seen the email?</p>
<p>Lasky: Yes. Is it a false injection?</p>
<p>Thrane: No! </p>
<p>Lasky: Did we just detect a gravitational wave?</p>
<p>Thrane: I think we did.</p>
</blockquote>
<p>And the rest, as they say, is history.</p>
<p>This is truly the dawn of a new age of discovery. The gravitational-wave universe has many untold stories to tell, and scientists across Australia are striving to tell the tale along with the rest of the world.</p><img src="https://counter.theconversation.com/content/54525/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 organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The discovery of gravitational waves involved a team of more than 1,000 scientists from across the globe, including Australia. So how does such an international collaboration work?Paul Lasky, Postdoctoral Fellow in Gravitational Wave Astrophysics, Monash UniversityLetizia Sammut, Postdoctoral research fellow in Gravitational Wave Astrophysics, Monash UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/545282016-02-12T00:09:33Z2016-02-12T00:09:33ZTimeline: the history of gravity<figure><img src="https://images.theconversation.com/files/111240/original/image-20160211-29180-13i1prz.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p><em>Our understanding of gravity has gone through a few permutations, from Newton’s equations through to Einstein’s general relativity. With today’s discovery of gravitational waves, we look back on how our grasp of gravity has evolved over the centuries.</em></p>
<hr>
<h2>1687: Newtonian gravity</h2>
<p>Isaac Newton publishes <a href="http://cudl.lib.cam.ac.uk/view/PR-ADV-B-00039-00001/1">Philosophiae Naturalis Principia Mathematica</a>, giving a comprehensive account of gravity. This gave astronomers an accurate toolbox for predicting the motions of planets. But it was not without its problems, such as calculating the precise orbit of the planet Mercury.</p>
<p>All planets’ <a href="https://en.wikipedia.org/wiki/Precession">orbits precess</a> – with the closest point of their orbit moving slightly with each revolution – due to the gravitational tugs from other planets. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111236/original/image-20160211-29172-17upuhr.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111236/original/image-20160211-29172-17upuhr.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111236/original/image-20160211-29172-17upuhr.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111236/original/image-20160211-29172-17upuhr.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111236/original/image-20160211-29172-17upuhr.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111236/original/image-20160211-29172-17upuhr.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111236/original/image-20160211-29172-17upuhr.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">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>The issue with Mercury’s orbit was that the amount of precession did not match what Newton’s theory predicted. It was only a small discrepancy, but big enough for astronomers to know it was there!</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111102/original/image-20160211-29198-to74rs.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111102/original/image-20160211-29198-to74rs.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=564&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111102/original/image-20160211-29198-to74rs.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=564&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111102/original/image-20160211-29198-to74rs.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=564&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111102/original/image-20160211-29198-to74rs.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=708&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111102/original/image-20160211-29198-to74rs.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=708&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111102/original/image-20160211-29198-to74rs.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=708&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>1859: Planet Vulcan</h2>
<p>To explain Mercury’s odd behaviour, <a href="http://www.britannica.com/biography/Urbain-Jean-Joseph-Le-Verrier">Urbain Le Verrier</a> proposed the existence of an unseen planet called [Vulcan](https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet), which orbited closer to the sun. He suggested that the gravity from Vulcan was influencing Mercury’s orbit. But repeated observations revealed no signs of Vulcan. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111103/original/image-20160211-29214-1q2fape.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111103/original/image-20160211-29214-1q2fape.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111103/original/image-20160211-29214-1q2fape.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111103/original/image-20160211-29214-1q2fape.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111103/original/image-20160211-29214-1q2fape.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111103/original/image-20160211-29214-1q2fape.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111103/original/image-20160211-29214-1q2fape.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">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>1905: Special relativity</h2>
<p>Albert Einstein shakes up physics with his <a href="https://theconversation.com/au/topics/special-relativity">special theory of relativity</a>. He then started incorporating gravity into his equations, which led to his next breakthrough.</p>
<h2>1907: Einstein predicts gravitational redshift</h2>
<p>What we now call gravitational redshift was first proposed by Einstein from his thoughts in the development of general relativity.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111104/original/image-20160211-29175-hjyke5.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111104/original/image-20160211-29175-hjyke5.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=248&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111104/original/image-20160211-29175-hjyke5.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=248&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111104/original/image-20160211-29175-hjyke5.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=248&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111104/original/image-20160211-29175-hjyke5.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=312&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111104/original/image-20160211-29175-hjyke5.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=312&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111104/original/image-20160211-29175-hjyke5.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=312&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Einstein predicted that the wavelength of light coming from atoms in a strong gravitational field will lengthen as it escapes the gravitational force. The longer wavelength shifts the photon to the red end of the electromagnetic spectrum.</p>
<h2>1915: General relativity</h2>
<p>Albert Einstein publishes <a href="https://theconversation.com/au/topics/general-relativity">general theory of relativity</a>. The first great success was its accurate prediction of Mercury’s orbit, including its previously inscrutable precession.</p>
<p>The theory also predicts the existence of black holes and <a href="https://theconversation.com/au/topics/gravitational-waves">gravitational waves</a>, although Einstein himself often struggled to understand them.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111105/original/image-20160211-29207-1udort5.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111105/original/image-20160211-29207-1udort5.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111105/original/image-20160211-29207-1udort5.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111105/original/image-20160211-29207-1udort5.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111105/original/image-20160211-29207-1udort5.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111105/original/image-20160211-29207-1udort5.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111105/original/image-20160211-29207-1udort5.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">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>1917: Einstein theorises stimulated emission</h2>
<p>In 1917, Einstein publishes a paper on the quantum theory of radiation indicating <a href="http://www.britannica.com/technology/stimulated-emission">stimulated emission</a> was possible.</p>
<p>Einstein proposed that an excited atom could return to a lower energy state by releasing energy in the form of photons in a process called spontaneous emission.</p>
<p>In stimulated emission, an incoming photon interacts with the excited atom, causing it to move to a lower energy state, releasing photons that are in phase and have the same frequency and direction of travel as the incoming photon. This process allowed for the development of the laser (light amplification by stimulated emission of radiation).</p>
<h2>1918: Prediction of frame dragging</h2>
<p><a href="https://en.wikipedia.org/wiki/Josef_Lense">Josef Lense</a> and <a href="https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4912">Hans Thirring</a> theorise that the rotation of a massive object in space would “drag” spacetime around with it.</p>
<h2>1919: First observation of gravitational lensing</h2>
<p>Gravitational lensing is the bending of light around massive objects, such as a black hole, allowing us to view objects that lie behind it. During a total solar eclipse in May 1919, stars near the sun were observed slightly out of position. This indicated that light was bending due to the sun’s mass.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111106/original/image-20160211-29190-rgd6ni.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111106/original/image-20160211-29190-rgd6ni.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111106/original/image-20160211-29190-rgd6ni.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111106/original/image-20160211-29190-rgd6ni.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111106/original/image-20160211-29190-rgd6ni.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111106/original/image-20160211-29190-rgd6ni.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111106/original/image-20160211-29190-rgd6ni.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">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>1925: First measurement of gravitational redshift</h2>
<p><a href="http://www.britannica.com/biography/Walter-Adams">Walter Sydney Adams</a> examined light emitted from the surface of massive stars and detected a redshift, as Einstein predicted.</p>
<h2>1937: Prediction of a galactic gravitational lensing</h2>
<p>Swiss astronomer <a href="https://en.wikipedia.org/wiki/Fritz_Zwicky">Fritz Zwicky</a> proposed that an entire galaxy could act as a gravitational lens.</p>
<h2>1959: Gravitational redshift verified</h2>
<p>The theory was conclusively tested by <a href="https://en.wikipedia.org/wiki/Robert_Pound">Robert Pound</a> and <a href="https://en.wikipedia.org/wiki/Glen_Rebka">Glen Rebka</a> by measuring the relative redshift of two sources at the top and bottom of Harvard University’s Jefferson Laboratory tower. The experiment <a href="https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment">accurately measured</a> the tiny change in energies as photons travelled between the top and the bottom.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111107/original/image-20160211-29202-6zzdjz.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111107/original/image-20160211-29202-6zzdjz.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111107/original/image-20160211-29202-6zzdjz.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111107/original/image-20160211-29202-6zzdjz.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111107/original/image-20160211-29202-6zzdjz.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111107/original/image-20160211-29202-6zzdjz.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111107/original/image-20160211-29202-6zzdjz.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">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>1960: Laser invented using stimulated emission</h2>
<p><a href="http://www.laserinventor.com/bio.html">Theodore H. Maiman</a>, a physicist at Hughes Research Laboratories in California, builds the first laser.</p>
<h2>1960s: First evidence for black holes</h2>
<p>The 1960s was the beginning of the renaissance of general relativity, and saw the discovery of galaxies that were powered by the immense pull of <a href="https://theconversation.com/au/topics/black-holes">black holes</a> in their centres.</p>
<p>There is now evidence of massive black holes in the hearts of all large galaxies, as well as there being smaller black holes roaming between the stars.</p>
<h2>1966: First observation of gravitational time delays</h2>
<p>American astrophysicist Irwin Shapiro <a href="http://www.relativity.li/en/epstein2/read/i0_en/i3_en/">proposed</a> that if general relativity is valid, then radio waves will be slowed down by the sun’s gravity as they bounce around the solar system. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111108/original/image-20160211-29207-173wfss.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111108/original/image-20160211-29207-173wfss.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=404&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111108/original/image-20160211-29207-173wfss.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=404&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111108/original/image-20160211-29207-173wfss.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=404&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111108/original/image-20160211-29207-173wfss.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=507&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111108/original/image-20160211-29207-173wfss.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=507&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111108/original/image-20160211-29207-173wfss.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=507&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>The effect was observed between 1966-7 by bouncing radar beams off the surface of Venus and measuring the time taken for the signals to return to Earth. The delay measured agreed with Einstein’s theory.</p>
<p>We now use time-delays on cosmological scales, looking at the time differences in flashes and flares between gravitationally lensed images to measure the expansion of the universe.</p>
<h2>1969: False detection of gravitational waves</h2>
<p>American physicist <a href="http://www.nytimes.com/2000/10/09/us/joseph-weber-dies-at-81-a-pioneer-in-laser-theory.html">Joseph Weber</a> (a bit of a rebel) claimed the first experimental detection of gravitational waves. His experimental results were never reproduced.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111109/original/image-20160211-29198-ynqrjq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111109/original/image-20160211-29198-ynqrjq.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=668&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111109/original/image-20160211-29198-ynqrjq.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=668&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111109/original/image-20160211-29198-ynqrjq.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=668&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111109/original/image-20160211-29198-ynqrjq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=839&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111109/original/image-20160211-29198-ynqrjq.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=839&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111109/original/image-20160211-29198-ynqrjq.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=839&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>1974: Indirect evidence for gravitational waves</h2>
<p><a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/1993/taylor-bio.html">Joseph Taylor</a> and <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/1993/hulse-bio.html">Russell Hulse</a> discover a new type of pulsar: a binary pulsar. Measurements of the orbital decay of the pulsars showed they lost energy matching the amounts predicted by general relativity. They receive the 1993 <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/1993/press.html">Nobel Prize for Physics</a> for this discovery.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111115/original/image-20160211-29202-1b945x2.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111115/original/image-20160211-29202-1b945x2.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=639&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111115/original/image-20160211-29202-1b945x2.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=639&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111115/original/image-20160211-29202-1b945x2.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=639&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111115/original/image-20160211-29202-1b945x2.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=802&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111115/original/image-20160211-29202-1b945x2.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=802&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111115/original/image-20160211-29202-1b945x2.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=802&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>1979: First observation of a galactic gravitational lens</h2>
<p>The first extragalactic gravitational lens was discovered, when observers <a href="https://en.wikipedia.org/wiki/Dennis_Walsh">Dennis Walsh</a>, <a href="http://www.ast.cam.ac.uk/%7Erfc/">Bob Carswell</a> and <a href="https://en.wikipedia.org/wiki/Ray_Weymann">Ray Weymann</a> saw two identical quasi-stellar objects, or “quasars”. It turned out to be one quasar that appears as two separate images.</p>
<p>Since the 1980s, gravitational lensing has become a powerful probe of the distribution of mass in the universe.</p>
<h2>1979: LIGO receives funding</h2>
<p>US National Science Foundation funds construction of the <a href="http://www.ligo.org/index.php">Laser Interferometer Gravitational-Wave Observatory</a> (LIGO).</p>
<h2>1987: Another false alarm for gravitational waves</h2>
<p>A false alarm on direct detection from Joseph Weber (again) with claimed signal from the supernova SN 1987A using his <a href="http://arxivblog.com/?p=1271">torsion bar experiments</a>, which consisted of large aluminium bars designed to vibrate when a large gravitational wave passed through it.</p>
<h2>1994: LIGO construction begins</h2>
<p>It took a long time, but the construction of LIGO finally began in Hanford, Washington, and Livingston, Louisiana.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111092/original/image-20160211-29180-1uw773l.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111092/original/image-20160211-29180-1uw773l.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=436&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111092/original/image-20160211-29180-1uw773l.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=436&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111092/original/image-20160211-29180-1uw773l.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=436&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111092/original/image-20160211-29180-1uw773l.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=548&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111092/original/image-20160211-29180-1uw773l.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=548&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111092/original/image-20160211-29180-1uw773l.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=548&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>
<h2>2002: LIGO starts first search</h2>
<p>In August 2002, LIGO starts searching for evidence of gravitational waves.</p>
<h2>2004: Frame dragging probe</h2>
<p>NASA launches <a href="https://einstein.stanford.edu/MISSION/mission1.html">Gravity Probe B</a> to measure the spacetime curvature near the Earth. The probe contained gyroscopes that rotated slightly over time due to the underlying spacetime. The effect is stronger around a rotating object which “drags” spacetime around with it. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111116/original/image-20160211-29192-xld1xm.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111116/original/image-20160211-29192-xld1xm.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=483&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111116/original/image-20160211-29192-xld1xm.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=483&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111116/original/image-20160211-29192-xld1xm.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=483&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111116/original/image-20160211-29192-xld1xm.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=607&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111116/original/image-20160211-29192-xld1xm.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=607&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111116/original/image-20160211-29192-xld1xm.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=607&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>The gyroscopes in Gravity Probe B rotated by an amount consistent with Einstein’s theory of general relativity.</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/111112/original/image-20160211-29185-3ttewq.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/111112/original/image-20160211-29185-3ttewq.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1270&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111112/original/image-20160211-29185-3ttewq.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1270&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111112/original/image-20160211-29185-3ttewq.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1270&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111112/original/image-20160211-29185-3ttewq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1596&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111112/original/image-20160211-29185-3ttewq.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1596&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111112/original/image-20160211-29185-3ttewq.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1596&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>2005: LIGO hunt ends</h2>
<p>After five searches, the first phase of LIGO ends with no detection of gravitational waves. The sensors then undergo an interim refit to improve sensitivity, called Enhanced LIGO.</p>
<h2>2009: Enhanced LIGO</h2>
<p>An upgraded version called Enhanced LIGO starts new hunt for gravitational waves.</p>
<h2>2010: Enhanced LIGO hunt ends</h2>
<p>Enhanced LIGO fails to detect and gravitational waves. A major upgrade, called Advanced LIGO begins.</p>
<h2>2014: Advanced LIGO upgrade completed</h2>
<p>The new Advanced LIGO has finished installation and testing and is nearly ready to begin a new search.</p>
<h2>2015: False alarm #3 for gravitational waves</h2>
<p>The indirect signature of gravitational waves in the early universe was claimed by the <a href="https://theconversation.com/scientists-at-work-building-up-bicep2-at-the-south-pole-to-make-discovery-of-the-year-24610">BICEP2 experiment</a>, looking at the cosmic microwave background. But it looks like this was <a href="https://theconversation.com/bicep2-gravity-wave-finding-clouded-by-interstellar-dust-32048">dust in our own galaxy</a> spoofing the signal.</p>
<h2>2015: LIGO upgraded again</h2>
<p>Advanced LIGO starts a new hunt for gravitational waves with four times the sensitivity of the original LIGO. In September, it detects a signal that looks likely to be from the collision between two black holes.</p>
<h2>2016: Gravitational wave detection confirmed</h2>
<p>After rigorous checks, the Advanced LIGO team announce the <a href="https://theconversation.com/gravitational-waves-discovered-the-universe-has-spoken-54237">detection</a> of gravitational waves.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/111113/original/image-20160211-29214-szegpl.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111113/original/image-20160211-29214-szegpl.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111113/original/image-20160211-29214-szegpl.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111113/original/image-20160211-29214-szegpl.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111113/original/image-20160211-29214-szegpl.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111113/original/image-20160211-29214-szegpl.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111113/original/image-20160211-29214-szegpl.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">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Wes Mountain/The Conversation</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure><img src="https://counter.theconversation.com/content/54528/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>It’s taken centuries for our understanding of gravity to evolve to where it is today, culminating in the discovery of gravitational waves, as predicted by Albert Einstein a century ago.Geraint Lewis, Professor of Astrophysics, University of SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/496462015-11-30T00:22:31Z2015-11-30T00:22:31ZEinstein’s folly: how the search for a unified theory stumped him to his dying day<figure><img src="https://images.theconversation.com/files/103578/original/image-20151130-11614-8zpwab.jpg?ixlib=rb-1.1.0&rect=0%2C304%2C4517%2C2934&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Albert Einstein wrestled with unifying gravity with electromagnetism and quantum mechanics until his dying days.</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/Category:Portraits_of_Albert_Einstein#/media/File:Albert_Einstein_1947.jpg">Oren Jack Turner/Wikimedia Commons</a></span></figcaption></figure><p>This month marks exactly <a href="https://theconversation.com/au/topics/general-relativity-centenary">100 years</a> since Albert Einstein submitted the first paper fully describing the <a href="https://theconversation.com/au/topics/general-relativity">general theory of relativity</a>. It was both breathtaking and revolutionary. </p>
<p>Simply stated, gravity is a geometric property of <a href="https://einstein.stanford.edu/SPACETIME/spacetime2.html#fourth_dimension">spacetime</a> that is allowed to be curved. It was like looking at Newton’s world through the bottom of a glass.</p>
<p>General relativity is based on Einstein’s field equations, which describe the relation between the geometry of a four-dimensional description of spacetime, and the <a href="https://theconversation.com/without-einstein-it-would-have-taken-decades-longer-to-understand-gravity-50517">energy–momentum</a> contained in that spacetime. </p>
<p>Spacetime curvature is caused by <a href="https://theconversation.com/explainer-what-is-mass-49299">mass</a>; the more mass, the more spacetime is curved. This curvature can induce deflections or delays in the propagation of light. </p>
<p>Even close to home, our sun – not that massive as stars go – will alter the path of light near it. Newton’s theory predicts a deflection of light of 0.875 <a href="http://astronomy.swin.edu.au/cosmos/A/Arcsecond">seconds of arc</a> at the limb of the sun, whilst relativity predicted a deflection of 1.75 seconds of arc. Observations during <a href="http://www.powerhousemuseum.com/collection/database/?irn=355470">total solar eclipses of background star fields</a> confirmed Einsteins value. </p>
<p>Even had Einstein died shortly after his work on general relativity, he would still be regarded by many today as the greatest physicist who ever lived, and perhaps even the greatest scientist. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=340&fit=crop&dpr=1 600w, https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=340&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=340&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=427&fit=crop&dpr=1 754w, https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=427&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/102406/original/image-20151118-14222-1kp2toe.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=427&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The first part of Einsteins defining GTR paper: Feldgleichungen der Gravitation (The Field Equations of Gravitation) Preussische Akademie der Wissenschaften, Sitzungsberichte, 1915.</span>
</figcaption>
</figure>
<h2>Towards a unified field theory</h2>
<p>However, whilst he continued to work on many problems up until his death in 1955, he is regularly described as failing in one particular area: the <a href="http://www.britannica.com/science/unified-field-theory">unified field theory</a>. </p>
<p>From the 1920s, Einstein tried to develop a <a href="http://www.aps.org/publications/apsnews/200512/history.cfm">unified theory</a> that melded general relativity and <a href="https://theconversation.com/let-there-be-light-celebrating-the-theory-of-electromagnetism-35723">electromagnetism</a>, representing the only two forces known to exist.</p>
<p>Such a theory would describe a single field in which all forces are mediated and the properties of all particles – which at the time were only electrons and protons, with the <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/particles/neutrondis.html">neutron</a> not discovered until 1932 – could be deduced.</p>
<p>Other players in the quest appeared. <a href="http://www-history.mcs.st-andrews.ac.uk/Biographies/Kaluza.html">Theodor Kaluza</a> showed that if spacetime had five dimensions, then four dimensions could reflect general relativity, and one could represent electromagnetism. In the burgeoning <a href="http://www.pbs.org/transistor/science/info/quantum.html">quantum world</a> of the mid-1920s, <a href="http://www-history.mcs.st-andrews.ac.uk/Biographies/Klein_Oskar.html">Oskar Klein</a> shrank Kaluza’s 5th dimension to be compact, in a sense offering a quantum mechanical interpretation. </p>
<p>Einstein drew upon other work if it could help his cause. He even looked at variations to the successful mathematical basis of general relativity. It is <a href="http://www.stmarys.ac.uk/news/news/ug-applied-physics/2014/09/physics-beyond-god-play-dice-einstein-mean/">widely reported</a> that he did not support quantum mechanics, but promoted it (suffered it?) being a derivative of an eventual unified theory. </p>
<h2>Strong developments</h2>
<p>In a way, his mathematical focus hindered his acceptance of ongoing, major discoveries in physics like quantum mechanics. The discovery of two new forces in addition to gravity and electromagnetism – the <a href="http://www.livescience.com/48575-strong-force.html">strong</a> and <a href="http://www.thestargarden.co.uk/Weak.html">weak</a> nuclear forces – also made his work of a unified field based only on two forces unattainable.</p>
<p>Protons and neutrons in atomic nuclei had to be held together by a strong attractive force. Mesons, the force carrying particles for the <a href="http://aether.lbl.gov/elements/stellar/strong/strong.html">strong nuclear force</a> were discovered experimentally in 1947. Enrico Fermi in 1933 tried to explain beta decay, which was a radioactive transmutation between protons and neutrons. It was related to a <a href="http://home.fnal.gov/%7Echeung/rtes/RTESWeb/LQCD_site/pages/weakforce.htm">weak nuclear force</a>. </p>
<p>Eventually Sheldon Glashow, Steven Weinberg, and Abdus Salam announced a unified theory of electromagnetism and the weak nuclear force in 1968. Their <a href="https://www.fnal.gov/pub/inquiring/matter/madeof/electroweakforce.html">electroweak theory</a> postulated the weak force carrier particles – W and Z bosons – which were then discovered in the 1980s. </p>
<p>We now know that all forces <em>apart from gravitation</em> are related mathematically, albeit with some differences in phenomena. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=750&fit=crop&dpr=1 600w, https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=750&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=750&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=943&fit=crop&dpr=1 754w, https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=943&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/102418/original/image-20151118-14222-8yj7fq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=943&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The rich galaxy cluster Cl 0024+17. Blue streaks near the centre are smeared images of very distant background galaxies. Their light is being bent and magnified by the intervening cluster, in an effect called gravitational lensing.</span>
</figcaption>
</figure>
<h2>Todays efforts at a unified field</h2>
<p>The major pathway to unification over the last three decades has been <a href="https://theconversation.com/explainer-string-theory-2983">string theory</a>. Two forms of string theory have ten and twenty one dimensions respectively. In a strange parallel, the miniaturisation or compactification of many dimensions in string theory is the modern day equivalent of the <a href="http://www.superstringtheory.com/experm/exper5.html">quantisation of a 5th dimension</a> by Klein. </p>
<p>Despite little predictive power, and critics attacking its relation to a <a href="http://www.scientificamerican.com/article/multiverse-the-case-for-parallel-universe/">multiverse</a>, no other areas towards unification theory appear as fruitful as string theory.</p>
<p>For thirty years a unified theory proved a worthy opponent of Einstein. He worked on it even on his penultimate day in Princeton Hospital. <a href="https://www.ias.edu/people/oppenheimer">J. Robert Oppenheimer</a> was later both unflattering,</p>
<blockquote>
<p>During all the end of his life, Einstein did no good. He turned his back on experiments […] to realise the unity of knowledge. </p>
</blockquote>
<p>…and <a href="http://www.hup.harvard.edu/catalog.php?isbn=9780674034525">envious</a>,</p>
<blockquote>
<p>Of course, I would have liked to be the young Einstein. This goes without saying.</p>
</blockquote>
<p>A consensus seems to exist: in later years, Einstein worked with mathematical blinkers, immune to relevant discoveries, and unable to change his method of investigation. </p>
<p>As James Joyce <a href="http://www.online-literature.com/james_joyce/ulysses/">wrote</a>:</p>
<blockquote>
<p>A man of genius makes no mistakes. His errors are volitional and are the portals of discovery.</p>
</blockquote>
<p>Failure and mistake are harsh words. They are often the precursors of discovery. The unified field was Einstein’s nemesis for a variety of reasons. Despite this, many envied his early genius and we should focus on this especially in this centenary year of the greatest physics revolution.</p><img src="https://counter.theconversation.com/content/49646/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Glen Mackie does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>After the triumph of general relativity, Albert Einstein spent the rest of his life chasing a unified theory, which eluded him right up until the end.Glen Mackie, Senior Lecturer in Astronomy & Astrophysics, Coordinator of Swinburne Astronomy Online, Swinburne University of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/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/505172015-11-22T19:06:14Z2015-11-22T19:06:14ZWithout Einstein it would have taken decades longer to understand gravity<figure><img src="https://images.theconversation.com/files/101683/original/image-20151112-9379-43next.jpg?ixlib=rb-1.1.0&rect=336%2C132%2C2186%2C1499&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">It's possible that had Einstein not conceived of general relativity, then we'd still be at a loss to explain gravity to this day.</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Einstein_1921_by_F_Schmutzer_-_restoration.jpg">Wikimedia</a></span></figcaption></figure><p>It was 1905, and Albert Einstein had just turned theoretical physics on its head by publishing a paper on what later became known as <a href="https://theconversation.com/from-newton-to-einstein-the-origins-of-general-relativity-50013">special relativity</a>. This showed that space and time could not be considered in absolute terms: time could speed up or slow down; standard lengths could contract; and masses could increase. </p>
<p>And, most famously, energy was equivalent to mass, proportional to each other based on the famous equation <em>E = mc²</em>.</p>
<p>Although there is no doubting Einstein’s genius in formulating special relativity, it is generally accepted that had Einstein not published the theory in 1905, some other physicist would have done so shortly thereafter.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/101677/original/image-20151112-9396-szcalk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/101677/original/image-20151112-9396-szcalk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/101677/original/image-20151112-9396-szcalk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=579&fit=crop&dpr=1 600w, https://images.theconversation.com/files/101677/original/image-20151112-9396-szcalk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=579&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/101677/original/image-20151112-9396-szcalk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=579&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/101677/original/image-20151112-9396-szcalk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=728&fit=crop&dpr=1 754w, https://images.theconversation.com/files/101677/original/image-20151112-9396-szcalk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=728&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/101677/original/image-20151112-9396-szcalk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=728&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 ‘Einstein cross’ is actually four images of the same distant quasar that have been bent around a closer galaxy which acts like a gravitational lens.</span>
<span class="attribution"><span class="source">NASA, ESA, and STScI</span></span>
</figcaption>
</figure>
<p>It was not until 1915 that Einstein’s unparalleled genius was demonstrated when he published his general theory of relativity. This theory made the claim that space–time curvature is proportional to, and is caused by, the “energy-momentum density”, that is, the energy and the momentum associated with all and any kind of matter in a unit volume of space.</p>
<p>This claim was subsequently confirmed when it was shown to fit with observations of <a href="http://physics.ucr.edu/%7Ewudka/Physics7/Notes_www/node98.html">Mercury’s unusual orbit</a> and the bending of starlight around the sun. </p>
<p>And over the course of the last century, general relativity has been tested to remarkable accuracy and has withstood critique each time. General relativity was such a giant leap forward that it is arguable that had Einstein not formulated the theory, it may have remained undiscovered for a very long time.</p>
<h2>Path to general relativity</h2>
<p>In 1907, Einstein had the “happiest thought of my life” when he was sitting in a chair at the patent office in Bern: </p>
<blockquote>
<p>If a person falls freely he will not feel his own weight. </p>
</blockquote>
<p>This led him to the postulate the “<a href="http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/grel.html">equivalence principle</a>”, which states that one cannot tell the difference between an accelerating reference frame and a gravitational field. For example, standing on the Earth feels the same as standing in a space craft accelerating at 9.81 meters per second per second, which is the acceleration due to gravity on the Earth’s surface.</p>
<p>This was the crucial first step in Einstein’s formulation of a new theory of gravity.</p>
<p>Next, Einstein believed that, “all physics is geometry”. By that he meant we can think of space-time and all the universe in terms of geometry. The most startling conclusion from special relativity – the dynamic nature of time and space – must have led Einstein to start considering “geometric” space-time to be in need of modification.</p>
<p>Einstein then embarked on a series of <a href="http://www.gutenberg.org/files/5001/5001-h/files/ch03.htm">careful thought experiments</a>, comparing observations made by observers in <a href="http://www.gutenberg.org/files/5001/5001-h/files/ch03.htm">inertial</a> and <a href="http://www.animations.physics.unsw.edu.au/jw/foucault_pendulum.html">rotating</a> frames.</p>
<p>Einstein deduced that for observers in a rotating frame, space time could not be Euclidean, i.e. it could not be like the kind of flat geometry we all learn about in school. Rather, we need to introduce “curved space” to account for the anomalies predicted by relativity. Curvature thus became the second key assumption that underpinned his general theory.</p>
<p>To describe curved space-time, Einstein called upon the earlier work of <a href="http://www.britannica.com/biography/Bernhard-Riemann">Bernhard Riemann</a>, a 19th century mathematician. And with the help of his friend <a href="https://en.wikipedia.org/wiki/Marcel_Grossmann">Marcel Grossman</a>, also a mathematician, Einstein spent several exhausting years learning the mathematics of curved spaces, or what mathematicians call “differential geometry”. As Einstein commented, “compared with understanding gravity, special relativity was mere child’s play”.</p>
<p>Now Einstein had the mathematical tools to finalise his theory. His equivalence principle stated that an accelerating reference frame is equivalent to a gravitational field. From his geometrical studies, he believed the gravitational field was simply a manifestation of curved space-time. Hence, Einstein could show that accelerating frames were represented by non-Euclidean space.</p>
<h2>It follows</h2>
<p>The third key step for Einstein involved resolving complications that had arisen when special relativity was applied to Newtonian gravitational physics. In special relativity, the constancy of the speed of light in all reference frames, and the conclusion that the speed of light set the universal speed limit, was in direct contradiction to Newton’s theory of gravity, which postulated the instantaneous effect of gravity. </p>
<p>Put simply: Newtonian gravity predicted that if the sun was removed from the centre of the solar system, the gravitational effect on the Earth would be instantaneous. However, special relativity says that even the gravitational effect of the sun disappearing ought to travel at the speed of light.</p>
<p>Einstein also knew that gravitational force between two bodies was directly proportional to their masses, from Newton’s equation F = GMm/r². So mass clearly determined the strength of the gravitational field. Special relativity tells us that mass is equivalent to energy, so the energy–momentum density must also determine the gravitational force.</p>
<p>Thus the three key assumptions that Einstein used to formulate his theory were:</p>
<ol>
<li><p><a href="http://math.ucr.edu/home/baez/physics/Relativity/SR/rigid_disk.html">Rotating frames</a> (non-inertial) imply non-Euclidean (or curved) space-time </p></li>
<li><p>The equivalence principle asserts that accelerating frames (i.e. non inertial frames) are equivalent to gravitational fields</p></li>
<li><p>From special relativity, mass is equal to energy, and from Newtonian physics, mass is proportional to strength of gravity.</p></li>
</ol>
<p>Hence, Einstein was able to conclude that energy-momentum density causes, and is proportional to, space-time curvature.</p>
<p>It is not clear when Einstein had the “light bulb moment”, the moment when he was able to put the puzzle together and link mass/energy with the curvature of space. </p>
<p>From 1913 to 1915, Einstein published several papers as he laboured to complete the general theory. Some of these papers contained errors and took Einstein down theoretical pathways that ultimately were not productive.</p>
<p>But the final result, that the energy–momentum density of matter curves space-time, like a bowling ball curves a flat sheet of rubber, and that the motion of a mass in a gravitational field is based on the curvature of space time, just as a bowling ball moves freely on a curved rubber sheet, is surely one of the greatest insights of human intellect.</p>
<h2>Head start</h2>
<p>How long would it have taken us to understand gravity without Einstein’s sheer genius? It is possible we would have had to wait many decades. </p>
<p>However, in 1979 the cat would have been out of the bag. In that year, astronomers discovered the “<a href="http://www.universetoday.com/108542/double-vision-these-twin-quasars-are-actually-the-same-thing/">twin quasar</a>”, QSO 0957+561, the first gravitationally-lensed quasar. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/102452/original/image-20151119-19375-dqt9fk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/102452/original/image-20151119-19375-dqt9fk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/102452/original/image-20151119-19375-dqt9fk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=421&fit=crop&dpr=1 600w, https://images.theconversation.com/files/102452/original/image-20151119-19375-dqt9fk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=421&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/102452/original/image-20151119-19375-dqt9fk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=421&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/102452/original/image-20151119-19375-dqt9fk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=530&fit=crop&dpr=1 754w, https://images.theconversation.com/files/102452/original/image-20151119-19375-dqt9fk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=530&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/102452/original/image-20151119-19375-dqt9fk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=530&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 discovery of the ‘twin quasar’ would likely have tipped off physicists to the curvature of space-time had Einstein not beat them to it.</span>
<span class="attribution"><span class="source">NASA/ESA</span></span>
</figcaption>
</figure>
<p>This astonishing discovery could only have made sense if space-time was curved. It would surely have attracted a Nobel Prize if it hadn’t been for Einstein’s genius. Maybe it still should.</p><img src="https://counter.theconversation.com/content/50517/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Nothing to disclose</span></em></p><p class="fine-print"><em><span>Darren Dougan does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Special relativity was inspired, but it took true genius to conceive of general relativity. Had Einstein not come up with it, it may have taken decades for us to figure it out.John K. Webb, Professor of Astrophysics and Director of the Big Questions Institute at UNSW, UNSW SydneyDarren Dougan, Co-Founder of BQI@UNSW and PhD Student, UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/500132015-11-09T19:20:46Z2015-11-09T19:20:46ZFrom Newton to Einstein: the origins of general relativity<figure><img src="https://images.theconversation.com/files/101028/original/image-20151106-16255-1h26x55.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">General relativity didn't happen overnight, but took several steps to come to fruition.</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>One hundred years ago in November 1915, Albert Einstein presented to the Prussian Academy of Sciences his new theory of <a href="https://theconversation.com/explainer-einsteins-theory-of-general-relativity-3481">general relativity</a>. It is fair to say the theory turned out to be a great success. </p>
<p>General relativity was built on Einstein’s <a href="http://www.fourmilab.ch/etexts/einstein/specrel/www/">special relativity</a>, which provided solutions to some of the greatest puzzles of the 19th century theoretical physics. </p>
<p>So in order to grasp the meaning and significance of general relativity, it is worth reflecting on the state of physics in the 19th century to see how Einstein came to realise that space, time and geometry are not absolute but depend on the physical environment.</p>
<h2>The beauty of invariance</h2>
<p>In the 17th century, Isaac Newton developed a <a href="http://www.livescience.com/47814-classical-mechanics.html">set of equations</a> that described the physical properties of the world around us. These equations were very successful, from a description of the flight of a cannonball, to the motion of the planets.</p>
<p>They also had a very appealing property: all observers, regardless of whether they are moving or not – i.e. regardless of which “<a href="http://newt.phys.unsw.edu.au/einsteinlight/jw/module1_Inertial.htm">inertial frame</a>” they are in – are equivalent when it comes to their description of the world around them. So two individuals moving in different directions would see events unfold in the same way. </p>
<p>Even though formally these individuals would see things in a different way – one might say that things move from left to right, whereas the other might say they move from right to left – still the fundamental description of the unfolding events would remain the same, and the laws of physics derived by these individuals would have literally the same form.</p>
<p>But in the 19th century, people started noticing that not everything plays accordingly to this rule. </p>
<h2>Problems with electromagnetism</h2>
<p>The 19th century was a time of extensive study of the phenomena of electricity, magnetism and light. In 1865 James Clerk Maxwell published a set of equations that combined all these phenomena into a single phenomenon of “<a href="https://theconversation.com/let-there-be-light-celebrating-the-theory-of-electromagnetism-35723">electromagnetism</a>”.</p>
<p>Soon after Maxwell’s discovery, people realised that there is something strange when it comes to his equations. Their form changes when we move from one inertial frame to another. So an individual who is not moving can observe distinctively different physical phenomena than a person who is moving. </p>
<p>All the beauty of invariance and irrelevance of observers that we had got used to in Newtonian physics was gone. It now looked like some frames were preferable to others when it came to describing events in nature. </p>
<p>Then, at the turn of the 20th century, a new mathematical transformation was discovered that could preserve the structure of Maxwell’s equations when moving from one frame to another. Although many people contributed to this discovery, we now refer to it as the “<a href="https://en.wikipedia.org/wiki/Lorentz_transformation">Lorentz transformation</a>”. </p>
<p>The Lorentz transformation was different from the standard transformation of inertial frames that had been used in the Newtonian physics. In Newtonian physics, length and time are absolute, so the length of an object in one frame is the same as the length of that object in another frame. Also, time passes in the same way in one frame as in the other frame. </p>
<p>However, if taken literally, the Lorentz transformation implies that time and length do actually change, depending on which frame of reference you are in. </p>
<h2>Principle of relativity</h2>
<p>This got Einstein wondering whether the transformation that preserved the structure of Maxwell’s equations was merely a mathematical trick or whether there was something fundamental about it. He wondered whether time and space were absolute, or whether the principle of invariance of the laws of physics should be paramount. </p>
<p>In 1905, Einstein decided that it is the invariance of the laws of physics that should have the highest status, and postulated the principle of relativity: that all inertial frames are equivalent, the observer’s motion (with constant velocity) is irrelevant, and that all laws of physics should have the same form in all inertial frames.</p>
<p>When combined with electromagnetism, this principle would require that the transformation from one inertial frame to another must have a structure of the Lorentz transformation, meaning that time and space are no longer absolute and change their properties when changing from one inertial frame to another.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/C2VMO7pcWhg?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">A visualisation of special relativity.</span></figcaption>
</figure>
<h2>What about gravity?</h2>
<p>In 1907 Einstein realised that his theory was not complete. The principle of relativity was only applicable to observers moving with a constant velocity. It also did not fit with the Newtonian description of gravity. </p>
<p>Einstein, being a patent officer, did not have access to laboratory equipment. To compensate, he had to engage himself in thought experiments. He considered various scenarios in his head and worked through them step by step. </p>
<p>These thought experiments showed to him that gravity is not different from acceleration. So standing stationary on the Earth feels just the same as standing in a rocket ship accelerating at a constant 1G.</p>
<p>It also showed that the accelerated observer would observe that fundamental geometrical properties change. For example, that the number π (a mathematical constant) could no longer be defined as a ratio of a circle’s circumference to its diameter.</p>
<p>So it was not just time and space that lost their absolute meaning, but Einstein realised that also geometry itself was not absolute and could be susceptible to physical conditions.</p>
<h2>The road to general relativity</h2>
<p>All this reasoning convinced Einstein that the geometry of the spacetime and the physical processes that take place in the spacetime, are related to each other and that one can affect the other. </p>
<p>It also led to a striking conclusion: what we perceive as gravity is just a consequence of the motion through the spacetime. The larger the curvature of the spacetime the stronger gravity is.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/E43-CfukEgs?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Experiment with falling objects. In the 17th century Newton concluded that objects fall because they are pulled by Earth’s gravity. Einstein’s interpretation was that these objects do not fall. According to Einstein, these objects and Earth just freely move in a curved spacetime and this curvature is induced by mass and energy of these objects.</span></figcaption>
</figure>
<p>It took Einstein eight years to find the relation between the geometry of spacetime and physics. </p>
<p>The equations that he presented in 1915 not only led to a completely different interpretation of events around us but also provided an explanation for some baffling or yet to be discovered phenomena: from the anomalous orbit of the planet Mercury, through the bending of light by the Sun’s gravity, to predicting the existence of black holes and expanding universe.</p>
<p>It was a bumpy road from Newtownian physics to special and then general relativity. But each step, driven by Einstein’s insight, drove inexorably towards a picture of the universe that persists to this day.</p><img src="https://counter.theconversation.com/content/50013/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Krzysztof Bolejko receives funding from the Australian Research Council and the University of Sydney.</span></em></p>This month is the centenary of the general theory of relativity. But how did we get from the absolutism of Newton to the relativity of Einstein?Krzysztof Bolejko, Cosmologist, 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.tag:theconversation.com,2011:article/161492013-11-21T06:16:05Z2013-11-21T06:16:05ZMight some of Doctor Who actually be possible?<figure><img src="https://images.theconversation.com/files/35599/original/7pgzy4vn-1384862732.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Bigger inside, says science.</span> <span class="attribution"><span class="source">Ian West/PA</span></span></figcaption></figure><p>As Doctor Who’s 50th anniversary looms, time travel is everywhere – on the screen, at least. Famously, the Doctor can whizz through the years using a “dimensionally transcendental” machine, the TARDIS, and make changes to the past as and when he likes. But what is time travel – and how much of “Doctor Who” might really be possible?</p>
<p>A handy definition of time travel comes from philosopher <a href="http://plato.stanford.edu/entries/david-lewis/">David Lewis</a>. Lewis says time travel involves a journey having different durations viewed from outside (in “external time”) or from inside (in “personal time”). Suppose you spend five minutes travelling aboard your machine, as measured by (e.g.) your watch and your memories. On arrival, you find 150 years have elapsed in the outside world. Congratulations, you have time-travelled. Five minutes of your personal time has covered 150 years of external time. </p>
<p>Odd as this sounds, Einstein’s theory of Special Relativity introduced such possibilities to physics in 1905. The theory says: the duration of a process varies with the relative velocity of the observer. The closer that relative velocity gets to the speed of light, the longer the travelling process takes.</p>
<p>Suppose you want to see the Earth a billion years hence, but worry you have only about 50 personal years left. Special Relativity specifies that if you travel very close to the speed of light relative to the Earth, your 50 personal years can cover one billion Earth years. </p>
<p>In backward time travel, personal and external time differ in direction, so journeys end in external time before, not after, they begin; you spend five personal minutes travelling 150 years into the external past. General Relativity suggests that the universe is essentially <a href="http://www.nature.com/news/curved-space-time-on-a-chip-1.13840">curved spacetime</a>, which might allow such divergences of external and personal time. </p>
<p>Relativity treats space and time as aspects of a single entity: “spacetime”. One of the more remarkable features of General Relativity is that it allows time and space axes to be interchanged, so one observer’s space axis can be another observer’s time axis.</p>
<p>In 1949, Austrian mathematician Kurt Gödel used General Relativity to describe a universe where intrepid voyagers can go anywhere in (past or future) time without travelling faster than light. Gödel’s universe has no boundaries in space or time, and all the matter in it rotates. But our finite, non-rotating universe is not Gödel’s. Despair not though – simply spin an ultradense, very (maybe infinitely) long cylinder very fast. Spacetime should curve around the cylinder so the direction of the local future partially points into the external past. Such devices are called “<a href="http://www.andersoninstitute.com/tipler-cylinder.html">Tipler Cylinders</a>”, after physicist Frank Tipler. </p>
<p>Better yet, quantum theory suggests that “wormhole” connections between different spacetime points spontaneously form and break all the time. The chances are that natural wormholes are tiny - vastly smaller even than an electron, (and a billion trillion electrons can fit in a teaspoon). But you could perhaps find (or create) a wormhole big enough and durable enough to let you slip through into the past. Difficult, but theoretically possible.</p>
<h2>No, you can’t kill your physics teacher</h2>
<p>So perhaps you could travel into the past. But what about paradoxes? What is to stop you assassinating your grandfather or yourself as infants? One answer says: logical consistency. </p>
<p>Classical logic says you cannot consistently kill in infancy someone who achieves adulthood. But, Lewis says, time travel need not involve doing the logically impossible – provided travellers’ actions in the past are consistent with the history whence they come. So you could try killing your baby grandfather, but something would foil you – you would sneeze, or your gun would jam. Lewisian time travel is therefore (classically) consistent, but might look very strange, since seemingly possible actions (like shooting an unprotected infant) would prove impossible.</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/35598/original/rs6m9cpk-1384860517.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/35598/original/rs6m9cpk-1384860517.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=398&fit=crop&dpr=1 600w, https://images.theconversation.com/files/35598/original/rs6m9cpk-1384860517.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=398&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/35598/original/rs6m9cpk-1384860517.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=398&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/35598/original/rs6m9cpk-1384860517.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=500&fit=crop&dpr=1 754w, https://images.theconversation.com/files/35598/original/rs6m9cpk-1384860517.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=500&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/35598/original/rs6m9cpk-1384860517.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=500&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">monkeyc</span></span>
</figcaption>
</figure>
<p>Another view says that backward time travel requires many worlds – that is, many different but equally real versions of physical reality. Physicist David Deutsch and philosopher Michael Lockwood argue that time travel must involve inter-world travel. If you travel backwards in time, you must arrive in a history different from your native one and so would be quite unfettered by your past once you get there. You could even kill this other history’s counterparts of your grandfather and yourself.</p>
<p>Both these concepts of backwards time travel may disappoint anyone wanting to change the “one and only” past. Conventional logic says time travellers would either help make history what it was (Lewis) or create a different history (Deutsch/Lockwood). However, quantum logic might let travellers change the actual (one-and-only) past.</p>
<p>Suppose we hold that quantum measurements determine (or change) quantities measured, even if those quantities lie in the past. Someone could travel back and “observe” history turning out differently from how it originally was, thereby retrospectively making actuality different from what it had been. What would happen to travellers who rebooted history is not clear, but this model seems closer to the time travel familiar from “Doctor Who” and other fictions. Beware, though, because quantum theory allows no predicting, and still less controlling, of the outcomes of changing the past. There would be no way to foresee the effect you would have on the present.</p>
<p>So classical logic, General Relativity and quantum theory all seem to permit time travel. Classical logic plus General Relativity suggest backward travellers face weird consistency constraints. Many-worlds travellers face no constraints, but get displaced into different histories. Quantum-logic travellers could change the (one and only) past without constraints, but they couldn’t predict or control what they would get.</p>
<p>So far, however, it seems only the Doctor knows how to change the past at will.</p><img src="https://counter.theconversation.com/content/16149/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alasdair Richmond received funding from the Arts and Humanities Research Council, to fund one semester of a two-semester leave period that covered the academic year 2008-2009.</span></em></p>As Doctor Who’s 50th anniversary looms, time travel is everywhere – on the screen, at least. Famously, the Doctor can whizz through the years using a “dimensionally transcendental” machine, the TARDIS…Alasdair Richmond, Senior Lecturer in Philosophy, The University of EdinburghLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/58912012-03-21T03:14:46Z2012-03-21T03:14:46ZWarp drives and reality: new hope for a Galactic Empire?<figure><img src="https://images.theconversation.com/files/8779/original/h26xdht2-1332209276.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Hyper-drives might be the stuff of science fiction, but they could be science fact too.</span> <span class="attribution"><span class="source">20th Century Fox</span></span></figcaption></figure><p>Fans of science fiction must be disheartened when introduced to <a href="http://www.upscale.utoronto.ca/GeneralInterest/Harrison/SpecRel/SpecRel.html">Einstein’s Special Theory of Relativity</a>. Dreams of <a href="http://en.wikipedia.org/wiki/Galactic_Empire_(Star_Wars)">galactic empires</a>, criss-crossed by roguish princesses and beautiful smugglers, go out the window with one simple rule: “thou shalt not travel faster than the speed of light”.</p>
<p>Even a rocket ship travelling just under the speed of light (roughly 1 billion km/h) would take more than 100,000 years to get from one side of the Milky Way to the other. That’s slightly longer than the fraction of a second required to traverse galaxies in science fiction staples such as Star Wars.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/8777/original/jgjtkp97-1332207541.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/8777/original/jgjtkp97-1332207541.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/8777/original/jgjtkp97-1332207541.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/8777/original/jgjtkp97-1332207541.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/8777/original/jgjtkp97-1332207541.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/8777/original/jgjtkp97-1332207541.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/8777/original/jgjtkp97-1332207541.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/8777/original/jgjtkp97-1332207541.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The closer to the speed of light you go (x-axis), the greater the time dilation effect (y-axis).</span>
</figcaption>
</figure>
<p>Of course, we are a long way from being able to build spacecraft that can travel at 1 billion km/h – the best we’ve been able to manage so far is <a href="http://www.lifeslittlemysteries.com/616-whats-the-fastest-spacecraft-ever.html">roughly 60,000km/h</a>, the speed being travelled by <a href="http://en.wikipedia.org/wiki/Voyager_1">Voyager 1</a> as it currently speeds away from Earth.</p>
<p>Even if we could build a near-speed-of-light spacecraft, the effects of <a href="http://en.wikipedia.org/wiki/Time_dilation">time-dilation</a> would pose a real problem. This dilation, according to Special Relativity, ensures clocks on our almost-light-speed rocket would run more slowly than those left behind on Earth.</p>
<p>When you returned from your galactic travels, even though only a few decades had passed for you, everyone you knew before would be dead, and even the civilisation from which you came would have probably passed into history. </p>
<p>While science fiction has come up with creative ideas to circumvent the speed-of-light barrier, such as travelling through <a href="http://starwars.wikia.com/wiki/Hyperspace">“hyper-space”</a> or zipping through a <a href="http://en.memory-alpha.org/wiki/Wormhole">wormhole</a>, science <em>fact</em> appears to be bound by Einstein’s light speed law.</p>
<p>And while this law can’t be broken, it seems it can be bent. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/j0GZ3qSV9s0?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>In 1994, Mexican physicist <a href="http://en.wikipedia.org/wiki/Miguel_Alcubierre">Miguel Alcubierre</a> used <a href="http://theconversation.com/explainer-einsteins-theory-of-general-relativity-3481">Einstein’s General Theory of Relativity</a> to <a href="http://members.shaw.ca/mike.anderton/WarpDrive.pdf">show</a> that if you wrap a rocket in a bubble of distorted <a href="https://theconversation.com/explainer-gravity-5256">space-time</a> (see image below), the rocket can travel at any speed through the universe. </p>
<p>It can achieve this by (in hand-wavy terms) compressing the space in front of it, and expanding the space behind it. Voila: a warp-drive worthy of the world of science fiction!</p>
<p>Within the bubble, rocketeers would comfortably float in a <a href="http://www.youtube.com/watch?v=OZY8279b7BU">weightless environment</a>, and would not suffer the time-dilation of high-speed rockets. This is a bonus for those who like to keep their diaries in check, and for those who want to see their nearest and dearest when they return to Earth.</p>
<p>The Alcubierre warp-drive looks like it solves the problems of building a Galactic Empire (although there are <a href="http://deathstarpr.com/2012/02/building-a-death-star-would-cost-852-quadrillion-worth-it/">some issues</a> that ensure building one will require technologies and knowledge far in excess of our own).</p>
<p>Putting practical considerations to one side, would there be any negative consequences of using such a warp-drive? This is a question I asked with two of my honours students, Brendan McMonigal and Phil o’Byrne.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/8778/original/jfxjjc4h-1332208841.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/8778/original/jfxjjc4h-1332208841.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/8778/original/jfxjjc4h-1332208841.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=278&fit=crop&dpr=1 600w, https://images.theconversation.com/files/8778/original/jfxjjc4h-1332208841.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=278&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/8778/original/jfxjjc4h-1332208841.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=278&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/8778/original/jfxjjc4h-1332208841.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=349&fit=crop&dpr=1 754w, https://images.theconversation.com/files/8778/original/jfxjjc4h-1332208841.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=349&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/8778/original/jfxjjc4h-1332208841.jpg?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">The warp field according to the Alcubierre drive.</span>
<span class="attribution"><span class="source">AllenMcC</span></span>
</figcaption>
</figure>
<p>Our starting point was the realisation that the space through which a warp-drive would fly is not truly empty. Rather it’s a tenuous <a href="http://csep10.phys.utk.edu/astr162/lect/cosmology/cbr.html">sea of atoms and radiation</a>, even in the space far away from galaxies. So what would happen when a warp-drive rocket ploughs through this sea?</p>
<p>Our key concern was for the rocketeers themselves. If the atoms and radiation make it into the rocket, and their energies are boosted by their journey through the warped space-time of the bubble, the resulting irradiation would fry the occupants. This would not make for a pleasant trip.</p>
<p>By using the equations of relativity, we traced the passage of the particles as they encounter the bubble, and found that, in general, the rocketeers would be fine. They would receive a dose of radiation no worse than usual space travel (although this <a href="http://science.nasa.gov/science-news/science-at-nasa/2004/17feb_radiation/">can be quite dangerous</a>).</p>
<p>But as we increase the speed of the warp bubble, pushing into “superluminal” (faster-than-light) travel, something quite interesting happens. As the ship runs into particles and radiation, these particles make their way into the bubble, but get stuck before reaching the rocket at the centre. As the ship progresses, it sweeps up more and more material which continues to accumulate in the bubble. </p>
<p>Even though these particles and radiation are fixed with regards to the rocket, their energy starts to grow, and grow, and grow. </p>
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<p>This is fine as long as the warp-bubble keeps moving. But clearly, if your aim is to establish a Galactic Empire, you’ll want to stop every now and then to explore your conquests. Unfortunately, in slowing down, all of the matter and radiation caught up in the bubble – whose energies have been boosted to extremely high levels – would be released in <a href="http://www.youtube.com/watch?v=djZFHTa6TfA">a deadly beam</a> in front of the ship.</p>
<p>So, while the rocketeer would remain safe throughout his or her superluminal journey, the energy burst released when they slow down would fry whatever they’ve stopped to admire. Such an energy blast could even sterilise the planet they were hoping to visit.</p>
<p>While this may disappoint the rocketeer (and the residents of the now-fried planet), it would be nothing compared to the fact Earth could be destroyed by the return journey.</p>
<p>The warp-drive has become a death machine!</p>
<p>From the outside, it might be difficult to separate science fact from science fiction, but all of this work is based within the framework of Einstein’s General Theory of Relativity – the most accurate description we have of gravity.</p>
<p>In 2016, Einstein’s work will be 100 years old, but the complexity of the equations means we have only started scratching the surface of what is truly possible. </p>
<p>We shouldn’t write off the Galactic Empire just yet.</p>
<p><strong>Further reading:</strong></p>
<ul>
<li><a href="http://members.shaw.ca/mike.anderton/WarpDrive.pdf">The warp drive: hyper-fast travel within general relativity</a> - Miguel Alcubierre</li>
</ul><img src="https://counter.theconversation.com/content/5891/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Geraint Lewis receives funding from Australian Research Council and holds a Future Fellowship.</span></em></p>Fans of science fiction must be disheartened when introduced to Einstein’s Special Theory of Relativity. Dreams of galactic empires, criss-crossed by roguish princesses and beautiful smugglers, go out…Geraint Lewis, Professor of Astrophysics, University of SydneyLicensed as Creative Commons – attribution, no derivatives.