tag:theconversation.com,2011:/us/topics/doppler-effect-133/articlesDoppler effect – The Conversation2023-10-16T12:30:12Ztag:theconversation.com,2011:article/2058102023-10-16T12:30:12Z2023-10-16T12:30:12ZWhy is space so dark even though the universe is filled with stars?<figure><img src="https://images.theconversation.com/files/538108/original/file-20230718-17-5jcl17.jpg?ixlib=rb-1.1.0&rect=26%2C6%2C996%2C676&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">This age old question has been dubbed Olbers' paradox.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/the-milky-way-appears-over-the-valle-de-la-luna-in-the-news-photo/1418507439?adppopup=true">John Moore via Getty Images News</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>Why is space so dark despite all of the stars in the universe? – Nikhil, age 15, New Delhi</strong></p>
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<p>People have been asking why space is dark despite being filled with stars for so long that this question has a special name – <a href="https://lambda.gsfc.nasa.gov/product/suborbit/POLAR/cmb.physics.wisc.edu/tutorial/olbers.html">Olbers’ paradox</a>.</p>
<p>Astronomers estimate that there are about <a href="https://theconversation.com/how-many-stars-are-there-in-space-165370">200 billion trillion stars</a> in the observable universe. And many of those stars are as bright or even brighter than our sun. So, why isn’t space filled with dazzling light?</p>
<p><a href="http://www.astrojack.com/">I am an astronomer</a> who studies stars and planets – including those outside our solar system – and their motion in space. The study of distant stars and planets helps <a href="https://scholar.google.com/citations?user=pF3HbeQAAAAJ&hl=en&oi=ao">astronomers like me</a> understand why space is so dark.</p>
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<p>You might guess it’s because a lot of the stars in the universe are very far away from Earth. Of course, it is true that the farther away a star is, the less bright it looks – <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/isq.html">a star 10 times farther away looks 100 times dimmer</a>. But it turns out this isn’t the whole answer. </p>
<h2>Imagine a bubble</h2>
<p>Pretend, for a moment, that the universe is so old that the light from even the farthest stars has had time to reach Earth. In this imaginary scenario, all of the stars in the universe are not moving at all.</p>
<p>Picture a large bubble with Earth at the center. If the bubble were about 10 <a href="https://exoplanets.nasa.gov/faq/26/what-is-a-light-year/">light years</a> across, it would contain about <a href="https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs">a dozen stars</a>. Of course, at several light years away, many of those stars would look pretty dim from Earth. </p>
<p>If you keep enlarging the bubble to 1,000 light years across, then to 1 million light years, and then 1 billion light years, the farthest stars in the bubble will look even more faint. But there would also be more and more stars inside the bigger and bigger bubble, all of them contributing light. Even though the farthest stars look dimmer and dimmer, there would be a lot more of them, and the whole night sky should look very bright.</p>
<p>It seems I’m back where I started, but I’m actually a little closer to the answer.</p>
<h2>Age matters</h2>
<p>In the imaginary bubble illustration, I asked you to imagine that the stars are not moving and that the universe is very old. But the universe is only about <a href="https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question28.html">13 billion years old</a>. </p>
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<a href="https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Image of lightly colored galaxies and stars against dark background" src="https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=613&fit=crop&dpr=1 600w, https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=613&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=613&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=770&fit=crop&dpr=1 754w, https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=770&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=770&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Galaxies as they appeared approximately 13.1 billion years ago, taken by the James Webb Space Telescope.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/image-released-by-nasa-on-july-11-2022-shows-galaxy-cluster-news-photo/1241872380?adppopup=true">NASA/ESA/CSA/STScI/Handout from Xinhua News Agency via Getty Images</a></span>
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<p>Even though that’s an amazingly long time in human terms, it’s short in astronomical terms. It’s short enough that the light from stars more distant than about 13 billion light years hasn’t actually reached Earth yet. And so the actual bubble around Earth that contains all the stars we can see only extends out to about <a href="https://science.nasa.gov/observable-universe">13 billion light years from Earth</a>.</p>
<p>There just are not enough stars in the bubble to fill every line of sight. Of course, if you look in some directions in the sky, you can see stars. If you look at other bits of the sky, you can’t see any stars. And that’s because, in those dark spots, the stars that could block your line of sight are so far away their light hasn’t reached Earth yet. As time passes, light from these more and more distant stars will have time to reach us. </p>
<h2>The Doppler shift</h2>
<p>You might ask whether the night sky will eventually light up completely. But that brings me back to the other thing I told you to imagine: that all of the stars are not moving. The universe is actually expanding, with the most distant galaxies <a href="https://starchild.gsfc.nasa.gov/docs/StarChild/questions/redshift.html">moving away from Earth at nearly the speed of light</a>. </p>
<p>Because the galaxies are moving away so fast, the light from their stars is pushed into colors the human eye can’t see. This effect is called the <a href="https://starchild.gsfc.nasa.gov/docs/StarChild/questions/redshift.html">Doppler shift</a>. So, even if it had enough time to reach you, <a href="https://svs.gsfc.nasa.gov/12856">you still couldn’t see</a> the light from the most distant stars with your eyes. And the night sky would not be completely lit up. </p>
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<figcaption><span class="caption">The Doppler shift, also known as the redshift, is a phenomenon in which light from objects that are moving away from an observer appears more toward the red end of the spectrum.</span></figcaption>
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<p>If you wait even longer, eventually the stars will all burn out – <a href="https://astronomy.swin.edu.au/cosmos/m/main+sequence+lifetime">stars like the sun last only about 10 billion years</a>. Astronomers hypothesize that in the distant future – a thousand trillion years from now – the universe will go dark, <a href="https://en.wikipedia.org/wiki/The_Five_Ages_of_the_Universe">inhabited by only stellar remnants</a> like white dwarfs and black holes.</p>
<p>Even though our night sky isn’t completely filled with stars, we live in a very special time in the universe’s life, when we’re lucky enough to enjoy a rich and complex night sky, filled with light and dark.</p>
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<p class="fine-print"><em><span>Brian Jackson receives federally funded research grants from NASA. </span></em></p>An astronomer explains why space looks so dark despite containing 200 billion trillion stars.Brian Jackson, Associate Professor of Astronomy, Boise State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2086492023-06-28T20:04:03Z2023-06-28T20:04:03ZAstronomers puzzled by ‘planet that shouldn’t exist’<figure><img src="https://images.theconversation.com/files/534458/original/file-20230627-17-bpm2lk.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C1920%2C1080&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Julian Baum</span></span></figcaption></figure><p>The search for planets outside our Solar System – exoplanets – is one of the most rapidly growing fields in astronomy. Over the past few decades, more than 5,000 exoplanets have been detected and astronomers now estimate that on average there is at least one planet per star in our galaxy.</p>
<p>Many current research efforts aim at detecting Earth-like planets suitable for life. These endeavours focus on so-called “main sequence” stars like our Sun – stars which are powered by fusing hydrogen atoms into helium in their cores, and remain stable for billions of years. More than 90% of all known exoplanets so far have been detected around main-sequence stars.</p>
<p>As part of an international team of astronomers, we studied a star that looks much like our Sun will in billions of years’ time, and found it has a planet which by all rights it should have devoured. In <a href="https://www.nature.com/articles/s41586-023-06029-0">research</a> published today in Nature, we lay out the puzzle of this planet’s existence – and propose some possible solutions.</p>
<h2>A glimpse into our future: red giant stars</h2>
<p>Just like humans, stars undergo changes as they age. Once a star has used up all its hydrogen in the core, the core of the star shrinks and the outer envelope expands as the star cools. </p>
<p>In this “red giant” phase of evolution, stars can grow to more than 100 times their original size. When this happens to our Sun, in about 5 billion years, we expect it will grow so large it will engulf Mercury, Venus, and possibly Earth.</p>
<p>Eventually, the core becomes hot enough for the star to begin fusing helium. At this stage the star shrinks back to about 10 times its original size, and continues stable burning for tens of millions of years.</p>
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Read more:
<a href="https://theconversation.com/explainer-how-do-you-find-exoplanets-24153">Explainer: how do you find exoplanets?</a>
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<p>We know of hundreds of planets orbiting red giant stars. One of these is called <a href="https://exoplanets.nasa.gov/exoplanet-catalog/7100/8-ursae-minoris-b/">8 Ursae Minoris b</a>, a planet with around the mass of Jupiter in an orbit that keeps it only about half as far from its star as Earth is from the Sun.</p>
<p>The planet was discovered in 2015 by a team of Korean astronomers using the “Doppler wobble” technique, which measures the gravitational pull of the planet on the star. In 2019, the International Astronomical Union <a href="https://www.nameexoworlds.iau.org/_files/ugd/6358ac_5eebee4eba4f41b7a9f6201123673a24.pdf">dubbed</a> the star Baekdu and the planet Halla, after the tallest mountains on the Korean peninsula.</p>
<h2>A planet that should not be there</h2>
<p>Analysis of new data about Baekdu collected by NASA’s Transiting Exoplanet Survey Satellite (<a href="https://www.nasa.gov/tess-transiting-exoplanet-survey-satellite/">TESS</a>) space telescope has yielded a surprising discovery. Unlike other red giants we have found hosting exoplanets on close-in orbits, Baekdu has already started fusing helium in its core.</p>
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<em>
<strong>
Read more:
<a href="https://theconversation.com/nasas-planet-hunting-spacecraft-tess-is-now-on-its-mission-to-search-for-new-worlds-94291">NASA's planet-hunting spacecraft TESS is now on its mission to search for new worlds</a>
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<p>Using the techniques of <a href="https://exoplanets.nasa.gov/news/1516/symphony-of-stars-the-science-of-stellar-sound-waves/">asteroseismology, which studies waves inside stars</a>, we can determine what material a star is burning. For Baekdu, the frequencies of the waves unambiguously showed it has commenced burning helium in its core.</p>
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<a href="https://images.theconversation.com/files/534474/original/file-20230628-19-njdov.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/534474/original/file-20230628-19-njdov.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/534474/original/file-20230628-19-njdov.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=331&fit=crop&dpr=1 600w, https://images.theconversation.com/files/534474/original/file-20230628-19-njdov.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=331&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/534474/original/file-20230628-19-njdov.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=331&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/534474/original/file-20230628-19-njdov.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=416&fit=crop&dpr=1 754w, https://images.theconversation.com/files/534474/original/file-20230628-19-njdov.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=416&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/534474/original/file-20230628-19-njdov.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=416&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Sound waves inside a star can be used to determine whether it is burning helium.</span>
<span class="attribution"><span class="source">Gabriel Perez Diaz / Instituto de Astrofisica de Canarias</span></span>
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<p>The discovery was puzzling: if Baekdu is burning helium, it should have been much bigger in the past – so big it should have engulfed the planet Halla. How is it possible Halla survived?</p>
<p>As is often the case in scientific research, the first course of action was to rule out the most trivial explanation: that Halla never really existed. </p>
<p>Indeed, some apparent discoveries of planets orbiting red giants using the Doppler wobble technique have later been shown to be illusions <a href="https://ui.adsabs.harvard.edu/abs/2018AJ....155..120H/abstract">created by long-term variations in the behaviour of the star itself</a>. </p>
<p>However, follow-up observations ruled out such a false-positive scenario for Halla. The Doppler signal from Baekdu has remained stable over the last 13 years, and close study of other indicators showed no other possible explanation for the signal. Halla is real – which returns us to the question of how it survived engulfment. </p>
<h2>Two stars become one: a possible survival scenario</h2>
<p>Having confirmed the existence of the planet, we arrived at two scenarios which could explain the situation we see with Baekdu and Halla. </p>
<p>At least half of all stars in our galaxy did not form in isolation like our Sun, but are part of binary systems. If Baekdu once was a binary star, Halla may have never faced the danger of engulfment. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/534475/original/file-20230628-23-52qepf.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/534475/original/file-20230628-23-52qepf.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=509&fit=crop&dpr=1 600w, https://images.theconversation.com/files/534475/original/file-20230628-23-52qepf.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=509&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/534475/original/file-20230628-23-52qepf.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=509&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/534475/original/file-20230628-23-52qepf.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=640&fit=crop&dpr=1 754w, https://images.theconversation.com/files/534475/original/file-20230628-23-52qepf.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=640&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/534475/original/file-20230628-23-52qepf.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=640&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">If the star Baekdu used to be a binary, there are two scenarios which can explain the survival of the planet Halla.</span>
<span class="attribution"><span class="source">Brooks G. Bays, Jr, SOEST/University of Hawai'i</span></span>
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<p>A merger of these two stars may have prevented the expansion of either star to a size large enough to engulf planet Halla. If one star became a red giant on its own, it would have engulfed Halla – however, if it merged with a companion star it would jump straight to the helium-burning phase without getting big enough to reach the planet. </p>
<p>Alternatively, Halla may be a relatively newborn planet. The violent collision between the two stars may have produced a cloud of gas and dust from which the planet could have formed. In other words, the planet Halla may be a recently born “second generation” planet. </p>
<p>Whichever explanation is correct, the discovery of a close-in planet orbiting a helium-burning red giant star demonstrates that nature finds ways for exoplanets to appear in places where we might least expect them. </p>
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<a href="https://images.theconversation.com/files/534470/original/file-20230627-35262-ufsunt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="An illustration of a planet in a ring of dust and debris around a star." src="https://images.theconversation.com/files/534470/original/file-20230627-35262-ufsunt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/534470/original/file-20230627-35262-ufsunt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/534470/original/file-20230627-35262-ufsunt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/534470/original/file-20230627-35262-ufsunt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/534470/original/file-20230627-35262-ufsunt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/534470/original/file-20230627-35262-ufsunt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/534470/original/file-20230627-35262-ufsunt.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>
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<span class="caption">The planet Halla may have formed from debris created by the merger of two stars.</span>
<span class="attribution"><span class="source">W. M. Keck Observatory / Adam Makarenko</span></span>
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</figure><img src="https://counter.theconversation.com/content/208649/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Daniel Huber receives funding from the Australian Research Council (ARC), the National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), and the Sloan Foundation. He is also affiliated with the University of Hawaiʻi. </span></em></p>The planet Halla looks like it should have been devoured by its host star, a red giant called Baekdu – but a secret in the star’s past may hold the answer to the planet’s present.Daniel Huber, Astronomer, University of SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1986942023-02-01T12:08:35Z2023-02-01T12:08:35ZSeti: alien hunters get a boost as AI helps identify promising signals from space<figure><img src="https://images.theconversation.com/files/507097/original/file-20230130-12-qfen8v.jpg?ixlib=rb-1.1.0&rect=672%2C272%2C2956%2C1999&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The new study analysed data gathered at the Green Bank Observatory in West Virginia.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/green-bank-west-virginia-october-15-762059119">Shutterstock</a></span></figcaption></figure><p>An international team of researchers looking for signs of intelligent life in space have used artificial intelligence (AI) to reveal eight promising radio signals in data collected at a US observatory.</p>
<p>The results of their research, <a href="https://www.nature.com/articles/s41550-022-01872-z">published in Nature Astronomy</a> are remarkable. The team hasn’t yet carried out an exhaustive analysis, but the paper suggests the signals have many of the characteristics we would expect if they were artificially generated. In other words, they are the kinds of signals we might pick up from an extraterrestrial civilisation broadcasting into space.</p>
<p>A cursory review of the new paper suggest these are indeed promising signals. They’re much more compelling than what is perhaps the most famous Seti candidate, <a href="https://astronomy.com/news/2020/09/the-wow-signal-an-alien-missed-connectio">the “Wow!” signal</a>, radio emission bearing the hallmarks of an extraterrestrial origin that was collected by an Ohio telescope in 1977.</p>
<p>Realistically, it’s most likely that these eight new signals were generated by human technology. But the real story here is the effectiveness of AI and <a href="https://en.wikipedia.org/wiki/Deep_learning">the techniques used by the team to</a> dig out rare and interesting signals previously buried in the noise of human-generated <a href="https://public.nrao.edu/telescopes/radio-frequency-interference/">radio frequency interference,</a> such as mobile phones and GPS.</p>
<p>Astronomers working in the field of <a href="https://www.seti.org/primer-seti-seti-institute">Seti (the search for extraterrestrial intelligence)</a> must filter out interference produced by radio communications here on Earth.</p>
<p>In this case, Peter Ma from the University of Toronto and his colleagues unleashed a set of algorithms on a mountain of data collected by the <a href="https://greenbankobservatory.org">Green Bank Telescope in West Virginia</a>, US. The data was gathered through a Seti initiative called <a href="https://seti.berkeley.edu/listen/">Breakthrough Listen</a>, established in 2015 by the investor Yuri Milner and his wife Julia. </p>
<p>Here are the characteristics astronomers look for in signals that could be artificially-generated: firstly they are <a href="https://en.wikipedia.org/wiki/Narrowband">narrow-band</a>, which means that where the radio transmission is confined to only a few frequency channels. They also disappear as the telescope is moved to another direction in the sky, and they exhibit <a href="https://en.wikipedia.org/wiki/Doppler_effect">“Doppler drifting”</a>, where the frequency of the signal changes in a predictable way with time. We would expect Doppler drifting because both the transmitter — on a distant planet, for example — and the receiver, on Earth, are moving.</p>
<figure class="align-center ">
<img alt="Artist's impression of exoplanets" src="https://images.theconversation.com/files/507060/original/file-20230130-22-kadncw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/507060/original/file-20230130-22-kadncw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=449&fit=crop&dpr=1 600w, https://images.theconversation.com/files/507060/original/file-20230130-22-kadncw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=449&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/507060/original/file-20230130-22-kadncw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=449&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/507060/original/file-20230130-22-kadncw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=565&fit=crop&dpr=1 754w, https://images.theconversation.com/files/507060/original/file-20230130-22-kadncw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=565&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/507060/original/file-20230130-22-kadncw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=565&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Any artificial signals from deep space need to be distinguished from radio interference here on Earth.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/planets-deep-space-cosmos-nebula-stars-2057080619">Shutterstock</a></span>
</figcaption>
</figure>
<h2>Buried in the noise</h2>
<p>The Breakthrough Listen project’s <a href="https://seti.berkeley.edu/blc1/">first candidate signal</a>, called BLC1, was first announced in 2020. But it was <a href="https://www.nature.com/articles/s41550-021-01508-8">later traced</a> to transmissions associated with cheap electronic devices on this planet. The application of AI techniques to the Breakthrough Listen observing programme, however, is a potential game changer for the field. Even seasoned Seti researchers are beginning to think that we might be on the cusp of a momentous scientific breakthrough.</p>
<p>This may explain renewed interest by groups around the world that are planning for Seti success. For example, a <a href="https://seti.wp.st-andrews.ac.uk">Seti post-detection hub</a> has been set up at the University of St Andrews in Scotland. This will study how humans should react if we discover we are not alone in the Universe.</p>
<p>The International Academy of Astronautics (IAA) <a href="https://iaaseti.org/en/">Seti permanent committee</a> oversees the <a href="https://iaaseti.org/en/protocols/">Seti post-detection protocols</a>, which outline what steps scientists should take in the event of detecting a genuine signal. The IAA has opted to update the text of the protocols sometime later this year.</p>
<p>But the new study highlights a problem with previous signals of interest. When the team took another look at the stars associated with the eight narrow-band transmissions, they could no longer detect the signals. </p>
<p>It would not be surprising if many, and perhaps the vast majority of bona-fide Seti signals, were isolated events. After all, what are the chances that we point our telescopes in exactly the right direction, at the right time and with the right frequency on multiple occasions?</p>
<h2>Missing ingredients</h2>
<p>As I <a href="https://theconversation.com/seti-new-signal-excites-alien-hunters-heres-how-we-could-find-out-if-its-real-152498">argued here</a> a few years ago, Seti surveys would greatly benefit from employing multiple radio telescopes, operating in a manner that’s known as a <a href="https://public.nrao.edu/ask/how-does-a-radio-interferometer-work/">classical interferometer network</a>. </p>
<p>These telescope arrays (groups of several antennas observing together) generate huge amounts of data. With AI onboard, the challenge is perhaps more manageable than previously thought. </p>
<p>Breakthrough Listen is already using telescope arrays such as <a href="https://www.sarao.ac.za/science/meerkat/about-meerkat/">MeerKAT in South Africa</a> for Seti searches. In Europe, researchers have been experimenting with <a href="https://www.evlbi.org">arrays that span the globe</a>.</p>
<p>This European approach would help us isolate signals from human-made interference, give us multiple independent detections of individual events, and permit us to localise signals to individual stars and possibly orbiting planets. </p>
<p>Among the future projects is the <a href="https://www.skao.int/en">Square Kilometre Array</a>, an international project to build the two largest telescope arrays in the world, which will be based in Australia and South Africa. Another upcoming project is the <a href="https://ngvla.nrao.edu">next generation VLA (ngVLA)</a>, a series of linked telescope facilities that will be spread across the United States. These radio telescope arrays will be even more sensitive than current instruments.</p>
<p>It’s my belief — and indeed hope — that somewhere out there intelligent beings are waiting to be discovered. The AI revolution might be the missing ingredient that previous endeavours have lacked. In particular, AI algorithms will eventually evolve into powerful tools that no longer suffer from <a href="https://www.nist.gov/news-events/news/2022/03/theres-more-ai-bias-biased-data-nist-report-highlights">human biases</a>. </p>
<p>Lord Martin Rees, chairman of the Breakthrough Listen advisory board and the astronomer royal, has proposed that if we do find aliens they are likely to be <a href="https://theconversation.com/seti-why-extraterrestrial-intelligence-is-more-likely-to-be-artificial-than-biological-169966">intelligent machines</a> operating in the depths of space, unconstrained by the biological limitations placed on humans. </p>
<p>If we ever do find a bona-fide signal, it could just be that it’s mediated by machines on Earth and in space.</p><img src="https://counter.theconversation.com/content/198694/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Michael Garrett is on the advisory board of the Breakthrough Listen initiative and the Seti Institute.</span></em></p>Can artificial intelligence transform the search for alien intelligence?Michael Garrett, Sir Bernard Lovell chair of Astrophysics and Director of Jodrell Bank Centre for Astrophysics, University of ManchesterLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1414642020-06-26T12:35:41Z2020-06-26T12:35:41ZCould we extract energy from a black hole? Our experiment verifies old theory<figure><img src="https://images.theconversation.com/files/343998/original/file-20200625-33550-15c78sw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Rotating black holes suck up anything that gets near enough.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/black-hole-elements-this-image-furnished-277133552">muratart/shutterstock</a></span></figcaption></figure><p>A rotating black hole is such an extreme force of nature that it drags surrounding time and space around with it. So it is only natural to ask whether black holes could be used as some sort of energy source. In 1969, mathematical physicist <a href="https://mathshistory.st-andrews.ac.uk/Biographies/Penrose/">Roger Penrose</a> proposed a method to do just this, now known as the “<a href="http://adsabs.harvard.edu/full/1974ApJ...191..231W">Penrose Process</a>”. </p>
<p>The method could be used by sophisticated civilisations (aliens or future humans) to harvest energy by making “black hole bombs”. Some of the physics required to do so, however, had never been experimentally verified – until now. Our study confirming the underlying physics has just been <a href="https://www.nature.com/articles/s41567-020-0944-3.epdf?sharing_token=7py-x7WbHwlmjJDNQ5ikwNRgN0jAjWel9jnR3ZoTv0NqwCPRuiiyfuz9sGetXz5zuoiDprbaJ7IKKUGtIfSUtrXJIsWiYfqgRALfvXrT5uvtwk0DcE1n6A52KUr1XonWs4U977zYI1mAhXuWl9odRVaGyC1FdfSXyQ7lSf5Asso%3D">published in Nature Physics</a>. </p>
<p>Around its event horizon (the boundary around a black hole beyond which nothing, not even light, can escape), a rotating black hole creates a region called the “<a href="http://hosting.astro.cornell.edu/academics/courses/astro201/ergosphere.htm">ergosphere</a>”. If an object falls into the ergosphere in such a way that it splits – with one part falling in to the black hole and the other escaping – the part that flees effectively gains energy at the expense of the black hole. So by sending objects or light towards a rotating black hole, we could get energy back.</p>
<p>But does this theory hold up? In 1971, the Russian physicist <a href="http://phys-astro.sonoma.edu/brucemedalists/zeldovich/index.html">Yakov Zel’dovich</a> translated it to other rotating systems that could be tested back on Earth. The black hole became a rotating cylinder made from a material that can absorb energy. </p>
<p>Zel’dovich imagined that light waves could extract energy from the cylinder and become amplified. For the amplification effect to work, however, these waves need to have something called <a href="https://www.wired.com/story/what-is-angular-momentum/">“angular momentum”</a>, which twists them into spirals.</p>
<p>When twisted light waves hit such a cylinder, their frequency should change because of something called the “<a href="https://theconversation.com/explainer-the-doppler-effect-7475">Doppler shift</a>”. You have most likely experienced this when listening to an ambulance siren. When it moves towards you it has a higher pitch than when it moves away from you – the direction of travel changes the pitch of the sound. In a similar way, changes in rotational speed alter the perceived frequency of a light wave.</p>
<p>If the cylinder rotates fast enough, the altered wave frequency should drop so low that it will become negative (which simply means that the wave spins in the opposite direction). </p>
<p>Positive frequency waves should be partly absorbed by the cylinder, losing energy. But the negative frequency waves would transform this loss into gain and instead become amplified by the cylinder. They would extract energy from the rotation, just like the object escaping from Penrose’s black hole. </p>
<p>Testing Zeldovich’s theory may appear simple. But the rotating object needs to spin at the same or higher frequency as the waves. To amplify visible light waves, which oscillate at a frequency of hundreds of trillions of times a second, you would need to rotate an absorbing object billions of times faster than anything that’s mechanically possible today. </p>
<h2>Breakthrough at last</h2>
<p>Light travels at about 300 million metres per second. So to make the theory easier to test, we opted to use sound waves, which travel roughly a million times slower, meaning we didn’t need the absorber to rotate so quickly.</p>
<p>To create a twisted sound wave, we used a ring of speakers all emitting the same frequency but starting at slightly different times, so the sound follows a spiral. For our rotating absorber we used a piece of sound-absorbing foam attached to a motor. Microphones placed inside the foam allowed us to record the sound after it had interacted with the rotating absorber. </p>
<p>We found that when the foam span slowly (at a low frequency), the sound we recorded was quieter because it had been absorbed by the foam. But when we spun the foam fast enough for it to Doppler shift the frequency of the sound waves enough to make them negative, the sound became louder.</p>
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<p>This can only mean that the sound wave had taken energy from our rotating absorber, finally proving the 50-year-old theory. </p>
<h2>Black hole bomb</h2>
<p>All this of course does not explicitly verify that Penrose’s idea for energy extraction will actually work for a black hole. Rather, our experiments verify the counter-intuitive underlying physics by showing that shifting wave frequencies from positive to negative results in the waves gaining rather than losing energy.</p>
<p>While we are not anywhere close to extracting energy from a rotating black hole, this doesn’t mean it couldn’t be done by a very advanced alien civilisation – or indeed our own civilisation in the distant future. Such a civilisation could build a structure around the black hole that rotates with it and then drop asteroids or even electromagnetic waves into it what would be reflected with more energy.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/343997/original/file-20200625-33557-10uhpwx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/343997/original/file-20200625-33557-10uhpwx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/343997/original/file-20200625-33557-10uhpwx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/343997/original/file-20200625-33557-10uhpwx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/343997/original/file-20200625-33557-10uhpwx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/343997/original/file-20200625-33557-10uhpwx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/343997/original/file-20200625-33557-10uhpwx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">First picture of a black hole.</span>
<span class="attribution"><span class="source">wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Even better, they could build a so-called black hole bomb by completely surrounding the black hole with a reflecting mirror shell. Light shone into the black hole would return amplified, and then reflected back by the mirror to the black hole to be amplified again, and so on. </p>
<p>The energy would increase exponentially in a back-and-forth runaway explosion. But by letting some of this amplified light out of the shell through a hole, you could control the process and produce essentially limitless energy.</p>
<p>Although this is still science fiction, in a very distant future when the universe has all but died and the only remnants of galaxies and stars are black holes, this method would be the only hope for any civilisation to survive. This would be a universe with immense, isolated sources of energy, shining bright in an otherwise completely black sky.</p><img src="https://counter.theconversation.com/content/141464/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Daniele Faccio receives funding from EPSRC, the Royal Academy of Engineering and EU Horizons 2020</span></em></p><p class="fine-print"><em><span>Marion Cromb receives PhD funding from EPSRC. </span></em></p>Twisted sound beams suggest an advanced civilisation may be able to harness immense amounts of power from a black hole.Daniele Faccio, Professor of Quantum Technologies, University of GlasgowMarion Cromb, PhD Candidate in Physics, University of GlasgowLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1008642018-08-03T14:19:25Z2018-08-03T14:19:25ZWhy starlight turns red escaping from black hole at heart of Milky Way<figure><img src="https://images.theconversation.com/files/230583/original/file-20180803-41369-1i14c89.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption"></span> </figcaption></figure><p>For the past 30 years, scientists at the <a href="http://www.eso.org/public/about-eso/esoglance/">European Southern Observatory</a> have been investigating the motion of stars near the massive black hole at the centre of our galaxy.</p>
<p>In July 2018, they <a href="https://doi.org/10.1051/0004-6361/201833718">revealed</a> that they have directly observed the subtle “<a href="http://astronomy.swin.edu.au/cosmos/G/Gravitational+Redshift">gravitational redshift</a>” effect predicted by <a href="https://www.newscientist.com/round-up/instant-expert-general-relativity/">general relativity</a>, the leading theory of gravity developed by <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1921/einstein-bio.html">Einstein</a>. </p>
<p>This is the first time that astronomers have detected the effect in light from stars near the <a href="https://www.newscientist.com/article/2133830-general-relativity-passes-test-at-milky-ways-central-black-hole/">central black hole in the Milky Way</a>.</p>
<h2>The redshift effect</h2>
<p>Redshift is a term that describes how light appears redder (that is, at a lower frequency) to an observer, compared to the point it was emitted. In general relativity, redshift arises for two main reasons. The first is the classic <a href="http://www.physicsclassroom.com/class/waves/Lesson-3/The-Doppler-Effect">Doppler effect</a>. This explains, for example, why an ambulance siren sounds a higher pitch (frequency) to someone when the ambulance is approaching them and a lower pitch when moving away. However, the siren always sounds the same to someone in the ambulance.</p>
<p>To understand this, consider different pulses of sound from the siren. If the ambulance is moving away from you, the time it takes a pulse to reach you is longer than the preceding pulse, because the ambulance has moved even further away in the time between the pulses. As a result, you hear the pulses less frequently than they’re emitted, making the siren sound at a lower pitch. In a similar way, the ambulance’s lights also appear (very slightly) redder to you than if the ambulance stopped.</p>
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<p>The second cause of redshift is gravity – and this gravitational redshift is the effect detected in the latest results from the international team led by Reinhard Genzel of the <a href="http://www.mpe.mpg.de/2169/en">Max Planck Institute for Extraterrestrial Physics</a> in Germany. Such observations are difficult for many reasons, including the thick layer of dust between us and the galaxy’s centre. With new infrared technologies, scientists have managed to obtain sharp images of a dozen stars at the very centre, after corrections for blurring caused by the atmosphere. The star with the fastest orbital speed has shown the gravitational redshift effect predicted by Albert Einstein.</p>
<p>Gravitational redshift forms the very heart of how general relativity works. To understand it properly, one first has to understand the <a href="http://www.einstein-online.info/spotlights/equivalence_principle.html">equivalence principle</a>. This says that if you’re inside a closed spaceship without windows, you can’t tell whether you’re just sitting on the ground and feeling the Earth’s gravity or if you’re accelerating upwards in deep space, with no gravity. In both cases, if you drop a ball in the spaceship, it will accelerate downwards at 9.8 metres per second over each second that you watch it.</p>
<p>Remarkably, the equivalence principle allows you to convert gravitational problems (like how does light behave near a massive object) to a non-gravitational problem (how does light behave in an accelerating spaceship without gravity). In this case, we can use the principle to calculate gravitational redshift. Consider light moving away from the black hole, say from a distance of 100m to 101m miles. Intuitively, we expect the light to lose some energy climbing away from the black hole. But how much?</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/230585/original/file-20180803-41344-11k465a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/230585/original/file-20180803-41344-11k465a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=366&fit=crop&dpr=1 600w, https://images.theconversation.com/files/230585/original/file-20180803-41344-11k465a.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=366&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/230585/original/file-20180803-41344-11k465a.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=366&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/230585/original/file-20180803-41344-11k465a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=460&fit=crop&dpr=1 754w, https://images.theconversation.com/files/230585/original/file-20180803-41344-11k465a.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=460&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/230585/original/file-20180803-41344-11k465a.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=460&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The motion of the S2 star passing through the extreme gravitational field near the supermassive black hole in the centre of the Milky Way.</span>
<span class="attribution"><a class="source" href="https://www.eso.org/">European Southern Observatory</a></span>
</figcaption>
</figure>
<h2>How starlight responds to a black hole</h2>
<p>Instead of thinking about light moving upwards against gravity, we can think of it as moving upwards in a giant spaceship that accelerates upwards at the same rate as the gravitational pull at that point. Suppose the spaceship is not moving initially, when the light is emitted upwards from the bottom. By the time it reaches the top, the top wall of the spaceship is already moving away. As a result, the light is received at a lower frequency at the top wall than when it was emitted at the bottom wall.</p>
<p>Extending this logic to very large distance leads to Einstein’s predictions for how much light from an object will appear reddened just because it’s close to a massive black hole and we’re not. This gravitational redshift is normally rather subtle and can be easily obscured by a small error in calculations of the Doppler effect – for example, if the exact speed isn’t known. In fact, it’s normally rather difficult to know how much each effect actually contributes as we usually only know the <em>total</em> redshift.</p>
<hr>
<p><em><strong>Read more: <a href="https://theconversation.com/einsteins-theory-of-gravity-tested-by-a-star-speeding-past-a-supermassive-black-hole-100658">Einstein’s theory of gravity tested by a star speeding past a supermassive black hole</a></strong></em> </p>
<hr>
<p>The breakthrough made by Genzel and his team came from imaging the central region of our galaxy at high resolution over 26 years, using telescopes at the European Southern Observatory in Chile. Of particular interest was the <a href="http://www.eso.org/public/news/eso0226/">star S2</a>, which orbits the massive black hole at the galaxy’s centre every 16 years. Its highly elliptical orbit took it within 0.002 light years of the black hole in May 2018. Such a small separation enhances the orbital speed and thus the Doppler effect – but the gravitational redshift is enhanced even more, making it easier to detect.</p>
<p>The images were combined with <a href="https://www.britannica.com/science/spectroscopy">spectroscopic measurements</a>, where light is split into different wavelengths to identify particular features. Their frequencies are compared with laboratory measurements – the difference is called redshift. But is any of it gravitational?</p>
<p>The exquisitely sharp images tell us precisely how the star S2 moves, allowing scientists to calculate the conventional Doppler effect that this causes. The small remaining redshift (presumed gravitational) is only 10% off the amount predicted by Einstein, within the margin of error on these very tricky observations. I like to think he would have been very proud that his theory passed such a precise test in the extreme environment near a massive black hole.</p><img src="https://counter.theconversation.com/content/100864/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Indranil Banik 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>It’s all about the strong gravitational field of the black hole.Indranil Banik, Postdoctoral research fellow, University of St AndrewsLicensed 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>
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<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/515032015-12-02T10:13:17Z2015-12-02T10:13:17ZWhy is the night sky black?<figure><img src="https://images.theconversation.com/files/103686/original/image-20151130-10288-rnactd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Space: it's full of stars ... isn't it?</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/bethscupham/7663247816/in/photolist-cFb9Am-binsBK-47Y5sq-cCg8gy-e1EQPs-oX69UY-ch8rR9-6hJyak-ffkErQ-cTb5j-ftodF-5CXKZs-dALkbK-9DTDX6-5Ub9MW-29hYMw-8dguTW-dALk2z-6jLwnX-9mmZWk-6aXAH3-8t96NE-rBrMEF-4BwBee-aqdh6h-aqdgpC-binyhX-bH2LpH-orSMhR-ch8rrJ-dALjfZ-HugSV-e4SNWk-aqaACe-8fRDRs-92BjFh-9Aus68-jzm1fa-5fsbNS-3Nt1cv-ch8rkL-8d5KvQ-amWseb-85NxvD-c8r8Fm-c9RNrm-afZPuJ-6hvK8R-sfo7nK-9jR7EH">Beth Scupham/flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>It sounds obvious. That’s what night is. The sun has set and when you look up at the sky, it’s black. Except where there’s a star, of course. The stars are bright and shiny.</p>
<p>But wait. Imagine you are deep in a forest. All around you there are trees. Wherever you look, you are looking at a tree. Maybe a big tree close up or a bunch of small trees further away. Surely it should be the same with stars. We’re deep in the universe and whatever direction we look in, there ought to be stars there – billions and billions and billions of them. You would have thought that they’d fill the whole night sky, with the more distant ones fainter but more numerous. </p>
<h2>Olbers’ Paradox</h2>
<p>This is called “<a href="https://en.wikipedia.org/wiki/Olbers%27_paradox">Olbers’ Paradox</a>” after a 19th-century astronomer, although the conundrum was around for a couple of centuries before him. And the answer – at least, now – is fairly clear.</p>
<p>The reason the night sky isn’t just a blaze of light is because the universe isn’t infinite and static. If it were, if the stars went on forever, and if they had been there forever in time, we <em>would</em> see a bright night sky. The fact that we don’t tells us something very fundamental about the universe we live in.</p>
<p>A limit to the universe may seem a natural explanation – if you were in a forest and you could see a gap in the trees, for example, you might surmise that you were near the edge. But it’s dark on all sides of us, which would mean not just that the universe is bounded, but that we’re in the middle of it, which is pretty implausible. </p>
<p>Alternatively, the universe could be limited in time, meaning that light from far-away stars hasn’t had time to reach us yet. </p>
<h2>Blame the Doppler effect</h2>
<p>But actually the explanation is neither of these. Light from the far-away stars gets fainter because the universe is expanding. </p>
<p><a href="https://www.spacetelescope.org/about/history/the_man_behind_the_name/">Edwin Hubble discovered</a> in 1929 that distant galaxies and stars are travelling away from us. He also found that the furthest galaxies are travelling away from us at the fastest rate – which does make sense: over the lifespan of the universe, faster galaxies will have travelled further. </p>
<p>And this affects how we see them. Light from these distant, fast-moving galaxies and stars is shifted to longer wavelengths by the <a href="http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/origins/redshiftrev1.shtml">Doppler effect</a>. In the case of these stars, the effect shifts visible light into invisible (to the human eye) infra-red and radio waves, essentially making them disappear. Indeed, the blackness of the night sky is direct evidence of an expanding universe.</p>
<p>So if you want evidence of the Big Bang, you don’t need the <a href="http://hubblesite.org/gallery/album/">Hubble Telescope</a> or the <a href="http://home.cern/topics/large-hadron-collider">Large Hadron Collider</a>. You just need your own eyes and a clear, dark night.</p><img src="https://counter.theconversation.com/content/51503/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Roger Barlow 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>If there are infinite stars – where is all the light?Roger Barlow, Research Professor and Director of the International Institute for Accelerator Applications, University of HuddersfieldLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/74752012-07-03T03:52:21Z2012-07-03T03:52:21ZExplainer: the Doppler effect<figure><img src="https://images.theconversation.com/files/12489/original/rcwc5yrd-1341200929.jpg?ixlib=rb-1.1.0&rect=0%2C37%2C668%2C438&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Ripples in a pond help to illustrate wave motion and the Doppler effect.</span> <span class="attribution"><span class="source">*˜Dawn˜*</span></span></figcaption></figure><p>When an ambulance passes with its siren blaring, you hear the pitch of the siren change: as it approaches, the siren’s pitch sounds higher than when it is moving away from you. This change is a common physical demonstration of the Doppler effect.</p>
<p>The Doppler effect describes the change in the observed frequency of a wave when there is <a href="http://www.school-for-champions.com/science/motion.htm">relative motion</a> between the wave source and the observer. It was first proposed in 1842 by Austrian mathematician and physicist <a href="http://en.wikipedia.org/wiki/Christian_Doppler">Christian Johann Doppler</a>. While observing distant stars, Doppler described how <a href="http://zebu.uoregon.edu/%7Esoper/Light/doppler.html">the colour of starlight changed</a> with the movement of the star.</p>
<p>To explain why the Doppler effect occurs, we need to start with a few basic features of <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/sound/wavplt.html">wave motion</a>. Waves come in a variety of forms: ripples on the surface of a pond, sounds (as with the siren above), light, and earthquake tremors all exhibit periodic wave motion. </p>
<p>Two of the common characteristics used to describe all types of wave motion are <a href="http://en.wikipedia.org/wiki/Wavelength">wavelength</a> and [frequency](http://encyclopedia2.thefreedictionary.com/Frequency+(wave+motion). If you consider the wave to have peaks and troughs, the wavelength is the distance between consecutive peaks and the frequency is the count of the number of peaks that pass a reference point in a given time period.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/12176/original/rwpnbpxb-1340622468.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/12176/original/rwpnbpxb-1340622468.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=237&fit=crop&dpr=1 600w, https://images.theconversation.com/files/12176/original/rwpnbpxb-1340622468.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=237&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/12176/original/rwpnbpxb-1340622468.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=237&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/12176/original/rwpnbpxb-1340622468.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=298&fit=crop&dpr=1 754w, https://images.theconversation.com/files/12176/original/rwpnbpxb-1340622468.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=298&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/12176/original/rwpnbpxb-1340622468.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=298&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Snapshot of a moving wave showing the wavelength.</span>
<span class="attribution"><span class="source">Gillian Isoardi</span></span>
</figcaption>
</figure>
<p>When we need to think about how waves travel in two- or three-dimensional space we use the term <a href="http://en.wikipedia.org/wiki/Wavefront">wavefront</a> to describe the linking of all the common points of the wave. </p>
<p>So the linking of all of the wave peaks that come from the point where a pebble is dropped in a pond would create a series of circular wavefronts (ripples) when viewed from above.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/12177/original/d6tcwpyr-1340622488.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/12177/original/d6tcwpyr-1340622488.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=281&fit=crop&dpr=1 600w, https://images.theconversation.com/files/12177/original/d6tcwpyr-1340622488.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=281&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/12177/original/d6tcwpyr-1340622488.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=281&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/12177/original/d6tcwpyr-1340622488.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=353&fit=crop&dpr=1 754w, https://images.theconversation.com/files/12177/original/d6tcwpyr-1340622488.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=353&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/12177/original/d6tcwpyr-1340622488.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=353&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Wavefronts emerging from a central source.</span>
<span class="attribution"><span class="source">Gillian Isoardi</span></span>
</figcaption>
</figure>
<p>Consider a stationary source that’s emitting waves in all directions with a constant frequency. The shape of the wavefronts coming from the source is described by a series of concentric, evenly-spaced “shells”. Any person standing still near the source will encounter each wavefront with the same frequency that it was emitted.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/12178/original/dw6f2mwp-1340622509.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/12178/original/dw6f2mwp-1340622509.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=441&fit=crop&dpr=1 600w, https://images.theconversation.com/files/12178/original/dw6f2mwp-1340622509.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=441&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/12178/original/dw6f2mwp-1340622509.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=441&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/12178/original/dw6f2mwp-1340622509.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=554&fit=crop&dpr=1 754w, https://images.theconversation.com/files/12178/original/dw6f2mwp-1340622509.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=554&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/12178/original/dw6f2mwp-1340622509.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=554&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Wavefronts surrounding a stationary source.</span>
<span class="attribution"><span class="source">Gillian Isoardi</span></span>
</figcaption>
</figure>
<p>But if the wave source moves, the pattern of wavefronts will look different. In the time between one wave peak being emitted and the next, the source will have moved so that the shells will no longer be concentric. The wavefronts will bunch up (get closer together) in front of the source as it travels and will be spaced out (further apart) behind it. </p>
<p>Now a person standing still in front of the moving source will observe a higher frequency than before as the source travels towards them. Conversely, someone behind the source will observe a lower frequency of wave peaks as the source travels away from it.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/12550/original/hbsmgt9g-1341284530.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/12550/original/hbsmgt9g-1341284530.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/12550/original/hbsmgt9g-1341284530.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=440&fit=crop&dpr=1 600w, https://images.theconversation.com/files/12550/original/hbsmgt9g-1341284530.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=440&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/12550/original/hbsmgt9g-1341284530.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=440&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/12550/original/hbsmgt9g-1341284530.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=552&fit=crop&dpr=1 754w, https://images.theconversation.com/files/12550/original/hbsmgt9g-1341284530.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=552&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/12550/original/hbsmgt9g-1341284530.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=552&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Wavefronts surrounding a moving source.</span>
<span class="attribution"><span class="source">Gillian Isoardi</span></span>
</figcaption>
</figure>
<p>This shows how the motion of a source affects the frequency experienced by a stationary observer. A similar change in observed frequency occurs if the source is still and the observer is moving towards or away from it. </p>
<p>In fact, any relative motion between the two will cause a Doppler shift/ effect in the frequency observed.</p>
<p>So why do we hear a change in pitch for passing sirens? The pitch we hear depends on the frequency of the sound wave. A high frequency corresponds to a high pitch. So while the siren produces waves of constant frequency, as it approaches us the observed frequency increases and our ear hears a higher pitch. </p>
<p>After it has passed us and is moving away, the observed frequency and pitch drop. The true pitch of the siren is somewhere between the pitch we hear as it approaches us, and the pitch we hear as it speeds away.</p>
<figure><a title="By Polygon data were generated by Life Science Databases(LSDB). (Polygon data are from BodyParts3D.[11]) [CC-BY-SA-2.1-jp (www.creativecommons.org/licenses/by-sa/2.1/jp/deed.en)], via Wikimedia Commons" href="http://upload.wikimedia.org/wikipedia/commons/9/90/Dopplerfrequenz.gif"><img width="440" alt="Frontal lobe animation" src="//upload.wikimedia.org/wikipedia/commons/9/90/Dopplerfrequenz.gif"></a><figure><a href="http://upload.wikimedia.org/wikipedia/commons/9/90/Dopplerfrequenz.gif"><img width="440"></a><figure><figcaption>Wikimedia.</figcaption></figure>
<p>For light waves, the frequency determines the colour we see. The highest frequencies of light are at the blue end of <a href="http://en.wikipedia.org/wiki/Visible_spectrum">the visible spectrum</a>; the lowest frequencies appear at the red end of this spectrum. </p>
<p>If stars and galaxies are travelling away from us, the apparent frequency of the light they emit decreases and their colour will move towards the red end of the spectrum. This is known as <a href="http://www.esa.int/esaSC/SEM8AAR1VED_index_0.html">red-shifting</a>. </p>
<p>A star travelling towards us will appear <a href="http://en.wikipedia.org/wiki/Blueshift">blue-shifted</a> (higher frequency). This phenomenon was what first led Christian Doppler to document his eponymous effect, and ultimately allowed <a href="http://en.wikipedia.org/wiki/Edwin_Hubble">Edwin Hubble</a> in 1929 to propose that <a href="http://en.wikipedia.org/wiki/Hubble's_law">the universe was expanding</a> when he observed that all galaxies appeared to be red-shifted (i.e. moving away from us and each other).</p>
<p>The Doppler effect has many other interesting applications beyond sound effects and astronomy. A <a href="http://en.wikipedia.org/wiki/Doppler_radar">Doppler radar</a> uses reflected microwaves to determine the speed of distant moving objects. It does this by sending out waves with a particular frequency, and then analysing the reflected wave for frequency changes. </p>
<p>It is applied in weather observation to characterise cloud movement and weather patterns, and has other applications in aviation and radiology. It’s even used in police speed detectors, which are essentially small Doppler radar units. </p>
<p>Medical imaging also makes use of the Doppler effect to <a href="http://en.wikipedia.org/wiki/Doppler_echocardiography">monitor blood flow through vessels in the body</a>. Doppler ultrasound uses high frequency sound waves and lets us measure the speed and direction of blood flow to provide information on blood clots, blocked arteries and cardiac function in adults and developing fetuses. </p>
<p>Our understanding of the Doppler effect has allowed us to learn more about the universe we are part of, measure the world around us and look inside our own bodies. Future development of this knowledge – including how to reverse the Doppler effect – could lead to technology once only read about in science-fiction novels, <a href="http://www.tgdaily.com/general-sciences-features/54517-reversing-doppler-effect-holds-promise-for-invisibility-cloak">such as invisibility cloaks.</a></p>
<p><br>
<em>See more <a href="https://theconversation.com/topics/explainer">Explainer articles</a> on The Conversation.</em></p></figure></figure><img src="https://counter.theconversation.com/content/7475/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gillian Isoardi 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>When an ambulance passes with its siren blaring, you hear the pitch of the siren change: as it approaches, the siren’s pitch sounds higher than when it is moving away from you. This change is a common…Gillian Isoardi, Lecturer in Optical Physics Science and Engineering Faculty, Queensland University of TechnologyLicensed as Creative Commons – attribution, no derivatives.