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Gravity is the curvature of spacetime, and its effects travel at lightspeed. However, space is expanding; eventually, light from distant galaxies will become more and more redshifted, and we will no longer be able to see them (source).
As such, there is a limit to how far we could ever possibly see, since light too far will never reach us due to the rapid expansion of space… or at least, if I'm understanding this correctly.
Now, gravitational waves travel at lightspeed. So, after enough time, when an object's light no longer reaches us, will its gravity no longer affect us either?
A better rephrasing is: at a certain point in time, will the gravity of any extremely distant object - even the most massive stars, black holes, or galaxies - simply not affect us whatsoever, in the slightest?
The answer here is very similar to if you were asking about light.
In principle gravitational waves might allow us to fractions of a second after the big bang. Electromagnetic waves can see back to where the cosmic background radiation formed, about 400,000 years after the big bang.
You are right, the universe has expanded. At the present epoch it is estimated that the observable universe, containing objects that have emitted light or GWs that may reach us now, is around 46 billion light years.
However, it seems quite likely that the universe continues well beyond this horizon, and sources beyond this horizon can never have emitted light or GWs that will reach us.
As the relevant section of wikipedia (https://en.m.wikipedia.org/wiki/Observable_universe) points out, the detection of GWs extends our view very slightly. We cannot "see" with electromagnetic waves beyond 45.7 billion light years because of the "fog" of the cosmic microwave background, but GWs can penetrate this fog allowing us (in principle) to see signals from objects currently 46.6 billion light years away.
How a new era of astronomy will ride on gravitational waves
The 2017 Nobel Prize in Physics went to a trio of physicists who detected subtle ripples in space-time, ushering in an entirely new way of observing cosmic events.
Some 1.3 billion years ago, in a galaxy far, far away, two massive black holes collided violently, setting off ripples in the fabric of space-time. These ripples, called gravitational waves, passed through Earth on Sept. 14, 2015. And for the first time ever, humans detected the nearly imperceptible motion of gravitational waves.
Albert Einstein's general theory of relativity had predicted gravitational waves a century earlier, but it wasn't until the Laser Interferometer Gravitational-wave Observatory (LIGO) was built at the turn of the 21st century that there was any chance of finding evidence of them.
LIGO found that evidence in 2015. And again, three months later. And again this past January. And again in August. Each of the four gravitational wave detections added further support for Einstein's theory, ensuring that the LIGO team would go down in history.
Tuesday, that legacy was etched in gold, as the Royal Swedish Academy of Sciences awarded LIGO architects Rainer Weiss, Kip Thorne, and Barry Barish the 2017 Nobel Prize in Physics for their work designing, building, and using LIGO.
But these first four detections and the honor bestowed upon the scientists are just the beginning. LIGO's success signals the dawn of a new kind of astronomy.
As Kamala Harris’ portfolio grows, so does the scrutiny
“This opens up a new window on the universe,” says Saul Teukolsky, a theoretical astrophysicist at Cornell University in Ithaca, N.Y. “And each time a new window has opened up, we’ve made incredible discoveries.”
Until now, astronomers have largely relied on electromagnetic radiation to observe the universe. Objects emit electromagnetic waves across a broad spectrum, some visible to the human eye as light, but all detectable by the telescopes currently in use on Earth or in orbit.
But gravitational waves allow astronomers to look at the universe in an entirely different way: through motion.
“Everything generates gravitational waves. You and I generate gravitational waves by opening our mouths and talking. Every time matter moves around, gravitational waves are generated,” explains Lawrence Krauss, a theoretical physicist and cosmologist at Arizona State University.
Just as electromagnetic radiation can travel in a spectrum of wavelengths, so can these gravitational ripples. Over centuries of telescope construction, astronomers have honed their ability to observe the universe across that electromagnetic spectrum, from radio waves to gamma rays. And now, astronomers aspire to build better and better detectors that can capture the full range of gravitational waves as well.
“This is a totally new type of astronomy,” says Manuela Campanelli, director of the Rochester Institute of Technology's Center for Computational Relativity and Gravitation, in Rochester, N.Y.
And incorporating gravitational wave observations with data from existing techniques could revolutionize astronomy.
Multi-messenger astronomy, as it is known, offers astronomers insights into cosmic events dating back to the beginning of the universe. By combining data from electromagnetic radiation, gravitational waves, neutrinos, and cosmic rays, scientists can assemble detailed pictures of collisions of black holes, neutron stars, and other massive objects.
“There is a possibility of learning a lot of what's going on about and around these sources,” Dr. Campanelli says, “because now you have many independent means to extract information.”
The spectacular fireworks of colliding neutron stars, for example, can currently be observed in electromagnetic wavelengths, so scientists know a bit about what they are made of, says Professor Teukolsky. But how they work, the nuclear physics of neutron stars, has yet to be determined. And gravitational waves might be able to add that key piece of information.
That's because gravitational waves, unlike electromagnetic waves, are not absorbed by other objects as they pass through the universe, Teukolsky explains. So, with gravitational waves, he says, “we're able to see things deep inside these violent explosions that are going on.”
Astronomers hope to answer questions about basic physics, nuclear physics, and continuous phenomena using gravitational waves propagating from collisions of massive objects such as black holes to the subtle ripples of steady motion in the universe. And, perhaps, gravitational waves will carry astronomers answers to questions about the origins of the universe itself.
Some of the discoveries that lie ahead could even be unfathomable for astronomers now.
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“This is just the beginning of a whole new wave of astronomy,” Dr. Krauss says. “I can't think of another time when we've opened up this vast new window and we haven't been surprised. What else creates gravitational waves? What other cataclysmic events are there in the universe that we might observe? Who knows? That is the great thing about discoveries: they're discoveries.”
The first ripples of a discovery
Gravitational waves were first proposed by Henri Poincaré in 1905 as disturbances in the fabric of space-time propagating at the speed of light. Ten years later Einstein’s theory of General Relativity formalised these ideas. The concept of perturbations in space-time, forbidden in the Newtonian interpretation of gravity, were fully permissible in a theory which treated the universe itself not as a stage on which cosmic events unfold but a player in those events.
Gravitational waves arose from the possibility of finding a ‘wave-like’ solution to the general tensor equations at the heart of general relativity. According to Einstein, gravitational waves should be generated en masse by the interaction of massive bodies such as binary systems of super-dense neutron stars and merging black-holes, in fact, they can be generated by any accelerating objects but Earth-bound accelerating objects cause perturbations that are far too small to detect. Hence why our investigations must turn to areas of space where nature provides us with objects that are far more massive.
Einstein predicted that those ripples, created by objects of great mass, gravitational waves, would be so minute they would be impossible to detect by any technological means imaginable at that time.
Fortunately, Einstein was wrong.
How LIGO made waves in astronomy
One of the predictions Einstein made that hadn't yet been observed were the gravitational waves, and this is what the LIGO team found this month. Graihagh Jackson was there for the announcement, and spoke to scientist Professor Norna Robertson about how they did it.
Norna - The event that we say was two black holes which were orbiting each other and they're also moving towards each other because they're losing energy as they orbit each other and so they speed up and go round and round, faster and faster until they finally merge. And it's that final inspiral and merger, which happens in a fraction of a second, that produce a big burst of gravitational waves.
Graihagh - And that's rippled across the universe to us. How long does it take to reach us though?
Norna - The event happened something like one billion years ago, a billion light years away, and it's take all that time to ripple across space towards us and pass through the Earth on September 14th, 2015.
Graihagh - It's quite remarkable really, isn't it?
Norna - It is remarkable. It's wonderful. There have been many, many people involved in developing detectors and developing all the analysis techniques and, for all of us, this is really a momentous occasion.
Graihagh - Alongside Norna, something like a thousand scientists across 16 countries have been working together for 25 years! And, like Norna, Sheila Rowan from Glasgow University has spent her whole career searching for them.
Sheila - I wanted to be a scientist and wanted to be a physicist, I think, since I was about nine years old. When I was young, I couldn't think of anything more exciting to do in life than spend it studying these big questions and the universe. When you go out and you look up, where did it all come from? What's out there? How far does it go? And I've been lucky enough that I've been able to spend my life working in this area and doing that.
Graihagh - Lucky enough to also see all her hard work come into fruition. But how did LIGO detect them?
Sheila - When they're produced, of course, there's a huge amount of energy as two black holes collide but then that's got to spread out and travel across the universe. So, by the time it gets to us here on earth, it's a tiny signal and that means it's hard for us to build instruments that are sensitive enough to do that. And the way we do it is we take light from a laser, we split that laser light into two and we send it out along two four-kilometer long paths. It hit mirrors at the end of those paths, those mirrors send laser light back, the light then adds up again there, and whether it adds up so that you get a bright spot or whether it cancels itself out and you get a dark spot, depends on how far the light has travelled on that four kilometer path. Now what a gravitational wave does is it changes the lengths of the arms, the paths that light has travelled and, fundamentally, it does that by shaking the mirrors that we've put down. The trouble is it doesn't shake them very much - it shakes those mirrors by about 1/10,000 of the size of a proton inside an atom.
Graihagh - So how would you ever measure that?
Sheila - It's a big challenge and that's one of the reasons it's taken decades of work to do this, and there are various things that are key. One incredibly important thing, of course, is to take those mirrors that the gravitational wave's going to shake and make sure that nothing else shakes them. So, we couldn't just sit them on the ground because the ground moves all the time. It shakes due to far away earthquakes, it shakes just due to people driving cars past, so we can't do that. Instead what we do is we take the mirrors and we actually hang them.
Graihagh - Now this isn't how you'd hang a mirror on the wall - no siree. Because a gravitational wave passing through would move a mirror by less than the width of a proton and all this other stuff that Sheila mentioned: seismic activity, cars even, would move the mirrors and could give us a false positive. So how do you make a motionless mirror - I hear you ask? One of the key things is what you hang the mirror with. LIGO have use ultra high tech glass or silica to hang it because silica molecules don't wobble around too much. You can think of this as kind of like the fanciest shock absorbers around. This makes the mirror almost motionless. The final key component is the fact that there are multiple devices that record the movement of the hundreds of components that all connect to the mirror. Knowing how much these various bits of machinery move it means that with great precision they can account for these tiny movements. Now that they've made these motionless mirrors and even detect one gravitational wave, when will they detect the next one?
Sheila - We don't know the answer to that yet. We do have more data, we just haven't had time to look in there yet and see what's in there. So, we don't know, you'll have to wait to hear back from us but we promise we're looking hard.
Graihagh - Watch this space then?
Sheila - Or as my colleague in Glasgow often says "watch this space time".
Will gravitational waves too far away ever reach us? - Astronomy
When you look up at the night sky, you see a very particular view of the Universe. You see electromagnetic radiation, light, at optical wavelengths from objects like stars. If your eyes could see radio waves, which are another wavelength of light, they would see a very different picture of the Universe. The sources of radio light are different than the sources of optical light. Astronomers want to build all different kinds of telescopes to see the entire spectrum of electromagnetic radiation. You can see a view of the Milky Way Galaxy at all different wavelengths of light here (from this page) and you might notice that the view you get is very different depending on what kind of telescope you build.
For almost the entire history of astronomy, we viewed the Universe through an electromagnetic window. For many decades, astronomers have been interested in viewing the Universe through an entirely separate window: a gravitational one. Unlike electromagnetic waves, gravitational waves are very slight changes in spacetime that cause objects to move closer or farther away from one another by miniscule amounts. They are predicted from Einstein's theory of general relativity, and so a detection provides further evidence in support of the theory. The sources of gravitational waves are very exotic, the most notable being two compact objects like neutron stars or black holes in a close orbit. As they orbit around one another, gravitational waves are emitted from the system. Since energy is leaving the system, the orbits shrink, until the two objects eventually merge in a violent event. Observations of gravitational waves will allow us to study the dynamics of these sytems on many different size scales.
On February 11th, 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration announced the detection of gravitational waves from a black hole binary. This is the first concrete detection of a double black hole system. Both black holes were the most massive stellar-mass black holes ever detected (over other candidate objects). They observed the mass of the merged object to be less than that of the sum, implying that the difference in mass was converted to an enormous amount of energy that was lost as gravitational waves in the merger event (as much as 5000 supernovae!). They also measured the spin of the final black hole, the rate of black hole mergers in the local Universe, and more. So much new understanding of physics came from a single gravitational wave event.
Ever since, several gravitational wave detections have been reported, most notably the first event involving the inspiral and merger of two neutron stars in 2017. For the first time, astrophysicists measured a gravitational-wave event that also had an electromagnetic counterpart, which was observed by several telescopes on Earth. Mergers of neutron stars are believed to be among the most energetic events in the universe, releasing energies that could potentially account for unique physical conditions where the heaviest elements --such as gold-- would be produced. The detection of a neutron star binary gave rise rise to an exciting era of multi-messenger astronomy, which will certainly bring much more exciting knowledge to us!
This page was last updated on January 28, 2019.
About the Author
Michael Lam is a Cornell University graduate student and a member of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) Collaboration. He works on improving the timing precision of an array of millisecond pulsars for the goal of detection and study of gravitational waves. He completed his undergraduate degree at Colgate University in Astronomy-Physics and Computer Science and is originally from New York City.
BIG NEWS: For the first time, astronomers detect gravitational waves from two neutron stars crashing together!
Long, long ago, a pair of neutron stars were on the brink of a cataclysm.
Formed from massive stars billions of years earlier, these two supremely dense objects had been dancing around each other since the Universe was young. As they did, they slowly leaked away energy in the form of gravitational waves, ripples in the very fabric of space and time. Slowly, oh so slowly, they edged closer, revolving faster bit by bit as their mutual orbit cinched tighter.
The waves they emitted were a whisper at first, barely affecting space around them. But, over the eons, the whisper grew, until the loss of energy was too much. In the final few milliseconds, whirling around each other at a substantial fraction of the speed of light, the whisper of gravitational waves turned into a shout, then a roar.
At that moment, the mutual and ferocious gravity of the two neutron stars grew overwhelming: They literally ripped each other apart. At the center of the maelstrom the gravity was so intense the material crashed inward, and the gravitational waves emitted reached a fever pitch. The material was under such force that they tore a hole in space and time: A black hole was born . and the shriek of gravitational waves was its birth cry.
As that chaos around the newly formed black hole began to organize itself, a phenomenally complex witch’s brew of forces also focused twin blasts of energy up and down from the chaos, huge pulses of light that screamed away into the blackness of space.
Artwork showing the moment of the neutron star collision, with beams of energy shooting out and gravitational waves shaking ripples in the space-time continuum. Credit: NSF/LIGO/Sonoma State University/A. Simonnet
The ripples in space from the gravitational waves moved outward, followed closely behind by the fierce beams of light. They traveled for 130 million years before reaching Earth. Dimmed by the length of their journey, the waves passed through our planet, stretching it ever so slightly. A mere two seconds later the light from the catastrophe descended on our planet as well.
The flashes of energy from the neutron star merger, sent across the Universe before T. rex strode the Earth, reached the instruments of astronomers on and above the Earth . and signaled not just the birth of a black hole, but also of a new kind of astronomy.
As you might expect, this is very big news. Astronomers have been waiting a very long time to see exactly this sort of event.
A neutron star is the ultra dense remnant of the core of a massive star after the outer layers explode in a supernova. If two massive stars orbit each other, both can become neutron stars. After billions of years they spiral together and merge, creating a catastrophic explosion called a gamma-ray burst. Tremendous beams of energy are blasted away from the poles, focused by the ridiculously powerful magnetic fields of the two stars.
This video shows computer models of the neutron stars merging, and how their magnetic fields coalesce to form those beams.
If you’ve been paying attention to astronomy news, you may know that four gravitational wave events have been positively detected by the Laser Interferometry Gravitational Wave Observatory, or LIGO. These are caused by the mergers of two enormous black holes as they spiral in together, they shake the space-time continuum, creating ripples in it that expand away a bit like waves on a pond when you toss a heavy rock in. These literally stretch and compress space, but by the time these waves reach us, they are extremely low amplitude and take incredibly precise measurements to see. LIGO is designed to detect them.
The first such event was detected in 2015. Three more have been seen, the last one in August 2017 when LIGO was joined by another facility, Virgo, in Europe. Together, their sensitivity was increased, allowing them to detect even fainter gravitational waves.
The hope for quite some time has been that these enhanced observatories would be able to detect neutron star mergers as well, which are lower-energy than black hole collisions, and therefore more difficult to detect.
That hope became reality at 12:41 UTC on August 17, 2017 (just three days after the last black hole detection). A faint signal was received at the detectors at that moment, matching the profile of a neutron star/neutron star collision. The astronomers dubbed it GW170817: a gravitational wave event seen on 2017 August 17. It was the first time the gravitational waves from such an event had ever been detected!
But what makes this even more exciting is that the Fermi gamma-ray telescope orbiting high above the Earth also detected a weak flash of gamma rays just two seconds later. Fermi has instruments on it designed to look for gamma-ray bursts (GRBs), the soul-crushingly powerful explosions emitted when black holes are born. Gamma rays are the highest-energy form of light, sent out in a fierce blast at the formation moment of a new black hole, and we’ve been detecting them from exploding stars and merging neutrons stars for decades now. The burst just seen, GRB 120817A, is one of hundreds already seen, and in fact a rather weak one at that.
But the faintness of the burst belies the extraordinary nature of the event: This is the very first time a GRB has been detected along with the gravitational waves from the black hole formation!
This is incredibly important. The direction to the source of gravitational waves is extremely hard to pin down using LIGO/Virgo, but Fermi’s detection of the gamma rays narrowed the location in the sky with far higher precision.
The Very Large Telescope was used to spot GW170817 in NGC 4993 it’s the dot just above and to the left of the galaxy’s core. Credit: ESO/A.J. Levan, N.R. Tanvir
And that’s where this gets better still: Astronomers around the world were immediately notified, and within hours they were scrambling to search the targeted spot in the sky. Images taken using the Henrietta Swopes 1-meter telescope at the Las Campanas Observatory in Chile were compared to ones taken earlier of the same region, and after looking at only nine images they hit paydirt: A new point of light located very near the center of the galaxy NGC 4993, an old but luminous galaxy 130 million light-years away.
That unassuming dot had vast import. For the first time, astronomers had found the visible afterglow of a neutron star merger that had also been detected by gravitational waves.
As an astronomer I can tell you that the nature of this is nothing short of a breakthrough. With the afterglow detected, a huge amount of data becomes available. The distance to the galaxy means we know how much energy was emitted. The rate at which it faded proved it not just a typical supernova, an exploding star those fade over weeks and months, where this one dropped like a rock in mere days.
The Dark Energy Camera captured GW170817 mere hours after the event, then again two weeks later, by which time it had faded to invisibility. Credit: M. Soares-Santos, D. E. Holz, J. Annis
The material blasted outward from the explosion could be examined. It consists of two components, one thinly spread and moving very rapidly — fully a third the speed of light! — and a thicker, slower one, moving outward at a more leisurely, but still staggering, 1/10 th light speed.
As more telescopes observed the event, more was learned. Radio waves and X-rays emitted showed that the beams of matter and energy emitted by the explosion were aimed slightly away from us, probably by about 30°. As that material slammed into matter inside the host galaxy it slowed and puffed outward the beams widened, spreading their aim, and we on Earth so very far away caught the edges of them. That’s why the gamma-ray burst was faint, even though this object was very close by in a cosmic sense relative to most GRBs we only saw the edge of the blast. The energy received was only about a thousandth as bright as other GRBs like it elsewhere had the beams been aimed directly at us this would have been a phenomenally bright event (to be clear, I mean bright to an astronomer it still would’ve taken a telescope to detect it).
Artwork depicting the moment of collision between two neutron stars. The resulting explosion is… quite large. Credit: Dana Berry, SkyWorks Digital, Inc.
And this gets better yet. Astronomers took spectra of the afterglow, breaking the light up into individual colors. This allows a lot of information to be gleaned about the source. What they found is that the way the material glowed and faded was consistent with the creation of what we call r-process elements: The explosion was so powerful that lighter elements were able to rapidly capture neutrons ricocheting around inside the blast wave, performing stellar alchemy and changing their atomic structure into heavier elements. Which elements? Ones like gold and platinum.
Are you wearing gold jewelry? Probably some of the parts of the computer on which you’re reading this article use gold and/or platinum to operate. Until now, we weren’t exactly sure how those elements were created in the Universe it was thought they were formed in normal supernova explosions, but the physics was saying they must be created somewhere else. Now we know where.
Do you see? These elements were created in the heart of the catastrophic merger of two massive neutron stars billions of years ago somewhere in our own galaxy, a tremendous but brief explosion that left behind a black hole and scattered those precious elements into space. They seeded a cloud of gas and dust, which itself collapsed to form the Sun, the planets . and Earth. 4.56 billion years later, we mined those materials from our planet, admired them, used them to adorn ourselves, and created machines that allowed us to understand how those elements formed in the first place.
The Universe has created the conditions in which it can study itself. That’s what this newest burst means.
LIGO Sees First Ever Gravitational Waves as Two Black Holes Eat Each Other
Better start shining up some new Nobel Prize medals: Scientists have reported that, for the very first time in history, they have detected gravitational waves.
And oh my yes, this is a very big deal. It will open up an entirely new field of astronomy, a new way to observe the Universe. Seriously.
Gravitational waves (not to be confused with gravity waves, which are a totally different thing) are ripples in the fabric of spacetime, caused when a massive object is accelerated. By the time they get here from distant astronomical objects, the waves have incredibly low energy and are phenomenally difficult to detect, which is why it’s taken a century to discover them since they were first predicted by Einstein’s Theory of General Relativity. Essentially every other prediction of GR has been found to be correct, but the existence of gravitational waves has been maddeningly difficult to prove directly.
Until now. And what caused the gravitational waves they detected at the Laser Interferometer Gravitational-Wave Observatory is as amazing and mind-blowing as the waves themselves: They caught the death spiral and aftermath of two huge black holes 1.3 billion light-years from Earth, merging together in a titanic and catastrophically violent event.
Mind you, we’ve had some good evidence such binary black holes existed before this, but this new result pretty much proves they exist and that, over time, they eventually collide and merge. That’s huge.
The black holes had masses of 36 and 29 times the mass of the Sun before they merged. After they merged they created a single black hole with a mass of 62 times that of the Sun. You may notice those masses don’t add up right there’s 3 solar masses missing. That mass didn’t just disappear! It was converted into energy: the energy of the gravitational waves themselves. And the amount of energy is staggering: This single event released as much energy as the Sun does in 15 trillion years.
I know. There is nothing about this story that isn’t incredibly cool.
The actual data received by the two LIGO facilities. The wiggles in the plot are due to the physical warping of space as gravitational waves emitted by the merging black holes passed through the Earth. Credit: Abbot et al. 2016
So, to understand all this better you’ll need a wee bit of background. This is all very mind-bendy stuff, but I promise it’s worth it.
What Is a Gravitational Wave, Anyway?
One of the outcomes of Einstein’s General Relativity theory is that space and time are two facets of the same thing, which we call spacetime. There are lots of analogies for it, but you can think of it as the fabric of space, a four-dimensional tapestry (three of space and one of time) in which we are all embedded. Remember, it’s not literally like this we’re using an analogy. But it’ll help you picture it.
We think of gravity as a force, pulling us toward an object. But Einstein revisualized it, seeing it as an outcome of the warping of spacetime. A massive object distorts the shape of space, and another object moving through that warped space gets accelerated. We see that as gravity. In other words, matter tells space how to bend, and space tells matter how to move.
Objects with mass warp space, which we feel as gravity. Credit: ESA/C.Carreau
Another outcome of the mathematics of GR is that if a massive object is accelerated, it will cause ripples, waves, to move away from itself as it moves. These are actually ripples in the fabric of spacetime itself! Spacetime expands and contracts in complicated ways as a wave passes, a bit like how ripples will move out from a rock dropped into a pond, distorting the surface of the water.
There are lots of ways to generate gravitational waves. The more massive and dense an object is, and the harder it accelerates, the sharper and more energetic the waves are. The Earth moves around the Sun once per year, accelerated by the Sun’s gravity. But the motion is too slow and the Earth’s mass too low to ever hope to detect the mushy waves emitted.
But if you have two much more massive objects—like, say, neutron stars, the über-dense cores of stars that have previously exploded—they do generate waves that we can see.
In fact, we have! Kinda. In 1974, a binary neutron star system was discovered by astronomers Joseph Taylor and Russell Hulse. These two massive objects orbited each other very rapidly, once every eight hours or so. As they do, they emit a tiny bit of energy in the form of gravitational waves. That energy comes from the orbital energy of the stars themselves, so as they emit gravitational waves, they lose orbital energy. The orbit shrinks, and the time it takes the two stars to revolve around each other drops. Over time, that “orbital decay” can be very precisely measured … and it was seen! Not only that, it matched the prediction of GR perfectly.
The measured orbital decay of the two neutron stars (red crosses) matches the mathematcial prediction (smooth line) extremely well. Credit: Inductiveload/Wikimedia
Taylor and Hulse won the Nobel Prize for this. And they only detected gravitational waves indirectly. They saw how the loss of energy by emitting the waves affected the stars’ orbits. But they didn’t detect the waves themselves.
So How Did LIGO Do It?
Gravitational waves come in many shapes and forms, but what they all do is infinitesimally distort the shape of space. But how do you measure that? It’s not like you can hold a ruler up between two objects and measure how their distance apart changes when a wave passes through …
… right? Oh, wait. It turns out you can.
Enter LIGO: The Laser Interferometer Gravitational-Wave Observatory. LIGO is actually two facilities, one located in Washington state and the other in Louisiana (jointly operated by Caltech and MIT). Neither is what you might think of as an astronomical observatory: They each consist of very long pipes arranged in an L-shape. At the far end of each 4-kilometer-long pipe is a mirror.
One of the LIGO facilities seen from the air. Credit: LIGO
A very powerful laser sits near the vertex of the L, where the pipes meet. It sends out a pulse of light into a special mirror that splits the beam, sending half of it down one pipe, and the other half down the other pipe. Each mirror reflects is beam back down the pipe, and then they’re recombined inside a detector.
Here’s a video (credit: NSF) describing how this works:
Let me add what’ll seem like a bit of a non sequitur to help make this clear: Have you ever sat in a tub of water and sloshed your body back and forth? If you time it just right, you can amplify the wave of water coming back at you, making it splash higher. You can also time it just right so that you move in a way to negate the wave coming at you, too.
The motion of your body sets up the first wave. When you move again, you make a second wave. It the crest of the first wave hits the crest of the second wave, they amplify each other. If the trough of the second wave hits the crest of the first one, they negate each other.
This is called interference. Where the waves amplify it’s constructive interference, and where they negate each other its destructive interference.
Light is a wave. If the laser and the two mirrors in LIGO are set up just right, then the two beams will interfere with each other when they reach the detector. Interference patterns, called fringes, can be seen when you do that, and the exact pattern seen depends, in part on the exact distance between the mirrors. If one mirror moves a tiny bit relative to the other, then the fringe pattern changes.
See where this is going? If a gravitational wave passes through LIGO, one mirror will move a teeny tiny amount relative to the other, and that will create a change in the fringe pattern. Fringes are sensitive to extremely small changes in mirror position, so this is a great way to look for gravitational waves.
How sensitive? A typical gravitational wave will move the mirrors by about 0.0001 times the size of an atomic nucleus! So yeah, they’re sensitive.
LIGO has two such setups located thousands of kilometers apart to help distinguish real astronomical sources from things like earthquakes, trucks driving by, and so on. LIGO first went into operation in 2002. Over nearly a decade it looked but found no gravitational waves. In 2010 it shut down for a significant upgrade, making it far more sensitive. This new configuration started observing in September 2015.
Apparently, all this time they were right on the threshold of detection. Once the more sensitive rig was employed, it didn’t take long before they hit paydirt: This signal was detected on Sep. 14!
What Did They See?
Now we’re ready to put all this together.
Imagine two black holes in a very tight orbit around each other. Both are massive, and whipping around each other at a large fraction of the speed of light. They’ll be pouring out gravitational waves, ripples in spacetime expanding away at the speed of light. It’s possible LIGO could detect something like that, but there’s more to this.
As the black holes whirl madly and emit gravitational waves, they lose orbital energy. Like the neutron stars that got Taylor and Hulse their Nobel, the orbit of the two black holes shrinks. They revolve around each other ever faster.
This change in their orbital rate affects the waves they emit. The frequency of the waves (how many are emitted per second) depends on how rapidly the two objects orbit each other. As the orbit of the black holes shrinks, they revolve around each other faster, and the frequency of the gravitational waves goes up. But, since the black holes are moving more rapidly, they emit even more waves, so they lose energy faster, so they emit even more waves.
This is a runaway effect. The black holes get closer and closer together, whirl around each other faster, emit more and stronger gravitational waves with a higher frequency … until the black holes eat each other! They merge, becoming one (slightly larger) black hole.
What LIGO sees when this happens is the signature of the gravitational waves, with the frequency going up all the time. Sound is also a wave, and the frequency of sound waves is what we interpret as its pitch. A higher frequency sound has a higher pitch it’s a higher note, if you prefer.
As the black holes get close to merging, their frequency rockets up. In the sound analogy, it’s like they’re singing a note, and as they get closer the note gets stronger and stronger and higher and higher. At the end, the increase in pitch is so rapid it goes way up extremely quickly: This is a chirp.
Literally, a chirp is a sound where the frequency increases rapidly (listen to one here). So the signature of two black holes (or neutrons stars, or even white dwarfs) inspiraling and merging is a chirp in the gravitational waves. If you catch that, you’ve witnessed the black holes at The Moment Of Truth, when two become one.
And one last bit that boosts confidence: The signal from the merging black holes was detected in the Washington state detector first, then in the Louisiana detector 7 milliseconds later. That delay was due to the waves moving at the speed of light across space!
This merger is simply astonishing. It’s one of the most catastrophic events in the Universe, and until just last year we were essentially blind to it.
With this detection by LIGO, a new era in astronomy begins. In many cases, the gravitational waves are emitted from objects we can’t see directly, like black holes merging, or binary neutron stars. Sometimes, though, these objects do emit visible light. A supernova—an exploding star—can emit gravitational waves. Even more dramatically, when two neutron stars merge and form a black hole, they release not just gravitational waves, but also a huge flash of energy in the form of gamma rays and even visible light. These gamma-ray bursts occur in the Universe every day, and we see them all the time. If we can also detect the emitted gravitational waves from them, it will help astronomers understand these bizarre and incredibly violent phenomena.
Even better, we’re not starting fresh. Last year, the European Space Agency launched LISA Pathfinder into space. LISA stands for Laser Interferometer Space Antenna, and is basically a super-LIGO in space. LISA Pathfinder is a benchmark mission to test the very sophisticated technology involved. If it works, then a full-up LISA may be launched in the coming years, which will consist of three separate detectors separated in space by millions of kilometers. Its sensitivity will be far, far higher than LIGO’s, and will rip the field of gravitational wave astronomy wide open.
Whenever we find a new window into the Universe—radio waves, gamma rays, even the invention of the telescope itself—immense wonders have been our reward. In the vast majority of cases we had no clue what was waiting for us once we peered outwards in a new way. Stars numbered beyond imagining, galaxies packed together clear across the cosmos, planets, nebulae, and even an eventual understanding of how the Universe came to be, how it changes, and how it will evolve in the future.
The treasures, the beauty, the knowledge, have fundamentally changed how we humans see ourselves and our place in the Universe. And here we stand, our hand on another window, ready to throw it open.
LIGO: Laser Interferometer Gravitational-Wave Observatory
The first successful identification gravitational waves occurred in 2015 with LIGO, an observatory consisting of two separate sets of equipment located far apart in the USA. Despite the idea being conceived in the 1980s and observations starting from 2002, the results were obtained only in 2015, when the technology was sufficiently advanced.
How does LIGO work?
LIGO is in principle a laser interferometer, as described in the previous section. However, given the weakness of gravitational waves, the set-up had to be huge enough and sensitive enough to detect the most minute results.
For this, each individual interferometer consisted of a 4 km long L-shaped tunnel, in which the laser beams travelled distances over a 1000 km by multiple reflections. Then, the sensors are sensitive enough to detect a change less than ten thousand times smaller than a proton’s width. For comparison, if the whole distance to the nearest star to the Solar system was used as a reference, the change was less than a hair’s thickness.
Two observatories were used, far apart, so that meaningless noise could be filtered out and only the actual signal would be recorded.
How LIGO works. The interferometer senses minuscule distortions in space caused by gravitational waves. (Source)
After over a decade of null results, LIGO obtained its first success in 2015. It observed the fusion of a pair of black holes, which first orbited each other before merging.
While the black holes themselves merged 1.3 billion years ago, it took so long for the waves from them to reach us and be detected. The gravitational wave carried away the mass lost by the black holes in the form of energy.
This was followed by further detection of black hole mergers in 2016 and 2017. In an even more startling step forwards, LIGO observed the merger of two neutron stars, which are visible, unlike black holes, in 2017.
Black hole merger, as visualized by an artist. Source.
The scientific community gave LIGO its due recognition. It, along with contributors, received the Special Breakthrough Prize Award in Fundamental Physics in 2016. The scientists who were part of its journey, Rainer Weiss, Kip Thorne and Barry Barish, received the Nobel Prize in Physics in 2017.
Astronomers Surprised by Lingering X-rays Years After Landmark Neutron Star Collision
It’s been three years since the landmark detection of a neutron star merger from gravitational waves. And since that day, an international team of researchers led by University of Maryland astronomer Eleonora Troja has been continuously monitoring the subsequent radiation emissions to provide the most complete picture of such an event.
Their analysis provides possible explanations for X-rays that continued to radiate from the collision long after models predicted they would stop. The study also reveals that current models of neutron stars and compact body collisions are missing important information. The research was published on October 12, 2020, in the journal Monthly Notices of the Royal Astronomical Society.
Researchers have continuously monitored the radiation emanating from the first (and so far only) cosmic event detected in both gravitational waves and the entire spectrum of light. The neutron star collision detected on August 17, 2017, is seen in this image emanating from galaxy NGC 4993. New analysis provides possible explanations for X-rays that continued to radiate from the collision long after other radiation had faded and way past model predictions. Credit: E. Troja
“We are entering a new phase in our understanding of neutron stars,” said Troja, an associate research scientist in UMD’s Department of Astronomy and lead author of the paper. “We really don’t know what to expect from this point forward, because all our models were predicting no X-rays and we were surprised to see them 1,000 days after the collision event was detected. It may take years to find out the answer to what is going on, but our research opens the door to many possibilities.
The neutron star merger that Troja’s team studied — GW170817 — was first identified from gravitational waves detected by the Laser Interferometer Gravitational-wave Observatory and its counterpart Virgo on August 17, 2017. Within hours, telescopes around the world began observing electromagnetic radiation, including gamma rays and light emitted from the explosion. It was the first and only time astronomers were able to observe the radiation associated with gravity waves, although they long knew such radiation occurs. All other gravity waves observed to date have originated from events too weak and too far away for the radiation to be detected from Earth.
Seconds after GW170817 was detected, scientists recorded the initial jet of energy, known as a gamma ray burst, then the slower kilonova, a cloud of gas which burst forth behind the initial jet. Light from the kilonova lasted about three weeks and then faded. Meanwhile, nine days after the gravity wave was first detected, the telescopes observed something they’d not seen before: X-rays. Scientific models based on known astrophysics predicted that as the initial jet from a neutron star collision moves through interstellar space, it creates its own shockwave, which emits X-rays, radio waves and light. This is known as the afterglow. But such an afterglow had never been observed before. In this case, the afterglow peaked around 160 days after the gravity waves were detected and then rapidly faded away. But the X-rays remained. They were last observed by the Chandra X-ray Observatory two and a half years after GW170817 was first detected.
The new research paper suggests a few possible explanations for the long-lived X-ray emissions. One possibility is that these X-rays represent a completely new feature of a collision’s afterglow, and the dynamics of a gamma ray burst are somehow different than expected.
“Having a collision so close to us that it’s visible opens a window into the whole process that we rarely have access to,” said Troja, who is also a research scientist at NASA’s Goddard Space Flight Center. “It may be there are physical processes we have not included in our models because they’re not relevant in the earlier stages that we are more familiar with, when the jets form.”
Another possibility is that the kilonova and the expanding gas cloud behind the initial jet of radiation may have created their own shock wave that took longer to reach Earth.
“We saw the kilonova, so we know this gas cloud is there, and the X-rays from its shock wave may just be reaching us,” said Geoffrey Ryan, a postdoctoral associate in the UMD Department of Astronomy and a co-author of the study. “But we need more data to understand if that’s what we’re seeing. If it is, it may give us a new tool, a signature of these events that we haven’t recognized before. That may help us find neutron star collisions in previous records of X-ray radiation.”
A third possibility is that something may have been left behind after the collision, perhaps the remnant of an X-ray emitting neutron star.
Much more analysis is needed before researchers can confirm exactly where the lingering X-rays came from. Some answers may come in December 2020, when the telescopes will once again be aimed at the source of GW170817. (The last observation was in February 2020.)
“This may be the last breath of a historical source or the beginning of a new story, in which the signal brightens up again in the future and may remain visible for decades or even centuries,” Troja said. “Whatever happens, this event is changing what we know about neutron star mergers and rewriting our models.”
Reference: “A thousand days after the merger: continued X-ray emission from GW170817” by E. Troja, H. van Eerten, B. Zhang, G. Ryan, L. Piro, R. Ricci, B. O’Connor, M. H. Wieringa, S. B. Cenko and T. Sakamoto, 12 October 12 2020, Monthly Notices of the Royal Astronomical Society.
Additional authors of the paper from the UMD Department of Astronomy are Faculty Assistant Brendan O’Connor and Adjunct Associate Professor Stephen Cenko.
This work was partially supported by NASA (Chandra Award Nos. G0920071A, NNX16AB66G, NNX17AB18G, and 80NSSC20K0389.), the Joint Space-Science Institute Prize Postdoctoral Fellowship, and the European Union Horizon 2020 Programme (Award No. 871158). The content of this article does not necessarily reflect the views of these organizations.
We knew that smaller black holes could merge and rebound like this, but this is the first time the aftermath has been observed with supermassive ones.
“The amount of energy that you need to kick a supermassive black hole out like this is equivalent to 100 million supernovae exploding simultaneously,” says Chiaberge. “Nothing else can really do that.”
If the black hole really is being propelled by gravitational waves, they got lucky in spotting it. “This is an extreme kick – right on the edge of what we’d expect – so it would be a very unusual system,” says Daniel Holz at the University of Chicago.
Such an unusual system might help provide evidence that supermassive black holes do merge in our universe, a phenomenon for which we only have circumstantial evidence so far. “It’s a big question: do two supermassive black holes actually merge, or do they stall and basically orbit each other for the age of the universe?” says Chiaberge. “Seeing this proves indirectly that they can merge.”
It is still possible, though, that the black hole wasn’t actually kicked out at all and is just located behind the galaxy to which it seems to belong. “It could be incredibly extreme physics or it could be pedestrian astronomy,” says Holz. “Time will tell.”
Journal reference: Astronomy & Astophysics, DOI: 10.1051/0004-6361/201629522