Photo illustration by Olena Shmahalo/Quanta Magazine; Kip Thorne via A.T. Service, Roger Penrose via Festival della Scienza

Gravitational-wave theorists (left to right) Robert Oppenheimer, Roger Penrose, Albert Einstein, Karl Schwarzschild, Arthur Eddington, Kip Thorne and Richard Feynman, whose work helped pave the way for LIGO’s big announcement last week.

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“There are no gravitational waves … ” … “Plane gravitational waves, traveling along the positive X-axis, can therefore be found … ” … “ … gravitational waves do not exist … ” … “Do gravitational waves exist?” … “It turns out that rigorous solutions exist … ”

These are the words of Albert Einstein. For 20 years he equivocated about gravitational waves, unsure whether these undulations in the fabric of space and time were predicted or ruled out by his revolutionary 1915 theory of general relativity. For all the theory’s conceptual elegance — it revealed gravity to be the effect of curves in “space-time” — its mathematics was enormously complex.

The question was settled once and for all last week, when scientists at the Advanced Laser Interferometer Gravitational-Wave Observatory (Advanced LIGO) reported that they had detected gravitational waves emanating from the violent merger of two black holes more than one billion light-years away. Picking up the signal — a tiny flurry of contractions and expansions in space-time called a “chirp” — required extraordinary technical finesse. But it also took 100 years for scientists to determine what, exactly, Einstein’s theory predicts: not only that gravitational waves exist, but how they look after crossing the cosmos from a coalescing pair of black holes — inescapably steep sinkholes in space-time whose existence Einstein found even harder to swallow.

Daniel Kennefick, a theoretical physicist at the University of Arkansas, began his career as a graduate student working with LIGO co-founder Kip Thorne to unravel the predictions of general relativity. Fascinated by the contentious history of gravitational-wave research, Kennefick began a sideline as a historian; he is the author of the 2007 book Traveling at the Speed of Thought: Einstein and the Quest for Gravitational Waves, and last year he co-authored An Einstein Encyclopedia. In discussions before and after Thursday’s big announcement, Kennefick recounted the journey leading up to it and explained where theorists must go from here. An edited and condensed version of the conversation follows.

QUANTA MAGAZINE: How exciting was last Thursday’s announcement for you?

Courtesy of Daniel Kennefick

Daniel Kennefick, a theoretical physicist and Einstein scholar at the University of Arkansas.

DANIEL KENNEFICK: I couldn’t believe how exciting it was. It’s great, given the very controversial history of the field, that it’s such an incontrovertible detection. They didn’t have to dig the signal out of the noise as many of us expected they would; you could really see it in the data with your own eyes. And from a theorist’s point of view, one is thrilled that the theoretical predictions were so close to reality. There was the signal, and there was their prediction of what the waveform from the merger of two black holes would look like overlying it.

How would you characterize the history of gravitational-wave research that led up to this moment?

There’s no doubt that a big characteristic has been controversy — a series of controversies. Controversy over whether gravitational waves exist. Do they really exist? Do they carry energy? Do they exist in a way that we can hope to detect? Even just ontologically: What is reality? Are you measuring something here or are you kidding yourselves?

And that’s been true from the very beginning. The first mention of gravitational waves that we have from Einstein is of him saying they don’t exist. Gravitational waves were a very bold, daring idea that started to enter people’s heads 100 years ago, and yet there’s always been that sense of uncertainty. One question will be answered but a new question will come up.

How does the phrase in your book title — “traveling at the speed of thought” — capture this uncertainty?

When Einstein wrote his paper [predicting gravitational waves] in 1916, he thought he had discovered three different kinds of gravitational waves. Earlier that year, when he thought the waves didn’t exist, he had been using the wrong coordinate system. He changed to a different coordinate system at the suggestion of a colleague, and that allowed him to see more clearly that there were waves. But this coordinate system is itself kind of wavy, and so it turned out that two of the waves he thought he was looking at were really just flat space seen in a wavy coordinate system; they’re not real waves at all.

[The English astronomer and physicist] Arthur Stanley Eddington responded to Einstein’s paper in 1922, and he was interested in the question: Do gravitational waves travel at the speed of light? The answer is that they do, as we now know for sure. Eddington did his calculation to show that, and he realized that the two other types of waves, the spurious ones, could travel at any speed depending on what coordinate system you use, and so he said these fake waves “travel at the speed of thought.” It’s a charming phrase because on the one hand it shows the skepticism — “traveling at the speed of thought” as something that’s not real. And on the other hand it shows the importance of skepticism, because after all, there aren’t three types of gravitational waves; there’s only one kind.

And then Einstein changed his mind again in 1936 and said gravitational waves don’t exist. What happened?

Einstein and his assistant Nathan Rosen set out to find an exact [rather than approximate] gravitational-wave solution, and they discovered a problem. No matter how they tried to set up their coordinate system, they always found a “singularity” somewhere in space-time. A singularity means a place where we can’t assign a number to how big the wave is there. Now the truth is, this singularity was only a coordinate singularity; it’s not a real problem with gravitational waves.

Courtesy of the Archives, California Institute of Technology

Einstein on the beach in Santa Barbara, Calif. (undated).

Think about the North Pole. If I ask you what is the longitude of the North Pole, you’ll say, “Well, all lines of longitude run through the North Pole.” Our system of measurement breaks down there, but that doesn’t mean the North Pole doesn’t exist or you can’t go there. Physically, it exists. So Einstein and Rosen were confused. They thought that since there was a singularity there, this provided a proof that gravitational waves couldn’t exist. So they wrote this paper and they sent it off to the Physical Review. And the referee wrote a 10-page report pointing out the possibility of a mistake, and that was sent back to Einstein. He reacted very angrily and just withdrew the paper.

And some people started arguing that even if gravitational waves did exist, it wouldn’t be possible to feel them.

In 1955, Nathan Rosen tried to argue that gravitational waves don’t carry any energy, so they’re just a formal mathematical construct with no real physical meaning. A good way to think about that is, if I’m out in the ocean and there’s an enormous ocean swell, I might not even be aware that it’s there, because I’ll rise up with the wave and then sink back down with it, and so will everything around me. If gravitational waves are like that deep ocean swell, do they really interact with us or do we all just move together up and down in the swell? That was a big debate in the ’50s.

How did that question get resolved?

Rosen’s argument was brought up at a conference in 1957 in Chapel Hill, N.C., and very fortunately a man named Felix Pirani, who sadly just passed away, came to the conference. He had decided to look at how general relativity works, using a very practical approach that got around this whole problem of the coordinate system, and he showed that the waves would move particles back and forth as they pass by.

Richard Feynman heard Pirani’s talk and said, in essence, “Well, since we know that the particles move, all we have to do is imagine a stick, and on the stick we can put some beads. As the wave passes by, the beads will move back and forth, but the stick will stay rigid because the electromagnetic forces in the stick will try to keep the atoms and electrons in the same positions as they were previously. So the beads will drag against the stick, and the friction will produce energy. And the energy must have come from the gravitational wave. So I conclude that the wave has energy.” So this famous “sticky bead” thought experiment convinced a lot of people that there wasn’t any reason for the skepticism that Rosen had advanced. And then people like Joe Weber started trying to detect gravitational waves shortly after.

But people still didn’t know whether there would be any astrophysical sources of gravitational waves strong enough to detect, right?

LIGO

Einstein showed in 1918 that dumbbell-like systems that rotate about two axes at once, such as binary stars, radiate gravitational waves.

Right. Einstein wrote that it was unlikely that anyone would ever find a system whose behavior would be measurably influenced by gravitational waves. He was pointing out that the waves from a typical binary star system would carry away so little energy, we would never even notice that the system had changed — and that is true. The reason we can see it from the two black holes is that they are closer together than two stars could ever be. The black holes are so tiny and yet so massive that they can be close enough together to move around each other very, very rapidly. Since Einstein didn’t believe in the existence of black holes, he just couldn’t conceive of a system that could behave in such a way that you would be able to see the gravitational waves.

Karl Schwarzschild found the black-hole solution to Einstein’s equations in 1916, the same year Einstein predicted gravitational waves. Why didn’t Einstein believe in black holes after that?

Black holes themselves have a very controversial and complex history, and LIGO’s detection was the first really complete proof of the existence of black holes. In 1916 Einstein thought Schwarzschild had just discovered a physical simplification: Just as one would treat the Earth as a point mass [with its mass concentrated to a point] for simplicity, they thought the “Schwarzschild solution” — what we now call a black hole — treated the sun as a point mass just for convenience. They didn’t think it would ever be a real thing, where you would have the mass concentrated to a point. They thought that was impossible, outrageous. By the 1930s it was beginning to dawn on people, “You know, it’s not entirely clear to us that the theory prevents that from happening.” Gradually, people like Robert Oppenheimer, the famous director of the Los Alamos Laboratory for the Manhattan Project, began to show that it was possible for a star to collapse into itself until it actually created something that really did look like the Schwarzschild solution. And that work was taken up in the 1960s by John Wheeler’s group, of which Kip Thorne was one of the students, and they and others developed the theory of black holes.

How did people then figure out what the gravitational waves produced by merging black holes would look like on Earth?

B. P. Abbott et al., Observation of Gravitational Waves from a Binary Black Hole Merger, Phys. Rev. Lett. 116, 061102

The gravitational-wave “chirp” observed by Advanced LIGO’s Hanford (top left) and Livingston (top right) detectors, compared to theoretical predictions (bottom row) of the chirp from two black holes of 29 and 36 solar masses, respectively, merging 1.3 billion light-years away.

A key problem was imposing the condition that there are no waves coming into the binary black hole system from infinitely far away, only waves going out to infinity. But that’s actually very hard to do, because you usually need a completely different mathematical formalism to describe the very distant gravitational field —at “infinity” or out here at Earth — than you need to describe the black holes themselves. People would try to do this calculation in the 1950s and ‘60s and they would get wrong answers. In some cases, they would get an answer that the black holes were gaining energy rather than losing it, because they made a mistake and had incoming waves bringing energy in from infinitely far away. So what happened in the course of the 1960s was that people like Roger Penrose, the great English relativist, did research on the structure of space-time. And Penrose discovered that there’s more than one infinity at the edge of space and time, and you have to pick the right infinity on which to impose your conditions. And then other people introduced techniques from fluid dynamics. These are just examples of many different conceptual and formulaic breakthroughs that had to be made.

And then the next step was predicting the particular signals that LIGO’s detectors might pick up.

At one of my very first group meetings in Kip’s group as a young student — this was 1991 or so — he came in with a big sheet of paper, and he had typed up everything that needed to be done on the theory side if LIGO was going to work. Because the whole reason you can detect the signal is that it has this characteristic sweep, and you filter the data against it. But you can only filter if you know what the signal looks like, and since you’ve never seen it before, you can only know what it looks like if the theorists tell you. And so Kip said, I want everybody in the group to work on this. And that’s what we did.

You’d like to have a prediction of the waveform from the beginning of where LIGO could conceivably see the signal to the final stage where the black hole has settled back down again and is not emitting any more waves. But there’s no single method that can give you the whole thing. For the first stage, you can use approximation methods that were already around at that time, but it was realized that several orders of magnitude more levels of approximation would be needed, and this was very daunting. And then when the black holes are merging, the gravity is insanely strong, and so you need numerical methods, where you do the calculation on a supercomputer. There were a whole bunch of groups who were trying to do that, and they were confronted with serious challenges. They couldn’t evolve the two black holes over more than a tiny amount of time, which wouldn’t help at all. And so a few years ago, they basically decided, “We just don’t have a choice. We’ll keep changing our coordinate systems until we find something that works that doesn’t crash on us.” And a guy called Frans Pretorius found a way to do it, and the methods took off from there.

There’s this hope that LIGO will “open up a new window on the universe” by detecting gravitational waves from previously unknown astrophysical objects. Considering the effort that went into recognizing the signal from a black-hole merger, how will we be able to see the unexpected?

Yes, the real excitement would be to find something we didn’t expect. One possibility is that the unexpected might help us out by being a very large signal. Our hopes for that have been dampened somewhat, because the original LIGO was online for quite a while and if the signal were very large it might have seen it. It does look like the unexpected is not going to be easy, so how do we dig the signal out of the noise?

One answer is that there are certain kinds of techniques that people have been looking at where you don’t commit yourself to knowing precisely what the signal looks like, but you just look for certain kinds of regularities — for instance, maybe this unexpected signal is at least a periodic signal. And LIGO is certainly doing that. They even have an “[email protected]” project, where they’ll send a piece of LIGO data to your home computer if you sign up for this, and your computer will help look for simple things like that. Another approach is to use machine learning to try to teach machines to look for signals. You start with what you know, but there is some hope that over time these techniques might grow and develop to where they become sufficiently flexible to catch things that aren’t what you expect.

What do you take away from this story?

I am struck by the collective nature of the endeavor. It had to be a collaborative effort; each step was sufficiently difficult that it had to link to the next step. And collective efforts come with vitriol and disputes. People shouted at each other. But the finer qualities of human nature won out. People got over their anger. Einstein got over his anger. People admitted they were wrong. And eventually, as a community, we got there.

• Dato says:

Fantastic. I didn't know that numerical simulation was that hairy. A great triumph overall.

• Avery says:

I've read a bit about what they did to detect this but the only thing I've understood so far was Feynman's explanation.

• Alfred Burdett says:

Any responses to the following no doubt naive questions would be much appreciated.

1. It has been reported that during the recently observed black hole merger, something like three solar mass equivalents of energy were emitted by the merging black holes in the form of gravity waves. Is there any known or postulated mechanism whereby this conversion of black hole mass to emitted energy takes place?

2. Does a black hole emit gravity waves when it is not merging with infalling mass?

3. What is the relation, if any, between the gravity waves that may be emitted by a body and the gravitational field that it generates?

• Alfred Burdett says:

And one other naive question, if I may:

Since gravitons possess energy, why are they not retarded by gravity as are photons, and thus prevented from escape from a black hole?

• Clark Magnuson says:

Back in the 1990s a friend, Tom, wanted me to help read Kip Thorne's books on gravity.
It seemed like work, and we were not getting paid. We were just trying to read the layman's stuff.
I think there have been gravity wave detectors for 20 years. They just keep building bigger ones. I would like to see a history with money spent on all the detectors, who spent it and what was learned. Didn't we always know there were gravity waves, just faint?

It seems the reporting has been huzza huzza hype emotion history, and not much $perspective. We are paying for this stuff. Don't treat us like suckers. • Some answers by the layman says: Some answers: > It has been reported that during the recently observed black hole merger, something like three solar mass equivalents of energy were emitted by the merging black holes in the form of gravity waves. Is there any known or postulated mechanism whereby this conversion of black hole mass to emitted energy takes place? There is no need for a particular mechanism. The field equations and initial conditions tell your simulation how the wobbly gravitational fields wobble, then settle, generating the wave. You then integrate to see how much energy is in the outgoing wave. Given that "black holes cannot shrink" (which still applies here, I think) that energy will come from the enormous angular momenta and kinetic energy of the infalling black holes. > Does a black hole emit gravity waves when it is not merging with infalling mass? No, only special arrangements of masses emit gravitational waves, in particular, two masses orbiting each other. > What is the relation, if any, between the gravity waves that may be emitted by a body and the gravitational field that it generates? The static field is whatever is left when the wave has wandered off into infinity. > Since gravitons possess energy, why are they not retarded by gravity as are photons, and thus prevented from escape from a black hole? For this, Quantum Field Theory is needed, in particular are quantization of the gravitational field, if not a quantization of spacetime. This is still some way off. Here we are talking about Classical General Relativity: there are no particles here. And, dear Clark Magnuson: > It seemed like work, and we were not getting paid. Woah there! Work w/o getting paid? So did you succeed or not? > I would like to see a history with money spent on all the detectors, who spent it and what was learned. Didn't we always know there were gravity waves, just faint? Google is your friend, please do your research. And no, "we did not always know". With that kind of attitude, we would still be trying to decide whether stones are edible. • Eduardo says: "…I am struck by the collective nature of the endeavor.[…] But the finer qualities of human nature won out. People got over their anger. Einstein got over his anger. People admitted they were wrong. And eventually, as a community, we got there." This may be, surely, the only and the greatest way of accomplish this kind of breakthrough. Keep up the great work guys. The doors and windows of the universe (who knows universes?) has undoubtedly been opened up more-and-more, and you have been the first ones to see inside them!!! Sincerely! • unimpressed says: While this undoubtedly is a technical tour de force, I do not feel this this particular 'inertia-driven' project [pun intended] was worth the more than a billion bucks that could have been better spent to teach thousands of kids some basic math skills–and help them secure a better job in the future. I would not feel the same if perhaps$10M or $20M had been spent, but a billion is just too much to *confirm* what probably 99% of the specialists in the field have been certain of anyway. The 'inertial team' and those who uncritically cheer that huge expense for so little payoff seem to have lost any perspective or are uninterested in performing basic cost-benefit analysis or a true social cost of such projects. • Matt McIrvin says: "Since gravitons possess energy, why are they not retarded by gravity as are photons, and thus prevented from escape from a black hole?" They are. But the gravitational field of a black hole extends outside of its event horizon, and the gravitational waves are not emerging from somewhere inside the event horizon; they're being emitted by the oscillating gravitational field of the whole system. • Alfred Burdett says: Some Answers, Thanks very much for for your helpful response. You say that the waves derived energy from the angular momenta and kinetic energies of the infalling black holes, so I take it that the waves originated outside the event horiz0n, so that the my question about how they escaped does not arise. But I'm still unclear about the relation between gravity waves and the static gravitational field. Do the waves create the field. or what? • Matt McIrvin says: It helps, to some degree, to make an analogy with electromagnetic fields. The relationship between gravitational waves and the static gravitational field is similar to the relationship between electromagnetic waves (light, radio waves, etc.) and the static electromagnetic field. Electromagnetic waves don't create the static field*. But if you were to take a source of the static field and shake it around, the changes in the static field would propagate outward at the speed of light, and these can be described as electromagnetic waves. Similarly, a pair of masses orbiting each other will emit gravitational waves. There are some differences that come from the fact that there are positive and negative charges, but only positive masses. But this is the basic idea. (*though the static interaction can be formally described in the quantum theory of electromagnetism using "virtual photons"–this is probably best regarded as just a mathematical trick. The equivalent quantum theory of gravity is nowhere near as well-developed, and here we're just talking about non-quantum general relativity.) • Matt McIrvin says: @unimpressed: There's actually no shortage of math-skills education in our society: what there <em>is</em> is a shortage of is good technical jobs for these kids to get once they've got the skills education. And that's one of the things that spending on scientific projects like LIGO can produce. This money doesn't just go down a black hole and disappear; it goes to researchers and technicians to do their jobs. Now, you might be able to demonstrate that some other kind of jobs project, or some other kind of science spending (general NSF grants, say), might have a larger economic multiplier, and that might be a valid argument depending on our social priorities. But the shortage of well-paying jobs in today's economy is more a demand-side problem than a supply-side problem. Increasing the number of highly-skilled people, without more spending on the services they can provide, will just create a lot of unemployed highly-skilled people. I've also heard people argue that the technical research into making LIGO happen actually has a lot of potential technological spinoffs in the area of precision instrumentation. I never know how seriously to take these kinds of spinoff arguments; I've heard all kinds of extravagant claims about spinoffs of, say, the space program that don't always hold up under close analysis. But I do think that if we want kids growing up today to have opportunities for good, fascinating jobs in science and engineering, a robust program of pure research is a good way to support that, regardless of whether it has obvious applications. • Matt McIrvin says: The other thing to realize about LIGO is that unlike some other experiments in general relativity (Gravity Probe B comes to mind, and it's been criticized on these grounds), this one *isn't* just an effort to confirm something everyone already knows. If it were just a project to verify that gravitational waves exist, it wouldn't be discovering much. But what this really is is a new eye on the universe. We're definitely going to find out how often these kinds of black-hole collisions happen, and some details of the black holes involved. That is something nobody knows today. There's been a suggestion that the first detection was correlated with a gamma-ray burst: if that holds up, which it may or may not, it's a genuine surprise, because nobody predicted that. With any luck, it's going to detect other gravitational-wave-emitting phenomena as well. • Lakshminarasimhan Viswanathan says: Gravitational waves (GW): In curved space-time measure, longitudinal or transverse? Mathematically, how are these characterized? Like light quanta (waves?) any interference, differction..? • unimpressed says: @Matt McIrvin: thank you for a measured response. It wasn't my intention to criticize all basic research; far from it. I also used math education as a general example, and I agree with your supply-and-demand. However, having watched the struggles of the LIGO project over the years, I am singularly unimpressed with getting just a confirmation for one billion bucks. I can only imagine how many great math and science courses, well-equipped labs and paid internships and scholarships could be had for the price of those two sets of wiggles. Again, this for a billion-plus dollars? • Steven Solomon says: One of the most interesting and lucid articles I've read. It combined my interest in history and physics. And a special shout-out to Matt McIrvin….thanks for you insights and clarifications. • Alfred Burdett says: Matt, Thanks for your comment on the relationship between gravity waves and the static gravitational field. In that connection, you said, <i>The relationship between gravitational waves and the static gravitational field is similar to the relationship between electromagnetic waves (light, radio waves, etc.) and the static electromagnetic field. Electromagnetic waves don't create the static field. But if you were to take a source of the static field and shake it around, the changes in the static field would propagate outward at the speed of light, and these can be described as electromagnetic waves.</i> So are you saying that gravity waves are responsible for <i>changes</i> in the static gravitational field, or that there are two kinds of gravity wave? Or something else? • Allan Lindh says: Does anyone know of the observed signals shown above have been corrected for the instrument response of the strainmeters? For instance, is the decreasing amplitude as the signals become shorter wavelength at the end of the signals due to instrument response? And if that is the collision of the two BH's, why is there no spike at the end, the moment of the final collision? • CW says: Matt McIrvin: "There's actually no shortage of math-skills education in our society: what there is is a shortage of is good technical jobs for these kids to get once they've got the skills education." Thanks for the making that point, as well as your excellent explanations. Corporate owners and venture capitalists are interested in productivity, not hiring people. Here "productivity" is defined as the feature of corporate resources (including "human resources") that generate financial returns to said owners, not solutions to important problems (whether scientific or practical). Bell Labs is long gone. But I digress… • Lars Simonsen says: I'm a bit puzzeld. Everybody talks about photons not able to escape a black hole. I mean, it's a singularity – photons is not produced, huh? • Impressed says: I am impressed by the deep understanding (or at least the will to understand) and passion to science you guys here have, Einstein is still alive! Well, in theory of course. @Matt McIrvin Thank you for your beautiful explanations, comments and responses. Regarding your words in response to why it is important to spend so much on LIGO "I've also heard people argue that the technical research into making LIGO happen actually has a lot of potential technological spinoffs in the area of precision instrumentation. I never know how seriously to take these kinds of spinoff arguments; I've heard all kinds of extravagant claims about spinoffs of, say, the space program that don't always hold up under close analysis." I would like to comment thst as far as I am concerned there is a huge practical benefit from identifying gravitational waves, isn't it just as important, or even more, than radio waves? Or any other form of waves that society enjoys from? (Why am I saying even more important than radio waves, because according to what I heard (I didnt yet verify it) gravitational waves are far more stronger and affective than any form of waves we already have, for example all sorts of waves that we know about so far are weakened by colliding with any physical object versus the new identified gravitational waves which weakens only from traveling..) So, won't the billion dollars spent pay off big time? According to my calculation a billion will be peanuts when it comes to cashing out the benefits. Thaks again! Sincerely yours • Ajay Taide says: Is it possible that these tiny ripples of space time called gravitational waves could be the reason for quantum behaviour on a sub atomic scale • Robert Oerter says: Lakshmi, gravitational waves are neither transverse nor longitudinal. They are a 3rd type, called "tensor waves." They stretch in one direction and simultaneously compress in the perpendicular direction. (You could think of them as doubly transverse waves.) Check out the graphic on this page: https://www.quantamagazine.org/20160211-gravitational-waves-discovered-at-long-last/ As far as the cost-benefit analysis, this is FUNDAMENTAL RESEARCH, folks. It's not about immediate benefits, it's about learning more about our universe. The$1 billion cost is about same as two space shuttle launches. Pretty cheap for opening up a new window on the universe: one that will likely lead to unexpected new insights into stars, galaxies, maybe the Big Bang itself.

When I was in college, no one thought General Relativity was of any practical importance, as its effects were so weak as to be barely measureable. Nowadays every GPS has to take GR into account, or its accuracy would be significantly diminished.

Besides, how many things can you get for \$1 per light-year?

• Wiley says:

Natalie,
Thanks for your interesting interview, especially asking the question related to how Einstein changed his mind about the existence of gravitational wave, and the link to the article of how Robertson's (as in "FRW metric") role of correctly identifying the coordinate singularity (rather than the physical singularity) in the wave solution. Very nice!

Robert Oerter,
Gravitational waves (GWs) are transverse. But the difference you point out is valid: there is no dipole wave like electromagnetic (EM) wave (this is a consequence of momentum conservation), the leading order is quadrupole.

Ajay Taide,
GW already exists in classical general relativity (GR).

Lars Simonsen,
Metric (what people usually refer as the fabric of space-time) in GR determines how a particle moves. There are different types of metric solution of the Einstein equation for a black hole, depending on whether it has angular momentum or electric charge, but in general you can think that the geometry of the fabric being warped into a way that all particles are trapped in a sphere (so-called event horizon). GW, on the other hand, is another metric solution for e.g., the merger of a binary system. In this case, it's the metric that is oscillating rather than any photon produced.

• marees says:

Alfred,

I am no physics expert but based on simulated graphics of Black Hole merger, I assume that when two black-holes move towards each other, their surface is continuously getting distorted (that is not in equilibrium) and hence this results in radiation of energy in terms of gravitational waves. the source for this energy can only be a loss of internal mass, which I try to explain by the change in the surface to before equilibrium & after equilibrium.

gravity is similar to electromagnetic field but there is no repulsive charge concept, purely attractive. because of this the di-pole moment cannot produce wave & only quadrapole moment (two massive objects orbiting-around/moving-fast towards each other) can produce the gravitational waves.

In terms of particle theory I try to imagine a graviton being fired whose vibration alternately distorts the horizantal & vertical density of mass in the directions perpendicular to its motion.

if I am wrong, somebody can correct me, here.

• Alfred Burdett says:

@ Marees,

<i>the source for this energy [of gravitational waves] can only be a loss of internal mass…</i>

Not so, according to Some Answers, above, who says the source is:

"the enormous angular momenta and kinetic energy of the infalling black holes."

But is there any evidence of the existence of the space time continuum? What if space is just that, i.e., the absence of anything? Michelson Morley showed that light waves traverse the vacuum without an ether to wave in. So why do gravity waves need a spacetime continuum to wave in?

Even Einstein seems to have some doubts:

"We may have to give up, by principle, the space-time continuum. It is not unimaginable that human ingenuity will some day find methods which will make it possible to proceed along such a path."
————
* quoted in George Musser "Spooky Action at a Distance." Scientific American/Farrar, Straus and Giroux, New York, 2015.

• Andy Briggs says:

Dear Unimpressed, it's not just about confirming the existence of gravitational waves. It's about the birth of a completely new branch of science, from which we stand to learn an enormous amount about the workings of the Universe. It's a new window on the sky. It *may* enable us to detect gravitational waves from the Big Bang itself.

A billion seems pretty cheap for that.

• agusser says:

Due to the extreme gravity of both black holes, ¿does time dilation affect gravity's wave frequency? I imagine that when both BHs are closer then the orbiting period we detected would be grater as they aproach each other and we wouldn't "see" the real fusion of the BHs.