# Why is the detection of gravitational waves so significant?

LIGO has announced the detection of gravitational waves on 11 Feb, 2016. I was wondering why the detection of gravitational waves was so significant?

I know it is another confirmation of general relativity (GR), but I thought we had already confirmed GR beyond much doubt. What extra stuff would finding gravitational waves teach us? Is the detection of gravitational waves significant in and of itself, or is there data which can be extracted from the waves which will be more useful?

• Obligatory reference: smbc-comics.com/index.php?db=comics&id=2088#comic – Ant Feb 11 '16 at 18:03
• PHD Comics has a nice take on it: phdcomics.com/comics/archive.php?comicid=1853 – muru Feb 11 '16 at 18:54
• I've always found the metaphor of space a sheet with objects on it perplexing - the reason it sounds intuitive is because of our notion of gravity pulling things down! – Nacht Feb 11 '16 at 22:52
• @Nacht : Obligatory xkcd. – Eric Towers Feb 11 '16 at 23:26
• @EricTowers Obligatory xkcd. – iceman Feb 12 '16 at 9:59

Gravitational waves are qualitatively different from other detections.

As much as we have tested GR before, it's still reassuring to find a completely different test that works just as well. The most notable tests so far have been the shifting of Mercury's orbit, the correct deflection of light by massive objects, and the redshifting of light moving against gravity. In these cases, spacetime is taken to be static (unchanging in time, with no time-space cross terms in the metric). Gravitational waves, on the other hand, involve a time-varying spacetime.

Gravitational waves provide a probe of strong-field gravity.

The tests so far have all been done in weak situations, where you have to measure things pretty closely to see the difference between GR and Newtonian gravity. While gravitational waves themselves are a prediction of linearized gravity and are the very essence of small perturbations, their sources are going to be very extreme environments -- merging black holes, exploding stars, etc. Now a lot of things can go wrong between our models of these extreme phenomena and our recording of a gravitational wave signal, but if the signal agrees with our predictions, that's a sign that not only are we right about the waves themselves, but also about the sources.

Gravitational waves are a new frontier in astrophysics.

This point is often forgotten when we get so distracted with just finding any signal. Finding the first gravitational waves is only the beginning for astronomical observations.

With just two detectors, LIGO for instance cannot pinpoint sources on the sky any better than "somewhere out there, roughly." Eventually, as more detectors come online, the hope is to be able to localize signals better, so we can simultaneously observe electromagnetic counterparts. That is, if the event causing the waves is the merger of two neutron stars, one might expect there to be plenty of light released as well. By combining both types of information, we can gain quite a bit more knowledge about the system.

Gravitational waves are also good at probing the physics at the innermost, most-obscured regions in cataclysmic events. For most explosions in space, all we see now is the afterglow -- the hot, radioactive shell of material left behind -- and we can only infer indirectly what processes were happening at the core. Gravitational waves provide a new way to gain insight in this respect.

• "if the signal agrees with our predictions, that's a sign that not only are we right about the waves themselves, but also about the sources" -- conversely, and just as importantly, if the signal does not agree with predictions then it shows that we are wrong about something and can consider which of the assumptions to discard. – Steve Jessop Feb 11 '16 at 15:37
• Also, it's a breathtaking technological achievement. – user1504 Feb 11 '16 at 15:38
• @Mew Oh, is that what you want to hear? In that case, "STAR TREK WARP DRIVE FINALLY IN REACH!!! In other news, the Bears still suck." – Luaan Feb 12 '16 at 8:30
• I've a dual feeling from the answer. On the one hand the answer is big, enthusiastic, and detailed. On the other hand… it explains almost nothing! I mean, it mentioned problems of detectors, possible waves sources, the past of the GR, and its relation to waves… But with relation to the question it basically says, that waves somehow could give additional information about an explosion. How? What kind of information? Reading the answer gave me nothing new, and I am not even a physicist by the way. – Hi-Angel Feb 12 '16 at 9:46
• @HopelessN00b Even if it's currently as useless as you say, the answer does explicitly mention future improvements ("as more detectors come online..."). Seems like the first instrument to detect a phenomenon is always going to be relatively basic; why not take it as a sign of what could be possible? I'm sure you'd have been pretty underwhelmed looking through the first telescope too. – Cascabel Feb 12 '16 at 20:20

Chris' answer provides an excellent explanation as to why gravitational waves are useful to detect in general. Here's my take (as someone who works in the theory of black holes) on what is particularly interesting about the signal that was announced yesterday. Many of my thoughts are taken from the official NSF press conference and from colloquia at my institution.

The Event Itself

Numerical analysis of the gravitational wave event that was measured on September 14, 2015, has revealed a great deal about the nature of the event that took place.

The following is a figure from the LIGO report which shows the gravitational wave signal:

(source)

The red line in each graph is the gravitational wave signal measured from the observatory in Hanford, Washington. The blue line is the gravitational wave signal measured from the observatory in Livingston, Louisiana. The top left graph shows the Hanford signal alone, the top right graph shows the Livingston signal overlaid with the Hanford signal (look how nicely they match up, proving that this was not a local source of noise but rather a signal being generated from some cosmic distance).

The left graph in the second row is most interesting. The light gray line essentially shows the signal, cleared of as much noise as possible (the equipment is so sensitive that all sorts of things can cause slight jitters in the waveform). The red line represents the waveform that would be predicted by the techniques of numerical general relativity for a system of two black holes spiraling into each other. It's no coincidence that the observed waveform (light gray) and predicted waveform (red) overlap so well.

There is, of course, a great deal of analysis that goes into checking the statistical significance of this data. Scientists at LIGO have found that within a statistically significant margin, this waveform was probably produced by a binary system of two black holes, each about thirty times as massive as the size of the sun.

Black Holes In General

Before yesterday, we had no direct evidence to show that black holes existed. We were fairly confident in the existence of black holes, but only through indirect measurements. This is the first ever direct measurement of a black hole—the objects in question are massive enough and compact enough that they almost surely must be black holes. What's more, the data fits perfectly our general relativistic predictions as to what kind of radiation will be released by a black hole merger. This is huge news—physicists never had complete evidence that black holes existed before yesterday, although the public might take it for granted. Black holes exist, and they work the way we thought they did. That's incredible!

Types of Black Holes

From an astrophysical perspective, this is quite interesting, because both of the inspiraling black holes were about 30 times as massive as the sun (henceforth referred to as having "30 solar masses"). Astrophysicists had no real compelling evidence for black holes in this mass range. It was assumed that we had black holes in the range of 3-20 solar masses, and the so-called "supermassive" black holes (which are millions, billions, of solar masses? I'm not an astrophysicist so I can't tell you). This is a fascinating astrophysical problem—the mass in a black hole needs to come from somewhere. What is the process by which a black hole of ~30 solar masses forms? From where does it take its matter? How massive is it when it first forms (from a star, perhaps?), and how much does it grow after it has already become a black hole?

Oh, and by the way, we haven't just confirmed the existence of two black holes of solar mass ~30. We've confirmed the existence of one black hole of 62 solar masses—the black hole remaining after the two have merged. Speaking of, let's talk a bit about that final black hole.

The collective mass of the two black holes before they merged was ~65 solar masses. The mass of the final black hole was ~62 solar masses.

What that means is that 3 solar masses were radiated away in gravitational waves as the black holes merged. Not impressed? Well, here's some perspective: according to the NSF conference given yesterday, the power output of gravitational radiation during the last moments of the black hole merger was more than the collective power output of every star in the universe combined.

That's a lot of energy, very fast. What happens once that energy is released? Well...

Ring-Down

This is my personal favorite, but it's also the thing about which we have the least information. If you look again at the figure I included earlier in this response, at, say, the second graph in the left column, you'll notice that the pattern goes as follows:

Slight vibrations, increasing in amplitude in frequency, suddenly oscillating very quickly at a high amplitude, and then dying down to almost nothing.

That sudden increase in frequency is called a "chirp," and it's what LIGO was looking for. That chirp tells us everything we need to know about the black hole merger.

But what about what happens afterward? The exponential decay of the signal corresponds to the resulting black hole (with 62 solar masses) settling down into a stable state. The question of black hole stability is incredibly interesting, and the process by which a black hole settles down after some major perturbation (e.g. merging with another black hole) is a fascinating object of study.

Basically, if you hit a black hole, it rings. When you perturb a black hole away from its stable state, you create something called quasinormal modes—mathematical descriptions of the perturbation from equilibrium—which decay exponentially over time as the black hole approaches equilibrium.

The experimental signal does not contain much information about the ring-down. We can't glean much information about exactly how the black hole settles into a stable state—the process doesn't generate very strong gravitational waves, for one thing, and it happens very quickly.

But that's okay. In the figure, we can see it happen. We see two black holes merge, release three solar masses of radiation, and then settle down into a stable final state. That alone is incredibly exciting.

Oh, by the way, one parting thought: this black hole merger happened about a billion years ago. We're only getting its signal now.

• What do you mean by "bottom left graph"? If you mean the one with "Residual" label, it's just a difference between measured (top) and predicted (middle), not the cleared signal. – Ruslan Feb 12 '16 at 16:49
• Silly question: do we know these waves are from Black Holes ? Is it correct to assume the assumption of black hole binary source is just because there is nothing else we know of in our current theoretical structure which allows such extreme energies? In principle, this could be a signal from something more exotic, beyond our standard gravitational example-set ? – James S. Cook Feb 13 '16 at 14:29
• This was a fun read, thank you. @JamesS.Cook: The fact that it fits the predictions so well makes it hard, I would think, to come up with something different. I assume the data is not interpretable without a rotational collapse event of 2 extremely dense 30 sun masses; for all we know these must be black holes. Something along these lines. – Peter A. Schneider Feb 16 '16 at 15:00
• 3 solar masses escaped of 65 total masses! That is very interesting given the conventional understanding of a black hole as being a singuarity with all mass squeezed to the center. We know it can't be infinite density, do we even have a clue what that core consists of? Normally stuff that goes in could never escape (save tiny Hawking radiation). So then, what is the nature of the escaped mass and what happened to the cores of these masses? – Alan Baljeu Feb 17 '16 at 15:39
• I think there was already very strong direct evidence of black holes before the detection of gravitational waves. The stars near the center of our galaxy are orbiting something in the very center. This central object is invisible, very compact, but when you analyze the trajectories of the stars, the central object mass is over 4 million solar masses: youtu.be/duoHtJpo4GY?t=57 Something so massive could hardly be so small and dark. – mpv Mar 1 '16 at 13:08

In additions to what Chris White lists, I'd like to point to the fact that, except for a few meteorites and some dust collected on the plates of satellites and rocks from Mars (and cosmic rays and a handful of neutrinos; thanks Ruslan and Kyle Oman), until now all information reaching us from the Universe — whether it is the Sun, the more distant planets, other stars, galaxies, CMB, etc, — has come to us in the form of electromagnetic radiation.

Gravitational waves is a whole new mode of gaining knowledge about the Universe. Both from objects where we also see radiation, but also for instance perhaps at some point inflation at the Big Bang, where using electromagnetic radiation we can't see further back than the CMB, 380,000 years after Big Bang (this is what the BICEP2 guys thought they saw two years ago, but it turned out to dust…).

• Not only EM radiation: we also have some neutrino detectors. – Ruslan Feb 11 '16 at 16:43
• And particles, e.g. cosmic rays. – Kyle Oman Feb 11 '16 at 21:12
• Yes, that's true. I forgot about those. – pela Feb 12 '16 at 9:05
• Next step: A gravity wave version of SETI – Steven Gubkin Feb 14 '16 at 0:22
• @StevenGubkin Detection of two solar mass aliens dancing? – Nathan K Feb 15 '16 at 15:03

Gravitational waves are not obscured by anything. If detectors are made to work at lower frequencies (in space) then they can "see" gravitational waves originating from beyond the cosmic microwave background right back to the inflationary epoch.

Another thing that has become clear today is that binary mergers give a chirp that yield the masses of the merging components, but also gives accurate, independent distance estimates. These events are the equivalent of standard candles for EM waves - "standard sirens".

• Are gravitational waves also not obstructed by gravitational wave detectors? – Vi. Feb 22 '16 at 1:03
• Yes. Of course it depends on the assumption that grav waves travel at the speed of light. If we can be more accurate in doing the measurements, i.e. From space with larger interferometers, we could also see dispersion if different freqs had different speeds. Also if we could catch some EM signature from the merger event (from plasma/gas also flowing in at near light speed we could compare speeds. Also if we could see optically some signal of where exactly it happened we'd get distance independently so could get speeds. Lots more fun to come. – Bob Bee May 21 '16 at 21:05

Gravitational waves are a major component of phenomena like black hole mergers

The GW150914 gravitational wave event is believed to be a merger of two black holes with estimated masses of 36+5/-4 and 29±4 solar masses. The final mass was 62±4 solar masses. If our current models are correct then the missing 3.0±0.5 solar masses (5.3%) were radiated away as gravitational waves, and that in only 0.2 seconds.

If we could not detect gravitational waves, then that 5% would be a major gap in our models. Now in this case we only know that the event happened because we detected the waves, but supposing we had observed some similar event in the electromagnetic spectrum, if we could not also detect the gravitational waves then it would be a big flaw in our observations of the event.

With Gravitational Waves(GW) one can "know" that the Objects is there - detect it without "seeing"-visually, it just because the object has a mass.

Anything moving and having a mass is emitting GW - current detectors are sensitive only for objects with masses equal to mass of many Suns $2\times 10^{30} ~\rm{kg}$ (2 with 30 zeros).

Imagine one day may we have sensors-detectors able to detect movement of any objects with mass without seeing it ....

• Moving is not enough. You need acceleration. – pela Feb 12 '16 at 11:50
• Moving somewhat implies that acceleration happened at some point in the past. Similarly, constant stable movement somewhat implies that deceleration hasn't happened yet. – user2338816 Feb 12 '16 at 13:14
• Is acceleration needed? For instance, if a charged object moved rapidly past you, would you experience an electromagnetic wave? Similarly, if a black hole went whipping through the Solar System at a substantial fraction of the speed of light, wouldn't we experience a (single) gravitational wave? – Daniel Griscom Feb 14 '16 at 17:33
• @DanielGriscom: In the charged-object case, yes, you'd notice a one-time rise and fall of electric field. The object wouldn't be converting any of its kinetic energy into electromagnetic radiation, though. That's why it's not a wave. As I understand it, grav waves work the same way. – Peter Cordes Feb 16 '16 at 19:41

One interesting implication is that gravitational waves are considered to be more evidence of Inflation Theory, which is used to help explain the homogeneousness of the universe. If Inflation Theory is correct and space-time experienced an exponentially explosive expansion, that expansion need not have occurred at the same rate at every point in space.

In fact, the chances of that happening are apparently so astronomical as to be almost nil. As a result, a single point in space could expand at a completely different rate than that of its surrounding space-points (I've heard it compared to blowing up a balloon with a defect in it, so that the defect forms into a bubble on the surface when inflated).

As I understand it, you'd end up with a vast multitude- possibly even an infinite number of alternate universes, completely separate but still "attached" to the other universes. And with each universe possessing its own laws (or lack of?) to describe force, space, time, etc., the multiverse could theoretically exist forever, with a finite beginning but no end. (Source- one of my Engineering Physics professors)

@ Martin Thanks for the feedback! Admittedly I am largely ignorant of Inflation Theory, but I should add that my professor’s professor was a member of Guth’s team who helped developed the mathematics of Inflation Theory. In any case, my understanding is that although there are a variety of viable inflation theories, most serious models require the presence of gravitational radiation resulting from the Big Bang (termed Primordial Gravitational Waves). According to Guth’s theory, Inflation occurred just before the Big Bang, and when it stopped the energy present in the inflaton field was converted into heat and the Big Bang (and the granddaddy of all gravitational waves).

Inflation Theory is still pretty new (only about 40 years old or so?), so it stands to reason that any new theory being proposed nowadays is probably going to incorporate GR due to GR’s success. So I guess the short answer is that current Inflation models, which incorporate GR, require Primordial Gravitational Waves, and if Primordial Gravitational Waves exist, then Gravitational Waves exist.

• This seems like a conflation of many different theories - especially I have never heard of your kind of multiverse being a consequence of any serious inflation theory. You might want to search for more sources. In addition, I don't see how gravitational waves, which are a simple consequence of perturbative general relativity should count as evidence towards a theory build on top of GR. It just doesn't make sense. – Martin Feb 13 '16 at 10:00
• In reference to the original question concerning the usefulness of these finding- unless I'm mistaken, isn't this also the first chance we've had to confirm that gravity travels at light speed? – masterblaster Feb 14 '16 at 5:52

As you know, there is Coulomb law, which states that static interaction between charged bodies behaves with distance $r$ as $r^{-2}$. It is long range law: if we touch one charge, then, according to this law, another one will feel the changing simultaneously. This point of view on electric interactions was completely changed when Maxwell have realized that the light, electric and magnetic interactions have the same nature; since the speed of light is finite, the Maxwell theory says us that if we touch one charge, then the information of changing of force - electromagnetic field - will propagate with finite speed - the speed of light.

This conception, conception of finiteness of all interactions, stays thus in all fundamental theories (according to modern point of view) independently on their nature; that is because this is the property of our space-time (this fact is fixed, for example, in explicit form of Lorentz transformations and causality principle and comes from the general axioms based on space-time symmetries).

General relativity theory, for example, is based on the statement that locally our space-time looks like Minkowski one, which forces the finiteness of gravitational interactions. In particular, General relativity theory equations on metric (Einstein equations), being linearized in absence of matter, formally coincide with those which we can obtain by constructing the free theory of massless particle helicity 2, starting from the global Poincare symmetry. The latter describes waves.

In the above point of view, detection of gravitational waves is something bigger than the checking of General relativity, opening the new method of astrophysical observations or one another way to check GR. It is checking the property of the spacetime which is the basis of all modern fundamental physics.

A non-exhaustive list for some of the prospects:

Neutron Star equation of state

Gravitational waves can be used to verify equation of state

Gamma ray bursts' inner structure

The dynamics of GRBs are still shrouded in mystery and nothing can really probe into the inner structure of GRBs like gravitational waves can

Speed of gravitational waves

One of the more obvious ones, but it still hasn't been established that the speed of gravitational waves is c (as it should be)

Testing theories of gravity

Technically, GR can be re-constructed by relaxing the assumptions and allowing for example torsions. Maybe Brans-Dicke theory is right?

Gravitons

Rather obvious

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