# Can gravitational waves pass through a black hole?

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As the title says, what happens when a gravitational wave approaches a black hole? I would presume that something interesting happens because of the way spacetime works near black holes but I have no knowledge to back it up.

No, gravitational waves cannot pass through a black hole.

A gravitational wave follows a path through spacetime called a null geodesic. This is the same path that would be followed by a light ray travelling in the same direction, and gravitational waves are affected by black holes in the same way that light rays are. So for example gravitational waves can be refracted by gravitational lenses just as light waves are. And just like light waves, if a gravitational wave crosses the event horizon surrounding a black hole it is then doomed to travel inwards to the singularity and can never escape.

There is one caveat to this. When we talk about a gravitational wave we generally mean a ripple in spacetime that is relatively small. Specifically it is small enough that the energy of the gravitational wave does not significantly affect the spacetime curvature. So when we calculate the trajectory of a gravitational wave near a black hole we take the black hole geometry as fixed, i.e. unaffected by the wave, and we compute the trajectory of the wave in this fixed background.

This is exactly the same approach as we use for calculating the trajectories of light rays. Since light rays carry energy and momentum then, at least in principle, they have their own gravitational fields. But for both the light rays and gravitational waves likely to exist in the universe the energy carried is too small to make a significant contribution to the spacetime curvature.

When you say in your question:

I would presume that something interesting happens because of the way spacetime works near black holes

I would guess you are thinking that the gravitational wave could change the geometry near a black hole, but as described above typical gravitational waves don't have enough energy to do this. It would be reasonable to ask what happens if we give the wave enough energy, but the answer turns out to be that it no longer behaves like a simple wave.

Gravitational waves exist in a regime called linearised gravity where they obey a wave equation that is basically similar to the wave equation light obeys. If we increase the energy so much that gravity becomes non-linear (as if the case for black holes) then the oscillations in the spacetime curvature no longer obey a wave equation and need to be described by the full Einstein equations. For example it has been suggested, but not proven, that really high energy gravitational (or light) waves could interact with each other to form a bound state called a geon. I confess that I'm unsure how much work has been done studying oscillations in this regime.

Gravitational waves should be lensed by massive objects in a very similar way to light.

Light rays (and by extension, gravitational waves) from a distant object, that pass within 1.5 times the Schwarzschild radius (for a non-spinning black hole) have trajectories that take then towards the event horizon. Waves on such trajectories cannot escape from the black hole, so the basic answer is no, gravitational waves cannot "pass through a black hole".

However, far from "hiding" a source of gravitational waves, an intervening black hole would cause the presence of lensed and magnified images. For perfect alignment of source, black hole and observer, there would be an intense "Einstein ring" at an angular radius that depends on the relative distances of the source and the black hole.

Of course gravitational waves cannot be imaged at present, so what would be detected is an abnormally strengthened gravitational wave signal.

All the above is in the geometric optics limit that the wavelength is small compared with the lens. If the black hole is small enough (which depends on its mass), or the gravitational wave wavelength is large enough, then the behaviour ought to be analogous to a plane wave encountering a small, opaque disc (Takahashi & Nakamura 2003).

In which case we would get a diffraction pattern and perhaps a "bright" Arago spot in the centre, though I am not aware of any such calculations in the literature.

This is not an unlikely scenario. For example, the gravitational waves detected by LIGO have relatively high frequencies of 10-1000 Hz and therefore wavelengths of 30,000-300 km, which are as big as the Schwarzschild radii of 10,000 - 100 solar mass black holes and certainly bigger than black hole remnants of stellar evolution.

## Ask Ethan: How Do Gravitational Waves Escape From A Black Hole?

Two merging black holes, particularly in the final stages of merger, emit tremendous amounts of . [+] gravitational waves. Image Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project (http://www.black-holes.org).

Perhaps the greatest discovery of all announced in 2016 was the direct detection of gravitational waves. Even though they had been predicted by Einstein's general theory of relativity 101 years prior, it took the development of a laser interferometer sensitive to ripples in space that would displace two mirrors separated by multiple kilometers by less than 10^-19 meters, or 1/10,000th the width of a proton. This finally came to pass during LIGO's 2015 data run, and two bona fide black hole-black hole merger events unambiguously popped out of the data. But how does physics actually allow this? Mārtiņš Kalvāns wants to know:

This question has puzzled me for a long time. Articles about LIGO discovery state that some percentage of black hole merger mass was radiated away, leaving [a] resulting black hole smaller than [the] sum of [the] original mergers. Yet it is accepted that nothing escapes black holes [. ] So my question is: how was energy radiated from black hole mergers?

This is a really deep question, and goes straight to the heart of black hole physics and general relativity.

Illustration of a black hole and its surrounding, accelerating and infalling accretion disk. The . [+] singularity is hidden behind the event horizon. Image credit: NASA.

On the one hand, we have a black hole. All of its mass/energy is concentrated together at a singularity at the center, and it's forever invisible to the outside observer thanks to the presence of an event horizon. Inside a certain region of space (defined by the event horizon), any path that any particle can take, whether massive or massless, regardless of speed or energy, will inevitably take it into the black hole's central singularity. This means that any particle that enters the event horizon, crosses into the event horizon or otherwise ever finds itself inside the event horizon will never be able to get out, and thus its energy is trapped inside forever. Once you're inside a black hole, you simply become part of the singularity's properties: mass, charge (of all different types), and spin. That's it.

The ripples in spacetime occur at the frequency of the black holes' mutual orbit, and are more . [+] intense in magnitude the closer in they get. Image credit: R. Hurt - Caltech/JPL.

On the other hand, Einstein's general relativity tells us that when two masses (of any type) orbit one another, it creates ripples in the fabric of space itself as the orbits themselves decay. These ripples, known as gravitational waves, move at the speed of light, cause space to expands-and-contract whenever they pass through it, and carry energy. Because of Einstein's most famous equation, E = mc 2 (or, as he wrote it originally, m = E/c 2 ), we know that one source of energy is mass and one source of mass is energy. They can be converted into one another mass is only one particular form that energy can take on.

The signal from LIGO of the first robust detection of gravitational waves. Image credit: Observation . [+] of Gravitational Waves from a Binary Black Hole Merger B. P. Abbott et al., (LIGO Scientific Collaboration and Virgo Collaboration), Physical Review Letters 116, 061102 (2016).

So when LIGO released the results of the event that occurred on September 14, 2015 in January of this year, it was only mildly surprising that they found two black holes -- of 36 and 29 solar masses -- merging together to create a new black hole of 62 solar masses. Where did the other 3 solar masses (about 5% of the total system's mass) go? In the energy of gravitational waves. With subsequent events that have been detected, roughly the same trend emerges: two black holes of comparable masses inspiral and merge together, and up to around 5% of their total initial masses gets radiated away in the form of gravitational waves.

But each black hole has an event horizon. Each of the pairs has one before the merger, the final post-merger black hole has one, and at no point during the merger does either singularity become "naked" or ever emerge from an event horizon. So, how does the mass get out?

Any object or shape, physical or non-physical, would be distorted as gravitational waves passed . [+] through it. Note how no waves are ever emitted from inside the black hole's event horizon. Image credit: NASA/Ames Research Center/C. Henze.

It's not just a tricky question it's a trick question! It's like asking where the mass goes when protons fuse into deuterium, helium-3 and then helium-4 in the Sun. Why is helium-4 less massive than the four protons that made it up? Because of nuclear binding energy. A bound state is more stable and has less energy (and hence, less mass) than the unbound state. When two black holes inspiral, coalesce and merge, these two black holes are becoming more bound -- more gravitationally bound -- than they were before. The energy they're losing is due to gravitational binding energy, not because either of the masses is exiting the event horizon.

Newton's law of Universal Gravitation has been superseded by Einstein's general relativity, but it . [+] is still an illustrative tool to look at quantities like force and energy. Image credit: Wikimedia commons user Dennis Nilsson.

You can see this just from Newtonian gravity. Imagine you have two masses of 1 kg each, each at rest and mutually separated by an infinite distance. They have a certain amount of energy inherent to them in this system: 1.8 × 10^17 Joules, which you can get from Einstein's equation, E = mc 2 . Now bring them in to one another, and bring the distance down.

• If they're now separated by only one kilometer, the whole system has lost 6.67 × 10^-14 Joules of energy.
• If you reduce that separation to one centimeter, the system loses 6.67 × 10^-9 Joules.
• If you bring that separation down to the size of a proton, at 10^-15 meters, the system now loses an incredible 6.67 × 10^4 Joules, or 66,700 Joules. (Now we're getting somewhere!)
• And so if you want to lose a really significant amount of energy, you can imagine taking the separation all the way down to 10^-27 meters, where you'll lose 6.67 × 10^16 Joules, or about 35% of the original energy!

Light and ripples in space as the light passes through non-flat space, it changes how an observer . [+] at any other location perceives the passage of time for the light. Image credit: European Gravitational Observatory, Lionel BRET/EUROLIOS.

Of course, our Universe obeys general relativity on these scales, not Newtonian gravity, but the picture is the same. It isn't that the black holes are losing mass it's that the total amount of energy in spacetime is transforming from one form -- in two well-separated, unbound masses -- to another form: a single, tightly bound mass plus gravitational radiation. The orbital properties and the masses of the original black holes determine what percentage of the total original mass becomes binding energy, but in all cases it's always true that the final mass is larger than either of the original masses but smaller than the combined raw masses. 5% is the amount that's radiated away in the maximal case, where the two masses are roughly equal. If they had an incredible amount of energy in their spins and their spins were aligned, that percentage can be bumped up all the way to about 11%. But if one of the masses is much greater than the other, the percentage drops a 1 solar mass black hole merging with a 1,000,000 solar mass one can only radiate away 0.0001% of its energy.

An artist's impression of two stars orbiting each other and progressing (from left to right) to . [+] merger with resulting gravitational waves. This is the suspected origin of short-period gamma ray bursts, and also a source of gravitational waves. Image credit: NASA/CXC/GSFC/T.Strohmayer.

The inspiral and merger doesn't result in anything from inside the black hole getting out, but rather in spacetime deforming to account for the gravitational potential energy as the two masses coalesce and merge. The ringdown phase -- which occurs at the end of the merger -- represents the event horizon reverting to its maximally efficient shape: either a sphere or a spheroid. It's the very last fraction-of-a-second of the merger where the most energy is released, but no particles from inside the black hole are getting out. Einstein's predictions are very clear, and this is why we were able to make the detections in the first place: because we had calculated what signal to look for. Our intuition may give us trouble, but that's why we have the equations. Even when our instincts are no good, the calculations will give us the scientific truth.

## Can gravitational waves pass through a black hole? - Astronomy

465 clicks posted to STEM » on 18 Jun 2021 at 8:54 AM (6 days ago) | Favorite | share:

Spectrum: Ethan Siegel has finally jumped the shark.

Yeah, but that shark had a black hole in it.

Dr. DJ Duckhunt: Spectrum: Ethan Siegel has finally jumped the shark.

Yeah, but that shark had a black hole in it.

Merltech: More importantly, does it convert a 2d world into 3d?

When they hit the surface of that black hole, they're going to wish their fathers had never met their mothers. That's for sure.

What happens when a wave passes over a drain?

My guess is that a very similar thing happens. The black hole is a feature of space-time curvature. So is the gravitational wave. Seems like you are asking what happens when to features in a medium interact, which doesn't strike me as that novel or interesting. But maybe some weird shiat happens when you do the math. Who knows.

Guessing they want to explore this to try and model Hawking Points in the MBR?

Spectrum: Ethan Siegel has finally jumped the shark.

I don't know about that, but he did use a lot of words to just to say, yes, black holes affect gravity waves the same way they affect they affect all other known massless radiation.

BeesNuts: What happens when a wave passes over a drain?

My guess is that a very similar thing happens. The black hole is a feature of space-time curvature. So is the gravitational wave. Seems like you are asking what happens when to features in a medium interact, which doesn't strike me as that novel or interesting. But maybe some weird shiat happens when you do the math. Who knows.

Guessing they want to explore this to try and model Hawking Points in the MBR?

You have to decide if you think gravity is a fundamental force, or not. Space-Time is also a construct that we have to decide whether exists, or not. When we dig down into how things work, if this is not an event-driven universe, these things become symptoms not causes, and flexibility suddenly creeps in.

I personally think quantum is the result of extreme situations where the hidden flexibility of probability that is knitted into the fabric of the universe is revealed.We've spent a lot of time and effort building constructs to explain everything, but I think we're getting ridiculous. I think we've gone too far, and need to take a big step back from it.It was fun being a part of the DØ and ICECUBE, and I love the ease of the Standard Model. But as far as what's really going on, I think those are more of a convenience for scientists to discuss processes and experiment plans or results, than they are a good window into what's real.

sxacho: I would guess that any gravity waves that would reach the event horizon would be lost to the black hole while any that don't reach the event horizon will be bent in proportion to the distance away.

Yeah, in the 'keeping it simple,' way of talking about things, if your space-time depression is deep enough to cause a topographical circle with respect to 3x10^8 m/s, I think you're done.

aungen: BeesNuts: What happens when a wave passes over a drain?

My guess is that a very similar thing happens. The black hole is a feature of space-time curvature. So is the gravitational wave. Seems like you are asking what happens when to features in a medium interact, which doesn't strike me as that novel or interesting. But maybe some weird shiat happens when you do the math. Who knows.

Guessing they want to explore this to try and model Hawking Points in the MBR?

You have to decide if you think gravity is a fundamental force, or not. Space-Time is also a construct that we have to decide whether exists, or not. When we dig down into how things work, if this is not an event-driven universe, these things become symptoms not causes, and flexibility suddenly creeps in.

I personally think quantum is the result of extreme situations where the hidden flexibility of probability that is knitted into the fabric of the universe is revealed.We've spent a lot of time and effort building constructs to explain everything, but I think we're getting ridiculous. I think we've gone too far, and need to take a big step back from it.It was fun being a part of the DØ and ICECUBE, and I love the ease of the Standard Model. But as far as what's really going on, I think those are more of a convenience for scientists to discuss processes and experiment plans or results, than they are a good window into what's real.

While agnosticism about somethings on the bleeding edge of physics is reasonable, I don't think General Relativity is up for grabs. It predicted Black Holes AND Gravitational Waves. And there's nothing quantum about working entirely within that framework.

The REALLY crazy shiat is whether that event is somehow encoded without the black hole. Does it "remember" the event, in a manner of speaking. At that point, we're *way* deeper in the weeds than asking how two gravitational disturbances interact with one another.

I will readily admit that doing the actual mathematics surrounding such an event probably requires some wacky synthesis of navier-stokes and general relativity or something nuts like that, and I'm not the physistician to try and pull that off.

sxacho: I would guess that any gravity waves that would reach the event horizon would be lost to the black hole while any that don't reach the event horizon will be bent in proportion to the distance away.

This presupposes that gravity is affected by gravity. But I tend to agree that it would distort the wave as it passed through. However, gravity waves can technically propagate faster than light, so it's *possible* for the entire wave to exit a black hole, with the most extremely affected portion of the gravitational wave getting stretched out by a factor of its velocity relative to the black hole divided by the speed of light. If it's propagating slower than light speed, I can kind of imagine it locally discombobulating the wave, breaking the causal relationship between two "severed halves" of this cosmic echo.

BeesNuts: aungen: BeesNuts: What happens when a wave passes over a drain?

My guess is that a very similar thing happens. The black hole is a feature of space-time curvature. So is the gravitational wave. Seems like you are asking what happens when to features in a medium interact, which doesn't strike me as that novel or interesting. But maybe some weird shiat happens when you do the math. Who knows.

Guessing they want to explore this to try and model Hawking Points in the MBR?

You have to decide if you think gravity is a fundamental force, or not. Space-Time is also a construct that we have to decide whether exists, or not. When we dig down into how things work, if this is not an event-driven universe, these things become symptoms not causes, and flexibility suddenly creeps in.

I personally think quantum is the result of extreme situations where the hidden flexibility of probability that is knitted into the fabric of the universe is revealed.We've spent a lot of time and effort building constructs to explain everything, but I think we're getting ridiculous. I think we've gone too far, and need to take a big step back from it.It was fun being a part of the DØ and ICECUBE, and I love the ease of the Standard Model. But as far as what's really going on, I think those are more of a convenience for scientists to discuss processes and experiment plans or results, than they are a good window into what's real.

While agnosticism about somethings on the bleeding edge of physics is reasonable, I don't think General Relativity is up for grabs. It predicted Black Holes AND Gravitational Waves. And there's nothing quantum about working entirely within that framework.

The REALLY crazy shiat is whether that event is somehow encoded without the black hole. Does it "remember" the event, in a manner of speaking. At that point, we're *way* deeper in the weeds than asking how two gravitational disturbances interact with one another.

Uh . Of course quantum and general relativity are up for grabs. The rivalry is literally the highest fruit on the tree. If you seal that breach, you're the goose and the golden egg, getting the Nobel, and going down in the history books. In 2020 we had some interesting studies on white dwarf measurements that may have provided the first real link between them. This is stuff being worked on.

General Relativity is absolutely settled if you're not worried about details. But if you are, you'd better be careful. There are holes in it. Big, black holes.

BeesNuts: sxacho: I would guess that any gravity waves that would reach the event horizon would be lost to the black hole while any that don't reach the event horizon will be bent in proportion to the distance away.

This presupposes that gravity is affected by gravity. But I tend to agree that it would distort the wave as it passed through. However, gravity waves can technically propagate faster than light, so it's *possible* for the entire wave to exit a black hole, with the most extremely affected portion of the gravitational wave getting stretched out by a factor of its velocity relative to the black hole divided by the speed of light. If it's propagating slower than light speed, I can kind of imagine it locally discombobulating the wave, breaking the causal relationship between two "severed halves" of this cosmic echo.

Under what circumstances can gravity waves travel faster (or slower) than the speed of light? No snark, just genuinely curious.

hawcian: BeesNuts: sxacho: I would guess that any gravity waves that would reach the event horizon would be lost to the black hole while any that don't reach the event horizon will be bent in proportion to the distance away.

This presupposes that gravity is affected by gravity. But I tend to agree that it would distort the wave as it passed through. However, gravity waves can technically propagate faster than light, so it's *possible* for the entire wave to exit a black hole, with the most extremely affected portion of the gravitational wave getting stretched out by a factor of its velocity relative to the black hole divided by the speed of light. If it's propagating slower than light speed, I can kind of imagine it locally discombobulating the wave, breaking the causal relationship between two "severed halves" of this cosmic echo.

Under what circumstances can gravity waves travel faster (or slower) than the speed of light? No snark, just genuinely curious.

Gravity waves travel at the speed of propagation. Light slows down as it passes through a medium.

aungen: hawcian: BeesNuts: sxacho: I would guess that any gravity waves that would reach the event horizon would be lost to the black hole while any that don't reach the event horizon will be bent in proportion to the distance away.

This presupposes that gravity is affected by gravity. But I tend to agree that it would distort the wave as it passed through. However, gravity waves can technically propagate faster than light, so it's *possible* for the entire wave to exit a black hole, with the most extremely affected portion of the gravitational wave getting stretched out by a factor of its velocity relative to the black hole divided by the speed of light. If it's propagating slower than light speed, I can kind of imagine it locally discombobulating the wave, breaking the causal relationship between two "severed halves" of this cosmic echo.

Under what circumstances can gravity waves travel faster (or slower) than the speed of light? No snark, just genuinely curious.

Gravity waves travel at the speed of propagation. Light slows down as it passes through a medium.

Light doesn't slow down through a medium, the pathway is lengthened by bouncing around inside the medium before it exits at the same constant speed it maintained.

Gravity is an artifact of the spacetime continuum itself, and the 'nothing is faster than light' rules of GenRev only apply to physical objects travelling thru said continuum. The difference is why cosmic inflation was able to happen.

mcmnky: Spectrum: Ethan Siegel has finally jumped the shark.

I don't know about that, but he did use a lot of words to just to say, yes, black holes affect gravity waves the same way they affect they affect all other known massless radiation.

Yeah, that's entirely on brand.

If we assume a spherical shark of uniform density traveling at a relative velocity in the ergosphere of a supermassive galactic black hole, he's always been jumping it.

AdrienVeidt: aungen: hawcian: BeesNuts: sxacho: I would guess that any gravity waves that would reach the event horizon would be lost to the black hole while any that don't reach the event horizon will be bent in proportion to the distance away.

This presupposes that gravity is affected by gravity. But I tend to agree that it would distort the wave as it passed through. However, gravity waves can technically propagate faster than light, so it's *possible* for the entire wave to exit a black hole, with the most extremely affected portion of the gravitational wave getting stretched out by a factor of its velocity relative to the black hole divided by the speed of light. If it's propagating slower than light speed, I can kind of imagine it locally discombobulating the wave, breaking the causal relationship between two "severed halves" of this cosmic echo.

Under what circumstances can gravity waves travel faster (or slower) than the speed of light? No snark, just genuinely curious.

Gravity waves travel at the speed of propagation. Light slows down as it passes through a medium.

Light doesn't slow down through a medium, the pathway is lengthened by bouncing around inside the medium before it exits at the same constant speed it maintained.

Gravity is an artifact of the spacetime continuum itself, and the 'nothing is faster than light' rules of GenRev only apply to physical objects travelling thru said continuum. The difference is why cosmic inflation was able to happen.

c is the maximum speed light can travel through spacetime. There's no actual limit on velocity of spacetime itself.

leeksfromchichis: AdrienVeidt: aungen: hawcian: BeesNuts: sxacho: I would guess that any gravity waves that would reach the event horizon would be lost to the black hole while any that don't reach the event horizon will be bent in proportion to the distance away.

This presupposes that gravity is affected by gravity. But I tend to agree that it would distort the wave as it passed through. However, gravity waves can technically propagate faster than light, so it's *possible* for the entire wave to exit a black hole, with the most extremely affected portion of the gravitational wave getting stretched out by a factor of its velocity relative to the black hole divided by the speed of light. If it's propagating slower than light speed, I can kind of imagine it locally discombobulating the wave, breaking the causal relationship between two "severed halves" of this cosmic echo.

Under what circumstances can gravity waves travel faster (or slower) than the speed of light? No snark, just genuinely curious.

Gravity waves travel at the speed of propagation. Light slows down as it passes through a medium.

Light doesn't slow down through a medium, the pathway is lengthened by bouncing around inside the medium before it exits at the same constant speed it maintained.

Gravity is an artifact of the spacetime continuum itself, and the 'nothing is faster than light' rules of GenRev only apply to physical objects travelling thru said continuum. The difference is why cosmic inflation was able to happen.

c is the maximum speed light can travel through spacetime. There's no actual limit on velocity of spacetime itself.

Particles that travel faster than light through a medium emit what is called Cherenkov radiation, which can be seen as a blue shockwave of light behind them. Muons do this, for example. And neutrinos can arrive before the light from a supernova. Both items are specifically used in large scale multinational experiments.

None of these particles travel faster than propagation speed (c). Bosons like light travel at propagation speed, and are slowed down by interactions and entanglements. Particles like neutrinos go slightly slower, but ignore matter.

So you get a neutrino pulse sooner than you get a light pulse from a dying star, almost every time. A neutrino wave can alert telescopes to turn in a certain direction to look for a boom.

aungen: leeksfromchichis: AdrienVeidt: aungen: hawcian: BeesNuts: sxacho: [snip for length]

Particles that travel faster than light through a medium emit what is called Cherenkov radiation, which can be seen as a blue shockwave of light behind them. Muons do this, for example. And neutrinos can arrive before the light from a supernova. Both items are specifically used in large scale multinational experiments.

None of these particles travel faster than propagation speed (c). Bosons like light travel at propagation speed, and are slowed down by interactions and entanglements. Particles like neutrinos go slightly slower, but ignore matter.

So you get a neutrino pulse sooner than you get a light pulse from a dying star, almost every time. A neutrino wave can alert telescopes to turn in a certain direction to look for a boom.

Sure, light can arrive later than neutrinos or gravity waves, because light interacts with the medium it travels through, but BeesNuts was talking about gravitational waves escaping black holes. The implication is that gravitational waves can travel faster than the propagation speed of light (they even specifically said "propagate faster than light").

Gravitational waves, which do not interact the same as light waves, should travel at propagation speed (c), and not slow down like light does.

The ONLY thing I wonder about, is whether a gravity wave can ripple that first topographical circle around a black hole relative to propagation speed. I think it can.

So are black holes bigger on the inside, like a tardis?

They're constantly warping space and that curved space ends up inside the event horizon.

hawcian: BeesNuts: sxacho: I would guess that any gravity waves that would reach the event horizon would be lost to the black hole while any that don't reach the event horizon will be bent in proportion to the distance away.

This presupposes that gravity is affected by gravity. But I tend to agree that it would distort the wave as it passed through. However, gravity waves can technically propagate faster than light, so it's *possible* for the entire wave to exit a black hole, with the most extremely affected portion of the gravitational wave getting stretched out by a factor of its velocity relative to the black hole divided by the speed of light. If it's propagating slower than light speed, I can kind of imagine it locally discombobulating the wave, breaking the causal relationship between two "severed halves" of this cosmic echo.

Under what circumstances can gravity waves travel faster (or slower) than the speed of light? No snark, just genuinely curious.

They are distortions in the fabric of spacetime, so they aren't really "moving", since they are kind of the medium through which they are moving. More like they are propagating through the medium. Nothing in special or general relativity gets in the way of that propagation speed exceeding the speed of light. In fact, the *reason* light can't escape a black hole is because spacetime itself is falling into the hole faster than light speed.

/super simplified, words only explanation.
//Leonard Susskind has a couple excellent lectures on wtf we believe to be happening on the other side of the event horizon that gives excellent background on how cosmologists think about these things.

SoundOfOneHandWanking: So are black holes bigger on the inside, like a tardis?

They're constantly warping space and that curved space ends up inside the event horizon.

If you really wanna cook your noodle, current theory is that space and time switch dimensional roles the moment you cross the event horizon. Meaning you can move freely in any direction on the time axis, but that you are inexorably dragged towards the spatial singularity.

aungen: Gravitational waves, which do not interact the same as light waves, should travel at propagation speed (c), and not slow down like light does.

The ONLY thing I wonder about, is whether a gravity wave can ripple that first topographical circle around a black hole relative to propagation speed. I think it can.

I'm not so sure. there's still a question about whether the event horizon is a physical . thing. It's external topology might be causally disconnected from the rest of spacetime, and more connected to its internal geometry, whatever the fark that might end up actually being. The *real* Holographic Principle says that the topology of the event horizon is actually a full informatic description of infalling and radiating matter and energy. If that's true, then the only way to impact that topology would be to leave something behind in the black hole.

If that's *not* true, then holy hell would that be a life affirming observation to hear about. It might actually imply that there is no true singularity inside black holes, which would have *huge* implications about deep time. It'd definitely throw a monkey wrench into Sir Penrose's Conformal Cyclic Cosmology.

I suspect there is no singularity, but someone else will have to figure out how to prove that. I think you're spot on with the assessment.

## Ask Ethan: How do gravitational waves escape from a black hole?

“I think there are a number of experiments that are thinking about how you could look in different frequency bands, and get a glimpse of the primordial gravitational wave background. I think that would be really revolutionary, because that would be your first glimpse at the very first instant of our Universe.” -Dave Reitze, LIGO’s executive director

Perhaps the greatest discovery of all announced in 2016 was the direct detection of gravitational waves. Even though they had been predicted by Einstein’s general theory of relativity 101 years prior, it took the development of a laser interferometer sensitive to ripples in space that would displace two mirrors separated by multiple kilometers by less than 10^-19 meters, or 1/10,000th the width of a proton. This finally came to pass during LIGO’s 2015 data run, and two bona fide black hole-black hole merger events unambiguously popped out of the data. But how does physics actually allow this? Mārtiņš Kalvāns wants to know:

This question has puzzled me for a long time. Articles about LIGO discovery state that some percentage of black hole merger mass was radiated away, leaving [a] resulting black hole smaller than [the] sum of [the] original mergers. Yet it is accepted that nothing escapes black holes […] So my question is: how was energy radiated from black hole mergers?

This is a really deep question, and goes straight to the heart of black hole physics and general relativity.

On the one hand, we have a black hole. All of its mass/energy is concentrated together at a singularity at the center, and it’s forever invisible to the outside observer thanks to the presence of an event horizon. Inside a certain region of space (defined by the event horizon), any path that any particle can take, whether massive or massless, regardless of speed or energy, will inevitably take it into the black hole’s central singularity. This means that any particle that enters the event horizon, crosses into the event horizon or otherwise ever finds itself inside the event horizon will never be able to get out, and thus its energy is trapped inside forever. Once you’re inside a black hole, you simply become part of the singularity’s properties: mass, charge (of all different types), and spin. That’s it.

On the other hand, Einstein’s general relativity tells us that when two masses (of any type) orbit one another, it creates ripples in the fabric of space itself as the orbits themselves decay. These ripples, known as gravitational waves, move at the speed of light, cause space to expands-and-contract whenever they pass through it, and carry energy. Because of Einstein’s most famous equation, E = mc2 (or, as he wrote it originally, m = E/c2), we know that one source of energy is mass and one source of mass is energy. They can be converted into one another mass is only one particular form that energy can take on.

So when LIGO released the results of the event that occurred on September 14, 2015 in January of this year, it was only mildly surprising that they found two black holes — of 36 and 29 solar masses — merging together to create a new black hole of 62 solar masses. Where did the other 3 solar masses (about 5% of the total system’s mass) go? In the energy of gravitational waves. With subsequent events that have been detected, roughly the same trend emerges: two black holes of comparable masses inspiral and merge together, and up to around 5% of their total initial masses gets radiated away in the form of gravitational waves.

But each black hole has an event horizon. Each of the pairs has one before the merger, the final post-merger black hole has one, and at no point during the merger does either singularity become “naked” or ever emerge from an event horizon. So, how does the mass get out?

It’s not just a tricky question it’s a trick question! It’s like asking where the mass goes when protons fuse into deuterium, helium-3 and then helium-4 in the Sun. Why is helium-4 less massive than the four protons that made it up? Because of nuclear binding energy. A bound state is more stable and has less energy (and hence, less mass) than the unbound state. When two black holes inspiral, coalesce and merge, these two black holes are becoming more bound — more gravitationally bound — than they were before. The energy they’re losing is due to gravitational binding energy, not because either of the masses is exiting the event horizon.

You can see this just from Newtonian gravity. Imagine you have two masses of 1 kg each, each at rest and mutually separated by an infinite distance. They have a certain amount of energy inherent to them in this system: 1.8 × 10¹⁷ Joules, which you can get from Einstein’s equation, E = mc2. Now bring them in to one another, and bring the distance down.

• If they’re now separated by only one kilometer, the whole system has lost 6.67 × 10^-14 Joules of energy.
• If you reduce that separation to one centimeter, the system loses 6.67 × 10^-9 Joules.
• If you bring that separation down to the size of a proton, at 10^-15 meters, the system now loses an incredible 6.67 × 10⁴ Joules, or 66,700 Joules. (Now we’re getting somewhere!)
• And so if you want to lose a really significant amount of energy, you can imagine taking the separation all the way down to 10^-27 meters, where you’ll lose 6.67 × 10¹⁶ Joules, or about 35% of the original energy!

Of course, our Universe obeys general relativity on these scales, not Newtonian gravity, but the picture is the same. It isn’t that the black holes are losing mass it’s that the total amount of energy in spacetime is transforming from one form — in two well-separated, unbound masses — to another form: a single, tightly bound mass plus gravitational radiation. The orbital properties and the masses of the original black holes determine what percentage of the total original mass becomes binding energy, but in all cases it’s always true that the final mass is larger than either of the original masses but smaller than the combined raw masses. 5% is the amount that’s radiated away in the maximal case, where the two masses are roughly equal. If they had an incredible amount of energy in their spins and their spins were aligned, that percentage can be bumped up all the way to about 11%. But if one of the masses is much greater than the other, the percentage drops a 1 solar mass black hole merging with a 1,000,000 solar mass one can only radiate away 0.0001% of its energy.

The inspiral and merger doesn’t result in anything from inside the black hole getting out, but rather in spacetime deforming to account for the gravitational potential energy as the two masses coalesce and merge. The ringdown phase — which occurs at the end of the merger — represents the event horizon reverting to its maximally efficient shape: either a sphere or a spheroid. It’s the very last fraction-of-a-second of the merger where the most energy is released, but no particles from inside the black hole are getting out. Einstein’s predictions are very clear, and this is why we were able to make the detections in the first place: because we had calculated what signal to look for. Our intuition may give us trouble, but that’s why we have the equations. Even when our instincts are no good, the calculations will give us the scientific truth.

## Astroquizzical: How Does Gravity Escape From A Black Hole?

If it only moves at the speed of light, and light can’t escape, how can gravity?

I’ve heard that gravity “moves” at the speed of light if it does, then how can a black hole’s influence extend outside of the event horizon? If the answer is that “it’s not light, it ‘moves’ in a different way that’s not subject to the same rules as light,” then why does it move at the same speed?

There’s a few things tangled up in here, but let’s see if we can untangle them.

The first is about how gravity affects space-time in general. Normally you see the gravitational distortion of, say, the Earth, represented like the following:

Or you might have run into the explanation that gravity acts like a bowling ball on a rubber sheet, with heavier objects causing deeper indentations in the sheet.

These are reasonable explanations, though simplified, since the indentations happen in all directions, so these ‘indentations’ are really three dimensional distortions, or condensations, of space itself. But these simplifications illustrate an important point — fundamentally, gravity is a distortion to space itself. Often we speak of this distortion in terms of a ‘well’, which goes back to this two dimensional sheet metaphor, since it’s easy to think of things rolling downhill into a divot, formed by the presence of a large amount of mass.

Mathematically, the influence of gravity is written out as directly proportionate to the mass of the object, and inversely proportional to the square of the distance between you and that object. This distance dependence is what creates the particular smooth curve away from the center of the object. If you’re very close to the object, the gravitational well is deep, and you feel a strong gravitational pull. If you’re further away, gravity can’t pull space that far out of shape, so you feel a much weaker gravitational force.

These distortions to space are always present — for instance, all the planets are in their own distortion of space, with moons that float in ellipses within this distortion. The Sun has its own, much deeper distortion that all the planets are circling. The Sun follows the distortion of the galaxy. And, critically, each distortion was present in a milder form before each of these objects collapsed into their current form, so these distortions didn’t spontaneously form at any stage — they simply contracted and deepened as density increased. A small gas cloud could have the same mass as a small planet, but the planet’s gravitational well will be steeper than the gas cloud’s. The steeper the gravitational well, the faster you need to go to escape it, but you need both extreme mass and extreme concentration of that mass before you make it to black hole territory.

The event horizon of a black hole, in this context, describes the location beyond which, the black hole’s gravitational well is so steep, that even light can’t escape it. If you take one of these rubber sheet diagrams, you could place down a circle to describe it’s location. But the event horizon itself doesn’t describe a physical boundary to the influences of gravity. The gravitational well itself exists continuously outside and inside of the event horizon — outside of the event horizon it’s just slightly less steep. You could place another circle outside the event horizon that describes the location at which you need to go half the speed of light to escape — there’s no change or boundary in the physical distortion at this circle, it’s just a descriptive line that might prove useful to understand or describe the object’s influence on space.

The stars which eventually turn into black holes are the most massive stars that form, which tend to burn bright blue, extremely hot, and burn out quickly. Typically the threshold for the mass of the star that can leave behind a black hole is given at around eight solar masses, though this is probably a slightly fuzzy boundary. These stars have had their own gravitational distortion since they were a cloud of dense gas, well before they were stars. The stars contract, and light up, and their gravitational twist of space gets more intense, but it’s not a sudden change. It’s more of a gradual shift towards a more and more dramatic gravitational well.

The ‘movement’ of gravity at the speed of light limit can be considered entirely separately while gravity has a broad extent across a wide swath of space, the speed of light imposes a limit on how quickly information about changes can travel across that space.

The common example is that if the Sun were to spontaneously vanish — not explode, but vanish completely — the Earth wouldn’t notice anything different until about 8 minutes later, when the sunshine would vanish. But the Earth would also continue to orbit around the physically nonexistent Sun for another 8 minutes, because the changes to the distortion of space due to the Sun’s presence also haven’t reached us yet. If you think of the speed of light more as an information speed limit than as a particle speed limit, this makes it slightly easier to think about.

Information doesn’t always travel at the speed of light, though — depending on the environment that the information is traveling through, and the form of that information (which is not always light), the speed of information can proceed at speeds that are much slower than the speed of light. The speed of light in a vacuum seems to be a hard upper limit that nothing can surpass, but if your information is in the form of a compression wave, like sound, then the information travels at the speed of sound in that medium.

Think of a lightning strike — unless you’re right underneath the strike, you hear the thunder several seconds after the lightning strike. Light travels faster through air than sound does, so even though they were created at the same time, it’s the light’s information that reaches you first, and this discrepancy grows the further away you are.
The speed of light in water is even slower than through air (1.3 times slower, in fact). Effectively, the more dense the material you’re working with, the slower information goes through it.

Going back to black holes, changes to the shape of the gravitational distortion are also information traveling through a medium, though the medium of space is generally almost a perfect vacuum, so we’re working close to our absolute speed limit. As far as we know, gravity doesn’t function in a way that would slow it down in a vacuum, so it should also pass along information at the speed of light.

Gravity is one of the least fundamentally understood forces of nature we have a very good descriptive understanding of when we expect it to be important, and a very accurate description of its strength, but we don’t know exactly how it functions. How does it interact with matter? Is there a particle mediating its interactions? We have to wait a bit longer to answer these questions, but hopefully we’ll have better answers as we build detectors able to observe tiny fluctuations in the gravitational field that surrounds us!

If you have your own questions you’d like Astroquizzical to cover, you can submit them at Astroquizzical’s ask page!

## Hear the ‘chirp’ of gravitational waves passing through Earth

Researchers have announced the third detection of gravitational waves—ripples in the fabric of space and time.

Albert Einstein predicted gravitational waves as part of his theory of general relativity more than 100 years ago, but it has taken astrophysicists more than 50 years of trial and error to find the direct evidence to support his theory.

The Laser Interferometer Gravitational-wave Observatory (LIGO) made the detection January 4, 2017. Gravitational waves pass through Earth and the extremely sensitive LIGO detectors can “hear” them.

#### Hear the “chirps” of the gravitational waves in this podcast episode:

The long-awaited triumph in September 2015 of the first-ever direct observation of gravitational waves completed Einstein’s vision of a universe in which space and time are interwoven and dynamic.

“It is remarkable that humans can put together a story and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us.”

This third and latest detection points to merging black holes that are twice as far away from Earth as the two earlier pairs—about 3 billion light-years away. And this time the two black holes were unequal in size, one significantly lighter than the other. They merged into a black hole whose size is in the middle of the other two merged black hole pairs.

“Our handful of detections so far is revealing an intriguing black hole population we did not know existed until now,” says Vicky Kalogera, a senior astrophysicist with the LIGO Scientific Collaboration (LSC), which conducts research related to the twin LIGO detectors, located in the US. She is also director of Northwestern University’s astrophysics center, CIERA, and professor of physics and astronomy.

“Now we have three pairs of black holes, each pair ending their death spiral dance over millions or billions of years in some of the most powerful explosions in the universe. In astronomy, we say with three objects of the same type you have a class. We have a population, and we can do analysis.”

“Once again, the black holes are heavy,” says Shane L. Larson, researcher associate professor of physics and astronomy at Northwestern and an astronomer at the Adler Planetarium in Chicago.

“The first black holes LIGO detected were twice as heavy as we ever would have expected. Now we’ve all been churning our cranks trying to figure out all the interesting myriad ways we can imagine the universe making big and heavy black holes.”

The third detection is the subject of a new paper accepted for publication by the journal Physical Review Letters.

##### So, why all the hubbub about gravitational waves?

“With the third confirmed detection of gravitational waves from the collision of two black holes, LIGO is establishing itself as a powerful observatory for revealing the dark side of the universe,” says David Reitze of Caltech, executive director of the LIGO Laboratory. “While LIGO is uniquely suited to observing these types of events, we hope to see other types of astrophysical events soon, such as the violent collision of two neutron stars.”

This three-dimensional projection of the Milky Way galaxy onto a transparent globe shows the probable locations of the three confirmed LIGO black-hole merger events—GW150914 (blue), GW151226 (orange), and the most recent detection GW170104 (magenta)—and a fourth possible detection, at lower significance (LVT151012, green). The outer contour for each represents the 90 percent confidence region the innermost contour signifies the 10 percent confidence region. (Credit: LIGO)

### About 50X the mass of our sun

The latest finding solidifies the case for a new class of black hole pairs, or binary black holes, with masses that are larger than researchers believed possible before LIGO.

“As was the case with the first two detections, the waves detected in our new paper were generated when two black holes merged to form a larger black hole. In the latest merger, the final black hole was some 50 times the mass of our Sun,” explains coauthor Ling Sun, a PhD student at the University of Melbourne’s School of Physics and member of the Australian Research Council Centre of Excellence for Gravitational Wave (OzGrav).

This fills in a gap between the masses of the two merged black holes detected previously by LIGO, which had solar masses of 62 (first detection) and 21 (second detection).

“We have further confirmation of the existence of black holes that are heavier than 20 solar masses, objects we didn’t know existed before LIGO detected them,” says David Shoemaker of MIT, who is spokesperson for the LSC. “It is remarkable that humans can put together a story and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us.”

### Powerful collisions

The new detection, called GW170104, occurred during LIGO’s current observing run, which began November 30, 2016, and will continue through the summer. Twin detectors, one in Hanford, Washington, and the other in Livingston, Louisiana, carry out LIGO’s observations.

##### Did gravitational wave detector find dark matter?

LIGO made the first detection of gravitational waves in September 2015 during its first observing run since undergoing major upgrades in a program called Advanced LIGO. The second detection took place in December 2015. (The LIGO detectors were offline for nearly a year, from January to November 2016.)

In all three cases, each of the twin detectors of LIGO detected gravitational waves from the tremendously energetic mergers of black hole pairs—collisions that produce more power during the instant before the black holes merge than is radiated as light by all the stars and galaxies in the universe at any given time.

The recent detection is the farthest yet, with the black holes located about 3 billion light-years away. (The black holes in the first and second detections are located 1.3 and 1.4 billion light-years away, respectively.)

### Two theories

There are two primary models to explain how binary pairs of black holes can be formed.

In one model, the black holes come together later in life within crowded stellar clusters. The black holes pair up after they sink to the center of a star cluster. In this scenario, the black holes can spin in any direction relative to their orbital motion.

“This is the first time that we have evidence that the black holes may not be aligned, giving us just a tiny hint that binary black holes may form in dense stellar clusters,” comments B. P. Sathyaprakash, professor of physics and of astronomy and astrophysics at Penn State and co-leader of the paper.

The other model proposes that the black holes are born in the same binary system: they form when each star in a pair of stars explodes, and then, because the original stars were spinning in alignment, the black holes remain mostly aligned, even if not perfectly aligned.

GW170104 hints that at least one of the two black-hole spins might be misaligned with the binary orbit, mildly favoring the formation theory of dense stellar clusters.

The LIGO Laboratory receives funding from the National Science Foundation (NSF) and is operated by Caltech and MIT, which conceived and built the observatory. The NSF led in financial support for the Advanced LIGO project, with funding organizations in Germany (MPG), the UK (STFC), and Australia (ARC) making significant commitments to the project.

More than 1,000 scientists and engineers from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. LIGO partners with the Virgo Collaboration, which has the support of the Centre National de la Recherche Scientifique (CNRS), Istituto Nazionale di Fisica Nucleare (INFN), and Nikhef, as well as Virgo’s host institution, the European Gravitational Observatory, a consortium that includes 280 additional scientists throughout Europe. A list of additional partners is available here.

## Just Keep Spinning

One of the things mostly lost in the media fanfare over GW190814 is what the signal revealed about the objects’ spins. Scientists estimate merging black holes’ spins based on their analysis of the gravitational waves’ shape, and they visualize them in half-moon plots that look like this:

This diagram shows the range of possible spins that each black hole in the GW150914 merger event might have had in relation to their orbit around each other.
B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration) / Phys. Rev. X 2019, CC BY 4.0 International

The coordinates around the circumference indicate how tilted a black hole’s spin axis is with respect to the dance floor it circles with its partner. Zero degrees means the black hole was pointing straight up, 90° that it was rolling on its side, and 180° that it was upside-down compared with the black holes’ mutual orbit. The color shows which values are compatible with the data. As you can see, there are a lot of options.

But the spin plots for the two objects involved in GW190814 look like this:

Plot of the tilt-angle and spin magnitude for the 23-solar-mass black hole (left) and 2.6-solar-mass secondary object (right). The tilt angles are 0° for spins aligned, and 180° for spins anti-aligned, with the orbital angular momentum.
R. Abbott et al. / Astrophysical Journal Letters 2020, CC BY 3.0

See how tiny the purple region on the left is? When scrolling through the paper, I did a double take at this plot: We’ve seen nothing like it out of LIGO or Virgo before. The concentrated dot tells us that the larger black hole basically wasn’t spinning.

Calculations and previous gravitational-wave detections indicate that most merger-made black holes wind up with a spin about 70% of their maximum allowed rate. “Low spin means that that 23-solar-mass black hole came from a single star,” Kalogera says. “That’s a firm conclusion.”

The low spin has important implications for stellar evolution. As a star ages and swells, its rotation slows. Astronomers had predicted that both the star’s outer layers and its core will slow together, linked by magnetic fields. If the core spins slowly, then the black hole it becomes when the star dies should spin slowly, too. The large black hole in GW190814 supports that picture.

But X-ray measurements from a few black holes paired up with massive stars suggest those black holes are spinning fast. Not enough gas has poured onto them from the stars to have spun them up, either. Now the question becomes, are those measurements correct?

Scientists can’t tell how fast the 2.6-solar-mass object spun prior to the collision the right-hand spin plot is a uniform lavender cloud. That’s because the larger object dominates the signal, like a booming voice drowning out a whisper. It also largely dictated the spin of the black hole made by the merger, which is just above zero.

Black holes detected by gravitational-wave observations (blue) are almost all significantly more massive than those detected through electromagnetic observations (purple). Neutron stars measured with electromagnetic observations appear in yellow, and the neutron stars detected through gravitational waves are in orange. The new event, GW190814, is highlighted in the middle
LIGO-Virgo / Frank Elavsky & Aaron Geller (Northwestern)

## New gravitational wave detector: can be used to find a black hole the size of a tennis ball

It is understood that the research team led by him has just developed an application of this type of detector to observe &ldquosmall&rdquo primitive black holes. Their research results have been published in &ldquoPhysical Review D&rdquo a few days ago. &ldquoUntil today, these primordial black holes are still hypothetical, because it is difficult to distinguish between the black holes produced by the implosion of the stellar core and the primordial black holes. If it is possible to observe a smaller black hole, that is, a black hole with a mass of a planet but only a few centimeters in size, the situation is It will be different,&rdquo the research team said. They continued: &ldquoWe are providing experimenters with a device that can detect them by capturing the gravitational waves emitted when they merge. The frequency of this gravitational wave is much higher than currently available.&rdquo

< p>But what is the technique? A gravitational wave &ldquoantenna&rdquo is composed of a specific metal cavity and is properly immersed in a strong external magnetic field. When gravitational waves pass through a magnetic field, electromagnetic waves are generated in the cavity. To a certain extent, gravitational waves make the cavity &ldquohiss&rdquo (resonance), not with sound but with microwaves.

This device is only a few meters in size, but it is enough to detect the fusion of primitive small black holes millions of light-years away from Earth. It is much more compact than common detectors several kilometers long like LIGO, Virgo and KAGRA interferometers. This detection method makes it very sensitive to very high frequency gravitational waves (of the order of 100 MHz, while LIGO / Virgo / Kagra gravitational waves are only 10-1000 Hz), and these gravitational waves are not caused by fusion, neutron stars or stellar black holes. And other ordinary astrophysical sources.

On the other hand, it is an ideal tool for detecting small black holes. The mass and size of the planet has changed from a small ball to a tennis ball. &ldquoOur detector solution combines what we have mastered and the technologies we have in everyday life, such as the magnetron in the microwave oven, nuclear magnetic resonance magnets, and radio antennas. But don&rsquot take your home appliances apart and start taking risks: read us first. Article, and then order your equipment, figure out the equipment and the output signal,&rdquo the researcher said with a smile.

Although this patented technology is still in the advanced theoretical modeling stage, it has all the necessary elements to enter a more specific stage, namely the construction of a prototype. In any case, it paved the way for basic research on the origin of our universe. In addition to primitive black holes, this type of detector can also directly observe the gravitational waves emitted during the Big Bang to detect physics with higher energy than that achieved in particle accelerators.

## Gravitational waves: a new era for astronomy

The first ever direct obser­va­tion of grav­i­ta­tion­al waves in 2015 by the LIGO Sci­en­tif­ic Col­lab­o­ra­tion in the US is undoubt­ed­ly one of the biggest sci­en­tif­ic dis­cov­er­ies of the last decade, or even this cen­tu­ry. Six years lat­er, what can we say about these waves, and why is it so impor­tant to study them?

Grav­i­ta­tion­al waves (GWs), first pre­dict­ed to exist by Albert Ein­stein in 1916, allow for a brand-new way of look­ing at the uni­verse. Before their detec­tion, astronomers could only observe the sky using vis­i­ble light, and oth­er types of elec­tro­mag­net­ic radi­a­tion (includ­ing infrared, ultra­vi­o­let and gam­ma rays).

While light is the prop­a­ga­tion of elec­tro­mag­net­ic fields vibrat­ing in space and time, GWs are com­plete­ly dif­fer­ent: they are rip­ples in the very fab­ric of space-time itself. They can thus be emit­ted by non-lumi­nous objects.

Study­ing black hole mergers

Cre­at­ing such rip­ples in the (rather rigid) fab­ric of space-time is not easy though. Indeed, GWs can only be pro­duced by accel­er­at­ing very small and extreme­ly mas­sive objects close to the speed of light. The best can­di­dates are there­fore black holes (which are the most com­pact objects in the uni­verse), and cer­tain very dense stars known as neu­tron stars (which are between 1.4 and 2.4 solar mass­es with a diam­e­ter of less than 20 km). To com­pare, the Sun has a diam­e­ter of 1.39 mil­lion kilometres.

In gen­er­al, an iso­lat­ed black hole does not pro­duce GWs. It needs a com­pan­ion to which it remains bound for a long time (much like Earth is bound to the Moon) to form what is called a bina­ry sys­tem. As they are extreme­ly dense, the black holes deform space-time in their vicin­i­ty as they orbit each oth­er, gen­er­at­ing GW rip­ples that prop­a­gate across the uni­verse at the speed of light.

As it emits these GWs, the bina­ry los­es some of the ener­gy that binds the black holes, which end up spi­ralling ever clos­er to each oth­er. This infer­nal waltz pro­duces more and more intense GWs (that can trav­el bil­lions of light-years across the uni­verse) until the black holes even­tu­al­ly merge. From time to time, one of these bina­ries pro­duces GWs with an ampli­tude that is just large enough to be detect­ed when it reach­es Earth, even though the sig­nal is extreme­ly weak.

The LIGO detection

The first sig­nal from such a bina­ry black-hole coa­les­cence, which occurred approx­i­mate­ly 1.3 bil­lion light-years from Earth, was detect­ed in Sep­tem­ber 2015 by an instru­ment called LIGO (for Laser Inter­fer­om­e­ter Grav­i­ta­tion­al-Wave Obser­va­to­ry) 1 2 . The coa­les­cence includes the “inspi­ral” (when the black holes become clos­er), the “merg­er” (when they touch) and the “ring­down” (when the new­ly formed, big­ger black hole relax­es into a steady state).

The two black holes in ques­tion, of about 36 and 29 solar mass­es, even­tu­al­ly merged to form a sin­gle black hole of 62 solar mass­es. The 3 solar mass dif­fer­ence was entire­ly con­vert­ed into grav­i­ta­tion­al ener­gy car­ried by the GWs.

LIGO is a col­lab­o­ra­tive project with over 1000 sci­en­tists and engi­neers from more than 20 coun­tries, and three of its mem­bers were award­ed the 2017 Nobel Prize in Physics 3 . It took near­ly 50 years of intense research to build the GW detec­tors, and in June 2016 the researchers announced that they had observed a sec­ondary bina­ry black hole coa­les­cence 4 . The obser­va­tion was made on 26 Decem­ber 2015, and this time, the black holes were about 1.4 bil­lion light-years away. Rough­ly 50 such merg­er events have been detect­ed since this time. All these dis­cov­er­ies great­ly advanced many research fields and kicked off the era of grav­i­ta­tion­al-wave astronomy.

Tiny length changes

Instru­ments like LIGO and oth­er ground-based GW detec­tors, such as Vir­go in Italy and Kagra in Japan, rely on an advanced sens­ing method called laser inter­fer­om­e­try. This tech­nique has long been used to detect dif­fer­ent sorts of sig­nals, but it had nev­er been pushed to the lim­it need­ed to detect the very weak sig­nals that GWs produce.

The LIGO facil­i­ty basi­cal­ly works by send­ing twin laser beams down two 4 km-long “arms” arranged in an L‑shape and kept under a near-per­fect vac­u­um. The beams are reflect­ed by mir­rors pre­cise­ly posi­tioned at the ends of each arm. As a GW pass­es through the obser­va­to­ry, it caus­es extreme­ly tiny dis­tor­tions in the dis­tance trav­elled by each laser beam. The instru­ment is thus able to mea­sure the local con­trac­tion and expan­sion of space-time caused by the GW.

The extreme sen­si­tiv­i­ty of the instru­ment means that it is prey to all sorts of exter­nal vibra­tions (such as those from planes fly­ing by and waves on a dis­tant shore). LIGO engi­neers there­fore had to design sev­er­al inge­nious noise-reduc­tion sys­tems that not only great­ly enhance the pre­ci­sion of the detec­tors, but also allow them to dif­fer­en­ti­ate between ter­res­tri­al arte­facts and the pre­cious GW signals.

By mea­sur­ing how long it takes for the laser beams to trav­el along an arm, researchers can extract infor­ma­tion such as the fre­quen­cy and ampli­tude of the GW from the sig­nal. These quan­ti­ties are of major impor­tance since they con­tain key phys­i­cal infor­ma­tion about the source of the wave, such as its dis­tance from Earth and its posi­tion in the sky, as well its mass and whether it is a black hole or a neu­tron star.

Future detec­tors

The char­ac­ter­is­tics of inter­fer­om­e­ters like LIGO makes them sen­si­tive only to grav­i­ta­tion­al waves with­in a cer­tain fre­quen­cy band, from about 10 Hz to 10 kHz, which cor­re­sponds to black holes of about 10 to 100 solar masses.

To extend this fre­quen­cy range, the most promis­ing future project is the Laser Inter­fer­om­e­ter Space Anten­na (LISA) 5 . This Euro­pean space-based obser­va­to­ry, due to come online in 2034, will tar­get fre­quen­cies in the low­er mil­li­hertz range to detect waves from the merg­er of much big­ger black holes. These “super­mas­sive” objects are found at the cen­tres of most galax­ies – includ­ing our Milky Way – and have mass­es that are mil­lions or even bil­lions of times that of the Sun.

LISA should also be able to observe “asym­met­ric” pairs, such as a neu­tron star orbit­ing a super­mas­sive black hole, and even the so-called “cos­mic grav­i­ta­tion­al-wave back­ground”, which is very impor­tant for cos­mol­o­gy since it con­tains infor­ma­tion about the pri­mor­dial GW cre­at­ed right after the Big-Bang 6 . Far more pre­cise than its ter­res­tri­al cousins, LISA will be a mil­lions-of-kilo­me­tre-long instru­ment con­sist­ing of three tiny robots posi­tioned in an equi­lat­er­al tri­an­gle pat­tern in solar orbit just behind Earth.