Astronomy

How can gravitational waves escape from the gravity of black hole?

How can gravitational waves escape from the gravity of black hole?


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I know that even light can't escape from a black hole's gravity and the velocity of light and gravitational waves are the same. How can only gravitational waves escape from its gravity?


I see this phrase all the time, and I have to say I've come to greatly dislike it because it's a very bad misnomer. Nine times out of ten, when someone is talking about a black hole, they describe it as an object with such strong gravity that "not even light can escape".

However, this unqualified statement presents a strong misconception as to what black holes actually are and how they work and accomplishes nothing but confusing innocent bystanders such as yourself. The gravity of a black hole is no more or less strong than any other object in the universe. Black holes are not cosmic vacuums which use their powerful gravitational forces to suck up all nearby matter, light, etc. In fact, if you replaced our Sun with a black hole of exactly the same mass, all the planets in our system would go about orbiting exactly the same way and wouldn't notice a difference at all (aside from the mass extinction on Earth due to no longer receiving any energy from the Sun).

That being said, let's paint a better picture of what a black hole is and how it works. A black hole is a clump of mass which has become so massive that the gravitational force of that mass on itself, trying to pull it together, actually collapses the mass into a singularity. The singularity is a point-like region of space where all the mass becomes contained. Slightly outside this singularity, the physics gets weird. For example, if you're right next to this singularity, and you calculate the speed needed to get away from that singularity (e.g., you need to travel ~11 km/s to get away from Earth) you find a speed which is much greater than the speed of light. That's the origin of the phrase "not even light can escape". But, if you start farther away from the singularity, you need less speed to escape it because you feel less gravitational pull from it (gravity decreases with distance). This means, at some distance from the singularity, the speed of light actually is fast enough to escape the black hole. This distance is so important that scientists have given it a special name, the event horizon. It can get a lot more complicated than the simple picture I've painted above, but that's the general idea.

If you put all that together, then that tells you that any light which is outside the event horizon has no trouble escape the black hole. It is only the light inside this event horizon which cannot escape. Likewise, any gravitational waves outside the event horizon can escape just as easily. This is what the answer by StephenG meant by saying they were "outside" the black hole. By outside, he meant outside the event horizon. And it is true that as long as the creation of the gravitational wave happens outside the event horizon it will escape the black hole.

And just for reference of size, the supermassive black hole in the center of our galaxy, which is 4,000,000 times more massive than our Sun, has an event horizon which only extends ~10,000,000 km. That's barely out to the orbit of Mercury if it were in the position of our Sun. So you can see, it's not very hard to be outside the event horizon as the event horizon isn't that big in astronomical terms.


Gravitational waves are a distortion of space-time outside the black hole. They don't have to escape, because they're already outside.


Thread: Can gravitational waves escape from a Black Hole?

Possibly if it was modeled with mathematical degeneracy as a single or multiple degenerate triangle(s). Wikipedia had a separate page on it many years ago but most of the details seem to have been removed when the pages were merged because their Triangles page appears to makes no mention of it now.

Google probably describes it in the simplest way.

Think of the 'pea and cup' trick where the pea can travel in a straight line but is technically in 2 cups at the same time at the moment it is being transferred from one to the other.

Gravitational waves emitted inside the event horizon will not escape - they'll propagate towards the singularity.
You can't have overlapping event horizons - the supermassive black hole's horizon will move outwards to merge with the event horizons of the infalling black holes.

This is kinda what I picture of the merging EHs (about equal in mass), though the circularity at the boundary is likely too circular. [Just bubbles afterall.]

I assume that the surface area of the merged BH will be less than the sum of the original two. Is this right? [I'm still curious how to address this dramatic entropy change without a touch of violence.]

Some very interesting shapes appear during a black hole merger. Page back and forth here, for example.
The surface area of the final black hole event horizon can only be equal to or greater than the combined area of the event horizons of the merging holes - how much greater depends on the mass lost in gravitational waves during the merger.

Those are interesting and a bit surprising in their structure. Thanks.

Is this also true given the significant mass loss in a BHs merger? I don't see how the net EH could not be less.

[Added: Nevermind. I had already worked this out after the LIGO discovery. The EH size from the 36 solar mass + 29 solar mass more than doubled their original combined sizes. The 3 solar mass loss to the GW only reduced it about 9%.]

Remember the area of the event horizon is proportional to M 2 . If we ignore all the constants, then a a pair of black holes, each with mass 1, would have a combined event horizon area of 2. But if losslessly merged, they would have an event horizon area of 4.

Yep, I tried to make the correction above before you posted.

Still, for that event, the 9% reduction to the EH size suggests the entropy was reduced and this is still hard for me to think there wasn't more than a nice sinusoidal gravity wave generated. A refrigerator's lowering of entropy produces even greater heat. "Where's the heat?" (a takeoff of "Where's the beef?" commercials, which you might not have seen).

I don't know why you think that. There's a large increase in event horizon area after the merger, which is a large increase in entropy.

I don't know why you think that. There's a large increase in event horizon area after the merger, which is a large increase in entropy.

The gravitational radiation is emitted while the black holes approach each other, and while the event horizon is asymmetrical (as the peanut shaped event horizon rings down towards symmetry around the rotation axis). So mass is lost in the form of gravitational radiation before, during and after the horizon merger, but not once the event horizon has reached its equilibrium shape.

Ok, so I guess another way to say it is that the wave (mass loss) transpires while the EH is enlarging, and not as if there is a last gasp in the form of a humongous wave pulse (lower entropy event). Logical, but I did so which for some fireworks. Oh well, it does better explain why I never got traction with this when the LIGO news created waves, so to speak. [Even my puns barely make a noise. ]

At least neutron mergers are more dramatic, especially if the accretion disks are significant.

Thanks for the answers guys! So for my little thought experiment, it would seem the answer is that gravitational waves will be produced when each BH crosses the event horizon of the SMBH, but no additional gravitational waves will be emitted when the 2 BH merge inside the event horizon of the big SMBH.

My reason for asking this is because we can obviously still feel the gravity of the singularity outside of the event horizon, so I wasn't sure if that meant that gravity waves could also get out of the event horizon too.

Thanks for the answers guys! So for my little thought experiment, it would seem the answer is that gravitational waves will be produced when each BH crosses the event horizon of the SMBH, but no additional gravitational waves will be emitted when the 2 BH merge inside the event horizon of the big SMBH.

My reason for asking this is because we can obviously still feel the gravity of the singularity outside of the event horizon, so I wasn't sure if that meant that gravity waves could also get out of the event horizon too.

Well, keep in mind that the EH is where causality and information transfer go to die. IOW the EH is itself, the last possible form of gravitational information before the cutoff: a "no exit" sign.

IMO asking if gravity borne information can "pass" an EH is kind of like asking, can light reflect off a rainbow. Not meant snarkily, it is a literal comparison. Rainbows are an artifact of light and water. EH is an artifact of information and gravity.


Using gravitational waves to catch runaway intergalactic black holes

As two black holes circle each other, they emit gravitational waves in a highly asymmetric way, which leads to a net emission of momentum in some preferential direction. When the black holes finally do collide, conservation of momentum imparts a recoil, or kick, much like when a gun is fired. If the black holes are rapidly spinning in certain orientations, the speed of the recoil can be as high as 5,000 kilometres per second, easily exceeding the escape velocity of even the most massive galaxies, sending the black hole remnant resulting from the merger into intergalactic space. Image credits: SXS Lensing. Researchers have developed a new method for detecting and measuring one of the most powerful, and most mysterious, events in the universe &mdash a black hole being kicked out of its host galaxy and into intergalactic space at speeds as high as 5,000 kilometres per second (11 million miles per hour).

The method, developed by researchers from the University of Cambridge, could be used to detect and measure so-called black hole superkicks, which occur when two spinning supermassive black holes collide into each other, and the recoil of the collision is so strong that the remnant of the black hole merger is bounced out of its host galaxy entirely. Their results are reported in the journal Physical Review Letters.

Earlier this year, the LIGO Collaboration announced the first detection of gravitational waves &mdash ripples in the fabric of spacetime &mdash coming from the collision of two black holes, confirming a major prediction of Einstein’s general theory of relativity and marking the beginning of a new era in astronomy. As the sensitivity of the LIGO detectors is improved, even more gravitational waves are expected to be detected &mdash the second successful detection was announced in June.

As two black holes circle each other, they emit gravitational waves in a highly asymmetric way, which leads to a net emission of momentum in some preferential direction. When the black holes finally do collide, conservation of momentum imparts a recoil, or kick, much like when a gun is fired. When the two black holes are not spinning, the speed of the recoil is around 170 kilometres per second. But when the black holes are rapidly spinning in certain orientations, the speed of the recoil can be as high as 5,000 kilometres per second, easily exceeding the escape velocity of even the most massive galaxies, sending the black hole remnant resulting from the merger into intergalactic space.

The Cambridge researchers have developed a new method for detecting these kicks based on the gravitational wave signal alone, by using the Doppler effect. The Doppler effect is the reason that the sound of a passing car seems to lower in pitch as it gets further away. It is also widely used in astronomy: electromagnetic radiation coming from objects which are moving away from the Earth is shifted towards the red end of the spectrum, while radiation coming from objects moving closer to the Earth is shifted towards the blue end of the spectrum. Similarly, when a black hole kick has sufficient momentum, the gravitational waves it emits will be red-shifted if it is directed away from the Earth, while they will be blue-shifted if it’s directed towards the Earth.

“If we can detect a Doppler shift in a gravitational wave from the merger of two black holes, what we’re detecting is a black hole kick,” said study co-author Davide Gerosa, a PhD student from Cambridge’s Department of Applied Mathematics and Theoretical Physics. “And detecting a black hole kick would mean a direct observation that gravitational waves are carrying not just energy, but linear momentum as well.”

Detecting this elusive effect requires gravitational-wave experiments capable of observing black hole mergers with very high precision. A black hole kick cannot be directly detected using current land-based gravitational wave detectors, such as those at LIGO. However, according to the researchers, the new space-based gravitational wave detector known as eLISA, funded by the European Space Agency (ESA) and due for launch in 2034, will be powerful enough to detect several of these runaway black holes. In 2015, ESA launched the LISA Pathfinder, which is successfully testing several technologies which could be used to measure gravitational waves from space.

The researchers found that the eLISA detector will be particularly well-suited to detecting black hole kicks: it will be capable of measuring kicks as small as 500 kilometres per second, as well as the much faster superkicks. Kick measurements will tell us more about the properties of black hole spins, and also provide a direct way of measuring the momentum carried by gravitational waves, which may lead to new opportunities for testing general relativity.

“When the detection of gravitational waves was announced, a new era in astronomy began, since we can now actually observe two merging black holes,” said study co-author Christopher Moore, a Cambridge PhD student who was also a member of the team which announced the detection of gravitational waves earlier this year. “We now have two ways of detecting black holes, instead of just one &mdash it’s amazing that just a few months ago, we couldn’t say that. And with the future launch of new space-based gravitational wave detectors, we’ll be able to look at gravitational waves on a galactic, rather than a stellar, scale.”


Why Can Gravity “Escape” a Black Hole?

To answer this question, let’s start by examining what a black hole is.

According to general relativity, a black hole forms when there is so much mass in a small enough area that space-time is warped to the point that no path in space-time exists to escape.

Put another way, space itself is accelerating toward the center of the black hole so rapidly that you would need to travel faster than the speed of light to escape. And in the world of relativity, travelling faster than the speed of light is equivalent to being in two places at once according to some observers, and travelling backward in time according to others.

This is equivalent to saying that there is no future in which an object leaves a black hole after it has entered it. It can only leave it before it enters it: an obviously nonsensical statement. You can’t travel outside of a black hole any more than you can travel to yesterday.

And this is why gravity doesn’t need to escape a black hole.

The event horizon of a black hole isn’t some wall that gravity needs to break through in order to reach the outside. It’s merely an imaginary line we draw. If you’re inside a black hole, then on the other side of that line, space itself is accelerating away from you so rapidly that you could never catch up to it. It’s forever out of reach.

Gravity doesn’t need to escape a black hole, because gravity is an emergent property of the behavior of space in local areas. The space inside a black hole doesn’t need to communicate with the space outside a black hole in order for the process to work.

Space just keeps warping as you ride it toward the center of the black hole, until at some point you realize there’s no going back.

For an intro to relativity, take a look at my post on relativity for kids. For more on black holes, see my discussion with Dr. Caleb Scharf, or take a look at what physicist Leonard Susskind has to say about it and the strange world of quantum mechanics:

The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics


Why do black holes have gravity?

I just got through reading Wheeler's "Krone Experiment" and it got me thinking.

We all know the mantra "nothing can escape a black hole, not even light". With LIGO's confirmation that gravitational waves exist, and "nothing can escape a black hole", how does the gravity escape?

p.s. I do plan on researching this but I thought I would toss the question out.

#2 Jim Davis

Because gravity is not affected by gravity.

We know light is made of photons, but we do not know what gravity is made of. General relativity describes gravity as warping space. So gravitational waves are waves in "space". When we get a quantum theory of gravity together, we might understand how that works.

#3 Napp

What we call gravity is just the warping of space-time by the mass of an object. It's not something emitted by the black hole.

#4 GamesForOne

The "waves" that LIGO has detected are changes in the distance between objects that varies in a wavelike manner. In other words, when the wave passes by the distance between objects changes with time. Plot that distance variation vs. time and you get a wave shape.

It is mind-bending to know that the distances around us are as malleable as a stretchy fabric, just on a smaller scale that we don't perceive.

As Jim says, we don't know what carries those waves! All we see is the result.

#5 Keith Rivich

Because gravity is not affected by gravity.

We know light is made of photons, but we do not know what gravity is made of. General relativity describes gravity as warping space. So gravitational waves are waves in "space". When we get a quantum theory of gravity together, we might understand how that works.

Ok, I have a few thoughts on this but I will address just one for now.

If we know how holes work (basically) and we know how they effect EM radiation, does that tell us what gravity cannot be?

#6 WarmWeatherGuy

We don't know how gravity works. One idea proposed over 300 years ago (and since discarded) says that there are particles moving around everywhere through space. They are so tiny, like neutrinos, that they pass through matter like it was a sieve. They are so numerous that each piece of matter is being bombarded by them from all directions all the time. Here on Earth you will be hit by more of these from above than from below. Gravity pushes you towards Earth. The Earth does not pull you.

Here is the theory on Wikipedia. Le Sage's theory of gravitation

If you scroll down you will see why it was discarded. Speed of gravity.

It was discarded because if you move quickly in one direction then the particles in front of you will be hitting you at a higher speed than the ones hitting you from behind. This theory was discarded before the special theory of relativity. If these particles are moving at the speed of light then they will be hitting you at speed c from behind and from in front no matter how fast you go. Still, you will be hit at a higher frequency so it could be discarded for that I suppose.

One consequence of this idea is that there is a maximum amount of gravity possible. If a black hole blocks every one of these particles then adding more mass won't make it have more gravity provided the size doesn't grow. Imagine we took two earths and squished them together until they were the same size as Earth. You would expect to weigh twice as much on that double earth. But the density would be double and there would be particles that couldn't block these neutrino like particles because another particle already blocked it. So you get diminishing returns in terms of adding mass to give you more gravity. You might only weigh 1.9999999x instead of 2x on this new planet.

I am wondering if this could account for the Allais Effect where a pendulum changes its rate of precession during a total eclipse. The Moon's gravity would have less effect on us when it goes in front of the Sun because it wouldn't be blocking as many of these particles from us because the Sun has already blocked some of them from that direction.


Perfect probe

Not only will they be able to investigate black holes and strange objects known as neutron stars (giant suns that have collapsed to the size of cities), they should also be able to "look" much deeper into the Universe - and thus farther back in time. It may even be possible eventually to sense the moment of the Big Bang.

"Gravitational waves go through everything. They are hardly affected by what they pass through, and that means that they are perfect messengers," said Prof Bernard Schutz, from Cardiff University, UK.

"The information carried on the gravitational wave is exactly the same as when the system sent it out and that is unusual in astronomy. We can't see light from whole regions of our own galaxy because of the dust that is in the way, and we can't see the early part of the Big Bang because the Universe was opaque to light earlier than a certain time.

"With gravitational waves, we do expect eventually to see the Big Bang itself," he told the BBC.

In addition, the study of gravitational waves may ultimately help scientists in their quest to solve some of the biggest problems in physics, such as the unification of forces, linking quantum theory with gravity.

At the moment, General Relativity describes the cosmos on the largest scales tremendously well, but it is to quantum ideas that we resort when talking about the smallest interactions. Being able to study places in the Universe where gravity is really extreme, such as at black holes, may open a path to new, more complete thinking on these issues.

  • A laser is fed into the machine and its beam is split along two paths
  • The separate paths bounce back and forth between damped mirrors
  • Eventually, the two light parts are recombined and sent to a detector
  • Gravitational waves passing through the lab should disturb the set-up
  • Theory holds they should very subtly stretch and squeeze its space
  • This ought to show itself as a change in the lengths of the light arms (green)
  • The photodetector captures this signal in the recombined beam

Scientists have sought experimental evidence for gravitational waves for more than 40 years.

Einstein himself actually thought a detection might be beyond the reach of technology.

His theory of General Relativity suggests that objects such as stars and planets can warp space around them - in the same way that a billiard ball creates a dip when placed on a thin, stretched, rubber sheet.

Gravity is a consequence of that distortion - objects will be attracted to the warped space in the same way that a pea will fall in to the dip created by the billiard ball.


Ask Ethan: How Do Gravitational Waves Escape From A Black Hole? (Synopsis)

"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

Black holes are remarkable entities that have puzzled and fascinated us since they were first postulated long before Einstein developed his theory of relativity. One of their fundamental but bizarre properties is the fact that once something crosses or winds up inside the event horizon, it can not only never escape, it heads inevitably towards the central singularity. At that point, the only “information” about the singularity is its mass, charge (of various types), and spin.

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

Yet when two merging black holes coalesced together, as seen multiple times by LIGO, the mass of the final black hole was approximately 5% less than the sum of the masses of the two black hole progenitors. If nothing massive or massless can escape through the event horizon, how did this energy 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.

More like this

I have a question about other merger observables. If we were so lucky as to be able to observe a merger from close by (say a lightyear), with large telescopes, could we see anything other than gravitational waves? I'm assuming neither has an accretion disk, although that case might be interesting as well.

In the case of the million solar mass versus 1, how much mass is "lost"?

We also have an end case that I would call perfect aim, i.e. the two BHs centers of mass are aimed directly at each other (i.e. zero angular motion). Rather than spiralling in they merge as fast as possible.

Just on the hardware/measurement end of things,
For LIGO to have made the discovery they claim, much of the theory used to support that claim can not also be true.
In hard fact there were no orbiting black holes actually detected/observed. There was merely a template made up of what they imagined what the signal of such a thing would be like, much like the colorful CGI Black holes and space time depictions Ethan favors to decorate his blog with. They didn't even actually detect their purported circling black holes first to get a signal template from, they made one up. A computer program then tried to find a close match between a very heavily processed (manipulated) signal and a list of imagined templates. To process the signal as claimed, the scientists would have had have known what the cause was for ALL other sources of noise/vibration etc. to a level of accuracy greater than .004 the diameter of a proton measured from a device almost four kilometers long, situated on the face of a heavily populated tectonically active planet. Can we presently do this to this level of accuracy with such a signal to noise ratio? No. You can't even subtract out the background heat (which we do know about) out of a measurement from a device 4km long down to the level of less than a proton much less everything else (which we don't know about). It is true you can make your processing algorithm make extrapolations (take guesses) if you like, but in doing so you introduce even more bias into your measurement which is now composed more of computer guesses than actual data.
.
On the theoretical side of things, there are a lot of problems as well.
Gravity 'waves' or some intermediary particle like 'gravitons' are not compatible with GR. GR makes the assertion that gravity IS the space time curvature due to mass/energy. The curved math makes the curved space, and that's it, there is no other mechanism, its just a tautology. There are also no carried forces for gravity in GR as there are in Newton's gravity equations. There is no aether or background particle in GR to be a wave of something. The tensor calculus in GR has no mechanism by which it can stretch or compress space short of vigorous hand waving. There are also no 'ripples' in space time to convey such momentary spatial compressions effecting matter as claimed by LIGO, because there is no movement in space time, it isn't possible (unless you are metaphysically sidestepping into meta time by moving your point of observation about) since the time variable has already been compressed into the spatial geometry of the space time itself, you don't get to pretend you can pop it back out and have things move about inside the mathematical. space when that degree of freedom is gone.
.
It is my strong suspicion that LIGO (much like BICEP2) will eventually have to retract or slowly back away from their initial claims of discovery due largely in part because of the entire experiment being an expensive exercise in confirmation bias. Before you sharpen knives and ridicule is leveled at me for my skepticism, some perspective please, does anyone remember a short time ago "the detection is at the 5–7 sigma level, so there is less than one chance in two million of it being a random occurrence" that BICEP2 claimed? Most of you wanted BICEP2 to be true, and based on past experience with this blog, probably ridiculed those who didn't share your enthusiastic conviction. How did that work out? Do you honestly think we lucked out that one in two million chance?
.

Just on the hardware/measurement end of things,
For LIGO to have made the discovery they claim, much of the theory used to support that claim can not also be true.
[. ]
On the theoretical side of things, there are a lot of problems as well.

Thank goodness you separated the three ends and sides of things for everyone.

Sarcasm is not an argument. I was pointing out that even for argument's sake, if LIGO actually could detect 'gravity waves' or detect that matter was actually being compressed and stretched by gravity in some undisclosed way, GR would not be able to explain the observation by the very nature of what you claim to be observing. If you want waves of gravity moving about and conveying physical forces in your theory, you can't have GR too.

It's one way of going about things when someone is so demonstrably both full of themselves and full of sh*t.

I was pointing out that even for argument’s sake, if LIGO actually could detect ‘gravity waves’ or detect that matter was actually being compressed and stretched by gravity in some undisclosed way, GR would not be able to explain the observation by the very nature of what you claim to be observing.

The tensor calculus in GR has no mechanism by which it can stretch or compress space short of vigorous hand waving.

Wrong. Proper distance can be well defined in locally flat spacetime. I'm not going to walk you through the derivation of the wave solution and the reduction to two physical degrees of freedom, which can be found in plenty of introductory explanations. Hint: It doesn't involve really involve "tensor calculus" so much as algebra.

Most of you wanted BICEP2 to be true

As has been pointed out the last time somebody trotted out this routine, BICEP2 has f*ck all to do with the LIGO detection. Which reminds me:

and based on past experience with this blog, probably ridiculed those who didn’t share your enthusiastic conviction.

You're starting to sound awfully familiar.

"Sarcasm is not an argument."

However your posts are too incoherent to be argued against, since rational discourse can't work when you are busy being irrational.

Mockery is the only answer to an incoherent ramble, and incoherent is your only stock in trade.

In hard fact there were no orbiting black holes actually detected/observed. There was merely a template made up of what they imagined what the signal of such a thing would be like,

Oh not this again. We could make that same statement about most of science. We don't observe individual atoms using AF microscopy, we merely have a template of what a signal from atoms would be like (and we observe that signal instead). We don't observe the EM force, we merely have a template of what a signal from an EM force would be like (and we observe that signal instead). We don't observe brain activity, we merely have a template of what a signal of brain activity might look like (and we observe that signal instead). For that matter, I don't have any direct evidence of you. I merely have a template made up of what I imagine a signal from another person looks like.

That's what science is, CFT: taking an hypothesis, analyzing what signals we would see in the world if that hypothesis were true, and going and looking at/for those signals. When such predictions come true, the hypothesis is considered more confirmed. When the observations don't match, the hypothesis is undermined. But in many many cases, we don't actually directly observe the hypothesized law of nature or phenomena, we simply observe some effect we predict it will have on the observable world (in fact, we never "directly observe" a law of nature, we only observe how objects interact).

I have to ask - are you a creationist or did you get your science education from a private religious school? Because those folks are very into 'Baconian' science. They stress the value of direct observation over indirect observations, and they generally think grand theory making should be avoided when at all possible. You sound a lot like them. Is that your background?

They stress the value of direct observation over indirect observations

Then again, LIGO was a direct observation. CFT seems to have departed, but if the proper-distance approach is unsatisfying, one is still stuck with proper time. It's linearized GR with a TT gauge, and it's just fine. The coordinates do not need to change for the effect to be detectable.

Then again, LIGO was a direct observation

I think he was saying it's not a direct observation of BH merger. Yes its a direct observation of gravitational waves, but those could (in theory) come from some other phenomenon. That's true. but that's always technically true in science there can always be another explanation. That's just Hume's problem of induction. Which scientists don't typically worry about, and which is particularly dismissable when someone is only using it selectively against the hypotheses and theories they don't like.

Aye, Zeus could have made LIGO wobble.

Nobody made it Zeus proof. Or looked to see he wasn't there.

I think he was saying it’s not a direct observation of BH merger.

Ah. I may have paid too much attention to the scare quotes used for "gravitational waves." Thanks.

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17 Answers 17

There are some good answers here already but I hope this is a nice short summary:

Electromagnetic radiation cannot escape a black hole, because it travels at the speed of light. Similarly, gravitational radiation cannot escape a black hole either, because it too travels at the speed of light. If gravitational radiation could escape, you could theoretically use it to send a signal from the inside of the black hole to the outside, which is forbidden.

A black hole, however, can have an electric charge, which means there is an electric field around it. This is not a paradox because a static electric field is different from electromagnetic radiation. Similarly, a black hole has a mass, so it has a gravitational field around it. This is not a paradox either because a gravitational field is different from gravitational radiation.

You say the gravitational field carries information about the amount of mass (actually energy) inside, but that does not give a way for someone inside to send a signal to the outside, because to do so they would have to create or destroy energy, which is impossible. Thus there is no paradox.

Well, the information doesn't have to escape from inside the horizon, because it is not inside. The information is on the horizon.

One way to see that, is from the fact that nothing ever crosses the horizon from the perspective of an observer outside the horizon of a black hole. It asymptotically gets to the horizon in infinite time (as it is measured from the perspective of an observer at infinity).

Another way to see that, is the fact that you can get all the information you need from the boundary conditions on the horizon to describe the space-time outside, but that is something more technical.

Finally, since classical GR is a geometrical theory and not a quantum field theory*, gravitons is not the appropriate way to describe it.

*To clarify this point, GR can admit a description in the framework of gauge theories like the theory of electromagnetism. But even though electromagnetism can admit a second quantization (and be described as a QFT), GR can't.

Let's get something out of the way: let's agree not to bring gravitons into this answer. The rationale is simple: when you talk about gravitons you imply a whole lot of things about quantum phenomena, none of which is really necessary to answer your main question. In any case, gravitons propagate with the very same speed as photons: the speed of light, $c$. This way we can focus simply in Classical GR, ie, the Differential Geometry of Spacetime: this is more than enough to address your question.

In this setting, GR is a theory that says how much curvature a space "suffers" given a certain amount of mass (or energy, cf Stress-Energy Tensor).

A Black Hole is a region of spacetime that has such an intense curvature that it "pinches out" a certain region of spacetime.

In this sense, it's not too bad to understand what's going on: if you can measure the curvature of spacetime, you can definitely tell whether or not you're moving towards a region of increasing curvature (ie, towards a block hole).

This is exactly what's done: one measures the curvature of spacetime and that's enough: at some point, the curvature is so intense that the light-cones are "flipped". At that exact point, you define the Event Horizon, ie, that region of spacetime where causality is affected by the curvature of spacetime.

This is how you make a map of spacetime and can chart black holes. Given that curvature is proportional to gravitational attraction, this sequence of ideas completely addresses your doubt: you don't have anything coming out of the black hole, nor anything like that. All you need is to chart the curvature of spacetime, measuring what happens to your light-cone structure. Then, you find your Event Horizon and, thus, your black hole. This way you got all the information you need, without having anything coming out of the black hole.

The problem here is a misunderstanding of what a particle is in QFT.

A particle is an excitation of a field, not the field itself. In QED, if you set up a static central charge, and leave it there a very long time, it sets up a field $E=k$. No photons. When another charge enters that region, it feels that force. Now, that second charge will scatter and accelerate, and there, you will have a $e^<->->e^<->+gamma$ reaction due to that acceleration, (classically, the waves created by having a disturbance in the EM field) but you will not have a photon exchange with the central charge, at least not until it feels the field set up by our first charge, which will happen at some later time.

Now, consider the black hole. It is a static solution of Einstein's equations, sitting there happily. When it is intruded upon by a test mass, it already has set up its field. So, when something scatters off of it, it moves along the field set up by the black hole. Now, it will accelerate, and perhaps, "radiate a graviton", but the black hole will only feel that after the test particle's radiation field enters the black hole horizon, which it may do freely. But nowhere in this process, does a particle leave the black hole horizon.

Another example of why the naïve notion of all forces coming from a Feynman diagram with two pairs of legs is the Higgs boson&mdashthe entire universe is immersed in a nonzero Higgs field. But we only talk about the 'creation' of Higgs 'particles' when we disturb the Higgs field enough to create ripples in the Higgs field&mdashHiggs waves. Those are the Higgs particles we're looking for in the LHC. You don't need ripples in the gravitational field to explain why a planet orbits a black hole. You just need the field to have a certain distribution.

I think it's helpful to think about the related question of how the electric field gets out of a charged black hole. That question came up in the (now-defunct) Q&A section of the American Journal of Physics back in the 1990s. Matt McIrvin and I wrote up an answer that was published in the journal. You can see it at https://facultystaff.richmond.edu/

As others have pointed out, it's easier to think about the question in purely classical terms (avoiding any mention of photons or gravitons), although in the case of the electric field of a charged black hole the question is perfectly well-posed even in quantum terms: we don't have a theory of quantum gravity at the moment, but we do think we understand quantum electrodynamics in curved spacetime.

While in many ways the question was already answered, I think it should be emphasized that on the classical level, the question is in some sense backwards. The prior discussion of static and dynamic properties especially comes very close.

Let's first examine a toy model of a spherically-symmetric thin shell of dust particles collapsing into a Schwarzschild black hole. The spacetime outside of the shell will then also be Schwarzschild, but with a larger mass parameter than the original black hole (if the shell starts at rest at infinity, then just the sum of the two). Intuitively, the situation is analogous to Newton's shell theorem, which a more limited analogue in GTR. At some point, it crosses the horizon and eventually gets crushed out of existence at the singularity, the black hole now gaining mass.

So we have the following picture: as the shell collapses, the external gravitational field takes on some value, and as it crossed the horizon, the information about what it's doing can't get out the horizon. Therefore, the gravitational field can't change in response to the shell's further behavior, for this would send a signal across the horizon, e.g., a person riding along with the shell would be able to communicate across it by manipulating the shell.

Therefore, rather than gravity having a special property that enables it to cross the horizon, in a certain sense gravity can't cross the horizon, and it is that very property that forces gravity outside of it to remain the same.

Although the above answer assumed a black hole already, that doesn't matter at all, as for a spherically collapsing star the event horizon begins at the center and stretches out during the collapse (for the prior situation, it also expands to meet the shell). It also assumes that the situation has spherical symmetry, but this also turns out to not be conceptually important, although for far more complicated and unobvious reasons. Most notably, the theorems of Penrose and Hawking, as it was initially thought by some (or perhaps I should say hoped) that any perturbation from spherical symmetry would prevent black hole formation.

You may also be wondering about a related question: if the Schwarzschild solution of GTR is a vacuum, does it make sense for a vacuum to bend spacetime? The situation is somewhat analogous to a simpler one from classical electromagnetism. Maxwell's equations dictate how the electric and magnetic fields change in response to the presence and motion of electric charges, but the charges alone do not determine the field, as you can always have a wave come in from infinity without any contradictions (or something more exotic, like an everywhere-constant magnetic field), and in practice these things are dictated by boundary conditions. The situation is similar in GTR, where the Einstein field equation that dictates how geometry are connected only fixes half of the twenty degrees of freedom of spacetime curvature.


Black hole bombshell: Gravitational wave echoes may prove Stephen Hawking theory correct

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Messier 87: A look at the black hole jet in the galaxy

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Gravitational wave echoes suggest a black hole&rsquos event horizon may be more extreme than thought. University of Waterloo research reports the first tentative detection of these echoes is caused by a microscopic quantum &ldquofuzz&rdquo surrounding newly-formed black holes.

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Gravitational waves are ripples in the fabric of space-time.

Stephen Hawking used quantum mechanics to predict that quantum particles will slowly leak out of black holes

Professor Niayesh Afshordi

These are caused by the collision of enormous but compact entities in space, such as black holes and neutron stars.

Niayesh Afshordi, a physics and astronomy professor at Waterloo, said: &ldquoAccording to Einstein&rsquos Theory of General Relativity, nothing can escape from the gravity of a black hole once it has passed a point of no return, known as the event horizon.

&ldquoThis was scientists&rsquo understanding for a long time until Stephen Hawking used quantum mechanics to predict that quantum particles will slowly leak out of black holes, which we now call Hawking radiation.

Black hole news: Gravitational wave echoes suggest a black hole&rsquos event horizon may be more extreme (Image: Getty)

Black hole news: Gravitational waves are ripples in the fabric of space-time (Image: Express)

&ldquoScientists have been unable to experimentally determine if any matter is escaping black holes until the very recent detection of gravitational waves.

&ldquoIf the quantum fuzz responsible for Hawking radiation does exist around black holes, gravitational waves could bounce off of it, which would create smaller gravitational wave signals following the main gravitational collision event, similar to repeating echoes.&rdquo

Professor Afshordi and his co-author Dr Jahed Abedi from the Max Planck Institute for Gravitational Physics have reported the first tentative findings of these repeating echoes.

The news provides experimental evidence black holes may be radically different from what Einstein&rsquos Theory of Relativity predicts, and lack event horizons.

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The researchers used gravitational wave data from the first observation of a neutron star collision, recorded by the LIGO and Virgo gravitational wave detectors.

The echoes observed by Professor Afshordi and Dr Abedi match the simulated echoes predicted by models of black holes that account for the effects of quantum mechanics and Hawking radiation.

The study&rsquos co-authors wrote: &ldquoOur results are still tentative because there is a very small chance that what we see is due to random noise in the detectors, but this chance becomes less likely as we find more examples.

Black hole news: Black holes may be radically different from what Einstein&rsquos Theory of Relativity predicts (Image: Getty)

Black hole news: The researchers used gravitational wave data from the first observation of a neutron star collision (Image: Getty )

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&ldquoNow that scientists know what we&rsquore looking for, we can look for more examples, and have a much more robust confirmation of these signals.

&ldquoSuch a confirmation would be the first direct probe of the quantum structure of space-time.&rdquo

The study Echoes from the Abyss: A highly spinning black hole remnant for the binary neutron star merger GW170817 was published in the Journal of Cosmology and Astroparticle Physics in November.


Gravitational Waves Could Collide Sucking Earth Into a Black Hole

Ever wondered how the world might end? According to physicists, one unnerving possibility could involve Earth being swallowed up by a black hole created by freak gravitational waves.

Gravitational waves are invisible ripples in space that travel at the speed of light. The most powerful of these waves occur when objects move very quickly, for instance when two big stars orbit each other or two black holes orbit one another and merge. Such waves are often compared to the circular ripples that emerge when a stone is dropped in water.

However, if a particle or object travels at the speed of light, flat gravitational waves can result.

So, what would happen if these waves ran into each other? Scientists at Princeton University and the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, set out to answer this question using numerical solutions of the Einstein equations. These 10 equations detail Albert Einstein's general theory of relativity.

The findings published in the journal General Relativity and Quantum Cosmology indicate that if the waves were big enough, such as collision could create a black hole: an area of space with such a strong gravitational field that even light can't escape from it.

The physicists believe such a freak gravitational wave could be powerful enough to tangle space-time. That in turn could create a black hole. The resulting black hole could swallow up 85 percent of the wave's energy, while some of the lingering ripples would orbit the hole forever.

Frans Pretorius, study co-author and a professor of physics at Princeton University, told New Scientist: "These particles have a lot of energy and produce curvature in space-time, and when the waves collide, that curvature wraps in on itself. Space-time is sort of sucking itself into a black hole."

Reassuringly, however, if small waves collided they would likely cross each other and dissipate.

Dr. David Garfinkle, a professor in the department of physics at Oakland University in Michigan, told New Scientist nothing in the known universe exists that could cause plane-fronted waves to form a black hole.

The researchers believe the methods used in their study could help to solve other problems relating to strong field gravity and cosmology that involve particle distributions of matter.

Earlier this year, astronomers found the fastest-growing black hole ever. It ate up a mass equaling that of the sun's every two days. The black hole grew at 1 percent every million years, 12 billion years ago.

"We don't know how this one grew so large, so quickly in the early days of the Universe," Christian Wolf, an astronomer at the Australian National University (ANU), said in a statement. "The hunt is on to find even faster growing black holes."

Wolf continued: "This black hole is growing so rapidly that it's shining thousands of times more brightly than an entire galaxy, due to all of the gases it sucks in daily that cause lots of friction and heat."