Astronomy

What happens to the information on the event horizons of two merging black holes?

What happens to the information on the event horizons of two merging black holes?



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What happens to the hairiness/information on the horizons of two black holes if they collide?

After Hawkin the information of the matter which has fallen into the blackhole is encoded on the surface of the event horizon. What happens to the information of both blackholes when the event horizons merge?


Not every scientist agrees that information is "encoded" on the surface of a black hole. Many scientists believe black holes actually destroy information. In fact, Stephen Hawking and Kip Thorne made a famous wager against John Preskill about whether information is destroyed by black hole.

The simplest (and in my opinion, most likely) answer to your question is that black hole event horizons don't encode any information at all. The black destroys it. Which is why we say "black holes have no hair". Once you make a black hole out of any material, you can no longer tell what went into it. If you make one of photons or neutrinos or neutrons or whatever, all you know after the black hole forms is the amount of mass/energy it contains.

So when two black holes collide, their event horizons still contain zero information. Zero from the first black hole plus zero from the second.


I'm not a scientist, just someone who likes to read and think about this stuff. I think mass would be transferred.

My reasoning is as follows:

  1. Black holes aren't holes, they are masses. Gravity comes from mass. Each black hole has immense gravitational pull as they likely have incredibly dense masses in them.
  2. However, they do not have equal gravity. So what happens when two masses with enormous gravity pull on each other? The stronger pull wins.
  3. They aren't static either. Black holes do grow as they pull in more mass, so that implies that they can grow and so can probably shrink.
  4. I don't think black holes are inescapable. Light cannot escape but light has a constant speed while the force of gravity is theoretically infinite. Think about it, they grow instead of becoming an infinitely dense singular point. That means that mass resists the pull to further contract into a denser ball. So, if a black hole's gravity can be resisted, why not from a bigger gravitational pull pulled matter away from a smaller gravitational pull?
  5. There's no such thing as empty space. We are slowly realizing that empty space is likely filled with dark matter and other hard to detect substances. So the space that appears black isn't empty. So your scenario is still mass acting upon mass.
  6. I believe it has been shown that black holes have merged, so that process likely started when two event horizons collided.

Again, I'm no scientist, but it seems quite likely that mass is exchanged.


How black holes encode or destroy information is an open question, which RichS touched on. However, while he provided an answer consistent with the No Hair Theorem, I will provide an answer derived from the holographic principle. I'd like to stress that both are equally valid, since we (as of yet) do not know enough about black holes.

In terms of the holographic principle, the information about the matter that formed the black hole is encoded in some manner (micro gravitational fluctuations maybe?) on the two-dimensional 'surface' of the event horizon. It has been theorized that one could reconstruct this information by measuring the outgoing Hawking radiation, since this process decreases the radius of the black hole, hence the surface area of the event horizon, and subsequently the amount of information. I would liken this to burning a book and then trying to reconstruct the book by measuring the properties of the ashes and radiated light.

When two black holes merge, they form a black hole of smaller mass than the combined masses. Once more the total surface area of the event horizons has decreased to that of the event horizon of the new black hole, so information must have been 'radiated'. Black hole-black hole mergers are not believed to have any optical counterpart (burst of light for sake of simplicity) and the merging process is derived from relativity, so Hawking radiation is not a component of consideration. Where could the information have gone? Well, luckily enough these mergers do radiate gravitational waves, now proven by LIGO's recent discovery. Thus the information, if it does in fact encode itself on the event horizon surfaces, could be radiated/lost by the gravitational waves created during the merger.

EDIT: The above description sounds very 'hand-wavey', so I will expand on proposed theoretical method behind it.

Gravitational radiation is generated by the changing quadrupole moment, caused by the two in-spiraling black holes. However, just as for the case of electromagnetic radiation emitted by oscillating charges, the quadrupole moment contribution is only one part of the greater multipole expansion of the oscillating masses. Oscillations of the the event horizon would then cause deviations of the system from the simple quadrupole approximation, and result in gravitational radiation from the high-order multipole terms. This radiation falls off with distance faster as one moves to higher-orders, making measurement of these contributions much more difficult.

Of course this is just one proposed solution to the black hole information paradox.


I will try to answer this question properly--- but the proper answer is difficult because we don't have an exact solution for merging black holes, except for the case where one of the two is infinite in size. We also don't have a full resolution of the Cauchy horizon problem in black holes.

The classical answer, from a physical point of view, is that the black hole horizons merge, and any observer in the interior of a black hole will do what it does normally, without noticing anything from the merger, because their time is pointing in a different direction, into the center. The problem with this answer is that it requires an answer to the question of what is at the center of a black hole--- a spacelike singularity that swallows everything up (as Penrose conjectured), or a wormhole-like Cauchy-horizon pair that leads the observer to turn around and come out of the same black hole (what I personally believe). You would only realistically be able to come out of a spinning or charged black hole, even if Penrose is wrong and I am right. I should point out that absolutely nobody in the world agrees with me regarding this, but they have no real argument. But that doesn't mean anything, it's always like this when you suggest something new.

For a neutral black hole, you will hit the singular center for sure--- there is no Cauchy horizon, or rather, it shrinks to a degenerate point.

The issue of coming out of the black hole complicates the answer, because you could come out inside a bigger black hole, which the original black hole fell into during the intervening time. There is no way to answer this question without knowing how stuff comes out, so I will from now on pretend that this is impossible--- that you can't come out of a black hole. The reason is that I have no idea how long you spend inside a black hole that you come out of, and you would have to sometimes come out antimatter and left handed (if you are righthanded normally).

Anyway, ignoring this, the black holes swallow observers that are killed independently of what happens to the black hole later. Black holes that get close merge, but by each one's interior mushing up on the surface of the new black hole that forms, like soap bubbles hitting together a high-pressure region. Physically, the black hole horizons join together to make a new horizon, but the process of connecting is not classically realizable (it takes an infinite time for each black hole to fall into the other, from the exterior point of view).

Physically all this doesn't matter--- the black holes merge into one like soap bubbles merging. The merging of soap bubbles is also discontinuous from a long-wavelength continuum description.

Preserving discussion regarding the accepted answer

This discussion was interesting, and maybe helpful to see where this idea of rotating BH emissions is coming from, and that it is not mainstream physics (or at least not yet).

RM: You make some not well supported claims that I think are false: 1. An observer will reach a singularity 2. the singularity of another black hole can reach the observer first. Number 1 is only correct for nonrotating unperturbed black holes, and when a black hole is falling into another, I only know how exactly to model the interior solution in the limit that the big black hole is infinite. The observer could just rebound out of the first black hole, going through a Cauchy horizon to the outgoing sheet. As for 2, it is holographically suspect. This smells like an open question.

AB: I might get wrong somewhere of course, but let me disagree with your comment: 1) Even for perturbed and rotating (astrophysical) black holes an observer shall reach the singularity sooner or later, unless the observer gets ejected during the merger. +It has nothing to do with your ability to model the interior solution. 2) Imagine you send and observer to fall into a black hole 1 from a static position, and throw right after him a relativistically moving black hole number 2, so that it is tuned to reach observer when he crosses the horizon of black hole 1. I see no cheating here.

. However, I find very interesting the question you have posed. How much do you know about the possibility of an observer getting outside an event horizon through any process, such as black hole collisions or whatever? Maybe you have any references on that?

RM: There is no evidence that observers can reach a singularity outside of the perfect spherically symmetric Schwartschild solution (which unfortunately is presented as the generic case in books). When a black hole is perturbed, if it is rotating or charged, perhaps also when it is deformed by a strong gravitational field, the central singularity turns into a Cauchy horizon surrounding a timelike singularity. An observer cannot hit a timelike singularity, the observer just bounces past the Cauchy horizon (with tidal forces of course), turns around, and comes back out.

. The problem is that the Cauchy horizon parts are unstable to infalling deformations, and you generically get a lot of crud on the Cauchy horizon which makes a wall of hard-radiation in the limit of an eternal black wall. Some people (meaning Penrose) speculate that this means that you can't cross a Cauchy horizon. If this speculation is correct (I am pretty sure it isn't), then everything will hit a singularity in a real black hole. Whether this speculation is right or not depends on the details of both classical and quantum gravity.

. The classical issue is exactly how singular the Cauchy horizon is. From papers I have seen, and my own seat-of-the-pants intuition, the Cauchy horizon is not terribly singular, it is just like a sudden potential step in time quantum mechanics. It will excite the infalling system, make some anti-particles, but not to infinite energy, and you might be able to survive going through. This is supported by the observation that in an empty universe, the Cauchy horizon is completely regular-- no problem crossing it at all.

Once you cross Cauchy 1, you are in the center region, where you see a timelike singularity. You can't reach this timelike singularity, because it pushes you away, but you can shine light on it. You then cross a second Cauchy horizon, and you end up in the past region of a black hole very similar to the one you fell into, and then you are ejected. This cycle is most pronounced in the extremal case, where you can make things bind to a black hole and make simple harmonic motion by going in and out again and again.

. The reason quantum gravity is needed to make sense of this is that classically, the outgoing region is disconnected from the infalling region--- they are separate universes. In the 1970s, people speculated that the black hole links to another universe for this reason. But we know better today--- the only place you can come out, if this story is correct, is in this universe. But this requires a gluing map which identifies the other universe with this universe, and this gluing map is very difficult to figure out (I tried and never got a sensible answer I trusted).

. You asked about references--- unfortunately I have no references, this is just something I've been thinking about. The closest thing to an argument in the literature is that if you make a stack of D-branes, slide one out, and push it so that it falls into the others, it should oscillate back and forth in a reversible way. Unfortunately, the only literature reference I know is to an article by Gubser, which I think is incorrect and the argument he gives is not sufficiently convincing to me, which says that the brane will not oscillate reversibly, but get trapped in the stack.

PS: There is something wrong with this answer. Consider the phrase "1) The observer is able to feel the collision, provided that he hasn't yet reached the singularity of home black hole." Since, from the point of view of an observer outside the black hole, the infalling observer never actually makes it beyond the event horizon, your "provided" clause is void. – Peter Shor Mar 22 at 11:07

AB(@PS): sorry, but either you are strictly wrong here, or me/you have been unclear. Let us call observer 1 the one who is falling into a black hole and observer 2 the one who is remote. The fact that the observer 2 will never see the observer 1 crossing the horizon does not imply at all that ther observer 1 will not experience crossing the horizon or hitting the singularity. Observer 1 can experience 1) crossing the event horizon, 2) hitting the singularity, 3) feeling the tidal field of another intruding black hole. It is strict, definite and I refer to classical textbooks on GR, say MTW.

AB: I really enjoyed reading your comment and thank you for sharing your ideas. However, I believe the most relevant physics to the question is the one which concerns astrophysical black holes in classical general relativity. Why not quantum? It has not been established yet. Why astrophysical? Because the are the only type of black hole which are known to form in the universe under known physics. I will go through a couple of points where I cannot agree with you in the comments that follow.

. 1) Astrophysical black holes are formed by a gravitational collapse. They don't contain any wormholes and their formation doesn't change the topology of space-time. If an unlucky observer happens to find himself inside the event horizon of such a black hole, he cannot escape it in any way other then just by crossing backwards the horizon, which is impossible for stationary black holes.

. 1.1) The question I asked you is if you actually know the following. Given that an observer has crossed an event horizon of any astrophysical black hole (further BH), possible nonstationary, can the observer escape it, under any processes, still in the GR picture.

. 2) All perturbed BHs, including rotating ones, are known to settle down by emitting gravitational radiation. This is supported by perturbation theory and numerical relativity. If you throw in an observer, considering him as a perturbation, the system will settle down in the end, and hence the observer shall get static and get absorbed in the black hole solution, hence find himself on a singularity.

AB: Peter, you are definitely right here, probably the time sequencing could be done more careful. However, the whole description can be shrunk to what observer 1 shall see, in a sequence, and what the observer 2 shall see. For the observer 1 the sequence remains valid: he falls into the small black hole, and starts feeling the tidal field of another black hole right after crossing the horizon, and then experiences getting hit by it.

RM: The comments you made are superficially convincing, but are known today to be wrong. Rotating black holes, which are formed astrophysically, settle into a wormhole state without a doubt. The fact that the horizon, at formation, is a pure past horizon is irrelevant. We know today that past horizons and future horizons are dual quantumly, and that the fact that one Penrose diagram says things can only go in doesn't mean that another equally valid Penrose diagram can only have things going out. This is a change in the classical picture due to Susskind complementarity.

AB: Thank you again for your comment, I shall ponder on it. Can you please give me a reference to a research paper, which shows that rotating astrophysical black holes of classical general relativity settle into wormhole states?

RM: I do not use authority to support my positions, but I can explain why. The way to see this is to look at the global structure of the Kerr solution, which is worked out in Hawking and Ellis. It is qualitatively identical to the far simpler Reissner Nordstrom solution, and, unless you declare that the Cauchy horizon is too singular to pass, it makes a wormhole to another disconnected universe. The only wormhole free solution is the Schwartschild, because it is too symmetric. Wormhole to another universe is nonsense, it makes information loss, so you need gluing. – Ron Maimon Mar 23 at 18:42

. I think, from reading over your comments, that you are under the impression that black hole solutions have a singularity which absorbs matter--- this is what people say in popular books, but it is absolutely false. Only unrotating, uncharged Schwartschild black holes have a spacelike singularity which you can hit. There is not a single generic black hole solution with a space-like singularity. The spacelike singularity is just an artifact of spherical symmetry. Generic Penrose singularities are timelike.

AB: Dear Ron, so is it correct to say, that according to Hawking and Ellis (one of the best books on GR, actually) in their book all rotating noncharged black holes, formed by gravitational collapse of ordinary matter, always form also a wormhole singularity?

. I feel great affinity to your position about being critical to scientific sources, such as scientific papers, for example. However, if you make a claim not supported by a sourse you imply that you can provide a scientifically grounded proof of what you say. In other words, you make and original statement with a corresponding grounding -> you are responsible for the correctness of it. If you refer to other persons research, he/she is responsible for the statement and the proof.

RM: Hawking and Ellis say only the following, both of which are correct: 1. the exterior solution is asymptotically Kerr 2. the interior solution of a Kerr solution is a multiple-universe connecting wormhole. They do not say that the wormhole will form during collapse, because the continuation past the Cauchy horizon is suspect, because the Cauchy horizon can become singular. This is well known. It suggests that any astrophysical black hole will link to another universe, and lead to information loss, which Hawking advocated for more than 20 years, because of this property.

. I agree that I am responsible for an argument that I will not cite. But the only original thing in the things I said above is the statement that if you go into a rotating black hole, you go through the Cauchy horizons and come out of the same black hole. This is physically required by unitarity, if the Cauchy horizon is traversible and not singular, but I never found the gluing map. There is no hint for what it should be from classical mechanics, and there are crazy things--- you need to glue time backwards in some places, and you have to make sure you always come out in the future.

AB: well, though you said not a single time, that this and that popular source is incorrect. So, Hawking and Ellis in the end don't ever actually state that the black holes formed by gravitational collapse produce wormholes, right?

. Concerning Hawking and information loss, the whole idea was about no-hair theorem only: you throw information into a black hole, it settles down to a stationary solution, and you find that information has disappeared.

RM: Hawking and Ellis don't take a position on it--- they simply explain that the Kerr solution is a wormhole, and that the Kerr solution forms. You have to check everything for yourself anyway, but they happen to be correct in all their points. Kerr black holes produced by gravitational collapse produce wormholes if their interior Cauchy horizons are not too singular. It is not known if they are or not (I say no). The main difference between what I am saying and what people in the 1970s said is that I am saying the wormhole is to this universe, and you exit the same black hole.

. The no-hair theorem applies at asymptotic times--- it tells you that the black hole will be stable only in a Kerr state. It doesn't imply that the information disappears, only that the maximum entropy end state is Kerr. This is a thermal state (when not extremal) so it is just hiding the information inside like any other thermal body. I am asserting baldly that for near-extremal black holes, the stuff that goes in in the exterior picture spreads out over the horizon, then goes around the hole, and recollects and comes back out, making a harmonic oscillation in and out.


What happens if a blackhole enters another blackhole's event horizon??

I really wonder can matter escape by the gravitational chaos??

I asked this question to the scientist in NASA haven't recieved the answer yet..But this is a question we can only answer by maths which we can't use inside a black hole..So this means we can never know??

Your first sentence implies that one BH is larger than the other, so the smaller one "enters". In this case the two would merge into a more massive BH with somewhat less than the combined masses. Same would be true for any two colliding BH's, similar size or not. There have been several recent observations indicating merging Black Holes.

There would be a large gravitational effect by an output of "gravity waves" like the LIGO & LISA experiments are set to detect, and there would be a large Gravitomagnetism effect that would affect any nearby objects.

Matter would not (directly) escape the original or newly-formed, and larger, event horizon, but the combined angular momentum along with the larger magnetic field would certainly produce more "virtual particle" pairs which could, and do, become real particles by way of energy obtained from the magnetic field. See: https://www.physicsforums.com/showthread.php?t=17594 below for more on this effect.

But, all the detectable effects, and mass loss, would be observable from outside the event horizon as with any BH now, so the "unknown math" of the state(s) inside the event horizon would be no more or less necessary to understand than we do today. That said, two solitary and non-accreting black holes would be rare, and a collision of two even more rare. About all you can find published today about BH mergers deals with colliding galaxies and with binary (accreting) BH's.


Shocking New Observation: Merging Black Holes Really Can Emit Light

This simulation shows two stills from the merger of two massive black holes in a realistic, gas-rich . [+] environment. If the gas density is high enough, a black hole merger could produce an electromagnetic (light) signal: something that may have been seen in a spectacular 2019 event in both gravitational waves and optical light.

On September 14, 2015, history was made as the NSF's twin LIGO detectors directly observed humanity's first gravitational wave. From over a billion light-years away, two black holes of 36 and 29 solar masses each merged together, creating the ripples in spacetime that arrived on that fateful day. In an unexpected twist, NASA's Fermi satellite observed a weak gamma-ray signal from an unidentified location just 0.4 seconds later.

In the subsequent 5 years, LIGO has been upgraded and joined by Virgo, where some

50 additional black hole-black hole mergers have been seen. In all those events, not a single one emitted gamma-rays, X-rays, radio waves, or any other gravitational wave signal. Until, that is, May 21, 2019, when the Zwicky Transient Facility saw an electromagnetic flare coincident with one of those mergers. If true, it could cause us to rethink everything. Perhaps merging black holes do emit light, after all.

For the real black holes that exist or get created in our Universe, we can observe the radiation . [+] emitted by their surrounding matter, and the gravitational waves produced by the inspiral, merger, and ringdown phases. However, light can only be emitted from outside a black hole's event horizon.

LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

When you think about what a black hole is, you'll immediately understand why it shouldn't emit light when two of them collide. A black hole isn't a solid, physical object like the other forms of matter in our Universe. They aren't composed of identifiable particles they do not interact or react with the particles in their environments they will not emit light when another object collides with them.

The reason for this, of course, is that black holes are defined as regions of space that are so severely curved — with so much matter and energy located inside such a small volume — that nothing, not even light, can escape from them. If you have two black holes that orbit one another, gravitational radiation will cause those orbits to decay. When the two black holes merge, their event horizons coalesce, but there's still no way that light should be able to escape.

When two compact masses merge, such as neutron stars or black holes, they produce gravitational . [+] waves. The amplitude of the wave signals are proportional to the black hole masses. LIGO and Virgo, combined, have now found candidate black holes both above and below the previously anticipated mass range, but black hole-black hole mergers do not typically generate an electromagnetic signal.

NASA/Ames Research Center/C. Henze

This is in stark contrast to the merger of pretty much every other class of astrophysical object. If two stars merge together, they will create a bright, flaring phenomenon known as a luminous red nova, owing to the interactions between the matter throughout the various layers of the two stars as they merge together. Two white dwarfs merging together will lead to an even more spectacular phenomenon: a type Ia supernova, where the subsequent runaway explosion will result in the destruction of both white dwarf progenitors.

And, as we first discovered in 2017, when two neutron stars merge together, they can create a kilonova event: a bright, violent gamma-ray burst that leads to the central creation of either a new neutron star or a black hole, while generating and ejecting a large amount of heavy elements back into the Universe.

Neutron stars, when they merge, should create an electromagnetic counterpart if they don't create a . [+] black hole right away, as light and particles will be expelled due to internal reactions in the interior of these objects. However, if a black hole forms directly, the lack of an outward force and pressure could cause total collapse, where no light or matter escapes at all to the outside observers in the Universe. The event horizon is key: inside it, nothing can escape outside it (or without one entirely), light is bound to be emitted.

Dana Berry / Skyworks Digital, Inc.

For black holes, however, this should not be the case. Once you rise above a specific critical mass threshold — somewhere between 2.5 and 2.75 solar masses — you can no longer have a dense, degenerate object made out of conventional particles. Anything that would have been a white dwarf or a neutron star can no longer exist they must inevitably collapse to form a black hole instead.

White dwarfs are held up by the degeneracy pressure between electrons: the fact that no two identical fermions (one of the two classes of fundamental particle) can occupy the same quantum state. Neutron stars are held up by that same phenomenon but between neutrons: they cannot occupy the same quantum state either. When the matter composing these objects gets too dense, it triggers a set of nuclear reactions, which produce the electromagnetic radiation (i.e., light) that we then observe.

In the vicinity of a black hole, space flows like either a moving walkway or a waterfall, depending . [+] on how you want to visualize it. At the event horizon, even if you ran (or swam) at the speed of light, there would be no overcoming the flow of spacetime, which drags you into the singularity at the center. Outside the event horizon, though, other forces (like electromagnetism) can frequently overcome the pull of gravity, causing even infalling matter to escape.

Andrew Hamilton / JILA / University of Colorado

No such reactions are possible when two black holes merge. That's because whatever internal structure they have — thought to be a point singularity for (unrealistic) non-rotating black holes and a circular ring singularity for (realistic) rotating ones — is hidden behind the event horizon. Nothing that crosses over to the inside of an event horizon can ever escape, so any reactions that occur interior to the event horizon will never get out.

In other words, even if there is an internal, non-trivial structure to black holes, anything that occurs upon a collision between two of them will never get out. There will never be particles, light, or any other signal emitted from their mergers that arise from anything occurring inside the event horizons.

The only hope we have of seeing everything must come from interactions external to the event horizon itself.

This artist’s impression depicts a Sun-like star being torn apart by tidal disruption as it nears a . [+] black hole. Only material from outside a black hole's event horizon can generate observable electromagnetic signals once anything crosses over to the inside, there is no way for it to generate light.

ESO, ESA/Hubble, M. Kornmesser

This is the only plausible mechanism by which merging black holes can generate an electromagnetic (light-based) signal: if the matter surrounding them interacts during the end-stages of the merger process. There are plenty of known examples in astronomy where matter interacts with black holes to produce light:

  • during tidal disruption events, where a star gets torn apart passing close to a black hole,
  • in X-ray binaries, where a giant star has mass siphoned onto its orbiting black hole companion,
  • in an active galaxy or quasar, where accreted material flows into and around the black hole,

and so on. In all of these cases, it's not that material from inside the event horizon is getting out it's that material from outside the black hole is interacting with the external environment, emitting light in the process.

Even though black holes should have accretion disk, the electromagnetic signal expected to be . [+] generated by a black hole-black hole merger ought to be undetectable. If there's an electromagnetic counterpart, it should be caused by neutron stars.

NASA / Dana Berry (Skyworks Digital)

So what could be happening to cause the emission of light when two black holes inspiral and eventually merge? It can only be due to the presence of matter outside the event horizons of both black holes. Even though most models of black hole environments predict only very small amounts of energy transfer to the surrounding material during a merger, it is possible — at least in some extreme cases — that black hole-black hole mergers could create a light-emitting event.

For the very first black hole-black hole merger seen by LIGO, the signal that arrived at NASA's Fermi telescope was weak and arrived without directional information. It was only a 2.9-sigma signal: potentially a false positive detection the 0.22% odds of a "false alarm" are very high by physics standards. The gamma-ray burst candidate occurred when the detector was poorly-oriented with respect to the event, and ESA's complementary INTEGRAL satellite saw no signs of any high-energy emission.

The original signal from NASA's Fermi GBM detectors show, relative to LIGO's gravitational wave . [+] signal, when the excess signal arrived in their detector. This was, until recently, the only evidence for an electromagnetic signal ever produced by a black hole-black hole merger.

V. Connaughton et al. (2016), arXiv:1602.03920

Of the dozens of black hole-black hole mergers that have subsequently been detected, NASA's Fermi has seen exactly zero signs of another gamma-ray burst candidate. Perhaps it was simply an unrelated coincidence, after all.

Until, that is, May 21, 2019. On that date, the LIGO superevent database recorded a whopping three candidate events, including one that was initially reported as being a likely black hole-black hole merger with 97% probability. Its signal was seen in all three operational detectors: LIGO Livingston, LIGO Hanford, and Virgo. It was localized to a quite narrow region of space (just

2% of the sky with 90% confidence), and appears to be both very massive (around 150 solar masses total) and very distant (perhaps 10-15 billion light-years away) compared to the more typical black hole-black hole mergers we've seen.

At left, the location of the LIGO alert system's sky map for where the gravitational wave signal . [+] from May 21, 2019 arose, along with the location of the candidate electromagnetic counterpart seen by the Zwicky Transient Facility. At right, the distance estimates from gravitational waves (blue) and electromagnetic signals (black) are shown.

M.J. Graham et al., Phys. Rev. Lett. 124, 251102 (2020)

But the biggest news about it is that the Zwicky Transient Facility appears to have detected a brief electromagnetic flare that is coincident in both time and space with what our gravitational wave detectors saw. What's very exciting is that, within that

2% region of sky, they found, identified, and measured the source of the transient emission, and found a spectacularly possible culprit: an active galactic nucleus. It was chugging along like normal, and brightened suspiciously in the days following the gravitational wave event, slowly fading away over the course of a month.

The best-fit scientific explanation is this: the black hole-black hole merger could have occurred in the central, gas-rich region of a galaxy whose supermassive black hole is currently feeding on matter. The flare was likely powered by an accretion tail, and was visible in the optical part of the spectrum: the first and only black hole-black hole merger to have an optical counterpart so far. Its color is relatively constant, and it should be among the brightest signals that merging black hole can produce: large masses, relatively low-speed kicks, in dense gas environments.

This artist's concept shows a supermassive black hole in an active galaxy, with a pair of merging . [+] binary black holes passing through the gas-rich environment feeding the central black hole. The resulting flare marks the first time that optical light has been observed from a black hole-black hole merger.

While hopes were initially high that merging black holes might produce light signals, that enthusiasm faded over recent years as merger after merger failed to turn up any signal at all. With this new event, excitement is now rekindled: perhaps black holes only need the right circumstances to flare when they merge, and that future observations will ultimately reveal the link between merging black holes and the emission of light.

As Dr. Eric Burns — who worked on the 2015 detection as part of the NASA Fermi team — put it:

If true this would give us another type of joint GW-EM detections, which could be detected much further into the universe and still enable a wealth of multimessenger science. I think this work, GW150914-GBM, and similar observational investigations are important to ensure our expectations stand up to reality. Future studies should resolve this question in the next few years.

The future of merging black holes has, quite literally, never been so bright.


What happens if the event horizon of two black holes touch?

Can one be ripped apart or will they be forced to combine completely? If that's the case how long would it take?

The two black holes will revolve around each other until the event horizons collide. As they get closer to each other they orbit faster and faster until they completely collide and become one 'larger' black hole.

I would like to add that the binary black hole merger is a pretty complicated problem and that the merging process happens even before the event horizons overlap. Treating both black holes as two seperate objects is only applicable to a certain extent. LIGO has a nice video about a merging process.

Could two event horizons overlap and then separate again? (even if this would never happen naturally, is it theoretically possible?)

I'm curious because it's said nothing can ever leave after it crosses the event horizon, but if another horizon enveloped that same matter, there's the question of which black hole it would go with. And as long as one singularity is outside the other hole's event horizon, it should be able to leave, I think.

If the event horizons for two black holes get close enough, they merge and form a larger event horizon.

To add to this, assuming that their singularities will eventually meet at the same point, is it possible that they could meet with such force that it could result in an explosion that could eject some of the matter/energy beyond the new event horizon?

Event horizons are not physical things, they are regions, and not even well-defined ones. By that I mean one observer can see an event horizon where someone else does not. So event horizons don't touch or interact anymore than Senate and Congressional districts interact. You could be within the event horizons of more than one black hole and that would mean that you can't escape from either one. I don't know if that implies that the black holes themselves must merge, but that's a different question.


What would happen if the event horizons of two black holes touched?

My understanding of black holes is that once matter passes the even horizon, it can never make it back out. However, my thinking is that if another black hole came by, would it be possible for it to "suck the other black hole in"? And if so, would that qualify as matter exiting the first?

This is an awesome video! Thanks for that, it explains a lot. The rest of the series seems pretty great as well!

One thing needs to be added here. If there is no stuff in a disk around the black hole, nothing will happen. All the mass of the black hole is concentrated in the singularity, the event horizon only matters for stuff. They should behave as point masses moving in a geometry described by general relativity.

Wait, so there is no matter (mass) between the event horizon and the singularity? Or are you saying that the mass in between is negligible in comparison to the singularity's mass and thus can be ignored?

I know that gravity acts as a point source, but I'm interested in what would happen to this matter (if indeed it exists) in between the singularity and the event horizon.

So then it isn't necessary for the black holes to merge, like if they're moving rather fast to begin with? What happens to the spaceship that happens to be simultaneously within the event horizon of both black holes?

However, my thinking is that if another black hole came by, would it be possible for it to "suck the other black hole in"? And if so, would that qualify as matter exiting the first?

Yes and no. The black holes could merge. Stuff inside one event horizon would never get out -- the event horizons may combine but matter never escapes to the universe at large.

They would merge together as one black hole or just orbit each other.

I asked a similar question before, but didn't really get a satisfactory answer, so I'll ask again.

Consider this situation where two black holes come near each other. I apologize for thinking in terms of classical gravity here, but there should be a region directly between them where their gravity largely "cancels out", which would suggest to me that this region would be outside their event horizon. So I would imagine their horizons warping a little before getting close enough to merge.

I would also imagine that a region just within the horizon of one of the black holes could be made to be outside the horizon due to this warping, and thus any matter that was captured but still hadn't fallen far could be "freed".

Can someone in the know please explain what is wrong with this?

So you mean like a Lagrangian point between two black holes which would happen to be within their event horizons? It makes sense from my perspective, but I'm also quite certain a physicist would say that isn't possible.

This is a case where you should "step-back, and unask the question, because it is based on false premises". The horizon of a black hole does not exist in the same way anything else does. Locally, an observer going through the horizon does not, can not, notice anything different than anywhere else. The horizon is something we define about the system in our analysis of it. It's the boundary past which things can't escape. To even get a rough idea of this, we need to know the entire future evolution of the system, including anything like other black-holes swinging by.

Maintaining this definition of the horizon during interactions means that when a black hole comes near another, the horizon will change, but any matter or light, any trajectories behind the horizon will remain behind the horizon -- if it didn't remain behind the horizon, then our definition for the horizon earlier was flawed. Stuff that fell through the horizon is not waiting right behind it waiting to get out -- it's fallen even further in. The upshot is that it's possible to alter where stuff will fall in, but not possible to rescue anything that has fallen in.


Black holes aren't the bottomless, inescapable pits as once thought of.

While true, anything that falls in to one won't see the light of space again, it has been discovered energy in the form of radiation can and does escape.

I have been trying to get my grey cells understanding Hawkins Radiation theorem and the " The Information Paradox ".

Black holes aren't the bottomless, inescapable pits as once thought of.

While true, anything that falls in to one won't see the light of space again, it has been discovered energy in the form of radiation can and does escape.

I have been trying to get my grey cells understanding Hawkins Radiation theorem and the " The Information Paradox ".

Indeed. But Hawking radiation occurs much too slowly to have caused this. Not that I think you're suggesting it did.

The accretion disk emits x-rays as it heats up. I imagine accretion disks must be rather dense, and therefore opaque, nearer to the black hole, rather like the inside of a star. Maybe the merger disrupted the disk, allowing a large release of the energy that was trapped within.

My understanding is these accretion disks are spinning at a rapid rate of knots something like 130 times a second.

And I'm wondering if this spinning, at that speed has its own gravitational pull and through some disruptive process, allowing Gamma to escape,---or creating it.

Is this a little evidence that blackholes with their event horizons don't actually form, and that they are objects in a continual, but time dilated, state of collapse?

If they were just balls of collapsing matter then that may account for the gamma ray burst. If Black holes were real, with their singularities and event horizon, then there would be no matter to generate a gamma ray burst, would there?

well, I'm not a believer in the event horizon.

As matter falls towards the event horizon its fall is slowed by gravitational time dilation, as appeared from the outside, and never appears to cross the event horizon. oh whatever..

I think the classic "everything falls through the event horizon" model will one day be seen as very simplistic, and wrong. I hope this gamma ray burst will be evidence for the non-eventhorizon model of a "black hole"

well, I'm not a believer in the event horizon.

As matter falls towards the event horizon its fall is slowed by gravitational time dilation, as appeared from the outside, and never appears to cross the event horizon. oh whatever..

I think the classic "everything falls through the event horizon" model will one day be seen as very simplistic, and wrong. I hope this gamma ray burst will be evidence for the non-eventhorizon model of a "black hole"

I've often wondered about that. Time (from an outside perspective) comes to a standstill within the event horizon. Surely this means the matter falling inwards wouldn't have time to become a singularity before that stage. Then as the black hole radiates matter away, the event horizon will shrink and any matter exposed by it would collapse further. Finally, the black hole will have lost enough mass that the internal pressure would overcome gravity and cause it to explode. So a singularity never forms.

Although, at the centre of the infalling matter, wouldn't the gravity would be zero because the mass is pulling in all directions? So maybe that leads to a singularity.

I just don't know enough about the physics involved.

I've often wondered about that. Time (from an outside perspective) comes to a standstill within the event horizon. Surely this means the matter falling inwards wouldn't have time to become a singularity before that stage. Then as the black hole radiates matter away, the event horizon will shrink and any matter exposed by it would collapse further. Finally, the black hole will have lost enough mass that the internal pressure would overcome gravity and cause it to explode. So a singularity never forms.

Although, at the centre of the infalling matter, wouldn't the gravity would be zero because the mass is pulling in all directions? So maybe that leads to a singularity.

I just don't know enough about the physics involved.

I don't either, although I wish I did. I can imagine a scenario in which matter never collapses into a singularity because it's essentially 'frozen' in time at the point of collapse.

I don't think even the Big Bang started with a singularity as it's commonly understood!

I don't either, although I wish I did. I can imagine a scenario in which matter never collapses into a singularity because it's essentially 'frozen' in time at the point of collapse.

I don't think even the Big Bang started with a singularity as it's commonly understood!


Time travel thought -- two black holes orbiting each other

The reason why I think it could happen is that spacetime is being curved really extremely in black holes and when you draw a chart of spacetime near and in black hole , you can see that time axis is being bend towards the center of black hole and that thing is happening from all sides of the black hole, so if you enter black hole from one side , the time axis goes straight to the center of the black hole and theoretically its coming on the other side of the black hole, but in opposite direction.

But of course stuff can't exit the black hole, not even the event horizon, but that is only from the perspective of the viewer outside of black hole and of course all the stuff ends up in the center of black hole because it can't continue its natural trajectory of time axis , because there is center of black hole blocking its path. (same reason why we cant orbit the earth by simply falling down, there is earth in a way).

But if there are two orbiting black holes and their event horizons are overlapping or touching, you could travel trough there, but from the point of view of viewer outside, you would just fall in a black hole and at the same time as the antimatter version of you would fall in at the same time as you, but from exactly opposite side of the black holes. But from your point of view you would come on other side (assuming you survive and are not ripped apart by spagetification) and on the other side you would see universe traveling backwards in time, because your time axis would be reversed.


Answers and Replies

It's not really fair to us to write something and then ask us what you mean.

GR says black holes whose EHs intersect merge. You say they don't. Who are we to believe?

I understand that, but that is for black holes that merge. The specific question here is a dynamic meeting where they don't merge. The idea being that a particle within the EH of one black hole could be pulled out by the dynamic influence of another. I've never read any discussions about this particular scenario where they don't merge.

An similar analogue might be the the escape of a star from a galaxy by the collision with another galaxy. Normally it could not escape, but with an external influence can it be different?

With enough kinetic energy KE, the singularities can get any distance together and still separate. The attraction between them is not infinite, and there is a finite amount of energy E needed to pull them apart. If KE > E, then a particle can be within the event horizon of one black hole and be released.

well, what can I say .
If you think that V50 and I are wrong, go and find a credible research paper that states otherwise and link to it here

there's your mission for the day

well, what can I say .
If you think that V50 and I are wrong, go and find a credible research paper that states otherwise and link to it here

there's your mission for the day

I am asking on here to get an answer, so that someone might be able to point me to a good paper/book/paragraph. Is that not what this forum is for?

Would you respond to my question with some backup discussion? It is a very specific scenario, and gets hardly any coverage online. What I have read so far has not provided me with a satisfying answer.

No you are not asking. You are repeating the same wrong statement again and again after being told repeatedly that it is wrong. You are trying to think in a Newtonian way about something inherently relativistic.
I suggest learning GR. Currently it seems as if all your ideas come from reading popular science. This is not a good way to learn actual physics. I would recommend the new texbook by Guidry, but note that textbooks are going to have prerequisites.

This means that you have not read any of the answers in this thread or you are not satisfied by the correct answer.

No! It is a popularised misunderstanding that ”the escape velocity is c”. It is the boundary a region from which there is no possibility to reach spatial infinity by a future directed non-spacelike curve. You cannot apply classical mechanics to something that is fundamentally based on GR. The notions of time and space fron Newtonian physics are simply not applicable

It is not a ”field”. A ”field” in physics is a quantity that has a value in every point, such as the electromagnetic field or pressure.

Also wrong, given that we use the appropriate definition of field and not a made up popsci or scifi concept, which we should not if we want to discuss actual science.

Hi,
As black hole horizon radius grows linearly with the mass, a black hole with mass M1 will have a radius R1,a black hole with twice that mass ( M2=2M1) will have an horizon with twice the radius R2=2R1.
For the two horizons to touch (not even to intersect, just to be tangent) the center of the black holes have to be at maximum at a distance R1+R2 that is equivalent to 3R1, but putting 3 times the mass of R1 in a sphere of radius R3 is exactly the mass you need to create a blackhole of mass R3.
As I mentioned in a previous post I am convinced that the horizon of each black hole recedes from the incoming one, but as soon as the two masses are close enough to eachother they are surrounded by a new event horizon whose radius is derived from the mass of the two (and an external observer will see that the two black hole merge).

You appear to have a mistaken understanding of what the "radius" of a black hole's horizon is. It is not "distance to the center" the "center" of a black hole (meaning the locus ##r = 0##) is not a place in space at all, it's a moment of time that's to the future of all other moments in the hole's interior.

The "radius" of a black hole's horizon is actually ##sqrt##, where ##A## is the area of the horizon.

This is incorrect because, as above, the "center" of the holes is not a place in space to begin with, and the radius of the horizon is not "distance from the center".

Also, this description implies that the horizons are also "places in space". They're not. They are outgoing null surfaces, i.e., they are made up of light rays that are radially outgoing. So falling through a black hole's horizon is not like "passing a point in space". It's like floating in space while a blast of light rays flies past you.

It shouldn't pretty much everything in that post was wrong (see my previous post).

Kinetic energy is frame-dependent, and the kinetic energy of a black hole in a particular frame has nothing to do with the size or behavior of the hole's horizon.

In other words, these two conditions from your OP.

. are inconsistent with each other. Either the holes merge, or they don't.

Also, black holes merging is not a matter of their event horizons "overlapping". In a spacetime in which two black holes merge into one, there is only one event horizon: it just is shaped like a pair of trousers rather than like a cylinder (heuristically speaking).

No, they can't. Black holes are not like ordinary objects. Your mental model of them is wrong.

More precisely, because if they merge, there is only one horizon, not two, and it is shaped like a pair of trousers, not a cylinder, as I said in a previous post. If the holes are moving very fast relative to one another, the pair of trousers will have "legs" that are not parallel, but twisted relative to one another, heuristically speaking.

No, it is neither of those things. It is an outgoing null surface--a surface made up of radially outgoing light rays (or the worldlines that such rays would follow if they were present--no actual light rays need to be present). In other words, in suitable global coordinates, an event horizon is a surface in which radially outgoing light rays stay at the same radial coordinate. Which, as I said in a previous post, means that your usual intuitions about how coordinates work are not valid. The radial coordinate at the horizon is no longer a "spatial" coordinate: it no longer labels a "place in space". It's a null coordinate, labeling the path of an outgoing light ray (or family of such rays). And the reason nothing can escape from an event horizon is that nothing can outrun a light ray, so if radially outgoing light rays stay at the same radial coordinate, anything else must fall inward, to smaller radial coordinates.

All of this applies just as well when two black holes merge: you just have to apply it along the surface of the trousers instead of a cylinder.


What would happen if two black holes collided?

Sorry if this has been covered before, but what world happen if two black holes collided? Would we be able to observe the collision? Recommended links for further reading. Thank you.

I've answered a similar question here, but it's slightly different so I'll add a bit to it. We expect this to happen, but haven't actually observed it yet. A big reason is that all of our instruments are built to study electromagnetic waves (light). The problem is that black holes, by definition, don't allow light to escape, so that two black holes merging will have very little EM signature. It won't quite be zero, since there's bound to be some gas and dust in the vicinity, but too small for us to notice, especially given that this should happen rarely enough that we can only expect to see it in other galaxies during our lifetimes.

However, two black holes merging should be extremely loud in gravitational waves, which propagate at the speed of light and distort space-time as they move through. GWs are weak (gravity is by far the weakest of the 4 fundamental forces of strong, weak, EM, and gravity), so in order to detect them, we need extremely sensitive instruments. This is about where the other comment I linked to picks up.

However, two black holes merging should be extremely loud in gravitational waves, which propagate at the speed of light and distort space-time as they move through.

It's worth pointing out that, as I understand it, gravitational waves are still purely somewhat theoretical. edit: strong theoretical prediction with strong indirect evidence for their existence, just not yet directly observed We (generally speaking) expect them to exist, but we haven't gotten the instrumentation up and running yet capable of detecting them. There have been some large, space-based interferometers planned and others built, meant to detect gravitational waves, but so far they have eluded detection. However, this isn't problematic for theories depending on gravitational waves yet, because we haven't yet set up an instrument sensitive enough to conclusively expect to detect them. I believe that the current state of trying to detect gravitational waves puts them at "perhaps slightly on the weaker/smaller side of the range we expected to find them in".

I think it's fair to say that, realistically, we generally expect we will find them, and if they're significantly outside of the range we expect to see them (ie much smaller or less significant in impact), that is probably an avenue for interesting new physics - even more so if we don't see them at all.