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

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?

• Ligo just picked up a signal from two merging black holes, so apparently the answer is yes. Commented Feb 24, 2016 at 14:49
• A similar question was asked here before (astronomy.stackexchange.com/q/13809/10437) you might find this useful.
– Dean
Commented Feb 24, 2016 at 14:53
• There were two black holes before, and one black hole after... Commented Feb 24, 2016 at 17:11
• Possible duplicate of Collision of 2 black holes Commented Feb 24, 2016 at 18:40
• This whole idea of storing information on the event horizon of a black hole or extrapolating to the idea of a "holographic" universe bothers me. I guess I need to get over it since this is mainstream thought including a favorite theorist of mine (Leonard Susskind). I suppose if you believe this then, if falling into a singualarity doesn't destroy information then colliding black holes shouldn't either and all would be stored on the resulting black hole. Commented Mar 1, 2016 at 3:12

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 doubt that gravitational waves can efficiently encoded this amount of information which is transfered. Of course this means that the concepts of gravity waves (General relativity), hawking radiation, or something else are flawed... It doesn't add up. Lets take an analogy. Put a fast and faster rotating object (as an analogy to gravity waves generated as the blackholes get close to each other) into a waterpool and observe the frequency of the waves...how much information can it encode? Not that much... Commented Sep 10, 2016 at 23:56
• First, consider how much information we encode using electromagnetic waves. Second, consider how much information gained from the LIGO wave detection. Finally, to counter your example, we could easily determine the presence of any surface impurities/deformities by observing microfluctuations in the waves. Learning about the surface of the rotating object, is similar to learning about the surface of the event horizon. Remember, this is like burning the book and then trying to reconstruct it; possible in theory, but not practical. Commented Sep 11, 2016 at 20:27
• Too much handwaving for me... Yes we encode a lot of information with electromagnetic waves, but this can be only done because the encoding is manmade and the frequencies are high. For the black hole merger the black hole has to emit most information with higher and higher frequency, doesnt make any sense to me me because it collapses microseconds after that... Commented Sep 12, 2016 at 0:53
• It's hardly handwaving, but rather a consequence of multipole radiation. Since the higher-order terms fall off more quickly, we can only hope to measure the lowest order (quadrapole), but, analogous to electromagnetic radiation, there are high-order terms. If information is encoded in the event horizons through gravitational potential deformations, then it would appear in the high-order radiation terms which describe the deviation of the system from the simple quadrapole approximation. Commented Sep 12, 2016 at 4:25
• ok i take the handwaving back, sorry for that! Commented Sep 13, 2016 at 16:52

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.

• If it takes an object apparently forever to fall into a black hole to an observer watching the object fall in, that's encoded information. In practice, it would be impossible to read fairly quickly, but a good argument can be made that the information is there, or, probably there. That information is, in my opinion, the simplest answer. That doesn't mean it's correct, but in terms of straight forward simplicity, I think preserving information is the simplest. Commented Jul 22, 2016 at 20:34

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.

• "while the force of gravity is theoretically infinite" Gravity is not an "infinite force". It has a specific strength, and is actually weaker than all 3 other forces. Commented Apr 6, 2016 at 7:13
• @RichS ... thanks for letting me know that it's weaker than the other three forces. Super relevant and totally new information! And so you're implying that gravity's force is constant, and doesn't increase proportional to mass? I'm no genius or even physicist but come on man, go color in a pretty picture. Thanks for the down vote
– Dave
Commented Apr 6, 2016 at 9:22
• No worries. The pull of gravity increases proportional to mass, and decreases with distance, but the force of a gravity is a constant. The pull of a specific object is different from the overall force of gravity. Neither the gravitational pull of any object nor the value of big G - the gravitational constant of the universe - is infinite. The universe would be vastly different if it was. en.wikipedia.org/wiki/Gravitational_constant Commented Apr 6, 2016 at 21:16
• Do you understand the question about hair and information? Because your answer didn't touch on the question. Commented Jul 1, 2016 at 4:07