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And what would happen if an antimatter black hole and a matter black hole would collide?

BananaRepublic
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    Within our current models, black holes are NOT made of any type of matter. They are simply holes in space. – Prahar Jul 15 '23 at 19:56
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    Related: https://physics.stackexchange.com/q/142551/123208 It's not an exact duplicate, but it mentions that we can't distinguish between a BH formed from matter from one formed from antimatter. – PM 2Ring Jul 15 '23 at 20:15
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    The holes would coalesce. – Ghoster Jul 15 '23 at 21:06
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    Possible duplicates: https://physics.stackexchange.com/q/25982/2451 and links therein. – Qmechanic Jul 15 '23 at 21:55
  • If there are primordial black holes, they would have (in most scenarios) formed from equal quantities matter and antimatter. – Sten Jul 15 '23 at 22:17
  • @Prahar What models are you referring to? The only validated theory so far is GR and based on it your statement is incorrect. Real astrophysical black holes are made of the collapsing star matter deeply time dilated just outside the horizon for as long as the universe exists. Only eternal black holes could be viewed as “holes in space”, but they don’t exist in reality. – safesphere Jul 17 '23 at 02:34
  • All “duplicates” are wrong. The no-hair theorem applies only to non-existing eternal black holes, but not to real astrophysical black holes. A black hole made of antimatter would interact with a normal black hole differently. See my comment below under the answer. – safesphere Jul 17 '23 at 02:42
  • @safesphere - Black holes might be created by collapsing matter or anti-matter but the final state of such a collapse is still simple a "hole in space" (in general relativity). You are absolutely wrong about the no-hair theorem. ALL black holes whether they are formed by collapse or are the non-existing eternal ones eventually stabilize to a stationary Kerr-Newman solution. There is literally no model that exists today that can tell us anything about how antimatter black holes are different from matter black holes. – Prahar Jul 17 '23 at 09:57
  • @safesphere - Note that I am not talking about the accretion disk that lies OUTSIDE of the black hole. This can of course be made of matter or anti-matter and their collisions would definitely be different depending on the type. I am talking of the black hole itself (without any accretion disk). – Prahar Jul 17 '23 at 10:02
  • @Prahar Your response doesn’t address my comment, but is just an opinion already criticized as incorrect. You still haven’t provided a reference to any specific model. As observed by us from outside, the matter or antimatter of a collapsed star always remains just above the horizon. Note that there is no information paradox in GR, the information about the star never disappears. Any falling particle conceptually can elastically bounce off something and return any time up to the eternity. So the no-hair theorem self-evidently does not apply. The “time to stabilize” that you mention is infinite. – safesphere Jul 18 '23 at 03:32
  • @Prahar To be fair, the difference between matter and antimatter black holes is conceptual in GR, but not in practical reality. This difference cannot be easily (if at all) revealed by experiment. The collapse is exponential where the collapsing matter gets within the Planck length of the horizon in seconds. What happens after that at the horizon is a mystery beyond GR. If matter is destroyed at the horizon, which is almost certain, then sure, matter and antimatter black holes are the same, but conceptually not for as long as GR still holds. – safesphere Jul 18 '23 at 03:47

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According to theory, a black hole resulting from the collapse of antimatter is indistinguishable from one resulting from the collapse of matter. If a difference were found, that would invalidate the theory, but we don't know what difference to look for.

There is very little antimatter in the universe, and it isn't observed in concentrations that could collapse to form black holes. Such concentrations should be very conspicuous, so we don't believe they exist. We thus believe that the histories of black holes that we observe involve matter, not antimatter.

If a black hole whose history involves antimatter collides with one whose history involves matter, the theory says that nothing special should happen: it's just like any other black hole collision.

John Doty
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  • It is a good answer in general +1, but the statement that nothing special happens in the collusion is not quite correct. In real astrophysical black holes, all matter of the collapsed star is “frozen” just outside the horizon. In a collusion, this matter will mix with antimatter and create spectacular fireworks with a lot of photons escaping as a burst of radiation, albeit heavily redshifted. The same will not happen in a collusion of two normal black holes. – safesphere Jul 17 '23 at 02:43
  • @safesphere - this is simply wrong. Matter is NOT frozen outside the horizon. It looks like it is frozen at the horizon for an external observer. For an infalling observer would see absolutely NOTHING special at the horizon (according to GR). Note that you'd have to be an infalling observer to even get close enough to the horizon for the black hole merger to occur so what you said about external observers is not relevant to the case at hand. – Prahar Jul 17 '23 at 10:00
  • @Prahar We study black holes by observing them from outside. Matter doesn’t “look like” frozen, it is frozen at the horizon for an external observer. The experience of an infalling observer is irrelevant to us, because he too is frozen at the horizon forever, as far as the entire external universe is concerned, including us. As a side note, your description of what happens to an infalling observer simply repeats the old flawed textbook propaganda, but I won’t expand on it since it is irrelevant anyway. The second part of your comment doesn’t make a grammatical sense to me, can you rephrase? – safesphere Jul 18 '23 at 04:02
  • @Prahar Per GR, a collision of matter and antimatter black holes would create a burst of radiation caused by the annihilation of matter and antimatter at the same radius above the horizon. However, since this radius is sub-Planck, this burst would be redshifted beyond detection and thus cannot be practically observed from outside despite conceptually existing. Also matter and antimatter may be at different radii in black holes of different age, so there may be no annihilation. Finally, as I mentioned above, matter (information) may be destroyed altogether at the horizon, but this is beyond GR. – safesphere Jul 18 '23 at 04:14