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Let's say we have 2 identical black holes, except that their charges are opposite in sign.

Can we consider one to be matter, and the other antimatter? After all, I heard that anti-particles are the same but have opposite charges.

Will they annihilate on contact?

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  • $\begingroup$ Hint: What happens with the black holes in the limit of vanishing charges? $\endgroup$
    – Qmechanic ♦
    Commented Nov 17, 2023 at 11:17
  • $\begingroup$ @Qmechanic I heard neutrons and antineutrons still annihilate. But I don't know enough to follow your hint :( $\endgroup$
    – Juan Perez
    Commented Nov 17, 2023 at 11:41

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Antimatter of charged particles have the opposite charge. But this does not say that opposite charged matter is antimatter. just take not a black hole but a normal piece of matter you can charge two of the same kind with opposite charge, and they are of cause no antimatter. Why should it be different with black holes?

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  • $\begingroup$ Just to play devils’s advocate, ordinary matter is defined by its microscopic information. It could be matter or antimatter, etc. In contrast, black holes are defined only by a handful of parameters per the No-Hair Theorem (e.g. with no difference for matter vs. antimatter). So no conclusions about black holes can be made based on ordinary matter arguments. For example, hypothetically, the upcoming theory of Quantum Gravity could potentially define some sort of “anti-black-holes” with properties that we cannot envision today based on the properties of ordinary matter. $\endgroup$
    – safesphere
    Commented Nov 17, 2023 at 15:42
  • $\begingroup$ This seems very far fetched. $\endgroup$
    – trula
    Commented Nov 17, 2023 at 15:53
  • $\begingroup$ Only the Quantum Gravity example, but the main argument stands based on the No-Hair Theorem. $\endgroup$
    – safesphere
    Commented Nov 17, 2023 at 16:04
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Black holes are objects of the classical theory of General Relativity. There are no quantum particles or antiparticles in this theory. So there is no such a thing as an “anti-black-hole”.

The No-Hair Theorem (or more generally conjecture) states that black holes don’t retain microscopic information, so there is no difference between black holes made of matter and antimatter.

Also the Hawking Black Hole Theorem predicts that black holes cannot become smaller or disappear (except through the Hawking radiation). So black holes cannot “annihilate”.

Then what happens when two oppositely charged black holes merge? Their charges cancel each other, their masses add up. The final result is just one uncharged and twice larger black hole.

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