Let's say you have two same-mass black holes, about 10 solar masses each, but while one is made of pure matter, the other is made of pure antimatter. Obviously no amount of energy produced could escape the gravity of a black hole; however, when matter and antimatter meet under normal circumstances, they turn into a combination of photons and neutrinos losing more than 99% of their mass. (Resulting is much less gravity?)

I'm wondering if there are any generally accepted theories that would allow black holes to even interact in such as way that a matter/antimatter reaction could occur... and if so, could the reduction in mass cause the black holes to explode, or at least evaporate very quickly or do something else more interesting than just merge as black holes normally like to do.

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    $\begingroup$ I think this ought to be migrated to physics.stackexchange. $\endgroup$
    – Qami
    May 3, 2021 at 19:39
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    $\begingroup$ It might be helpful to talk about the worldbuilding aspect here, to make this not a pure question of physics/astronomy. $\endgroup$
    – HDE 226868
    May 3, 2021 at 19:45
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    $\begingroup$ @Qami I've slightly reworded my question to clarify that I am looking for a model of physics that would allow some sort of reaction. There is obviously no single canonical theory when it comes to how black holes behave; so, if there are any generally accepted theories that say they might explode or react in some other interesting way, I will lean more in that direction for world building reasons, but if they pretty much all agree, that you just get more black hole, I will probably go with J Dutch's explanation of just getting more black hole, and nix the idea. $\endgroup$
    – Nosajimiki
    May 3, 2021 at 20:01
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    $\begingroup$ Black holes aren't composed of baryonic matter. They are their own antimatter (as is the case with some other particles). $\endgroup$
    – John O
    May 3, 2021 at 21:40
  • $\begingroup$ No, because even light cannot escape from a black hole. The matter and antimatter would annihilate to produce gamma rays which would not travel fast enough to reach the escape velocity of a black hole, which by definition, is greater than the speed of light. $\endgroup$ May 3, 2021 at 21:57

4 Answers 4


This question has been answered here

Antimatter is exactly the same as regular matter, except everything is backwards. Electrical charges, spin directions, and configuration of all the sub-particles that make it up. It's all backwards.

Everything is opposite, except for mass. An anti-electron has the exact same amount of mass as electron.

Here's the part you care about. When equal amounts of matter and antimatter collide, they are annihilated. But not disappeared or canceled out. They're converted into pure energy.

As Einstein explained to us, mass and energy are just different aspects of the same thing. You can turn mass into energy, and you can turn energy into mass.

Black holes turn everything, both matter and energy, into more black hole.

Imagine a regular flavor and an antimatter flavor black hole with the same mass slamming together. The two would be annihilated and turn into pure energy.

Of course, the gravity of a black hole is so immense that nothing, not even light can escape. So all energy would just be turned instantaneously into more black hole. Want more black hole? Put things into the black hole.

If these two objects came together, you'd end up with a black hole with twice the mass that you had before. [...]

The bottom line is: If a regular black hole and an antimatter black hole got black-hole-married in space, they wouldn't vanish.

Feeding in antimatter won't do any good, it's just like regular matter or energy.

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    $\begingroup$ @Nosajimiki if you're looking for something almost equally dense and exotic as a BH, but want to have a spectacular kaboom, try neutron stars. Granted, their mass is limited at about 2 suns, but I still wouldn't want to be anywhere closer than a few ten thousand lightyears when it happens. $\endgroup$
    – Neinstein
    May 4, 2021 at 5:59
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    $\begingroup$ @Martijn - you're perhaps missing the point; the mass inside a black hole is no longer matter as we understand it, so "anti" or "normal" is meaningless. An observer looking at the two bodies independently would be unable to distinguish between them. $\endgroup$
    – jdunlop
    May 4, 2021 at 8:14
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    $\begingroup$ Ah, then I'm definitely missing the point :') $\endgroup$
    – Martijn
    May 4, 2021 at 8:18
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    $\begingroup$ I think you could still have your kaboom, if you count the gammaray burst from the initial annihilation - the collaps of resulted energy into a newer bigger black hole would probably not be instantanious. Still, try to enjoy the show from a distance ;) $\endgroup$
    – jena
    May 4, 2021 at 9:18
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    $\begingroup$ @Martijn we don't know what happens inside. What happens viewed from outside is that from two black holes we get one, biggger one. Neither the initial black holes, nor the resulting bigger black hole, carry any properties that can reveal what went in. $\endgroup$
    – fraxinus
    May 4, 2021 at 11:59

Nothing special would happen, as per the famous no-hair theorem, because the material that formed a black hole is irrelevant to an outside observer.

The no-hair theorem says that the three key properties of a black hole are its mass, charge, and angular momentum. When studying black holes, we don't care about the specific type of matter that formed them - protons, neutrons, neutrinos, photons, etc., whether they be matter or antimatter. The only important thing is that there is a certain amount of mass-energy contained within a certain volume. Therefore, a black hole formed from matter is indistinguishable from a black hole of the same mass, charge and angular momentum but formed from antimatter.

This means that nothing unusual would happen if the two black holes interacted or even merged into a more massive black hole. We can't really speculate about what happens beyond the event horizon, but if you had, say, a proton and an antiproton annihilate within the Schwarzschild radius, the black hole would remain unchanged - while energy would be released in the form of photons, those photons are still bound by the speed of light and would be unable to escape the black hole. It's a consequence of special relativity that changing mass into energy doesn't do anything.

One way you could see some sort of matter-antimatter reaction would be if the black holes have accretion disks, one of matter and one of antimatter. As the two bodies came closer and closer together, the disks would interact, leading to matter-antimatter annihilation and the emission of high-energy photons; electron-positron annihilation, for example, usually leads to the emission of gamma rays. This would lead to some interesting effects; the disks might disperse from the resulting radiation pressure.

This setup is admittedly a bit unrealistic. We don't expect matter and antimatter to exist in the same place in significant quantities, as they'd almost immediately annihilate. It really depends on whether you're willing to handwave anything.

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    $\begingroup$ Just to play devil's advocate, wasn't the theoretical singularity that preexisted the Big Bang just the great-grandmother of all black holes? And yet something made it go boom in a way that resulted in low-density matter (e.g., hydrogen) escaping its event horizon. What's the speculation into how that happened (or is there any)? Or am I comparing apples to corn-on-the-cob? $\endgroup$
    – JBH
    May 4, 2021 at 0:59
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    $\begingroup$ @JBH I'd say it's no worse than comparing apples to oranges - not quite as far as apples to corn on the cob. Anything trying to describe that period truly is speculation - perhaps conjecture at best; there are numerous miscellaneous models, but no known experiments can probe that far back in time. Some involve something sort of akin to a singularity (which isn't necessarily the same thing as a black hole), while others don't. $\endgroup$
    – HDE 226868
    May 4, 2021 at 1:09

There's no special law of physics that says that matter and antimatter explode when combined, or turn into photons or neutrinos when combined.

Any collection of particles can turn into any other collection of particles as long as the various quantum numbers are conserved. A collection of 100 electrons is stable because it has an electric charge of −100, and there's nothing else it can turn into that has the same charge and a low enough total energy. A collection of 90 electrons and 10 positrons is unstable because there are other things it can turn into with the same charge and total energy, such as 80 electrons and a bunch of photons.

It may be reasonable to describe two black holes with equal masses and equal and opposite electric charges as antiparticles of each other. If you combine them, you'll get a black hole with a charge of 0. It will eventually decay by Hawking radiation into photons, neutrinos, etc., but it will take a while. It just follows its usual rules, like the electrons and positrons do; there's no special rule for matter-antimatter combinations.


We don't know and you can make up the effect you want. The answer given above correctly sums up our current understanding of black holes and gravity. However, currently we can't reconcile our understanding of gravity (which explains black holes) with our understanding of quantum mechanics (which explains mater/antimatter interaction). In other words, "nothing will happen" ist not something we know but something we'd need to test in an experiment.

If you want a spectacular effect, you can make one up: During matter/antimatter annihilation, the mass/energy stops to have gravity for short moments (due to quatnum gravity effects we have not been able to observe yet, because they are to weak when we do these experiments with single atoms in a collider). This allows some mass and or radiation to escape (either in direction with the spin of the BH, or as a jet along the axis of rotation). Also the on/off sends strong gravitational waves. Some of the emitted matter/energy lumps may be dense enough to form black holes themselves, they may loose momentum when interacting with the accretion disks and fall back into the large BH ...

The shape will be weird - wikipedia has this to say about a black hole merger: "As two black holes approach each other, a ‘duckbill’ shape protrudes from each of the two event horizons towards the other one. This protrusion extends longer and narrower until it meets the protrusion from the other black hole. At this point in time the event horizon has a very narrow X-shape at the meeting point. The protrusions are drawn out into a thin thread. The meeting point expands to a roughly cylindrical connection called a bridge."

One question that a reader with a good grasp of physiscs will have is when this cataclysm will happen - I think our best guess according to current physics is that time will stop inside the event horizon due to gravitationl time dilation. Short 'hiccups' in gravity will also stop this dilation.

For story purposes it would be a bad idea if the affair becomes a completly naked singularity (20 SM matter annihiliation?), however I think you can make up a mechanism with that only partially ot for short moments 'opens' the event horizon.

For scientists this will be very exciting because it is the first chance to look at a naked singularity. Problem is: Noone can quite predict what will happen, so noone knows the safe distance to observe the event!

  • $\begingroup$ FWIW, the OP did not ask for hard science and even if - it is up to experiment what would happen ... $\endgroup$
    – mart
    May 4, 2021 at 13:57

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