# Antimatter Black Hole

## Main Question or Discussion Point

okay, this might be a very silly question but whatever.

If a black hole were composed of antimatter, would we be able to tell? If the black hole were created by anti-matter collapsing and becoming dense enough to turn it into a black hole, then if regular matter fell into it, it wouldn't annihilate, correct? Or rather, would we be able to tell, as a distance outside observer, if the black hole was annihilated?

What I'm assuming is basically that the anti-matter black hole is there from the beginning, and then a black hole of similar mass in regular matter were to collide with it.

I know that if two black holes of regular matter collide, then they become a single black hole with the mass of the two original ones added together.

But if they were two black holes of regular and anti-matter, then wouldn't the matter at the center be annihilated? But wouldn't we never be able to find out? The black hole would remain a black hole, yes? Would the black hole's mass increase, even though it should annihilate? I am pretty sure that we would be able to notice if the mass of the black hole increased or went away if it were annihilated.

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Drakkith
Staff Emeritus
First, anything beyond the event horizon is not observable. Whether the mass of the black holes was originally composed of normal matter or antimatter doesn't matter. Once beyond the EH it makes no difference.

As for what happens inside the BH, well that depends on what exactly is going on behind the event horizon. If everything is sucked into a singularity then there is nothing to annihilate. Otherwise even if matter and antimatter annihilate into photons and other particles, none of it can ever be observed since it is behind the EH.

If two black holes merge, the result is simply a higher mass black holes. It matters not whether one is antimatter. We would only see the merger of two black holes.

Maybe that's where all the anti-matter went!

First, anything beyond the event horizon is not observable. Whether the mass of the black holes was originally composed of normal matter or antimatter doesn't matter. Once beyond the EH it makes no difference.

As for what happens inside the BH, well that depends on what exactly is going on behind the event horizon. If everything is sucked into a singularity then there is nothing to annihilate. Otherwise even if matter and antimatter annihilate into photons and other particles, none of it can ever be observed since it is behind the EH.

If two black holes merge, the result is simply a higher mass black holes. It matters not whether one is antimatter. We would only see the merger of two black holes.
but so if there was a black hole that was *known* to be made of antimatter and a black hole *known* to be made of regular matter, and they collided, then wouldn't the results tell us something about what happens beyond the EH?

bcrowell
Staff Emeritus
Gold Member
http://en.wikipedia.org/wiki/No-hair_theorem

If you're comparing electrically neutral matter to electrically neutral antimatter, then the properties of the black hole end up the same. If you dump in 1 coulomb of protons versus -1 coulomb of antiprotons, the resulting black hole has different charge.

Nabeshin
but so if there was a black hole that was *known* to be made of antimatter and a black hole *known* to be made of regular matter, and they collided, then wouldn't the results tell us something about what happens beyond the EH?
To emphasize bcrowell's point, from a classical standpoint a black hole has only three properties:
1) Mass
2) Charge
3) Spin

No matter what I put into it, these are the only three bits of information that survive. All black holes with these same three properties are identical, whether they are composed of matter, antimatter, or anything else.

another black hole question:
if one were to use a cloud on neutral hydrogen gas surrounding a black hole and fire protons directly at the center of the black hole (to avoid changing BH angular momentum) and the associated electrons in the opposite direction. Since, "anything beyond the event horizon is not observable" the charge inserted remains masked, not attracting counter balancing charge. Continue this process until the the static electric force due to charge build up exceeds the gravitational attraction. What effect will it have on the black hole? What will be observable?

Nabeshin