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I've always wondered what would happen if a hypothetical scenario, two black holes meet, with similar proportions:

Maybe one swallow the other one?

Or would make a bigger one between them.

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This is actually a very nice example of why black holes are such extreme objects, they even break the basic rules arithmatic:

Black hole math: $$1 \text{ black hole} + 1 \text{ black hole} = 1 \text{ black hole}$$

(this is a little insider science joke)

A black hole is a region of space from which nothing, not even light, can escape. To achieve that it must have very strong gravity, so a lot of mass in a small region of space. So we may call it a "hole in spacetime", but it still has a mass.

In physical sense black holes can be described by only three quantities:

  • Its mass M (how much stuff was used to make it)
  • Its total angular momentum, J (how fast is it spinning)
  • Its electric charge, Q (does it have an electric/magnetic field)

All these three properties can change if the black hole is fed with 'stuff'. If you add matter the black hole mass increases. If that matter has (aligned) angular momentum, the black hole will spin faster and if the matter is charged, then the black hole charge changes accordingly.

When dealing with merging black holes, it is the mass property that is important. If I repeat the black-hole-math for its mass you get

$$ 1 \text{ black hole mass} + 1 \text{ black hole mass} = 2 \text{ black hole mass} $$

So a merger of two identical black holes just gives a single black hole with twice the mass.

Such black hole mergers are probably less hypothetical than you might think. They are considered to be likely method of creating supermassive black holes and are also considered as a potential source for gravitational wave emission.

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