What happens when black holes collide? Does one black hole “consume” the other? Do they become a larger black hole? Does it get more dense or just larger?
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I don't know how to address the "consume" question. If you were pulling on a piece of fabric and two tears in it grew until they met each other to become one tear... would you say that the larger one consumed the smaller?
Wait, really? So if you had a super massive disk that was just 1 electron away from having enough mass to become a black hole... and then an electron popped into existence due to quantum randomness... then it would become a sphere instantly? Wouldn't that violate the speed of light or something?
The event horizon is the imaginary line across the river which once passed, even if you paddle as quickly as you can, you won't be able to get away from the waterfall. Once you pass that line, you're bound to reach the waterfall eventually.
Now, thanks to Maxwell and Einstein, we know there's a maximum speed that anyone can paddle, the speed of light, and so we define the event horizon to be relative to this speed.
You can calculate the event horizon for just about anything. The main difference between a black hole and everything else, is that for a black hole the event horizon is larger than the object itself.
For example, the event horizon of a neutron star with a mass of 1.4 solar masses and a radius of 10km is about 4.1 km, well inside the neutron star. Thus you don't get the "black hole effect", since once you pass the surface of the neutron star the matter above you pulls you away from the center.
The river analogy is actually not far off what they try to use as an analog for testing black hole predictions, effectively a large water tank with a drain hole. Sixty Symbols did a video on this way back[1], and this thesis[2] goes into the details. Some are going beyond water using liquid helium to simulate quantum black holes this way[3].
[1]: https://www.youtube.com/watch?v=kOnoYQchHFw