Consequences of Overlapping Event Horizons in Supermassive Black Hole Mergers

[FredrikJ]

What happens if two supermassive black holes pass each other so close that their event horizons briefly overlap each others? What will happen with matter and energy that is present in the overlapped region when the event horizons no longer overlap?

 

[phinds]

If they get that close, they will merge. There are numerous threads on this forum that discuss that. I suggest a forum search.

 

[Andrea Panza]

I always thought that two event horizons cannot rally cross and black holes don't merge, but get wrapped in a bigger horizon that encloses them both

 

[Nugatory]

black holes don't merge, but get wrapped in a bigger horizon that encloses them both

It's a matter of taste whether you say that they "merge" or "get wrapped in a bigger horizon". Either way, you end up with one larger event horizon that encloses all the mass previously contained in the two; the distinction is mostly due to the ambiguity that crops any time we use natural language to describe a physical phenomenon best described mathematically.

 

[Chronos]

By the time two black holes approach near enough for their event horizons [EH] to overlap, a merger has already begun. It's not like an event horizon extends outwards for light years. For example Sag A, the supermassive black hole in the Milky Way center has a mass of around 4 million suns, but an EH only about the size of Mercury's orbit. Remember the EH is where escape velocity = c. So obviously nothing can approach that closely and avoid being eaten, regardless of its mass or velocity.