[ELI5] if a (relatively) small black hole is orbiting a more massive black hole, as they eventually merge, would the matter be drawn back out of the singularity of the smaller?

Question

I understand that they consume matter and grow, and that black holes merge to form larger ones. But I’m curious if scientist know whether or not this takes place bit by bit, or whether because the matter can’t escape the singularity of the smaller one, it would be a case of the entire thing being eaten up all at once?

If the latter is the case, would this happen in an instant with a reaction, or just a slow process as it all gets enveloped?

To clarify: I’m aware that in some cases, ultramassive black holes have other black holes orbiting them, because I watched the kurzgesagt video on it, but that’s the extent of my knowledge

Many thanks

In: Planetary Science

Answer 1

>But I’m curious if scientist know whether or not this takes place bit by bit, or whether because the matter can’t escape the singularity of the smaller one, it would be a case of the entire thing being eaten up all at once?

ELI5 – the merger of a smaller black hole with a larger one happens quickly once they reach the final stages, resulting in a highly energetic event that produces detectable gravitational waves. The smaller black hole is not “eaten bit by bit” but rather merges entirely in one powerful event.

TL;DR – Whilst he merger itself is an “all at once” event in the sense that once the event horizons touch, they quickly coalesce into a single black hole – the lead-up to this event involves a gradual process of orbital decay over potentially millions or billions of years where both gradually lose energy through gravitational waves.

For two supermassive blackhole there is also the Final-Parsec Problem – how close can supermassive black holes get before their gravitational wave emission is no longer efficient at bringing them closer – but for stellar-mass black holes merging into more massive black holes, the final-parsec problem is not as significant because their orbits decay efficiently via gravitational wave emission up to the final merge.

Stellar-mass merging itself has been observed ([GW150914](https://en.wikipedia.org/wiki/GW150914)) – and in that event about three solar masses were converted to gravitational radiation in a fraction of a second – with a peak power of about 200 solar masses per second. To give some context, this is about 50 times the total output power of all the stars in the observable universe.