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
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The short answer is a firm “no”.
The longer answer is that black holes merge at the event horizon, and the singularities (if they actually exist) are never exposed. In fact from our point of view it’s much better to model black holes as their event horizons, ignoring the interiors completely.
Think of two drops of water merging, they don’t suddenly turn inside out at the last instant. Instead as they approach each other, their exteriors suddenly overcome mutual surface tension and you just have one drop of water. In the case of black holes it’s very similar, and you will have a slightly “wobbly” event horizon for fractions of a second, as the new black hole settles down into a new stable equilibrium.
Relevant fun fact: when two black holes merge, the diameter of the resulting black hole is the _sum_ of the individual diameters. So a 1 km and a 2 km black hole form a 3 km one. That is very different from merging water drops where the _volumes_ add up, not the diameters!
So they are already merged when they would barely touch. But it’s not as simple as staying spherical until they touch, then suddenly popping into a larger sphere. Instead they keep circling each other and each forms a elongated shape. After the merger they will still have lots of rotational energy left over, so the final sphere is wider at the equator.
Disclaimer: by diameter, shape and all that I speak about the event horizon. The singularities themselves are essentially just punctures in space.
But, if time slows down as one approaches the event horizon, wouldn’t they never actually touch?
>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.
Question about what happens when the event horizons touch and the 2 black holes merge into a single black hole that is “wobbly” for a short time before it settles down into a new stable state. Let’s just suppose that singularities exist and each black hole has a singularity at its center. Can we calculate how quickly the 2 singularities move towards each other to form the new more massive singularity of the merged black hole? And is this speed still bounded by c?
For example, if the 2 black holes both have event horizons with a diameter of 0.1 light seconds, when the event horizons first touch the centers / singularities would be at a distance of 0.1 light seconds from each other. Does this mean that it would take at least 0.1 seconds (from our point of view) for the new black hole to stabilize, since the centers / singularities can’t move towards each other to form the new center / singularity faster than c?
Not a physicist so please excuse my very likely total misunderstanding of how astrophysics works.
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