Can black holes “eat” matter indefinitely or is there a limit? Do they ever have trouble absorbing large masses or is it always the same?

Question

Can black holes “eat” matter indefinitely or is there a limit? Do they ever have trouble absorbing large masses or is it always the same?

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Answer 1

These are excellent questions: first of all there is no meaningful limit on how large a black hole can be. There are some practical limits like: it can’t be any bigger than the amount of matter in its direct vicinity, but generally speaking they can be anywhere from the mass of a large star to the mass of multiple galaxies. (As a side note: Some really old black holes can be absolutely enormous because the universe used to be a lot more matter-dense)

But yes, they do definitely have some limitations regarding how much mass they can absorb at any given time: if the matter they’re absorbing is highly energetic for instance, compressing it down further and further, and giving it all the kinetic energy of falling down into a gravity well can cause some pretty epic (for lack of a better word) explosions. You can think of it as a sort of series of naturally ocurring multi-thousand ton hydrogen bombs, because that’s essentially what happens when a black hole eats stellar matter. Hydrogen and helium that are already at fusionable temperature and pressure, and then are put under many times more pressure and many times more temperature

Well this kind of detonation can often expell some of the matter the black hole was drawing in closer, so in practice, black holes only eat a certain percentage of a star’s mass, and ejects small amounts of it at extremely high velocity periodically.

This means there isn’t really a limit on how much a black hole can eat theoretically, but there is a limit on how quickly and efficiently they can do so.

Also black holes can merge which is its own cup of physics tea, but I am not qualified to discuss the details of this sort of interaction so I would recommend someone else, but I do know this kind of merger can release a LOT of energy, and that the merger is not 100% mass efficient (the two black holes don’t keep the entire sum of their two masses)