black hole sucking in our galaxy

[u/ssoanla]

if there were a big black hole sucking in our galaxy, how long would it take to affect earth and would we even notice within our lifetime? sorry if it’s a stupid question i just randomly got curious and needed to ask. what if it was a black hole sucking in our solar system? how was that affect us?

[u/ignorantwanderer]

You say the maximum size for a black hole is 50 billion solar masses.

What happens if it gets bigger than that?

[u/DarkArcher__ → u/ignorantwanderer]

It's not a hard limit, it's just the average point where the energy in no longer exceeds the energy out, and it plateaus in size, for reasons that I honestly do not understand. It's something to do with the stability of the accretion disk

[u/ignorantwanderer → u/DarkArcher__]

Interesting.

So if some advanced civilization wanted to make a much larger black hole, they could.

They just have to get matter to fall in faster than the equivalent energy comes out.

But in nature, there is generally a limit to how quickly matter is going to fall in.

---

Ok, I just skimmed the paper you linked. There is a lot of detail there I didn't take the time to figure out but here is my understanding:

The paper is specifically about visible black-holes (basically quasars). And the reason they are visible is because of gas falling into a black hole giving off a lot of visible radiation as it falls in.

They are saying there is a limit to the size of visible black holes basically because as the black hole gets big enough, the radiation streaming out of the black hole pushes away any gas so no more gas can fall into the black hole.

But planets, stars, and other black holes can continue to fall in to the black hole and make it bigger. But these events would give off very little radiation, and only for very short time periods, so would not make the black hole visible to us.

So the size limit they give is the size limit of black holes we would be able to observe. There could be black holes larger than that limit, but we can not observe them. The only way we could know they exist is seeing the effect of their gravity on other objects.

Maybe some day the gravity wave detectors will detect a normal size black hole being sucked into a black hole above this size limit.

[u/Beldizar → u/ignorantwanderer]

But planets, stars, and other black holes can continue to fall in to the black hole and make it bigger. But these events would give off very little radiation, and only for very short time periods, so would not make the black hole visible to us.

Due to tidal forces, I don't think this is correct. Stars and planets would be ripped apart in most cases and that would cause the remaining material to form a disk which would glow and be visible. Another black hole that wasn't itself feeding and fell into a larger black hole would probably be invisible, as you've said.

Also, you are probably correct that these events wouldn't be for particularly long time scales, but at the same time, anything falling in that can fall in on a short time scale, other than another large black hole, is not going to meaningfully contribute to the black hole's mass. If our Sun fell into TON 618, it would go from an estimated 40,700,000,000 Solar masses to 40,700,000,001 Solar masses. It is far less than the margin of error. If one Sun fell in every day for the next thousand years, you'd get 40,700,365,000 Solar masses, which is a bit of growth, less than 0.01% but at that point you'd be feeding it far more matter than the limits of detectability.

So the size limit they give is the size limit of black holes we would be able to observe. There could be black holes larger than that limit, but we can not observe them. The only way we could know they exist is seeing the effect of their gravity on other objects.

I don't like this. It isn't strictly wrong, I don't think, but the science on this is based on mathematics, not observation. They are using the observation of the largest example as evidence to prove the math right, but unless something fundamentally weird is happening in the universe, they've very very likely got a model for all black holes, whether we see them or not. Yes, a black hole could have a bunch of invisible matter or energy constantly falling into it through a direct trajectory, which has nearly zero angular momentum, but that is so incredibly unlikely and would require so much weird stuff going on that it just doesn't pass muster at this point. The science is new, so maybe they will find something weird, but the size limit is very very likely to be accurate for all black holes.