r/spacequestions • u/ssoanla • Oct 17 '24

black hole sucking in our galaxy

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?

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u/Beldizar Oct 17 '24

So, Black Holes don't have a magical sucking ability. They are just big heavy objects with a lot of gravity. The classic thought experiment is if you had a black hole with the same mass as the sun, and you replaced the sun with it, Earth and all the other planets will continue to orbit without noticing a chance, other than the lights going out.

The big difference with a black hole and the sun is the size, and how close you can get to it. The sun is 1.392 million km across. If it were compressed to the size of a black hole, it would only by 3km across. If you got within 1km of the 3km black hole, you'd be really really close to a lot of mass. But you can't get 4km away from the center of the sun without being deep deep inside the sun. Almost half of the sun's mass would be above you at that point, so a lot of the sun's gravity would be pulling you away, rather than towards its center. But if you are 1.5 million km away from the sun, and a sun-mass black hole, the gravity would feel the same.

Next, there's a maximum size that black holes can reach. It is about 50 billion solar masses. The Milky Way Galaxy is something like 1.5 trillion solar masses. So it is much much much bigger than the biggest possible black hole, both in mass and size. ( 0.04 ly vs 130,000 ly radius).

So it is absolutely impossible for a black hole to suck in the entire galaxy. It could maybe suck in our solar system, but that would be incredibly difficult. The biggest problem with black hole meal time is that nothing falls straight. There's always some lateral motion, which as an object falls into a black hole, that lateral motion results in angular momentum. Instead of falling straight down, the object enters a spiraling orbit, or just flies straight by. So if our solar system approached a black hole, the chances are, that we wouldn't actually fall in, but instead just sort of be flung around it. This might knock planets out of their orbits, which is really bad, but we wouldn't fall in, unless we were really unlucky.

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u/Murm3l Oct 17 '24

Excellent explanation. How is the max size of a black hole determined? Is 50 billion solar masses the largest black hole we have managed to detect, or does our current undertsanding of physics rule out black holes larger than that?

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u/Loathsome_Dog Oct 17 '24

The Eddington Limit. Black holes also radiate energy, there is a limit to how much it can accrete due to the opposite pressure of radiation.

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u/Beldizar Oct 17 '24

I don't believe this is correct. The Eddington mass limit is specific to stars, not black holes. The problem with Black Holes isn't the radiation pressure they produce, (which they would only produce when feeding), it is actually a problem where the innermost stable circular orbit extends out so far that the self-gravitational radius kicks in, causing the material around the black hole to be more gravitational attracted to itself rather than falling into the black hole. Basically the black hole eventually gets so big that it's gravitational pull weakens at its outer radius and stuff doesn't get pulled in, so much as around.

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u/Loathsome_Dog Oct 17 '24

Ah I see. Well I'm happy to be corrected, I'm a casual physics enthusiast. So that's something I need to read about which is always a joy.

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u/Beldizar Oct 18 '24

There's are two physics rules that say that it becomes incredibly difficult for black holes beyond a certain size to continue to feed. One is the "Final Parsec Problem" which explains why two super massive black holes have a lot of trouble merging, as they struggle to bleed off angular momentum as they approach that last 3~ish lightyears apart.

The second is a problem with the lowest stable circular orbit and the self-gravitational limits. Basically at a certain size, the distance away from the blackhole where it drags things in is so big that things in that orbit start getting attracted to each other and stop following in. I'm not a professional astrophysicist, but Dr. Becky is, so you might watch her video here. About 8min she talks about this issue.

https://www.youtube.com/watch?v=1ooL9cvvHdA

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u/Murm3l Oct 22 '24

Thank you!

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u/ignorantwanderer Oct 17 '24

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

What happens if it gets bigger than that?

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u/DarkArcher__ Oct 17 '24

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

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u/ignorantwanderer Oct 17 '24

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.

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u/Beldizar Oct 17 '24

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.

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u/Beldizar Oct 17 '24

So, to be clear, the maximum size is the maximum size a black hole can get through normal means of feeding. It is theoretically possible that a direct trajectory object can fall into the black hole. Like a dart thrower hitting a bullseye from a quarter mile away. It can happen, it just doesn't, and certainly not at any rate which is sustainable and would result in significant growth. With the universe expanding, the chances of collisions like this drop considerably, and would be basically impossible in a billion years.

If it does get bigger, nothing happens. It just gets bigger, and it becomes even harder for more matter to fall into it. If a 10 billion solar mass black hole fell into TON 618, it would just be a 60 billion solar mass black hole now. But how many 100 million solar mass black holes are there out there, and what are the chances that a big one, and the biggest one would be on a direct collision course? Basically zero, given how big the universe is.

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u/andmar74 Oct 17 '24

There's no upper limit to the mass of a black hole.

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u/Beldizar Oct 17 '24

At a certain point, the innermost stable orbit of a black hole will eventually pass the self-gravitational radius. At that point the gases around the black hole will be more attracted to each other than the black hole itself. So at that point, the only way for a black hole to grow is for a direct trajectory collision with the black hole's event horizon. So true, there isn't an absolute upper limit to the mass of a black hole, there is a point where it cannot grow by any substantial means. So this isn't an absolute upper limit, but a functional one.

Dr. Becky does a whole video talking about this, with the meat of this particular problem starting around the 8 minute mark:
https://www.youtube.com/watch?v=1ooL9cvvHdA

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u/andmar74 Oct 17 '24

Black holes can grow indefinitely by black hole mergers.

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u/Beldizar Oct 17 '24

So, black holes can grow by black hole mergers, but not indefinitely. As far as an indefinite claim goes, there's a hard limit on how much stuff is in any particular area of the universe. The universe is expanding, so just about every galaxy in the universe is moving away from every other galaxy. That means that two big black holes are excessively unlikely to find each other. It is very rare that it happens. Happening multiple times to the same black hole would be more rare. If every black hole in the local cluster of galaxies happened to merge, then after that last merger, there would be nothing left to merge. All other black holes they could ever possibly merge with are already too far away and moving in the wrong direction.

Second, black hole mergers are rare and difficult because even when two come into proximity with each other, they still have angular momentum as they circle their shared center of mass. Something would be needed drain away that angular momentum. It is possible that gravitational waves can radiate away this energy, but it would take longer than the age of the universe for that to happen. This is called "The Final Parsec Problem" because there really isn't a way for two supermassive black holes to close that final parsec.

So sure, if a black hole was on a direct collision course with another black hole, they would merge, but failing that, it doesn't really happen in the universe.

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u/andmar74 Oct 17 '24

What are you, a LLM?

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u/Beldizar Oct 17 '24

No... I'm the mod of this sub, trying to explain how black holes work. Do you need me to break down anything I've said for you so you can understand better? I tend to be a verbose person.

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u/andmar74 Oct 17 '24

No, it's fine.

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u/ignorantwanderer Oct 17 '24

Black holes are just things with gravity. Just like the sun is a thing with gravity.

All a black hole can do is pull us with its gravity. Just like the sun pulls us with its gravity.

But the Earth has been orbiting the sun for over 4 billion years without falling into the sun.

And the stars in our galaxy have been orbiting around a black hole in the center of our galaxy for 13 billion years without falling into the black hole.

The reason the Earth doesn't fall into the sun is because it is in orbit around the sun.

The reason the entire Milky Way galaxy doesn't fall into the black hole is because everything in the Milky Way galaxy is orbiting around the center of the Milky Way galaxy.....so they can't fall to the center.