Since a black hole’s volume is directly proportionate to its mass, the density is always constant.
You can actually think of a black hole as already being infinitely dense. The only reason it has a size is because of its gravity. The actual singularity has no volume, just mass. It’s an infinitely small point.
Using singularities is actually pretty common in freshman physics problems, though it’s just glossed over. Take the simplest gravity problem, finding the field around a planet A. Planet A is said to have mass M, so it creates a gravitational field around it. But in the simplest case, Planet A is considered a point mass, where all the mass is concentrated at a point of zero size. Since it has zero size, none of the space in this problem is taken up by this object. So in a sense, the mass doesn’t “exist”, though the gravitational force emanating from it does, and that’s what’s important. If you instead wanted to treat it like a real planet and give it some size, you’d have tidal forces and it’d be a more complicated problem. The nice thing is that as long as you’re far enough away from a planet, you can pretend it doesn’t exist and that the gravitational field is emanating from a single point, and the calculations will work out fine.
It’s the same thing in general relativity, except instead of a gravitational field, you have curvature of spacetime. For a simple schwarzchild black hole, curvature accelerates objects toward a certain point, which we call the center of the black hole. Since it’s a point source of zero size, it doesn’t really exist in a traditional sense, but the curvature around it does. What’s strange is that, as far current physics tells us, after something collapses into a black hole there’s nothing able to stop it from collapsing any further, so its size gets smaller and smaller until it hits 0. It actually becomes the simplified zero-size point mass of freshman physics problems. It’s likely that quantum gravity will fix this somehow, but for now, all we can do is treat the center of a black hole like a real live singularity.
Also, the whole non-existent thing is carried further by the fact that anything beyond the event horizon cannot possibly interact with the rest of the universe, so for all intents and purposes it doesn’t exist anymore. The curvature outside the horizon certainly exists, and we can tell ourselves that it’s being created by something inside it, but for practical purposes what’s actually inside the event horizon doesn’t matter.
It’s all in how you look at it, and how you answer the question that if something occupies zero space, does it actually exist?
OK, this is what I was looking for.
So when people refer to the “size” of a black hole, they’re actually referring to the area of its event horizon?
A black hole’s radius is directly proportional to its mass. Volume = 4/3 pi r[sup]3[/sup]. If the mass of a black hole doubles, its volume becomes 8 times as large, reducing its density by a factor of 4.
Yes, when they refer to the radius of a black hole. The other common way to refer to the size of a black hole is by its mass.
Black holes don’t vacuum stuff up. The only force pulling anything into a black hole is gravity. There’s no other mysterious suction force. Stuff falls into a black hole and can’t get out.
A black hole could swallow a star, but the star would have to collide with the black hole for that to happen. Stellar collisions are rare in our part of the galaxy. Think about it- the nearest star to the Sun is Proxima Centauri, 4.3 light-years away. The radius of the Sun is about 9 light-seconds. That means that light takes 9 seconds to cross the Sun, and 4.3 years to go from the Sun to the nearest star. Stars are tiny in relation to the distance between them.
If you had one tennis ball in Philadelphia, and another one in Indianapolis, that would be an approximately proportionate model, in distance and size, between the Sun and Proxima Centauri. Stars are just not going to collide when they’re that far apart. Since the only way a black hole can swallow up a star is for the black hole and the star to collide, that isn’t going to happen in our part of the galaxy either.
The black holes in this part of the galaxy are fairly small, too. Supermassive black holes, the ones that can be the size of the solar system, are only found in the centers of galaxies. The black holes in this part of the galaxy come from supernovae, and are smaller (in radius, not less massive) than the Sun. We’re no more likely (probably much less likely) to collide with a stellar-mass black hole than we are to collide with another star.
The center of the galaxy is another story, but we’re not there, and we’re not going there any time soon. Even if the Sun were headed into the center of the galaxy at the speed of light (which it isn’t), it would take us about 25,000 years to get there. For comparison, we have recorded history back to about 4500 BC, which is only 6500 years. It would take more than three times as long as all of recorded history for the Sun to get to the center of the galaxy if it were traveling straight there at the speed of light. The Sun isn’t moving toward the center of the galaxy anywhere near that fast.
I thought that rotating black holes were predicted to have a ring-shaped singularity (and that most black holes would be rotating). Is that outdated?
If we can predict that it is a ring, then can we predict its radius? And doesn’t that mean that there is some “there” there?
Black holes that are at the centers of galaxies have large amounts of matter surrounding them and responding to the powerful gravitational pull. That creates all sorts of spectacular effects visible in various wavelengths. And that’s how most black holes are spotted.
Yes, most black holes are believed to be rotating (and in fact, rotating at very close to the maximum possible speed). When physicists talk about non-rotating holes, it’s mostly for the sake of simplicity, since most of the features are qualitatively the same. The singularity of a rotating hole is ring-shaped, but there’s still no “there” there: Trying to discuss the location of the singularity is roughly analogous to trying to discuss a latitude of 95 degrees: The language is adequate to say that, but it doesn’t correspond to any actual location.
One other point, in response to one of the other posts: I said that there is no matter in a black hole, not that there’s no mass. In ordinary physics, the two terms are often used synonymously, but here, there’s a distinction. By “matter”, what I strictly speaking mean is the stress-energy tensor, and one of the first calculations to be done in GR is to show that it is indeed zero everywhere for a Schwarzschild black hole. But there’s still mass as an emergent property of the topological defect of the singularity.
I also hear it’s more correct nowadays to call them “Singularity-Americans.”
I’m having trouble visualizing the difference between the event horizon of a smaller black hole and a huge black hole. Is the event horizon of a smaller BH really a noticeable demarcation? What exactly is noticeable about the small BH’s event horizon that is not noticeable with a giant BH. Is it the tidal forces that have been mentioned? I don’t see any reason to think they would be significantly different at the event horizon than say a small distance past or closer. So is there anything else special about the event horizon other than it marks the point of no return for light?
Think about escape velocity.
The closer you get to a massive object the more energy you need to escape from its gravitation well. If the object is massive enough, that energy translates to needing to go faster than the speed of light at a certain point. That point is the event horizon. Within the event horizon, no amount of energy is sufficient for escape.
Is it noticeable? Theoretically, yes. The volume enclosed by the event horizon is truly black, because no electromagnetic radiation escapes. There’s all sorts of stuff going on with the matter close to the horizon, as in the links I gave above, that would make it impossible to sit just outside and watch, but the demarcation would be awesome.
As for tidal forces, someone else can do the math, but the difference in gravitational potential over a very few feet would be significant for many black holes. See Spaghettification.
Gravity is felt everywhere, right?
I mean, right now, I’m exerting a teeny tiny, itsy bitsy amount of gravitational pull on my laptop, correct?
So wouldn’t that mean that eventually everything in a given galaxy will be pulled into the black hole at its center?
Also: Do all galaxies have black holes at their center? Like, that’s what keeps the galaxy in a cluster?
Probably not. Look upthread.
Gravity is very weak. You can lift your laptop against the entire gravitational pull of the earth. The earth will remain in orbit forever, essentially, against the gravitation pull of the sun. The stars in the galaxy will remain in orbit around the black hole in its center essentially forever, unless they are perturbed by an outside force. They would orbit around the center even if there wasn’t a black hole there. The universe as a whole is expanding so even galaxies won’t squash together in the end (although local groups of galaxies very close together may do so).
As also said earlier, black holes are not some magic suction device. They have exactly as much gravity as any other object of that mass. In the context of an entire galaxy, that much mass is trivial.
Most, but not all, I believe is the current understanding. Depends on the size and evolution of the galaxy.
No, no more than all the planets will eventually crash into the Sun. You can have stable orbits around a gravitating body. You can have situations like the Earth-Moon system, where tidal forces cause the Moon to move away from the Earth, not toward it.
Or stars could get thrown into intergalactic space as a result of galaxy collisions. These galaxies are colliding, and you can see the long tails of stars and stuff thrown out of them. Galaxy collisions are much more frequent than collisions between stars.
Is it still a popular theory among scientists that our big bang was the creation of a black hole in another universe? (I’m not even sure if this was ever a popular theory among scientists. But I vaguely remember reading that it was.) And the matter pulled into black holes in our universe is being spewed out into other universes?
Veering further off-topic.
Assuming tidal forces didn’t rip it into tiny shreds, is there any reason that a spacecraft could not maintain an orbit as long as it wished inside a black hole’s event horizon? How freely could it adjust its orbit? Or is even the maintenance of an orbital velocity impossible inside the event horizon?
It is not a popular theory.
If this were the case we should see “white holes” in our universe spewing out matter but we do not see anything like that out there.
You would suddenly be awash in light that was unable to pass the event horizon, correct? In theory, the inside of the event horizon of a black hole could be brightly lit, yes? Or am I thinking about this wrong?
I am not an astrophysicist, but that doesn’t sound right to me. It’s not as if there are a lot of additional light sources inside the black hole - well, actually, there might be, as gases sucked into the hole are still going to be heated by collision and tidal stresses just like outside. But beyond that - why would the sky one foot “beneath” the event horizon look any different than the sky one foot “above”?
You’re thinking about it wrong. It’s not like the event horizon is a barrier bouncing light back into the interior. Rather it means that once you cross the event horizon there is no path you (or light) can follow that leads to the “outside”.
Once you’re inside the event horizon, there are no paths that will increase your distance from the singularity. This is true no matter where you are inside the event horizon. So if you drop a flashing beacon into a black hole and then immediately follow it, you won’t be able to see the beacon. It’s closer to the singularity and it’s light can’t climb back out to where you are even though you’re inside the event horizon as well.