# Physical size of a black hole?

Note: To begin with, I’ll say that I know our information on black holes is sorely limited… but there are a few things I’m wondering…

Most science articles I’ve read have described a black hole’s physical size (when size is mentioned) as only being a few miles across. Then again, others define black holes as being singularities (single geometric points). Which is it?

Does the former definition refer instead to the event horizon, and not the “size” of the hole itself? In other words, if you were to freeze the universe for a moment and nullify gravity, would you find a big blob of mass where a black hole is, or would you find a near-infinitely dense chunk of mass too small to be seen?

Further, what’s the largest a black hole can get? Take the theoretical black hole at the center of the Milky Way (how much evidence is there for its existence, anyway?)… how big would that black hole (or, rather, its event horizon) be?

(The reason I ask is because I’ve been thinking of a sci-fi story that details a ship’s travel - via reality-altering FTL drive - through the edge of a black hole’s event horizon. Yes, I know it’s impossible… but so is hyperdrive and phasers. :))

And, yes, I know I’m asking, like, fifty questions at once. When I make a GQ thread, I like to get my money’s worth.

Nah, it’s simple. You have it right. The few km figure is the radius of the event horizon. Some people assume that the matter inside is in a singularity, but we really don’t know. All we know is that the radius of the matter distribution is less than the radius of the event horizon.

You too can compute even horizon radii. R = 2GM/c[sup]2[/sup]. For M = M[sub]sun[/sub], R ≅ 3 km. And as you double the mass, you double the radius, etc. So, for M = 1 Million M[sub]sun[/sub], R ≅ 3 Million km.

In short it’s hard to tell because space and time act so differently inside a BH. Also there are ring singularities too.

There is no theoretical limit to the size of a BH. The one at the center of the Milky Way is about a million times the mass of the sun, IIRC. That’s small compared to some active galaxies like quasars.

Inside the BH there is no “lump of matter”, just a singularity. It lies in the future - you can’t escape it for the same reason you can’t go backward in time. (Firing your rocket engines in any direction only makes you fall faster!)

I don’t think a quasar counts as a galaxy. They are supposedly about the size of our solar system and maybe as big as a few light months across (making them pretty big) but I think they are a single object.

I also thought that at least one explanation for a quasar was actually the energy being released by a LOT of matter in-falling on a super-massive black hole at the center of a galaxy making a quasar not really an independant object in its own right (afterall quasar is short for quasi stellar object).

A quasar is the core of a galaxy, but the actual bright thing we see is very small (solar-system sized or so, or smaller). It’s pretty much universally accepted now that the source of energy is matter falling into a black hole.

The black hole at the core of our Galaxy weighs in at 2.6 million solar masses, so it has an event horizon radius of about 7.8 million kilometers. It’s believed that the largest holes are about ten billion solar masses, but there’s no known upper bound on how big they can be. Note, of course, that if such an ultramassive hole were anywhere in our vicinity, we’d notice it. If you want your ship to stay close to home, your only choices for a supermassive hole would be the Milky Way’s, or maybe the one in the Andromeda Galaxy (which is probably a little bit bigger).

To cross an event horizon going in, by the way, you don’t need any sort of special physics: It’s only going out that’s tricky (read impossible, so far as we know). The stretching out into spaghetti business that you may have heard of is due to tidal forces, but the larger your hole gets, the less you’ll need to worry about that. For the hole in the center of our Galaxy, it’d be quite safe to cross the horizon (until you reach the singularity at the center and get destroyed, of course).

Once you’re inside the horizon, things start to get rather weird. For instance, time is now a spatial direction, but the direction to the center of the hole is a temporal direction. The center is always in your future, and you can no more avoid reaching the center than you can avoid reaching tomorrow. You can get there quicker if you’d like, but no matter what you do, you’ll reach the center in very short order (for one of those hypermassive black holes, it might be as long as a day or so).

What you’ll find at the center is not quite certain. According to classical (i.e., non-quantum) general relativity, the center is a singularity, a point (or ring, if the hole is rotating) with zero thickness, but with quantum gravity, it’s reasonable to suppose that there’s a lump of… something… there, of size of order the Planck length. In any event, whatever is in the center, it probably isn’t much fun to run into it.

I would prefer to think of it as a super dense neutron star – on the same principal that dictates how matter is squashed together to form degeneracy and thus approaching the Heisenberg limit.

If a neutron star were to have a mass limit to where it would collapse under its own weight, then perhaps it would simply squash to a smaller size (approaching but not exceeding the Heisenberg limit). It would have the mass of a black hole, not emit light as its gravity would be too strong, and have all other typical characteristics.

The only solution then is that of a certain mass range, it would simply be a black star. Its inherent gravitic lensing effect would make it appear to be a hole when it actually is not.

Understood that increasing mass would decrease the size, and perhaps exceeding the Heisenberg limit would cause the mass to become a singularity, the whole thing becomes a new entity altogether – an actual black hole and nothing more than a massive, sizeless, geometric point in space.

Back to the first note, when a neutron star finally loses energy and cools off enough, might it too become a ‘black hole’? it would still retain its mass and inherent lensing effect… Also, it would absorb mass that got near enough to it.

Worse, one could be coming this way and no one would see it (and those that might would in fact mistake the thing for a black hole…) :smack:

I think I get that space becomes a temporal thing in a black hole. I think you explained that well. But what I’m not getting is how time becomes space-ish. Does this mean that in a black hole, you can go back in time? How does that work out?

I heard that the more mass a black hole has, the less dense the physical mass at the center gets, in other words, the proportion of the physical object becomes larger in relation to the event horizon. Is there any truth to this?

It’s how coordinates behave, the practical implications are just that everything moves towards the singularity. You can move across an event horizon without noticing it (particualrly for a supermassive black hole where the tidal forces at the event horizon are minimal).

It becomes less dense in the sense that 4M/3pi*R[sup]3[/sup] becomes less the greater that M is where M is the mass parameter of the black hole and R is the Schwarzchild radius. This is as R= 2GM/c[sup]2[/sup] (where G is the gravitational constant and c is Einstein’s constant).

Arguably the singularity itself doesn’t have a density, but for most purposers you can assume it to be always infinite.

Note that there are theoretical analyses of what is in the center of a black hole that suggest a blobby lump of mass of very unstable shape. This is very small in proportion to the event horizon though.

eta: Just saw that the book I was getting my info from was published in 1995, so may be outdated.

Not to be obtuse or anything, but if the center is always in my future, how would I ever get to it? Or did you just mean it’s inescapable?

I once read that if/when the material universe enters a black hole, time will be stretched, and though it won’t BE eternity, it will FEEL like eternity. Must say, I can’t quite grasp this. All I can think of is being at the end of a long line at the supermarket, with Olivia Newton John songs on the PA.

If there are denser states of matter than degenerate neutrons then there could be such things as Quark stars. No one knows because the actual physical state of even neutron stars is a bit speculative.

Some radical theories claim that singularities in fact don’t form and what we would consider to be a black hole is actually a super-dense shell of mass just outside what would otherwise be the event horizon. See Black Star for one such theory.

It’s always in the finite future i.e. you going to hit it in a finite (proper) time.

Extreme time dialtion effects occur around black holes, but time dialtion is just comparing clocks, what actually matters to someone falling in to a black hole is that they will hit the singularity in a finite period of time.

General relativity which predicts black holes states that at the centre of a (Scwarzchild i.e. non-rotating) black hole there lies a point-like infinitely dense singularity (that said the topic of singualrities is actually quite complex, so this is a slight simplification due to the fact that most would not have the singularity as a spacetime event).

There are new theories such as string theory and loop quantum gravity which seem to prdict that the black hole singualrity may not be an actual singualrity (I’m pretty sure both these theories don’t have any kind of spacetime singualrities in them).