Do black holes lose mass? And if they do, how is it that their mass is infinite (if indeed it is, which is what one scientist/talking head fellow said on a recent NatGeo special) if it can be decremented? Infinity minus any real number is still infinity.
And my second question is whether dark matter and dark energy are subject to the law of conservation of mass and energy. Thanks.
ETA: Dammit. Cold a mod please move to General Questions? Thanks.
I thought that was the gist of Hawking radiation. Pairs of virtual particles are constantly being formed and self-annihilating, but occasionally one forms inside the event horizon and the other is not poofed out of existence. To keep balance and order in the universe, a bit of the black hole poofs out as well. Or something like that. Since leaving college, most of my science information comes from XKCD.
Black holes have infinite density, not mass.
But isn’t density a function of mass and size? If, for example you crunched a star of a particular mass to the size of a golf ball, you might end up with a quasar. So doesn’t infinite density imply infinite mass?
In any case, are black holes subject to entropy? Don’t they expend energy by their warping of spacetime?
If you have a finite amount of mass occupying a single point, that will get you infinite density.
I don’t think black holes have infinite density or infinite mass or zero volume or any of that. I think they just have enough mass packed tightly enough to make it impossible to escape without exceeding the (finite) speed of light.
Is there any controversy over the, um, matter?
ETA: And what about DM and DE: are they conserved?
Moved from The BBQ Pit to General Questions. Sorry for the delay.
Gfactor
Pit Moderator
The mass of a black hole is not infinite – it’s exactly the same as whatever it was that collapsed (most commonly, a star) had before it collapsed.
Conservation of energy generally gets a little hairy when you’re doing cosmology, I believe – think of redshift in an expanding universe, which would appear to lower its total energy (a photon’s energy is directly proportional to its frequency, so, if the frequency gets lower, as it does in an expanding universe, its energy gets lower, too). Now, if you drop dark energy into the mix, I suspect it depends on your model of it – the as far as I know most prevalent one, a cosmological constant, in principle amounts to adding a constant energy density to the vacuum, which would appear to imply that, as its volume grows larger, the total energy content of the universe rises. (However, I don’t think the two could ‘even out’, since I’d expect the redshift effect to depend linearly, and the dark energy effect cubically on the universe’s expanse.)
Take this just as a WAG though, I could be wildly wrong with everything here; if so, I hope others more versed in the topic will correct me.
It’s considered as near a guarantee as you can get in unknown physics that they do. Conservation of energy is as near as sacred a rule as science has; nothing is known or believed to violate it.
Actually, yes. I understand quite a few physicists consider the idea that there’s a singularity at the center to be due to our lack of understanding of such extreme conditions. They think that the collapse is stopped at some point and that the core of a black hole is extremely dense exotic matter not a singularity with infinite density.
Mathematicians have as a general rule of thumb that when you get “infinity” as the answer to a calculation, you’ve made a mistake somewhere. The rules of General Relativity predict that matter that undergoes gravitational collapse contracts into a point with zero volume and infinite density- which everyone is certain means that General Relativity is not completely correct.
The huge thing about Hawking radiation and the fact that black holes are therefore not completely black is that it means that black holes are in thermal equilibrium with the rest of the universe. That means that thermodynamics (including entropy and conservation laws) apply to black holes as they do to all other objects in the universe. It turns out that a black hole is at the same maximum entropy as thermal radiation. In a universe that was at maximum entropy you would have black holes that (VERY) slowly evaporated into radiation and thermal radiation that (VERY) infrequently collapsed into black holes.
That seems an odd line of reasoning. “I shall define quantity C to be A divided by B. Oh, but that means C would be infinite if B were zero while A were non-zero. Hm… well, I suppose this means there’s something incorrect with any theory that allows A to be zero while B isn’t zero”. You could dismiss all sorts of things out of hand with this. Can’t have vertical lines, what with their infinite slope. (And can’t have horizontal lines either, what with their infinite co-slope…)
I think it should be physicists have problems with infinity. Also, I’m not sure about dark matter and dark energy, but there was a bet between physicists about whether black holes conserve information or destroy it. Hawking conceded the bet, but Kip Thorne didn’t. There still isn’t a real consensus.
If light can’t get away, then things that are slower moving than light will do even worse, and actually fall towards the center. (So would light once it’s in closer than the event horizon.) According to GR anything will hit the singularity in a finite time. So any of that mass that’s “packed tightly” inside the event horizon eventually ends up at the singularity, leading to infinite density at that point.
As mentioned above, GR is considered to break down at that point, so who knows what really happens?
[Second straight physics thread I’ve responded to with “Who knows?” Nobody promised we’d win the fight against ignorance . . . . ]
But that’s all right. “We don’t yet know” is a perfectly legitimate answer. As Arthur Eddington famously said, “Something unknown is doing we don’t know what.”
I think the problem with this line is in the dividing by the b = zero thing. What you get when you divide by 0 isn’t infinity, it’s indeterminate. So C wouldn’t be infinite, it would meaningless.
Talking about anything inside the event horizon of a black hole is really more the province of philosophy than physics. Physics as we know it works exactly the same way no matter what is inside the horizon, so long as it’s spherically symmetric. The simplest extrapolation of the equations that work outside the horizon would give you a singularity of infinite density in the center, but that’s just an extrapolation. It’s still quite possible that there’s something weird that happens really close to the center that prevents a singularity from forming.
Where things get interesting is in the case where the event horizon itself falls into that “really close to the center” range. A very small black hole, presumably at around the Planck mass, would start running into that issue. Of course, what makes the questions so interesting is that nobody knows the answers yet.
On conservation of energy, by the way: Conservation of energy still holds in cosmology, but it’s a local law, not a global one. Basically, if you have a small box, then the rate of change of the amount of energy in that small box is equal to the rate of energy flow through the walls of the box. But even though that holds true for every small box in the Universe, it does not necessarily hold true for the Universe itself.
If you like, but then you’d have to say the same about the density of a bunch of mass all wrapped into a single point.
Wouldn’t the warping of spacetime be the manner in which energy is conserved for the black hole? In other words, things going in beyond the event horizon eventually are attracted to the core, whether it’s a point or not. Doesn’t the energy spent by that crash into whatever is there necessitate the conservation of energy by the black hole itself, in the form of warped spacetime?
Why does the subject change to Black Holes so quickly when the question is about Dark Matter?
Would all of the Laws of Physics be assumed to be true of Dark Matter based on what is true in 4% of the known universe of non-Dark Matter? I have no opinion on this. I am asking what position science would hold at this time of not knowing much of anything about DM.