Physicists, humor me and my coffee-table understanding of physics for a moment. According to my doubtless over-simplified thoughts, black holes must increase in mass for all eternity.
A black hole is a star whose density is so great that its gravity can crush neutrons. No force in the universe can prevent the star’s relentless contraction. It is crushed tighter and tighter until it eventually becomes a singularity.
Now stars, like planets, rotate. When our incipient black hole starts collapsing, then the star, by the laws of angular momentum, must rotate faster, just like a whirling figure skater pulling in her arms. But since nothing can stop the star from collapsing ever further inward, nothing can stop its rotational speed from growing ever higher.
Eventually, the rotational speed of our collapsing black hole must approach the speed of light. And like anything that approaches the speed of light, its mass must increase from relativistic effects. The increased mass may temporarily slow the black hole’s rotation, like a man bear hugging a whirling figure skater. But since added mass also means added gravity, the collapse must resume and the angular velocity must again approach lightspeed, whereupon the mass increases still more . . . and so on indefinitely.
But this process can’t go on forever, can it? There used to be a law called conservation of energy; that the amount of mass/energy in the universe was fixed. Mass can turn into energy, and energy into mass, but the sum of the two always equals the same constant. At present, the black hole is on course to absorb all the mass/energy in the universe, just by relativistic physics. What then?
I bet neutron decay would get rid of all the matter in the universe (outside of black holes) before the black holes could absorb it all, but who knows?
Relativity calculations are based not merely on speed, but on velocity (speed in a specific direction).
Rotational velocity (with respect to an internal axis) is not necessarily analogous to translational velocity (with respect to the external universe). An atom moving northward on one side of the mass is offset by an atom moving southward on the other side of the mass. The vectors cancel each other out, and the net velocity (for purposes of the relativity calculations) is zero.
also, black holes are constantly radiating. It’s called Hawking radiation, and it’s either electrons or a mixture of electron and positron emission. I think.
But I do know that rate of emission increases the more radius decreases. so a black hole evaporates faster and steadily faster if not being fed.
jb
Actually, I think that if you define the edge of a BH as the event horizon, they don’t radiate per se (though the difference is a bit pedantic).
As I understand it (/insert standard caveats), the so-called vacuum is, at a quantum level, a seething froth of energy (zero point energy). This is made up of the formation of vast numbers of particle/antiparticle pairs. They come into existence, and the minutest fraction of a second later they annihilate each other (+ meets - and they disappear). The net energy in say a cubic centimetre of this stuff is zero.
Now, very near the event horizon of a BH, three things can happen to a newly-created particle § and antiparticle (AP) pair. The first is that both get sucked in. Not interesting. The second is that they both escape (and presumably annihilate very shortly after). Also not interesting. The third - you’ve guessed it - is that one particle escapes and one gets sucked in.
In this scenario, the escaped particle now exists in our universe and has mass/energy. The particle that was pulled in must, in order that the conservation of mass/energy is preserved, have negative mass/energy (which becomes ‘part’ of the BH). Hence the radiation (the escaped particles) and the loss of mass of the BH.
There is a difference between mass and relativistic mass. Hopefully someone here can explain that better than I could. Also, maybe someone could explain the specifics of a rotating black hole. (a rotating point?) Anyway…
The gravity from a black hole is like the gravity from anything else…a weak force that dissipates quickly with increasing distance. So, give yourself a bit of distance from a black hole, and you can maintain a safe orbit. Most big galaxies have black holes in their center and most of hte galaxy can go on without fear of being sucked in.
The black hole will consume stuff in the immediate vicinity but after that is gone, things get a lot quieter.
Black holes do slowly evaporate (Hawking Radiation), but this is so slow that black holes will around long after the rest of the stars/galaxies in the universe have fizzled out.
a) I believe from reading “Black Holes and Warped Spacetime” when I was 12 or so that a rotating black hole forms a ‘ring singularity’, where instead of a point the singularity is a ring. This would stop the contraction at a non-zero radius.
b) rotational motion has an acceleration to it, so special relativity does not apply. This goes directly to your ‘infinite mass’ argument.
Since I dropped out of General Relativity in college (seriously), I missed out on the groovy tensor calculus that would let me be more specific about this.
There is a limit. See kip Thorne’s Black holes and Time Warps, probably page 51.
I don’t think rotating black holes develop a ring singularity. The gravitational field itself has energy and momentum, so even though all the matter falls to the singularity, the angular momentum of the black hole can be conserved. Also, if string theory is correct, I believe the matter won’t fall to a point, but to within a Planck length region about the center. Could this be where the “ring singularity” you are remembering came from?
Here’s a link to an earlier related discussion. In it Dr. Lao provides a link to a site which claims there is a ring singularity. I don’t trust the site, however, so I’m not going to retract what I said about there not being a ring singularity.
Something about a black hole that I just thought about - the event horizon is the shell beyond which light at that point couldn’t make it up the gravity well up to my eyes.
I think of it as a person below me who can throw a ball upwards just so fast, and below some point he won’t be able to hit me with it. But if I go down further into the gravity well myself, his upper level where he couldn’t hit me also would go down.
So if I approach a black hole, as I get farther and farther down into its gravity well, would the event horizon shrink, from my point of view? Or does the speed-of-light-is-constant-for-all-observers rule make my ball analogy not apply?
From what I have heard it is possible for a Black Hole to consume too much matter too quickly and increase its Schwarzchild radius rapidly enough so that it remanifests in our time space. I tend to think that this would be accompanied by some ferocious pyrotechnics as so much compacted matter is reliberated.
In theory, you might be able to accellerate a sufficiently large quantity of matter towards a black hole to swamp its ingestive capabilities. This is unlikely though because of the engineering required to intentionally propel stars towards a target.
I would tend to think that this sort of thing does occur naturally. If a black hole is on a trajectory that intersected with the core of a galaxy, it could suddenly encounter a sufficient surplus of matter that I might gorge itself overquickly and thus remanifest.
There is also a theory that our universe is the byproduct of the mutual anihilation of a matter and antimatter set of black holes. The matter black hole was the more massive and therefore the residue is the matter universe that we know and love. Anyway, for fun read David Brin’s book “Earth”. Humans are forced to figure out how to extract a black hole that has fallen into the earth’s core.
CurtC, that is a fantastic question. I wish I had an answer for you. You should post this doozy as a separate thread- btw, I don’t know if constant-c invalidates your thought experiment- replace his ball with a flashlight.
So as he got closer and closer to the event horizon, you would see the flashlight dim. But your time would slow down the stronger the gravity around you got, so it may not dim as much (if you slow your time down, more photons/second are hitting your eye from any source- I think it would seem brighter).
I wonder what happens, though, once you pass the event horizon yourself. Do you go through it, or is it a limit? I don’t know…
jb
If the event horizon is the point that light can’t escape the pull of gravity and escape, then shouldn’t there be a ‘light orbit’ at the event horizon (or slightly larger then that) where light just continues to circle until it hits a incomming partical?
A while ago, on this website, folks who know more than me convinced me that all the mass in a black hole is contained in a single, dimensionless point. Spinning? What is the significance of a black hole spinning if all the mass is in a single point? How would you measure it?
What do you mean by “faster”? If you mean a higher angular velocity, you are correct. But if you are talking about translational velocity, you’re incorrect.
Actually, there’s an even more basic problem. Suppose you have a black hole that has fully collapsed. Now you want to pull it appart. To do this, you have to accelerate each part past the speed of light, which takes an infinite amount of energy. The fact that it takes an infinite amount of energy to go from a collapsed star to a non-collapsed star means that an infinite amount of energy must have been released when the start collapse. The answer to this problem is that as it collapses, the velocity increases, which causes time dilation. As a result, the star doesn’t ever actually finish collapsing.
No, it’s not. The momentum cancels out, but the velocity doesn’t.
Yes there is, but the slightest disturbance, say from a stray photon, would cause it to spiral into the hole. It’s an unstable equilibrium geodesic.
The speed of light is not constant in general relativity. In fact when you throw your ball at him it would wind up where you would have never suspected it would. It’s a very complicated question as the horizon itself is collapsing at c for a local observer.
Well the developers of string theory would argue with that point but if the singularity is spinning then so to would be the horizon.
Relativistic mass is no longer an acceptable term - It’s confusing with respect to rest mass and it would have different transverse values and and longitudinal values. Plus relativistic mass could not cause any kind of gravitational effect or the whole universe would collapse to a black hole.
