A star has so much mass, that the internal pressure creates fusion reactions, thus producing massive amounts of energy. This energy is released into the universe as electromagnetic radiation.
As a star becomes more and more massive, the reactions may vary, but basically the same thing is going on…until one day…
The star is so massive that photons cannot escape its gravity well.
Why do we say the laws of physics “break down” inside a black hole, but not within a star? What is the big difference? Black holes are just stars with enormous mass, right?
The physics breaks down at the singularity where the curvature of space would become infinite. Equations get divided by zero - klablooey! The problem is that we don’t have a good quantum theory of gravity.
What they really mean when they say that “the laws of physics break down” is that no modern theory of physics can predict what happens inside a black hole. An analogy might be the breakdown of Newtonian physics when you go to a very small scale, except that we have Quantum physics to predict what happens then.
There’s a considerable difference between a star and a black hole. As long as the nuclear reactions in the core continue, the star can still support itself against final collapse. If the fuel runs out, then a middling-mass star will collapse to a white dwarf, which is supported by electron degeneracy pressure instead of by heat. If it’s more massive than that, then the electrons and protons merge to form neutrons or some other, more exotic particles, and it’s supported by neutron degeneracy pressure. All of these are processes which we understand fairly well. However, there is no known way to support a star, below neutron degeneracy pressure, so if a star is too massive for that, too, then it’ll collapse completely.
It’s not quite accurate to say that the laws of physics fail inside the event horizon. We can make reasonable predictions about what goes on inside the horizon based on the behavior outside. Unfortunately, there’s no way for a person outside the hole to make any sort of measurements to check those predictions, and going inside to check is probably unwise.
One of the predictions made in this way is that there’s a singularity at the center, a point where the curvature of space is infinite. About that point, we know almost nothing, since the laws of physics we know really don’t handle singularities of that sort. Even there, though, the laws of physics apply perfectly well; it’s just that we don’t know what the relevant laws are.
The laws of physics make sense when space-time curves, etc. When said space-time is mushed into an infintely small point… physics don’t work so good. A black hole is not merely a star that’s so massive that light can’t escape (i.e, that its volume is the same as any normal star), but one that was so massive that all of its mass got squished into a dot. The confusion may arise from the fact that the event horizon (the point of no return for light) takes up a certain star-like volume in space.
One other thing, by the way: Black holes don’t necessarily have high mass or density. The only way for a black hole to form nowadays is through stellar collapse, which requires a mass of about three times that of the Sun (not a particularly extraordinary mass, as stars go), but they could also have been formed during the Big Bang, at almost any mass. And the density of a black hole decreases as the mass increases, so a supermassive black hole such as is found in the core of most galaxies might have a density less than that of water. What you actually need is for some amount of mass to be contained in a sphere with radius equal to that mass’s Schwartzschild radius.
One other thing to consider-- beyond the divide-by-zero problem of the naked singularity-- is that by definition nothing can escape a black hole (disregarding Hawking radiation). Therefore there’s really no way to compile data on what happens beyond the point of no return…
Well, we can also say that the black hole’s event horizon defines the boundary of the observable universe. We can make conjectures based upon what we believe might happen on the other side as was alluded to before, but for all intents and purposes, it is ultimately not known what shape the “Laws of Physics” take beyond GM/c^2 = r.
You might just as well say that we don’t know if what shape the Laws of Physics take on the opposite side of the Milky Way galaxy, since we can’t see that region of space because our view is blocked. Or that we don’t know if the laws of biology work on Io since we’ve never been there.
There’s no reason to think that the laws we know stop working at the Schwarzschild radius, as Chronos said.
As far as I remember, the process of collapse of the star takes a finite amount of time. After that time, there is a point in space where spacetime curvature equals infinity, whatever that means.
This link, posted by scotth in another thread, describes “gravastars” which might be a way to avoid the inifity weirdness of BHs.
Since this is the second thread I’ve seen today mentioning gravistars, I just thought I’d pop in and mention that the idea is still rather “out there”, and most physicists still think that the evidence is for black holes.
The difference between the event horizon and the far side of the Galaxy, as far as continuity of the laws of physics is concerned, is that there is no way, even in principle, to determine anything about the laws inside the event horizon, whereas it is possible to observe the far side of the galaxy in other wavelengths, or in reflection off interstellar dust, or by other methods. If the laws of physics changed over in the other arm, then there would be some sort of observable interface between the region of “our” laws and “their” laws. Since, by those observations, “their” laws would be able to influence us, that’d really also mean a difference in our physics, as well. A black hole is not like this, so in principle, the laws inside could be anything at all (well, at least, anything that doesn’t let information out). It’s just Occam’s Razor that tells us to extend the equations analytically.
I would note that black holes were considered “out there” for a long time. I haven’t read too much about gravistars, but the general idea makes more sense to me then some idealized mathematical construct where the math can’t even be done. Quantum physics is “out there” but a verified fact. When I was reading these threads earlier today, I though about a BEC. After reading the preprint about gravistars on the LANL server, I saw that they referred to a GBEC (gravitational Bose-Einstein condensate).
What confuses me is how the idealized singularity of a black hole could give rise to irregular or spinning black holes. That’s another question, but if anyone has an answer, I’d appreciate it.