UrbanXrisis
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my physics teacher said that there is an actual size for the black hole and this size can be calculated. I thought a black hole is a point mass.
Warren's explanation is spot-on. Let me point out in common English the dichotomy that he laid out. In mathematical terms, the BH singularity is a dimensionless point. Pretty neat, isn't it, taking 3-D physical objects and embedding them in a dimensionless entity? In the physical model, the effects of a BH in our universe extend to the event horizon, which has a radius that can be calculated, and any object that ventures to that radius is committed to join the BH. The size of a BH varies infinitely between none and some depending on whether you are a mathemetician or a physicist.UrbanXrisis said:my physics teacher said that there is an actual size for the black hole and this size can be calculated. I thought a black hole is a point mass.
That is classical mistake people make. BH has surface, ofcourse the problem can not be solved in SM. You need more fancy tools like quantum information theory!I thought a black hole is a point mass.
Uh, what? You apparently don't know what you're talking about.Marjan said:That is classical mistake people make. BH has surface, ofcourse the problem can not be solved in SM. You need more fancy tools like quantum information theory!
Yes, but is BH modeled entirely by the GR enough to describe real BH? I know that SM isn't at all. We wouldn't need QG if GR would be enough!chroot said:The classical black hole is modeled entirely by the general theory of relativity.
Of course, but such complexity is not necessary to describe the event horizon. Please attempt to keep responses at a level appropriate for the original poster.Marjan said:Yes, but is BH modeled entirely by the GR enough to describe real BH? I know that SM isn't at all. We wouldn't need QG if GR would be enough!
No.mapper said:What I would like to know is if there is a min or max density a black hole can get.
The two coalesce, and generate a lot of gravitational radiation. The merger of two black holes is actually a very well-studied scenario. The various gravitational-wave detectors (LIGO, etc.) are, in fact, looking for neutron star mergers or black hole mergers, since these are the most prolific generators of gravitational radiation in the universe.Also, what happens if a black hold meets another one? If black holes truly do exists, then this would also be a possible scenario.
mapper said:What I would like to know is if there is a min or max density a black hole can get.
There'd be no max density according to general relativity, but there might be one according to quantum gravity, probably on the order of one Planck mass per Planck volume.chroot said:No.
mapper said:I would think that there is a min. =/
chroot said:A classical black hole is a point mass, but that point is surrounded by a mathematical surface called the event horizon. The event horizon is the "point of no return," in the sense that anything that ventures inside the event horizon cannot come back out. The event horizon has a radius, usually known as the Schwarzschild radius:
r_s = \frac{2 G M}{c^2}
This is the figure most people use when describing the "size" of a black hole.
- Warren
chroot said:No.
The two coalesce, and generate a lot of gravitational radiation. The merger of two black holes is actually a very well-studied scenario. The various gravitational-wave detectors (LIGO, etc.) are, in fact, looking for neutron star mergers or black hole mergers, since these are the most prolific generators of gravitational radiation in the universe.
- Warren
chroot said:Yes, kleinwolf, there are coordinate systems like the Finkelstein-Eddington coordinates which are only singular at the singularity, not at the event horizon.
- Warren