# Tag Info

13

That's a surprisingly subtle question, and it depends on what you mean by torn. You've probably seen the rubber sheet analogy for spacetime, and it's tempting to think that because a rubber sheet will snap if you stretch it too far maybe the same thing will happen to spacetime. However this is taking the analogy too far. Spacetime isn't an object, it's a ...

7

It's tempting to think of gravity as some kind of interaction between the two bodies involved - maybe some form of signal (gravity wave?) sent between the two bodies. If this were the case then you would indeed have to allow for a propagation delay as the signals were sent between the two bodies. However this is not how gravity works. A massive object ...

6

Black holes are not "infinitely heavy"; all black holes have a finite mass. Nevertheless, this mass is indeed concentrated, as far as we can tell, at a single point, so that a static black hole can be said to be infinitely dense. Within the framework of classical General Relativity, black holes contain a singularity which can be said to be a "tear" in ...

5

I don't think I can rigorously prove that simulation engines don't need to worry about the (possibly? I don't know if there's a reliable measurement) finite speed of gravity, but I can offer some lines of thought that point in that direction. I'll start with your question 3. Suppose that gravity does have finite speed equal to $c$. Your question seems to be ...

4

See Why is a black hole black? It's quite true that the Schwarzschild coordinates misbehave near the horizon, but there are plenty of other coordinate systems we can use. My answer to the question I've linked above uses Gullstrand-Painlevé coordinates, but you can also use Eddington-Finkelstein or Kruskal-Szekeres coordinates. The conclusion is the same in ...

3

You are a little confused in your stellar evolution model. After the ignition of hydrogen fusion in the core of a star, it will next progress to helium fusion, then to carbon/oxygen fusion via the triple-alpha process (I've skipped a lot of steps and details there, if you want the details you can look at either Hansen & Kawaler's Stellar Interiors text ...

3

The answer is that we can't see a black hole, which is I would guess what you were leading up to. There are only two ways we can detect a black hole using light (of various wavelengths): if it occludes something behind it if it's surrounded by an accretion disk Point (1) is actually quite complicated because a black hole doesn't simply mask whatever is ...

3

The other person is likely thinking about the concept of escape velocity. There are people who claim no man-made spaceship could ever have gone into space because they cannot reach the escape velocity for the earth. What they fail to acknowledge is that the escape velocity for the earth is not a constant, it depends on how high above the surface you are. So ...

3

When you're asking a question about general relativity you need to state what coordinates you want to use. This isn't just a mathematical nicety - as you'll see shortly, the different coordinate systems attached to different observers will describe very different behviours. The obvious interpretation of your question is to ask what happens when an observer ...

3

There are a couple of issues you might want to consider. Firstly there is the slightly boring one that we physicists measuring the mass of the black hole are outside it, and from this position the photon never reaches the event horizon let alone crosses it. I don't want to go into this here since the subject has been flogged to death in numerous questions ...

3

Please tell me what I did wrong It takes General Relativity (GR) to describe black holes and, in GR, energy conservation is, well, subtle. From John Baez's Relativity FAQ "Is Energy Conserved in General Relativity?": In special cases, yes. In general — it depends on what you mean by "energy", and what you mean by "conserved". So, in general, ...

2

In the good old Newtonian world the gravitational acceleration is just: $$g = \frac{GM}{r^2}$$ The equation you give is just a rewriting of this. If you substitute: $$r_s = \frac{2GM}{c^2}$$ into: $$\frac { r^2 }{r_s} \frac {g}{c^2} = \frac {1}{2}$$ you'll find it simplifies to the first equation. So there is nothing especially meaningful in this ...

2

The answer can be found in the nature of gravity. It is a force that arises due to curvature of spacetime which underlies everything. A black hole is a specific configuration of spacetime where nothing can leave by definition, not even light. Since there is no concept comparable to curved spacetime underlying the other forces, we observe no such phenomena.

2

Similar questions have cropped up on this site many times, and the debate surrounding them is usually fractious because people misunderstand each other's use of words like exist. One of the lessons of General Relativity is that any observer has to choose a locally convenient coordinate system that may not be globally convenient. We on Earth (quite sensibly) ...

1

It's a matter of what you mean by "see". Even for a distant observer, it will take a small amount of time for the gravitational redshift effect to become essentially infinite. If your collapsing gas star redshifts to the point where it won't emit a single photon in the age of the universe, it may not have yet technically "redshifted to zero", but it has ...

1

Does it gain infinite momentum before it crosses the horizon? Momentum is frame dependent so, when asking for the momentum, one must specify according to whom? Since the Schwarzschild metric is independent of time, the time component of the four-momentum of freely falling particle is constant. $$p_0 = -E$$ Now, imagine that the particle is at some ...

1

The density of a black hole is defined simply as the mass within the event horizon divided by the volume within the event horizon. This gives an average density, but doesn't imply that the density is uniform within the event horizon. So when you hear statements like the density of a supermassive black hole is the same as water don't take this too literally. ...

1

If we accept that black holes do not indeed destroy information and that they follow the second law of thermodynamics (this is how the entropy-is-proportional-to-area formula was derived, after all) then we can forget about their being black holes and simply think of them as some object radiating black-body radiation. From this standpoint, the entropy of the ...

1

It isn't true that the entropy of the black hole must always increase. Prior to the discovery of Hawking radiation there was a second law of black hole thermodynamics: $$\frac{dA}{dt} \ge 0$$ and because the entropy is proportional to the area this means the entropy must always increase. However since the discovery of Hawking radiation this has been ...

1

Black Holes and Big Bang seed are both singularity (and, may look similar), but they are fully different. Black Hole is singularity in Time (meaning, at singularity, space component from Spacetime vanishes and if you fall inside a Black Hole, singularity would be in your direct future), but Big Bang seed is singularity in Space (in rough words; Big Bang seed ...

1

First, black holes are not infinitely heavy. They tend to have well-defined masses, even locally. http://science.nasa.gov/astrophysics/focus-areas/black-holes/ By locally, I mean that the Newtonian limit tends to work, and you don't really need the ADM definitions. http://en.wikipedia.org/wiki/Mass_in_general_relativity Perhaps you mean the singularity, ...

1

The causal structure of space time, here that of the Schwarzschild-metric, does not depend on the choice of coordinates. What you say is in fact tautologically true. The event horizon is defined as the boundary of the region from within which you cannot escape. I recommend you look at the Penrose-Carter diagram of the Schwarzschild-metric, which depicts ...

1

To observe "nothing special" near a black hole, you would have to be staring very intently away from it. If you could see it as more than a "missing" point source, you could see the entire universe wrapped around it in perfect Einstein rings. The closer you approached the black hole, the more it would intrude into your remaining field of vision. Around ...

1

I agree that for a spacetime that is exactly Schwarzschild, the infalling observer does not see the entire history of the universe. However, this turns out not to be the generic case you would expect for an astrophysical black hole, which formed from collapse of some approximately spherical distribution of matter. This topic is actually being actively ...

1

Paolo Pani and Avi Loeb had two recent papers claiming to rule out the remaining window of primordial black holes as dark matter because they would either distort the CMB or destroy neutron stars: arXiv:1307.5176 and arXiv:1401.3025.

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