# Thread: Black Hole Physics Question

1. ## Re: Black Hole Physics Question

Originally Posted by halfeye
They are talking about an entirely different thing.

I am not talking about sound being faster than c. I am talking about light being slower than c, which happens in air, and in glass, and in diamond, for visible light.

As the density of the medium goes up, the speed of light in that medium goes down. As the density (or is it hardness?) of the medium goes up, the speed of sound in that medium increases. So the idea that at some point the graphs of declining light speed and increasing sound speed cross seems to me to be entirely natural.
I could also re-read your earlier post and realize that you weren't saying what I thought you were saying.

2. ## Re: Black Hole Physics Question

It is a distortion of space-time. Space is flat. Space-time is curved.
Spacetime is curved, and that's what's most relevant, but even if you just pick some reference frame and look at a single slice of space, it'll generally still be curved.

And there are some exotic materials which have been produced in laboratories on Earth in which the speed of light in that material is only a few miles per hour. I don't know what the speed of sound is in those materials, but I would not be at all surprised to find that it's faster.

3. ## Re: Black Hole Physics Question

Originally Posted by Chronos
Spacetime is curved, and that's what's most relevant, but even if you just pick some reference frame and look at a single slice of space, it'll generally still be curved.
If you were to use curved space in EFQ, you would get the wrong answer. If you use flat space, you get the correct answers and they tell you space-time is curved.

If you look at a slice of space, it is flat, not curved.

And there are some exotic materials which have been produced in laboratories on Earth in which the speed of light in that material is only a few miles per hour. I don't know what the speed of sound is in those materials, but I would not be at all surprised to find that it's faster.
The speed of light is considered to be a fundamental constant of our universe. The propagation of light in materials may be less than the speed of light.

4. ## Re: Black Hole Physics Question

Regarding the speed of light in materials, does the frequency of the light influence its speed? Do, say, gamma rays move faster in water (or Your Favorite Material) than radio waves?

5. ## Re: Black Hole Physics Question

Originally Posted by shawnhcorey
The speed of light is considered to be a fundamental constant of our universe. The propagation of light in materials may be less than the speed of light.
That is a failed quibble, the speed of light is obviously the speed at which light travels, wherever it is. Given that c is the speed of light in empty space, and space is never empty, does light ever travel at c?

Originally Posted by Lord Torath
Regarding the speed of light in materials, does the frequency of the light influence its speed? Do, say, gamma rays move faster in water (or Your Favorite Material) than radio waves?
Yes. This is the cause of rainbows.

6. ## Re: Black Hole Physics Question

Originally Posted by halfeye
That is a failed quibble, the speed of light is obviously the speed at which light travels, wherever it is. Given that c is the speed of light in empty space, and space is never empty, does light ever travel at c?
The speed of light is a fundamental constant.

And yes, light does travel at the speed of light in a vacuum.

7. ## Re: Black Hole Physics Question

Originally Posted by shawnhcorey
The speed of light is a fundamental constant.
The speed of light is the speed of light. c is a fundamental constant, which is the theoretical speed of light in a total vacuum.

And yes, light does travel at the speed of light in a vacuum.
Where? There are intergalactic vacuums where there is a density of matter of about one hydrogen molecule per centimetre, maybe even one hydrogen molecule per metre, but I am unaware of the existence of any total vacuums. It is possible that light couldn't cross such spaces due to the sparcity of the electric charges in them.

8. ## Re: Black Hole Physics Question

Originally Posted by shawnhcorey
If you were to use curved space in EFQ, you would get the wrong answer. If you use flat space, you get the correct answers and they tell you space-time is curved.

If you look at a slice of space, it is flat, not curved.
If you have non-flat spacetime but flat space, then you can obtain a spacetime with curved space by a change of coordinates (even something as simple as a Lorentz boost). If the EFQ only worked in a frame in which space is flat, they wouldn't exactly be a useful part of a theory of relativity.

Flamm's Paraboloid is also a counterexample, as it demonstrates that an embedding in 3D of a 2D slice of the Schwarzschild metric, with constant time and in a plane passing through the origin, isn't a flat surface.

9. ## Re: Black Hole Physics Question

it's possible the singularity could be infinitely dense if it is also infinitely small.

10. ## Re: Black Hole Physics Question

Originally Posted by opus
it's possible the singularity could be infinitely dense if it is also infinitely small.
Well, yes, that's pretty much the only way that *could* happen, given that infinite mass is an impossibility.

11. ## Re: Black Hole Physics Question

Originally Posted by opus
it's possible the singularity could be infinitely dense if it is also infinitely small.
That is what is alleged to be the case.

12. ## Re: Black Hole Physics Question

Ok, a few things that really need to be said firmly:

1. For any discussion about science we do have to assume that information taken from peer-reviewed journals is legit, unless someone can provide a sound mathematical or experimental proof otherwise. In such a case I recommend publishing the results in properly esteemed journal and not just this humble forum.

2. Gravity waves and gravity itself are two different things just as much as EM waves and EM field. One needs to travel, the other just exists everywhere already as far as classical physics goes.

3. The most general classic model of a black hole includes charge and angular momentum. It has also been proven that within GR a black hole can only have mass, momentum, angular momentum and charge. It cannot have any internal structure or other qualities.

4. Again, within GR it has just as well been proven that after a mass collapses into a black hole, singularity is inevitable. Proof was done by Penrose back in 1965. Whether it is true for a quantum gravity model is a different question but it still would not affect overall outside behaviour of a black hole.

5. Photons or gravitons are concepts from a quantum description of reality. At any point it would be good to distinguish whether one is talking about GR itself or a connection of GR with some quantum models just to be clear.

The last point is specifically important to the claim mentioned here that event horizons do not exist and I would want to elaborate on that a bit. A classical event horizon destroys information, since nothing can come out so we can never tell, what was a given black hole made of or when. A first crack in that happened when Hawking has shown that black holes can radiate, if one consideres a black hole in a quantum vacuum instead of a classical one. Naturally it did bring about a question, if this radiation contains any information about the history of the black hole and things it was created from. In other words: do black holes actually destroy information or not? For a long while it was considered that was exactly the case. More recent and more careful studies shown that information is not lost in a black hole and the Hawking radiation technically does emit it back into the rest of the universe. The thing is, it doesn't really change anything significant, since whatever comes out is just pure noise.

In short: classical event horizon destroys information and prevents anything from escaping a black hole. Quantum event horizon allows escape, but is the ultimate information scrambler, so do not expect to safely take a peek inside either way.

13. ## Re: Black Hole Physics Question

I think there’s an important distinction between arbitrarily dense and infinitely dense.

14. ## Re: Black Hole Physics Question

Originally Posted by danzibr
I think there’s an important distinction between arbitrarily dense and infinitely dense.
Or, as in this case, "we lack the math to properly describe how" dense. Without some unified theory, we run into what amounts to a divide by 0 error.

15. ## Re: Black Hole Physics Question

Originally Posted by halfeye
That is a mistake. It's true of light, but it's not true of sound. There are almost certainly materials (probably neutronium for one) where the speed of sound is higher than the speed of light through that material.

Most of the rest of the article seems okay to me, I'd prefer they didn't use that rubber sheet analogy, and drew a proper catenary for it if they must use it, but it gets the job done I suppose.
It doesn't help that what happens with light literally depends on frame of reference (and the same will be true with high speed sound waves). And on top of it how [generic] you are treating the material/waves on mass makes things ropey.

But I think normally when the speed of light is slower then it's because of reflections, absorbtion, secondary emissions and the like, and conversely changes in speed also cause refraction and reflections. You also have conductivity/permitivity model which ties into that (and is also frequency dependent).

In which case, for visible light, cardboard might almost be an example (although I suspect it's not really opaque enough), and it would need a certain point of view.

16. ## Re: Black Hole Physics Question

Originally Posted by halfeye
That is a mistake. It's true of light, but it's not true of sound. There are almost certainly materials (probably neutronium for one) where the speed of sound is higher than the speed of light through that material.

Most of the rest of the article seems okay to me, I'd prefer they didn't use that rubber sheet analogy, and drew a proper catenary for it if they must use it, but it gets the job done I suppose.
I forgot about this tangent before: the mistake in the article is that they mix up light propagation speed with the physical constant c, which coincides with light propaation in vacuum. You can totaly have particles travelling faster then light in a given medium with very neat consequences in form of Cherenkov radiation. Aside from that, there are materials, where the phase speed for EM waves is higher then c. Still, the group speed is always equal or lower then c, so causality is never broken.

17. ## Re: Black Hole Physics Question

One very minor nitpick:
3. The most general classic model of a black hole includes charge and angular momentum. It has also been proven that within GR a black hole can only have mass, momentum, angular momentum and charge. It cannot have any internal structure or other qualities.
In addition to electric charge, a black hole can also have magnetic charge. Where it would get it from is an interesting question, since nobody has (probably) ever observed a particle with magnetic charge (there was one detection back in the 80s, but it's never been repeated, and is widely assumed to have been some sort of glitch with the apparatus). But if you ever found such a particle, and threw it into a hole, the hole would retain the magnetic charge. And if nothing else, a sufficiently insanely strong magnetic field could pair-produce a pair of microscopic black holes with opposite magnetic charges.

18. ## Re: Black Hole Physics Question

Originally Posted by Chronos
a sufficiently insanely strong magnetic field could pair-produce a pair of microscopic black holes with opposite magnetic charges.
You probably don't want to do that. The half life of a 1kg black hole is supposed to be 10^-19 seconds. If the holes that appear have appreciable mass, they will go very bang!

19. ## Re: Black Hole Physics Question

But that's for neutral black holes. The black holes produced by magnetic vacuum polarization would presumably be extreme Reissner-Nordstrom holes (that is, with a charge of the maximum possible value given their mass), and would thus have zero temperature.

At least, under current models. But such a black hole would also be within an order of magnitude or so of the Planck mass, and so quantum gravity (however the heck that works) would almost certainly be relevant, so we don't really have any good idea of how such a hole would behave.

And all that said, any magnetic field strong enough to do this would probably also have a plethora of other effects which would be even more destructive than the resulting black holes themselves.

20. ## Re: Black Hole Physics Question

Originally Posted by Chronos
Yep, thanks, fixed.

21. ## Re: Black Hole Physics Question

Originally Posted by Chronos
One very minor nitpick:

In addition to electric charge, a black hole can also have magnetic charge. Where it would get it from is an interesting question, since nobody has (probably) ever observed a particle with magnetic charge (there was one detection back in the 80s, but it's never been repeated, and is widely assumed to have been some sort of glitch with the apparatus). But if you ever found such a particle, and threw it into a hole, the hole would retain the magnetic charge. And if nothing else, a sufficiently insanely strong magnetic field could pair-produce a pair of microscopic black holes with opposite magnetic charges.
Well, assuming that something like a magnetic charge exists. I was talking about a nice, clean classical black hole and as far as I know classical electrodynamics does not have magnetic charges. Black holes can have magnetic moment though since they both have charge and angular momentum.

edit: in the same manner black holes will preserve all quantum numbers (barionic, leptonic, color etc.)

22. ## Re: Black Hole Physics Question

Originally Posted by shawnhcorey
Einstein assumed that gravity travels at the speed of light. This is necessary for his field equations to work. Of course, if you had read about his work, you would already know this.

Schwarzschild used Einstein's field equation to speculate about massive objects and concluded that light does not travel fast enough to escape them. Of course, if you had read about his work, you would already know this.

Therefore super-massive objects have no gravity, it cannot travel fast enough to escape them. Of course, if you understood logic, you would already know this.
Logic requires you to know all your assumptions, and the precise meaning of all parts. Einstein never said that gravity travels at the speed of light, because he never said that gravity travels at all. Change to curvature certainly does, but that is very different.

You seem to assume that gravity is something constantly radiated, and that constant communication is required, but conservation of mass makes this unnecessary. If we form a closed surface around something, know how much mass is inside to start, and are able to perfectly account for all mass that crosses the surface, we can know exactly how much mass is inside the surface at all times, even if we cannot get information about the things inside. As something crosses into the surface, it leaves some curvature behind, and then if something crosses back it removes it again.

Consider a building where people have to check in and out. There are 2 ways to see how many people are in the building. One is to yell that everybody has to report to front desk. Lets say that 8 people appear. What you don't realise is that 2 people are locked in storage units, and hence cannot get there. You are assuming that gravity works a bit like this, and that because they cannot check in on demand, they are not counted. Closer is that gravity works like the check in system. Even though those two are stuck and cannot give information, the fact that they are in the building is recorded in the check in system. They don't suddenly get checked out when they lose contact with the front desk.

It is one of my quibbles with the "what would happen if the sun suddenly vanished" question, and why I don't think it is helpful for illustrating this point. One answer is that the earth would keep orbiting*, because the sun vanishing would not affect spacetime the same way that it moving away would. It would be functionally equivalent to the sun being replaced by an object that is physics-less of the same mass as far as gravity is concerned, and this is basically what happens when a star collapses into a black hole. It might as well vanish, but as far as the accounting of the universe is concerned, that region of space still has the same numbers, so the fields don't change.

* As for what would happen at where the sun used to be, where spacetime is curved in a region of no mass, I can only speculate. My best guess is that conservation of mass would breakdown again, and that this weird spacetime 'error' would result in stuff production similar to hawking radiation. Spacetime would continue to spontaneously generate mass until the curvature made sense again. Essentially, The sun vanishing would result in the fast creation of the same amount of mass in less 'friendly' forms. You would have a sun sized explosion instead.

23. ## Re: Black Hole Physics Question

Originally Posted by shawnhcorey

If you look at a slice of space, it is flat, not curved.
Strange. Say I look at a slice of space at a single time near a spherically symmetric massive object. If I construct a triangle whose interior contains the center of the massive object, the interior angles of the vertices of the triangle will add up to a bit more than 180°.

That doesn't fit the definition of "flat."

24. ## Re: Black Hole Physics Question

@Fat Rooster: that was incredibly intuitive and helpful. Thanks!

25. ## Re: Black Hole Physics Question

Originally Posted by halfeye
I am not talking about sound being faster than c. I am talking about light being slower than c, which happens in air, and in glass, and in diamond, for visible light.
Among other things.

Originally Posted by Lord Torath
Regarding the speed of light in materials, does the frequency of the light influence its speed? Do, say, gamma rays move faster in water (or Your Favorite Material) than radio waves?
Speed of light in materials tracks to index of refraction (hence the links above), which is wavelength dependant. There's a fair bit of data on it as well, though mostly in UV through microwave spectrums.

26. ## Re: Black Hole Physics Question

Originally Posted by Knaight
Speed of light in materials tracks to index of refraction (hence the links above), which is wavelength dependant. There's a fair bit of data on it as well, though mostly in UV through microwave spectrums.
Just as a fun fact: X-ray refractive lenses are a thing.

27. ## Re: Black Hole Physics Question

Originally Posted by Knaight
Among other things.

Speed of light in materials tracks to index of refraction (hence the links above), which is wavelength dependant. There's a fair bit of data on it as well, though mostly in UV through microwave spectrums.
I might be wrong, but I once read that light still travels at c through all those materials. It just gets bounced around so much that the time it takes from entering to leaving the material makes it look like it was moving slower . It was described to me as "you walk at, say, 1 km/hr in a straight line. But if I throw you into a hedge maze, and measure your speed from entering to leaving, it might take you 30 minutes you move the 10 meters (in a straight line) between the entrance and the exit, despite you still having been walking through the maze at 1 km/hr, because you can't move through it in a straight line".

Grey Wolf

28. ## Re: Black Hole Physics Question

Originally Posted by Grey_Wolf_c
I might be wrong, but I once read that light still travels at c through all those materials. It just gets bounced around so much that the time it takes from entering to leaving the material makes it look like it was moving slower . It was described to me as "you walk at, say, 1 km/hr in a straight line. But if I throw you into a hedge maze, and measure your speed from entering to leaving, it might take you 30 minutes you move the 10 meters (in a straight line) between the entrance and the exit, despite you still having been walking through the maze at 1 km/hr, because you can't move through it in a straight line".

Grey Wolf
While it might be a description of what is happening, it is not very useful for understanding the process. For one, this bouncing arround is more akin to scattering then orderly propagation and refractive index is unrelated to transparency and quality of a given material.

If I were to devise an analogy for changes in propagation speed, then I would rather look at how you run on a track in comparison to a sandy beach or shallow water.

29. ## Re: Black Hole Physics Question

Originally Posted by Grey_Wolf_c
I might be wrong, but I once read that light still travels at c through all those materials. It just gets bounced around so much that the time it takes from entering to leaving the material makes it look like it was moving slower . It was described to me as "you walk at, say, 1 km/hr in a straight line. But if I throw you into a hedge maze, and measure your speed from entering to leaving, it might take you 30 minutes you move the 10 meters (in a straight line) between the entrance and the exit, despite you still having been walking through the maze at 1 km/hr, because you can't move through it in a straight line".

Grey Wolf
That really doesn't explain how we can see through glass and water. The light comes through in a straight line, but slower; if it bounced around, it wouldn't come in a straight line, and we couldn't see through the material.

30. ## Re: Black Hole Physics Question

Originally Posted by halfeye
That really doesn't explain how we can see through glass and water. The light comes through in a straight line, but slower; if it bounced around, it wouldn't come in a straight line, and we couldn't see through the material.
The description that Grey_Wolf mentioned is similar to how electrons behave in an eletrical current. Although it bounces around, it tends to move in a certain direction, so it's perceived as moving only in that direction, but slower. Think of it as coming in a straight, slower line as the average path that the photons/electrons take, as it were.

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