# Thread: Black Hole Physics Question

1. ## 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
This is a bad model for electromagnetic waves. Among other things, it does not do a good job of explaining reflection, transmission, and refraction. Modeling it as speeds, on the other hand, does about 90% of the work for you.

Originally Posted by Gray Mage
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.
A better model of electron/crystal interactions shows that the actually moving electrons are actually moving at about the clip you expect, they just aren't all moving. Or rather, slightly more than half are moving in the direction of current flow, but only the fastest ones are not having their movement cancelled out by charge carriers moving the opposite direction.

2. ## 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 mystery that really baffled physicists pre Einstein was actually the opposite. Light having a speed in a material made sense if electrons were able to move to convey EM waves. How fast the wave moves in space could be consistent when referenced to the material it was in. What was really weird was that vacuum was able to transmit EM waves, given that there were no special reference frames for it, and so nothing to tell it how fast to go. Then when they actually measured the speed they got a weird constant result, which was impossible based on any existing model.

A better analogy for light slowing in materials is a string with waves going along in. When you put charges in front of an EM wave it is a bit like putting weights on that string. Those weights will change how waves move along the string. If the weights are heavy compared to the string, the waves relative to the weights will behave the same even if the weights are sliding along the string. The speed of the waves is constant relative to the weights, not the string.

Understanding how waves can propagate in a material is pretty intuitive, as we have mass and force and movement; all things which are very classical. It is actually a pretty bad description, as the sections of 'string' between the masses are actually more important than the masses in almost all cases, but it can work even if forces work instantaneously. Vibrations on a massless string though do not make any sense classically, hence EM waves in a vacuum being confusing.

3. ## Re: Black Hole Physics Question

edit: in the same manner black holes will preserve all quantum numbers (barionic, leptonic, color etc.)
Black holes do not preserve baryon or lepton number. Color (by which we mean the QCD property, not the visual phenomenon), I suppose they would, but only in the sense that a black hole would always be colorless (because the only way to feed a black hole, say, something red would also leave either something green and something blue or something antired in the immediate vicinity of the hole, and which would also immediately be eaten).

And the fact that you could pair-produce magnetically charged black holes from a sufficiently-strong magnetic field is itself proof of the existence of magnetic monopoles. There may or may not be other magnetic monopoles of more manageable mass, but if nothing else, there's the holes. As my old advisor used to put it, we know that monopoles exist; there just might be a very small number of them in the Universe, like maybe zero.

4. ## Re: Black Hole Physics Question

Speaking of strings, I faintly recall there being a variant of string theory that does away with singularities. Rather than being infinitely dense, a black hole is a hyper-dense volume of strings, with the event horizon actually corresponding to its surface. Sadly, this and other predictions unique to that brand of string theory are untestable with current means.

5. ## Re: Black Hole Physics Question

Originally Posted by Chronos
Black holes do not preserve baryon or lepton number. Color (by which we mean the QCD property, not the visual phenomenon), I suppose they would, but only in the sense that a black hole would always be colorless (because the only way to feed a black hole, say, something red would also leave either something green and something blue or something antired in the immediate vicinity of the hole, and which would also immediately be eaten).
Hmm... this I did not expect. Well, on the classical level there are no baryonic or leptonic numbers whotsoever, but on the quantum level it has been established that black holes perserve information and quantum numbers are conserved quantities. I would not expect the black hole to retain such qualities (same goes for color) for long since it is constantly radiating particles out, but in principle it should not be lost entirely unless the relevant symmetry is broken.

Originally Posted by Chronos
And the fact that you could pair-produce magnetically charged black holes from a sufficiently-strong magnetic field is itself proof of the existence of magnetic monopoles. There may or may not be other magnetic monopoles of more manageable mass, but if nothing else, there's the holes. As my old advisor used to put it, we know that monopoles exist; there just might be a very small number of them in the Universe, like maybe zero.
Could you point me toward some sources on this, because this is really interesting. The thing is, if black holes can be magnetic monopoles, then due to Hawing radiation and inevitable evaporation of a black hole, there have to be some particles with magnetic charge. The only case of magnetic monopoles I know of comes up in 2D spin ice systems and only look as such within the plane. I kind of remember that someone did obtain equivalents of black hole in 2D magnetic systems, but I don't remember the details.

6. ## Re: Black Hole Physics Question

If there are no other sorts of monopoles, then a magnetically-charged black hole would gradually lose mass (but keep the same charge) until such time as the mass equaled the charge (in appropriate units), and then decay no further. In general, for any sort of conserved property, the lightest particle that has a nonzero value of that property must be stable, and magnetically-charged black holes are no exception.

Further than that, though, even for electric charge (for which there's a convenient lightweight particle to carry it away), an extreme Reissner-Nordstrom black hole (one with charge equal to its mass) has zero temperature, and therefore still won't decay. At least, not until it loses some charge in some other way, which it probably won't have a difficult time doing, because all it needs to do is eat something of the opposite charge, which tend to be pretty easy to come by.

And don't be too disturbed about them not conserving baryon or lepton number. Most Grand Unified Theories (i.e., that unify the Strong and Weak Forces) predict that baryon number is is only approximately conserved, and hence that the proton will eventually decay (albeit the experimental bounds put the lifetime as very long). And if the Majorana model of the neutrino is correct (which says that the neutrino is actually its own antiparticle, and that what we call neutrinos and antineutrinos are distinguished only by their spin), then lepton number isn't even conserved (or, indeed, well-defined) even in the Standard Model.

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