Starboy

That is interesting. SR was derived by assuming Maxwell was right and now E&M is derived assuming Einstien is right.

Starboy

Jesse

Feather:
As for the other, no charge is ever actually at rest except within its own inertial frame. This applies to electric charges now, and would likely apply to magnetic monopoles as well.

What do you mean by "never actually at rest"? If relativity is correct then the only meaningful definition of "rest" is with respect to a particular inertial frame. The only way for this not to apply to magnetic monopoles is if we discover new laws of physics that pick out a preferred reference frame, and some magnetic monopole is at rest in that frame--but there's no reason the discovery of a preferred reference frame would be expected to have anything to do with magnetic monopoles in particular, it's just that once you have a preferred reference frame you can assign a universal meaning to "rest" (for charged particles just well as magnetic monopoles). So I don't think your comment that it "would likely apply to magnetic monopoles as well" makes sense--the fact that rest can only be defined with respect to an arbitrary reference frame either applies to all particles (including both charged particles and hypothetical magnetic monopoles), or it applies to none of them.

Jesse

Starboy:
That is interesting. SR was derived by assuming Maxwell was right and now E&M is derived assuming Einstien is right.

Yeah, that is pretty interesting actually. I can't quite get my head around whether it's surprising or if it's somehow inevitable that it would work out that way--I'm not even sure how to frame the question.

Feather

Correct. However, there is no particular reason that the concept of "no absolute rest" should be true. Hence if it is (for some odd, strange reason) observed that magnetic monopoles have a preferred rest frame, then it would certainly be possible that magnetic monopoles could be absolutely at rest.

Hence, because nothing else observed so far has a preferred reference frame, it is likely that any particle discovered in the future will not also, and hence never be "at rest" in any absolute sense.

That's all I meant.

Jesse

Feather:
Hence, because nothing else observed so far has a preferred reference frame, it is likely that any particle discovered in the future will not also, and hence never be "at rest" in any absolute sense.

OK, I gotcha. My point was just that the question of whether a magnetic monopole is at rest wrt a preferred reference frame would have no necessary connection to a discovery about magnetic monopoles themselves (it could just as easily be the behavior of some completely different phenomenon that defines a preferred ref. frame), but I guess you weren't saying anything different.

Answerer

Talking about reference frame, I'm seem to have a problem with the principle of equivalence lately as well. It was known that an uniform gravitional field can't be replaced by an accelerated frame over a large region, but why is this so, can anyone please help?

<img src="confused.gif" border="0">

Jesse

Answerer:
Talking about reference frame, I'm seem to have a problem with the principle of equivalence lately as well. It was known that an uniform gravitional field can't be replaced by an accelerated frame over a large region, but why is this so, can anyone please help?

Well, the equivalence of physics in different inertial reference frames is an idea from special relativity, which is a little different from the equivalence principle in general relativity, which is about the equivalence of accelerating in the absence of gravity and standing still in a gravitational field. Basically the idea is that if you are in a windowless elevator and you are experiencing G-forces, there's no experiment you can do that will tell you whether you're accelerating at a constant rate in deep space or if the G-forces are due to gravity. But the equivalence principle would only work perfectly in an ideal uniform gravitational field where the direction and magnitude of the force is the same everywhere; in a real gravitational field the force vectors are arranged in a sphere around the center of mass, and the force becomes stronger the closer you are to the center. So if you had really sensitive equipment you might be able to detect "tidal forces" in the elevator, such as the fact that the pull on the bottom is stronger than the pull on the top (in an intense gravitational field, like that near a black hole, tidal forces can become very strong--your feet might weigh much more than your head, pulling you apart like spaghetti). This is why the equivalence principle only holds exactly in the limit as your "elevator" (ie the region of space you are making measurements in) goes to an arbitrarily small size, so the tidal forces become arbitrarily small as well. For a larger region the equivalence principle only holds approximately, and the discrepancy is worse the larger the region you're considering.

[ September 30, 2002: Message edited by: Jesse ]</p>

NOGO

Relativity itself was discovered through electricity and magnetism. If magnetism is not a relativistic effect of electricity then relativity itself may go out the window.

I do believe, however, that all of this is one very firm ground.

On a CRT electric currents are sent through wound wires. These currents are then varied in order to deflect an electron beam that hits the screen. What deflect the beam since the wound wires are electrically neutral?

[ September 30, 2002: Message edited by: NOGO ]</p>