ARRELL wrote:

Michael Gmirkin wrote:

But I'll take the implication to be that you agree magnetic fields track back to electric currents.

Yes and no... for instance if you try to explain the magnetic moment of the electron classically as the electron physically spinning, then you get an equatorial velocity of the spinning electron that exceeds the speed of light...

Hmm, that is interesting. Implication being that classical mechanics is insufficient to explain somethings on its own. Will have to look that up. Thanks. :)

ARRELL wrote:

If you are discussing large systems like the sun, you have to consider the movement of the charged plasma and its interaction with the magnetic field

Granted. Having read a little bit on a number of instabilities in plasma (such as the "pinch effect"), I can certainly understand that the magnetic fields generated by electric currents can have a feedback effect on the currents themselves (or, rather, can affect the flow or direction of the current leading to changes in geometry, strength, etc.). So, I'd agree with the above. It would be stupid not to.

However, it seems like there is always talk of field lines being "dragged" along with the plasma (the "fluid" of magnetohydrodynamics), as though plasma is a permanent magnet. Wouldn't it be more correct to say that there is a persistent current in the moving plasma? A net repeating motion of charged particles in a circuit... As opposed to magnetic fields that simply persist of their own accord and get "dragged about"?

Reeling it back in from "large, complex systems" to the simpler case of electricity in wires, it's generally the net motion of charges (current) that determines the magnetic field (or lack of one, if there is no net motion). Cannot the current, then, be said to be of primary importance (IE, if you take away the current, except in the special circumstance of permanent magnets, the magnetic field ceases to be)? Granting that in complicated systems there may be many currents interacting via many magnetic fields (the forces between the electric currents).

If yes on the above points, can magnetic fields be used to "diagnose" electric currents?

Sure, its something that was in lab courses I taught- so what?

Well, I ask this on account of the magnetic fields seen ubiquitously in space. Not just on the surface of the sun, or the magnetosphere. I mean, if we acknowledge that electric currents are the root source of magnetic fields (as in the current-in-wire example above), then what does finding magnetic fields everywhere mean (simply as a hypothetical question)? I mean light, thin, sparse space plasma isn't a permanent magnet, right?

Granted, as you say, "it's more complicated than that," and I understand that. However, complication isn't an excuse for out-of-hand dismissal. IE, it can be that there are a hundred currents all interacting, producing magnetic fields (a force between the currents) and feeding back on themselves, but that doesn't negate the existence of the currents, just means that their interaction is complicated.

So, I guess that my question is whether we can use deductive reasoning with respect to the notion that magnetic fields can be diagnostic for electric currents (strength, direction, etc.). IE, if we detect and verify a long, thin rope-like magnetic field extending through space, and coincident with that we find two spiraling streams of plasma. Is it okay to reason that perhaps the two streams of plasma are currents "generating" the magnetic fields we have observed? Or, regardless of whether or not we can observe the streams of plasma, if we see a long rope-like, twisted magnetic field strung out through space (say a few orders of magnitude stronger and in a different direction than the ambient weak magnetic field of that portion of the galaxy or intergalactic space) can we work backwards to consider the possibility that such a ropelike magnetic field implies that a similar rope-like current exists in the same region of space (assuming we don't think that sparse space plasma is a permanent magnet), and then try to locate / verify its existence (though synchrotron radiation, or whatever means is most efficacious for the task)?

I mean, is that a reasonable deductive step? I'm just wondering.

Regards,
~Michael Gmirkin