# EM-fields are Real?

1. Apr 7, 2007

### Swapnil

I was thinking asbout the physical existence of E and B fields. I would assume that most devoted physicists would say that they are as real as the chairs we sit on (I forgot the exact quote and the person who said it). But doesn't relativity disprove this fact? For example, consider the following thought experiment:

Say there are two observers A and B. Say observer A is on a train which is moving at some speed relative to observer B who is outside on the ground watching observer A pass by. Now, say there is positive charge infront of observer A. Accorrding to observer A there would be no magnetic fields emanating from the charge since the charge is stationary in his frame. However, according to observer B there would magnetic fields emanating from the charge because there charge is moving according to him.

Most physicists would resolve this apparant paradox by saying that the combination of E-fields, M-fields, length contraction, time delation etc would make it such that the net effect of that charge on its surroundings would be the same and hence there is no paradox.

However, doesn't this imply that magnetic fields are not real (since one person "sees" it the the other doesn't). And only their combined effect(i.e. forces) are real?

2. Apr 7, 2007

### Stingray

That's why relativistically, the electromagnetic field is really one thing. It's mathematically represented as a rank-2 antisymmetric tensor field (think 4x4 antisymmetric matrix). The standard electric and mangetic fields may be derived from this, but they are observer-dependent. I don't think this diminishes their physical "reality" at all (as much as that means anything).

3. Apr 7, 2007

### reilly

If I'm in an airplane over Manhattan, and my son is waiting for a train in Penn Station, then we see quite different views. But they are both views of Manhattan.

B and E are real in the sense that they do real things -- move charges, carry TV images, make the hair on your head stand up,.... when you examine the end result of whatever you do with E and B, your confusion should be eased.
Regards,
Reilly Atkinson

4. Apr 7, 2007

### Andrew Mason

Special Relativity may explain why the magnetic force is a relativistic effect caused by relative motion of negative and positive charges. Your question seems to assume that the electric field is more "real" than the magnetic field. While the concept of "field" is a useful mathematical model that we can use to predict results, we should not get hung up on whether any field is "real".

The concept of the "field" was invented because we had a conceptual difficulty with Newton's "action at a distance". It is a successful model that gives us consistent results that fit with all known observation. Whether there is anything "there" or not does not matter to the scientist, and may be unknowable.

AM