In alt.engineering.electrical John wrote: | |>
|> Take two electrons, separated in space, stationary relative to some |> observer. There's an electric field, obviously, but no magnetic field. |> Now take another observer moving perpendicularly to the line joining the |> electrons. This observe sees the electrons in motion. Electrons in |> motion are an electric current, and an electric current produces a |> magnetic field, so for that observer there is a magnetic field present. |>
|> So one observer finds a magnetic field present where another observer |> finds none. The notion that a magnetic field has a concrete existence is |> clearly problematic. This paradox doesn't appear in the theory itself, |> because it simply tells you what will happen (or more exactly, what your |> measurements will show). It doesn't say anything about what is "really" |> there. |>
| This is all a bit over my head, but presumably the first observer (the one | who doesn't see the magnetic field because it doesn't exist for him) sees | something else; whatever the second observer sees as a magnetic field | manifests itself somehow for the first observer? Don't conservation laws | say that elements might vary, but the total sum must be the same? | Probably not...
If there is motion between the observer and the electrons, the observer might THINK he sees a magnetic field. But is it really there? That lies in the ability to observe. How do you tell if a magnetic field is there or not? You measure it by seeing how it acts on something. One way is with a coil and ampmeter. But now that's electrons again. The net effect is that something which is electrically changed has ultimately caused an electrical current where movement is involved.
On the other hand, how do we know there is an electron there? Maybe it only looks like it because that would explain a magnetic field, which is what we are measuring. Maybe what we sense as an electric/static charge is really a sensation of a magnetic current because we have to move in some way to know there is a charge and where it is.
One cannot be without the other, but the real question is whether there are really two things or not. I suggest that the answer is that what is really there is one thing that simply is characterizsed both ways.