# Relative Magnetic Fields: What is Reality?

• abrogard

#### abrogard

According to page 61 'Einstein' by Andrew Robinson an electric charge at rest produces no magnetic field.

But: For an observer moving East with uniform motion the charge will appear to be moving West uniformly and B, using a sensitive compass, will detect a magnetic field around the 'moving' charged object.

This boggles my mind.

This charge has simultaneously a magnetic field and no magnetic field?

So that if the field were to produce an output of some sort it would simultaneously not produce and produce an output?

Well, a magnetic field IS an output, isn't it?

I envisage whole electric motors both switched on an switched off at the same time...

This charge has simultaneously a magnetic field and no magnetic field?
I wouldn't use the word "simultaneously" since it has a specific meaning that doesn't work here. But yes, at every event the field is purely electric in one frame and is electric plus magnetic in other frames.

So that if the field were to produce an output of some sort it would simultaneously not produce and produce an output?
No, the outcome of any measurement is frame invariant.

Well, a magnetic field IS an output, isn't it?
How would you measure the magnetic field? Whatever way you can think of, you are guaranteed that the measurement will come out the same in any reference frame. The laws of EM are covariant.