This is one of the most elegant results in modern physics: the magnetic field is not an independent force, it is what an observer moving relative to a distribution of charges “sees” in place of a purely electric force.
Imagine a current-carrying wire in which, hypothetically, no electron actually circulated and the current were produced some other way. An observer at rest would see only a magnetic field. But an observer moving with the wire’s electrons would instead see a Lorentz contraction that breaks the balance between positive and negative charges, creating a net charge imbalance, that is, an electric field. What is a magnetic force for the first observer is an electric force for the second: the same physics, described differently.
What we call “magnetism” is thus the relativistic reflection of electricity. Einstein, in his 1905 paper, starts precisely from this observation: the electromagnetic field is a single thing, seen differently by different observers (Feynman 1963; Einstein 1938).
One reality, two names
The distinction between “electric field” and “magnetic field” is not absolute: it depends on the observer’s state of motion. Only the electromagnetic field, as a whole, is a well-defined physical quantity independent of the reference frame.
Collegamenti
Argomenti: Magnetismo Concetti: Campo magnetico · Forza di Lorentz
Esercizi collegati: True or false about the magnetic force · Magnetic field from a moving charge · Northern lights