The Special Theory of Relativity 137
with the same velocity as the charged particles, there will only be an
electric force between the charges. The observer will not observe a
magnetic force because, with respect to the observer, the charges are
both at rest. The interaction of the charged particles is identical for the
two observers because of the principle of relativity. Hence, the electric
and magnetic force observed by the stationary observer is equivalent
to the electric force observed by the moving observer or, in other words,
the electric and magnetic forces are the same. The magnetic force is just
an electric force in motion.