When discussing non-steady current, why is Newton's 3rd Law valid only if momentum carried by the Electromagnetic field taken into account?

Question

I can't understand what the underlying statement implies -

'When currents vary with time, Newton's third law is valid only if momentum carried by the electromagnetic field is taken into account'.

Answer 1

There are a lot of ways of establishing this with increasing mathematical sophistication, but most obviously, radiation can scatter off of particles without causing a back-reaction on the source of the radiation (for instance, the source could be turned off at the time of the scattering). Either you have to say "the radiation itself is carrying momentum" or you have to say "momentum is not conserved"