Abstract:  Electromagnetic fields carry energy, momentum, and even angular
momentum. The momentum density is ε0(E X B), and it accounts (among
other things) for the pressure of light. But even static fields can harbor momentum,
and this would appear to contradict a general theorem: if the center
of energy of a closed system is at rest, then its total momentum must be zero.
Evidently in such cases there lurks some other momentum, not electromagnetic
in nature, which cancels the field momentum. But finding this “hidden momentum”
can be surprisingly subtle. I’ll discuss a particularly nice example.