Hidden Momentum

Synopsis

Electromagnetic fields carry energy, momentum, and even angular momentum.  The momentum density is epsilon_0 (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.