  In electromagnetism, as in Newton's mechanics, action is always equal to
reaction. The force from the electromagnetic field on matter is balanced by an
equal and opposite force from matter on the field. We generally speak only of
forces exerted by the field, not forces exerted upon the field. But, we should
not be hesitant to speak of forces acting on the field. The electromagnetic
field closely resembles a relativistic fluid and responds to forces in the same
way. Analyzing this analogy sheds light on the inertial role played by the
field's mass, the status of Maxwell's stress tensor, and the nature of the
electromagnetic field.
