A basic principle of physics states that elementary physical processes can be reversed in time. For example, a movie of two billiard balls colliding and moving off at different speeds in different directions will look consistent with Newton’s laws whether you run it forward or backward. At least it will if the frictionbetween the billiard balls and the surface they are rolling on is negligible. The friction arises from interactions involving microscale processes that cannot be reversed under typical experimental conditions. Friction thus breaks time-reversal symmetry. And if the friction is significant, the movie of billiard ball collisions will look decidedly odd when run backward: The balls that normally slow down while rolling freely on the table will instead pick up speed.
Maxwell’sequations for the propagation of electromagnetic waves in vacuum exhibit the same time-reversal symmetry as frictionless mechanics. But when matter is present, interactions between radiation and the medium...
