In special relativity, the causality principle says that the speed at which information propagates from one place to another cannot exceed a finite maximum speed, usually called the speed of light. In classical field theory, this means that if two solutions of the field equations have initial data that differ only within a bounded region of space, then any differences between the two solutions should remain contained within a bounded region that does not grow any faster than a finite maximum speed. This article reviews proofs of causality in simple cases, namely free scalar fields and the free electromagnetic field, and also describes some intuition suggesting that it still holds when the equations of motion are nonlinear.
