The requirement of time-reversal symmetry, in general, is difficult to reconcile with the principle of causality. Maxwell's equations and the equations for electromagnetic waves have, in general, two possible solutions: a retarded (delayed) solution and an advanced one. Accordingly, any charged particle generates waves, say at time t 0 = 0 and point x 0 = 0 , which will arrive at point x 1 at the instant t 1 = x 1 / c (here c is the speed of light), after the emission (retarded solution), and other waves, which will arrive at the same place at the instant t 2 = − − x 1 / c , before the emission (advanced solution). The latter, however, violates the causality principle: advanced waves could be detected before their emission. Thus the advanced solutions are usually discarded in the interpretation of electromagnetic waves.