The Time Arrow of Radiation

Abstract

After a stone has been dropped into a pond, one observes concentrically outgoing waves. Similarly, after an electric current has been switched on, one finds a retarded electromagnetic field. Since the laws of nature which successfully describe these events are invariant under time-reversal, they are equally compatible with the reversed phenomena in which, for example, concentrically focussing waves would eject a stone from the water. Small deviations from the exact time-reversal symmetry of the laws modify this argument only in detail. Such ‘reversed processes’ have however never been observed in nature. The absence of ‘focussing solutions’ in high-dimensional configuration space may in fact similarly describe the time arrows of thermodynamics or of quantum theory (if applied to wave functions).