Fig. 3 A plane-wave transaction between emitter and absorber through the exchange of advanced and retarded waves (see text). Waves from emitter are indicated by red lines and waves from absorber by blue lines. Retarded waves are shown as solid, and advanced waves are shown as dashed. Relative phase of waves is indicated schematically by sinusoids inscribed on lightlike world lines. Double timelike world lines for emitter and absorber indicate higher energy state. Wave amplitudes have the value 
+*=2Re() at emitter and absorber loci and are therefore real.