I wasn't sure whether this question belonged in classical physics or here. It is something I never worried about before, but it is now bothering me. Given the reversibility of quantum processes why is one direction preferred over another? Sophiecentaur suggesting looking at Huygens Principle. This and later work (Kirchoff?) seems to suggest the backwards moving waves must suffer a self-cancelling effect, but that would imply that all wave propagation is fundamentally QM in nature.