A laser beam is split into two identical beams which impinge on a half-silvered surface from opposite sides. The reflected wave is out of phase with the transmitted wave. There is destructive interference on both outputs. Where does the energy go? See my avatar - I can't upload my own picture, not sure if it's possible without a URL. Yes it was triggered by reading about the HOM effect, but as far as I can see it should be true for classical waves - even radio or sound waves. Normally when someone asks this question of an interference experiment I am the first to say "wherever there's destructive interference somewhere there's constructive interference somewhere else" and waffle on about normalisation and conservation. But I can't see how it would work in this case. If there were random phase jitter it would "all average out" too, but this doesn't occur with a split laser beam. And I am assuming that the surface is symmetrical with inversion at both reflections. That's why I specified "half-silvered" which in principle could be a thin, self-supporting, film. Or an array of fence posts if you want to use sound. I'd even settle for non-inversion all round and get twice the energy out as I put in, but that probably has a flaw somewhere too. I dare say the answer is obvious - when it's pointed out. So if you want to me - please do. I will consider it a favour.