I've been thinking a lot about conservation of energy recently, and in addition to my other thread, I have another (quite different) seeming "paradox" that I can't resolve. Suppose you have a laser shooting into a Michelson interferometer with distances such that the split beam exactly destructively interferes with itself. If the laser takes, say, 1 joule to power for a second, where did that energy go? If the interference is perfectly destructive, then the effect can't propagate outside of the apparatus, so I would like to say that the energy went into the mirrors. However, this seems unsatisfactory since I cant see why the mirrors would absorb all the energy when the light is in a certain relative phase, but none of the energy when it is in another relative phase (imagine moving the distance a half wavelength - then the interference is constructive and all the energy must stay in the EM field).