I've been rewatching the Feynman Auckland lectures and have a question about using partial reflection as a communication medium.
Say you had two glass lenses in a vacuum, positioned several miles apart.
It seems that by adjusting the exact distance between the two, you could effectively alter the proportion of photons reflected off of the first surface to 0.
Thus, an observer with one of the mirrors could use the reflection probability of a photon from the front surface to infer the position of the far mirror.
Does this allow for FTL comms? Or does the change in partial reflection propagate at the speed of light?