Question about a single photon's wave Assume you have a single photon generator in empty space surrounded by a spherical array of many detectors each of which is 10 light seconds from the generator.  A single photon is generated.  I assume that the wave equation would describe a spherical propagation of the wave outward from its source.  If a photon is detected by one element in the detector array, does the wave function instantly collapse?  If a spherical wave is generated, how does the wave on the opposite side of the array instantly "know" the photon has been detected? Does the answer involve quantum entanglement?
 A: ("Collaps of the wave function" is a concept in the Copenhagen interpretation of quantum mechanics, so I will stick to that. In other interpretations - such as many-worlds - the explanation would be different, though all results are of course the same)
Yes, the wave function collapses when you measure the particle at one point. This does indeed mean that the wave function on the opposite side immediately "knows" that it isn't there. This does seem to violate special relativity, because this "signal" traveled faster than light. In fact it does not violate anything, because this signal does not carry information from one side to the other (after all, the location where the photon is measured is random). But still, it is quite weird and means that physics is not really "local" in a strong sense. This is called the "EPR paradoxon" (google it. Its quite fascinating).
Usually, the word "entanglement" is used if the states of two (or more) particles are correlated with each other. So this setting with a single photon would not be called entanglement, even though its essentially the same phenomenon. And you can easily get an entangled state out of this setting: If you let the photon be detected by one of the detectors, but you dont look at them (impossible in practice, but fine as a thought experiment). Then the states of the detectors (which are both in a superposition between "I have detected the photon" and "I have not detected the photon) are entangled with each other. This collapses once you look at one of the detectors to see which it is.
A: Wave function collapse is not a physical phenomenon. It does not happen. In this example there is a spherical wave, which describes the probability that photons reach the detectors. If the wave intensity is low and you don't integrate over along time, then it may be that only one photon is detected, with equal probability, by any of the detectors.
