Hi all, I am reading the book "Emperor's New Mind" and have a question related to time asymmetry in state vector reduction (p.458) in quantum mechanics. Consider the following situation, as presented in the book: Suppose I have closed room with a lamp L, which emits light in some fixed direction which is to be detected by a photon detector P placed in that direction. Between the photon detector P and the lamp L we have placed a half silvered mirror, which is tilted at an angle of 45 deg. to the path connecting L and P. The mirror reflects some amount of light and transmits some of the light. Whenever a photon is emitted by the lamp L it would be detected by the photon detector P with probability 0.5. Now suppose we take the reverse time situation: Suppose the light has reached the photon detector. When we evolve the wavefunction in reverse direction we see that it would bifurcate as it reaches the mirror and would reach L with "amplitude" 1/SQRT(2) and reach the top point B with the same amplitude (figure in attachment). The author then contends that the corresponding probabilities (square of these amplitudes) of 0.5 are the probabilities of following events: 'Given that L registers, what is the probability that P registers?' 'Given that the photon is ejected from wall at B, what is the probability that P registers?' I do not understand why the above probabilities correspond to these events (above). Since we are considering a time reversed situation where we are assuming that the photon has reached P, shouldn't these probabilities correspond to the following events: 'Given the photon reached P, what is the probability that L registers, i.e. it came from L?' 'Given the photon reached P, what is the probability that it came from B?'