Dan said: > Let me answer that question with another question: which slit did the > photon go through? :-) The real answer to that question is what I > consider the real answer to this question.
I'm not sure about that. The interesting behaviour of Young's Slits comes down to how we have to add amplitudes for experimentally indistinguishable histories. A better example of the failure of classical notions of the causal structure of spacetime would be Einstein-Podolsky-Rosen type experiments. (Although I suppose that's an example of the breakdown of classical ideas of causality itself - the link between entangled particles is something stronger than causal disconnection but weaker than full causal connectiton.) > One doesn't have to assume determanism to give this answer, BTW. One > can simply assume that spacetime is a limited concept, inadaquate for > the job. It's interesting that it's possible to build things like temporal paradoxes even without time travel in deterministic universes, because in such universes the past and future are implicitly contained within the present. For example, imagine a box with a light that can be either red or green and a prediction machine that uses the time-evolution rules to predict the colour of the light one minute in the future. Now wire the box to the prediction machine so that it switches the colour of the light to be exactly opposite to that predicted. Rich, who often used to daydream about quantum mechanics emerging naturally from general relativity with time machines.
