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Feb24-05, 02:10 AM
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Quote Quote by caribou
Talking of erasing the past, in Roland Omnes's Understanding Quantum Theory, he mentions the ideal von Neumann experiment.

A particle is emitted and can go down one of two channels. There is a detector in one channel. If, after a certain time, the particle is detected then you know it went along the channel with the detector and if the particle is not detected then you know it went along the channel without the detector.

So far, so simple.

Now here comes the interesting part as, if you arrange it so that the two channels curve and later join up into the same direction, then when the particle reaches the point at which the channels join up then the detector result will go from "detected" or "not detected" into a superposition and "don't know" Schrodinger Cat state even if the particle is now a huge distance away from the detector!

It's EPR and Schrodinger's Cat in one simple, fun package.

Decoherence keeps this lunacy from happening in the real world, thankfully.
This doesn't seem correct to me. The detector should go into a superposition of "detection" and "non-detection" even if you don't join the channels.