Re: Quantum Computers

John Clark (
Wed, 18 Aug 1999 00:55:36 -0400

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Doug Jones <> Wrote:

>There's your fallacy- detectors at the slits perturb the wave
>function of the electrons, and destroy the interference pattern.

It's possible to put detectors near each slit and tell which one the electron went through, but if you do that the interference pattern disappears. Copenhagen explains this by saying that the "measurement" (nobody knows exactly what that is) collapses the wave function, many worlds explains it by saying that the two universes do not converge as they would if the detectors were not there because now there is a difference between the two, there is a physical difference in the brain of the observer, one has a memory of the electron going through slit A and the other has a memory of slit B. Both theories give the same correct results so we can't distinguish between them with this experiment.

So let's change things a little, use quantum erasure so both brains forget the results of the measurement, both brains have the same memory and so are now physically identical; the two universes are now identical too. If Copenhagen is right the wave function has already collapsed and so the quantum erasure will have no effect on the result of the experiment, there will still be no interference pattern. If many worlds is right then the two universes will converge and you will see evidence that the electron went through slot A and evidence that it went through slot B, in other words you will see an interference pattern. There is a clear cut difference between the two theories that can be tested for.

>It's the detector, not whatever looks at the *output* of the
>detector, that matters. Erasing the recording of the output does
>not eliminate the interaction that the detector had with the
>electrons earlier.

I can see that you're absolutely positively 100% certain of that, but we'll have to actually perform the experiment to see if you're also correct.

John K Clark

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