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In another book called "The Ghost In The Atom" Deutsch does give what he considers a definitive test of the Many World Interpretation and a Quantum Computer is part of it, but it must be intelligent.
In the Deutsch test a conscious quantum computer shoots electrons at a metal plate that has 2 small slits in it. It does this one at a time. After leaving the plate the electrons hit some photographic film, but do not look at the photograph until later. The quantum mind has detectors near each slit so it knows which slit the various electrons went through. The quantum mind now signs a document saying that it has observed each and every electron and knows what slit each electron went through. It is very important that the document does not say which slit the electron went through, it only says that they went through one slit only, and the mind has knowledge of which one. Now the mind uses quantum erasure to completely destroy his memory of the experiment. The only part remaining is the document. Now develop the photographic plate and look at it. According to Deutsch if you see interference bands then the many world interpretation is correct. If you do not see interference bands then there are no worlds but this one and the conventional interpretation is correct.
Deutsch thinks an experiment like this will actually be performed in the first half of the next century. The reason it's so difficult to test is not many world's fault, the reason is that the conventional view says that conscious observers obey different laws of physics, many worlds says they do not so to test who's right we need a mind that uses quantum properties.
In the Copenhagen interpretation when the results of a measurement enters the consciousness of an observer the wave function collapses, in effect all the universes except one disappear without a trace so you get no interference. In the many worlds model all the other worlds will converge back into one universe when the electrons hit the photographic film but their influence will still be felt, you'll see indications that the electron went through slot A only and indications that it went through slot B only, and that's what causes interference. Time will tell who's correct.
John K Clark firstname.lastname@example.org
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