On 5/3/2018 4:03 PM, Bruce Kellett wrote:
The problem, of course, is that this unitary operator is formed in the multiverse, so to form its inverse we have to have access to the other worlds of the multiverse. And this is impossible because of the linearity of the SE. So although the mathematics of unitary transformations is perfectly reversible, measurements are not reversible in principle in the one world we find ourselves to inhabit.
I think we need a more precise term than "in principle" which could confuesed with "mathematically". You really mean reversal is /nomologically/ impossible even though it's /mathematically/ reversible. It's more impossible that /FAPP/ or /statistically/ but not /logically/ impossible. :-)
So even Deutsch's quantum brain is likely to run into difficulties, since it has to communicate with the real world.
That's a general problem with quantum computers; they need to decohere produce a result. I think Saibal Mitra wrote a paper on this point.
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