From: *Brent Meeker* <[email protected] <mailto:[email protected]>>
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. :-)
Not doable "in principle" just means that there is no conceivable way in
which it could be done. It is not just a matter of difficulty, or that
it would take longer than the lifetime of the universe. It is actually
impossible. Quantum mechanics does not imply that all things that are
logically possible are nomologically possible, or could be achieved in
practice. That is why Saibal's claim that there exists a unitary
operator that does what he wants is rather empty -- there are an
infinite number of unitary operators that are not realizable in
practice. And this limitation is a limitation "in principle".
Bruce
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.
Brent
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