On 5/3/2018 4:51 PM, Bruce Kellett wrote:
From: *Brent Meeker* <[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.
Well maybe it's that I speak American English instead of Aussie; but "in
principle" leaves me wondering which principle: logically, mathematical,
physical, economic,...
Brent
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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