On 5/3/2018 9:07 PM, agrayson2...@gmail.com wrote:
On Thursday, May 3, 2018 at 11:52:00 PM UTC, Bruce wrote:
From: *Brent Meeker* <meek...@verizon.net <javascript:>>
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
*If you take the view that quantum reality is irreducibly random, it
MEANS that there is no process in nature that can explain how a random
event could occur, for if such a process existed, it would contradict
"irreducibly random". Bruce seems to take the view that all
measurements are irreversible in principle. That might not be true.
For example, suppose the temperature of a system decreases. Isn't it
hypothetically possible to imagine a time reversal of all the IR
photons which caused the cooling, to reunite with the original system
and restore the previous higher temperature? If so, the cooling
process in this example is reversible albeit hugely improbable --
which I refer to as statistically reversible, or irreversible FAPP. *
*As long as there is no interaction with the outgoing photons, they are
just the potential particles of a spherical EM wave. An ideal spherical
mirror could then "in principle" reflect them back into a converging
spherical wave. This is what quantum erasure experiments do on a small
(one photon) scale. *
*I think Bruce can give an example of a measurement which is time
irreversible in principle, that is, impossible to time reverse. AG*
*If the EM wave interacts with something, then it's at a particular
place. In CI terms the spherical wave collapses to photon(s) at that
place. In general this "collapse" can happen at different places which
in MWI means the probability flows to orthogonal subspaces, quantum
erasure would mean reversing things in different (almost orthogonal)
subspaces.**
**
**Brent*
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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