scerir wrote:

>Wigner later (1983) changed opinion and wrote
>that decoherence forbids superposition of states like
>      c1 |s 1> |friend 1> + c2 |s 2> |friend 2>
>After that in QM the "conscious" being  - i.e. the "friend"
>who tells that  he already "knows" whether the outcome is
>|s 1> or |s 2> - plays no role.

But can decoherence really "forbid" macroscopic superpositions in principle, 
or only in practice? To build quantum computers, people have to figure out 
clever tricks to keep fairly large systems in quantum coherence, even though 
under normal circumstances decoherence would prevent this. Is there a limit 
to how far we could take this? If quantum computing follows something like 
Moore's law, it seems concievable that we could eventually simulate 
something as complex as a cat-in-a-box, which we'd have to treat as being in 
a superposition of states as long as coherence was maintained among the 
elements of the computer itself. And we don't need something so complex to 
get these sorts of "paradoxes"--at the bare minimum, we just need a system 
complex enough to keep internal records of its own state (analogous to the 
cat's brain or the videocamera in the box) so that we must treat the system 
as being in a superposition as long as coherence is maintained, but once 
coherence breaks down we can look at the internal records and see that the 
system *appears* to have been in a definite state all along (Is there any 
well-defined notion of what it means for a system to keep records of its own 
history, BTW? Maybe this is related to Maxwell's demon and the 
thermodynamics of computation?) Then we could then extend this to a 
"nested-box" scenario where one subsystem is in coherence relative to a 
larger system, which is itself in coherence relative to the outside world, 
and then decoherence occurs between the system and the subsytem while the 
system as a whole remains in coherence from an "outside" point of view 
(which would be like Wigner's friend looking in the box while both the box 
and the friend were still sealed in a room).

Von Neumann, London,
>Bauer, Wigner thought (and many more still think) that
>consciousness was able to "collapse" a superposition
>(E.P.W., Collected Papers, VI, pages 225-244)

Did Wigner only believe this until his change of opinion in 1983, or did he 
continue to think this way afterwards?


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