I'd like to take advantage of having a bona fide physicist on the list to
ask a question about decoherence and its implications for the MWI.

Paddy Leahy wrote:
> The crucial point, which is not taught in introductory QM 
> classes, is the theory of Quantum decoherence, for which see the wikipedia 
> article and associated references (e.g. the Zurek quant-ph/0306072).
> This shows that according to QM, the decay time for quantum decoherence is 
> astonishingly fast if the product ((position shift)^2 * mass * 
> temperature) is much bigger than the order of a single atom at room 
> temperature. Moreover, the theory has been confirmed experimentally in 
> some cases.
> Since coherence decays exponentially, after say 100 decay times there is 
> essentially no chance of observing interference phenomena, which is the 
> *only* way we can demonstrate the existence of other branches. "No chance" 
> meaning not once in the history of the universe to date.

I understand that there is research into attempts to measure decoherence,
using special conditions and experimental arrangements.  As you say, in
ordinary situations, environmental influences make observing any effects
from other branches effectively impossible.  But it is possible to set
up experiments that decohere "gradually" and where they can measure any
residual interference effects, comparing the results to the predictions
of QM.

As far as I know, these experiments so far are consistent with QM theory.
Of course it's always possible that departures would show up eventually,
which is part of why they do the experiments.  But I would assume that
most physicists would not be astonished to find that QM continued to
work correctly no matter how far out these experiments were pushed.
In fact, that would presumably be the expected result, and any confirmed
departures from QM predictions would be surprising and even revolutionary.

If this is true, then how can a physicist not accept the MWI?  Isn't that
just a matter of taking this decoherence phenomenon to a (much) larger
degree?  Either you have to believe that at some point decoherence stops
following the rules of QM, or you have to believe that the mathematics
describes physical reality.  And the mathematical equations predict the
theoretical existence of the parallel yet unobservable branches.

Of course, given that they are in practice unobservable, a degree of
agnosticism is perhaps justifiable for the working physicist.  He doesn't
have to trouble himself with such difficult questions, in practice.
But still, if he believes the theory, and he applies it in his day to
day work, shouldn't he believe the implications of the theory?

To me, it almost requires believing a contradiction to expect that
decoherence experiments will follow the predictions of QM, without also
expecting that the more extreme versions of those predictions will be
true as well, which would imply the reality of the MWI.  You either have
to believe that a sufficiently accurate decoherence experiment would
find a violation of QM, or you have to believe in the MWI.

Don't you?

Hal Finney

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