🚀 go-pugleaf

MRG3@psuvm.psu.edu writes:
>A quick comment on the role of the wavefunction/density matrix
>in consistent histories (ala Hartle & Gell-Mann):
>There is an initial condition (i.e. initial density operator) to be specified
>in the decoherence functional.  The initial conditions will  determine in part
>which histories decohere, and are quasi-classical.  The specific dynamics, and
>choice of coarse graining will also be important.

This is interesting.  I assumed that the initial density operator was
in fact a partial description of some pure state, unknown because of
our ignorance of the microscopic variables-- like an ensemble of experiments
repeatedly performed in which controllable variables interact with a
complicated environment.  You get a diagonal density matrix for the
macroscopic variables after decoherence, but in this view the state
after any one measurement would actually be some unknown superposition of
the various eigenstates of the measured observable, with the density
matrix giving average results.  Someone recently mentioned that in 
quantum cosmology, though, there might be such a thing as a truly mixed
state-- that the universe might be describable only by a density matrix,
not a single state vector or wave function.  Is this related to the idea
of degrees of freedom disappearing behind event horizons?  What does
it mean?  Am I babbling?

-- 
Matt McIrvin   mcirvin@husc.harvard.edu   Long live short .sigs