On Mon, Feb 16, 1998 at 07:20:55PM -0800, Hal Finney wrote:
> Mitchell Porter sent this only to me, by accident, and asked me to forward
> it to the list:
> > Everett quantitatively defines correlation early on in his
> > thesis, in the section on information. I will confess that I
> > haven't sat down and compared his definitions to those of the
> > decoherence theorist Wojciech Zurek. But it is certainly my
> > impression that a decoherent state is one in which the correlation
> > between two systems, in the sense of 'correlation' defined by
> > Everett, is very low.

I think decoherence is a relationship between branches, not between
subsystems, so it doesn't have anything to do with correlation. To quote
Price's FAQ:

Worlds, or branches of the universal wavefunction, split when different
components of a quantum superposition "decohere" from each other [7a],
[7b], [10]. Decoherence refers to the loss of coherency or absence of
interference effects between the elements of the superposition. For two
branches or worlds to interfere with each other all the atoms, subatomic
particles, photons and other degrees of freedom in each world have to be
in the same state, which usually means they all must be in the same place
or significantly overlap in both worlds, simultaneously. [end quote]

It seems to me that the modern MWI has more to do with relationships
between branches (i.e. decoherence) than with relationships between
subsystems (i.e. correlation or relative state), even though Everett
originally stressed the concept of relative state. 

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