**From:** Mitchell Porter (*mitchtemporarily@hotmail.com*)

**Date:** Tue May 27 2003 - 01:59:02 MDT

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A much-delayed reply to these posts... I've been discussing another

interpretation

(John Cramer's "transactional interpretation") elsewhere, and have come to

the

conclusion that to make any progress on this question (the truth about QM),

one

has to think about all the various interpretations (hidden variables, many

worlds,

time-symmetric, spontaneous collapse) at once, because they have all sorts

of

unexpected connections... and that this is going to be slow hard work,

because

two things have to be done:

(i) Get the technicalities of each interpretation right.

(ii) Get them to 'talk to each other' - establish a common frame of

reference.

I will spare you all my current thoughts about those connections, since they

are a work in progress, but interested parties might want to look at the

arxiv.org

preprints hep-th/0302111 (which I think is pointing the way towards an

Everett-derived "one-history" interpretation) and quant-ph/0305089.

D. Goel said

*> > Well, the coefficients of "mismatch states" like |x=1>|you=2> should
*

*> > become very small, compared to those of "veridical states" like
*

*> > |x=1>|you=1>. But I don't see how that addresses the basis problem.
*

*>
*

*>The mismatch state's coefficient was 0 in the first place. I had
*

*>added and subtracted a term like that to both sides.
*

I was describing an effect of decoherence (mismatch coefficients become

small). In your argument (see http://sl4.org/archive/0305/6592.html),

you are basically taking this effect for granted and setting those numbers

equal to zero. Which is OK as an approximation, precisely *because* of

decoherence, but there should in actuality be an infinitesimal (but still

nonzero) amplitude for cross-terms in your equation (3).

In any case, I think the crucial step is this:

*>Again, note that (2) can be put into (4) in many many ways. But the
*

*>issue is to put the time-dependent equation (3) into form
*

*>(4)----viz. we want an "interpretation" that is "stable" with time.
*

Who says? Where does this condition come from? This looks to me

like an "extra postulate".

Amongst the experts, I think the most popular way to pick the many

worlds out of the wavefunction is via 'decoherent coarse-grained

histories'. Each history is specified by a time series of projection

operators,

and it's required that each history decoheres from each other history,

where this is calculated using a 'decoherence functional' constructed

from the Hamiltonian. There is a brief exposition of this in the second

paper above, by Hartle. But an important point is that there is no

canonical set of decoherent histories associated with a particular

wavefunction, not even a canonical 'maximal set' (no extra histories

or finer graining of operators possible). So it's the basis problem again,

at a more sophisticated level.

Lee Corbin said

*>I suggest that you post your question to
*

*>the Fabric of Reality list, where Deutsch himself often answers
*

*>queries.
*

I tried posting there years ago, a question about Deutsch's "shadow

photons". Early in his book he says that the double-slit diffraction

pattern can be explained by the effect on photons in our world of

"shadow photons" in the worlds next door. My question was along

the lines: Can you mathematically describe this in terms of interactions

between individual shadow photons and individual photons in this

world? I don't think he could, in which case this way of speaking is

misleading. As I recall, I posted my question, it never showed up,

the moderator hadn't seen it, and I just lost patience and stopped

trying. I'll probably have another go eventually, but first I want to

work through my own version of many-worlds, in the context of

step (ii), back at the start of this post.

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