Re: MWI, Copenhage, Randomness

From: Brent Meeker <meekerdb.domain.name.hidden>
Date: Thu, 05 Sep 2002 00:26:07 -0700

On 04-Sep-02, Tim May wrote:

> On Wednesday, September 4, 2002, at 02:44 PM, Hal Finney
> wrote:

>> Tim May wrote:

>> In weaker forms of the MWI, where it's the early state
>> of the Big Bang (for example) which are splitting off
>> into N universes, De Witt and others have speculated (as
>> early as around 1970) that we may _possibly_
>> see some evidence consistent with the EWG interpretation
>> but NOT consistent with other interpretations.

>> I'm not familiar with the details of this. But I know
>> that much of
>> the impetus for increased acceptance of MWI models comes
>> from the
>> cosmologists.

> It was in DeWitt's article, "Quantum mechanics and
> reality," Physics Today, September 1970, reprinted in the
> collection "The Many-Worlds Interpretation of Quantum
> Mechanics," edited by Bryc DeWitt and Neill Graham, 1973.

> "Moreover a decision between the two interpretations may
> ultimately be made on grounds other than direct laboratory
> experimentation. For example, in the very early moments of
> the universe, during the cosmological "Big Bang," the
> universal wave function may have possessed an overall
> coherence as yet unimpaired by condensation into
> non-interfering branches. Such initial coherence may have
> testable implications for cosmology." (p. 165 of the
> reprint volume).

But to maintain strictly unitary evolution the branches are
only non-intefering when measured by macroscopic operators.
 For some (impossible to realize operators) there will
still be cross terms.

....
> By the way, issues of observers and measurements are
> obviously fraught with "Chinese boxes" types of problems.
> In the Schrodinger's Cat pedantic example, if the "cat
> alive or cat dead" measurement is made at the end of one
> hour by opening the sealed box, what if a video camera had
> been also sealed inside the box, and had seen the cat
> breathe in the cyanide gas at 10 minutes into the
> experiment? Does this imply the "wave function collapsed"
> at the time of the measurement by the human observers, at
> the one hour point, or at the time the video camera
> unambiguously recorded the cat's death?

Alive and dead are very macroscopic operators (average of
lots of micro-states) and so the cats interaction with it's
environment (the box) will very quickly diagonalize the
Alive X Dead density matrix. To introduce observers as
having a special effect seems to introduce an aphysical
mysticism.

> One could arrange a thought experiment involving literally
> a series of boxes within boxes, each being opened at, say,
> one minute intervals after the cyanide was released or not
> released. One set of observers sees the cat either alive
> or dead at the end of the canonical one hour period. But
> they are sealed inside a box. After one minute, their box
> is opened, and the observers in the next-larger box then
> see the "collapse of the wave function at the 61-minute
> point." After another minute, their box is opened and a
> new set of observer sees "the collapse of the wave
> function at the 62-minute point."

> And so on. (I don't know if I'm just reinventing a thought
> experiment someone developed many decades ago...it seems
> like a natural idea.)

> Seen this way, the "collapse of the wave function" in the
> Schrodinger's Cat thought experiment is seen as a problem
> of knowledge, not something quasi-mystical about an
> instantaneous collapse of some psi-squared function.

> (More interesting are the delayed choice experiments.)

In Bohm's QM the universal wave function determines
everything and the apparent randomness we see it just like
the randomness of statistical mechanics, it comes from our
inability to take into account all the non-local effects of
all the rest of the universe. It solve the "collapse of
the wave function problem" as you suggest by making it a
collapse of our uncertainity and it's perfectly
understandable then that different observers "collapse it"
at different times. The problem is that BQM isn't Lorentz
invariant - but people are working on that.

...

Brent Meeker
The concept of 'measurment' becomes so fuzzy on reflection
that
it is quite suprising to have it appear in physical theory
at the
most fundamenatal level.
      -- J. S. Bell
Received on Thu Sep 05 2002 - 00:26:35 PDT