From: Hal Finney (hal@finney.org)
Date: Sat Feb 22 2003 - 23:45:30 MST
Reason writes:
> The transactional interpretation of QM uses retarded waves. Worth a look,
> as it very elegantly kicks MWI, observer privilege and whole bunch of
> other annoyances out of the window. As I understand it, it falls down
> because a lot of other physical theories (outside my area of specialty,
> unfortunately) demand that retarded waves don't exist or don't exist in
> large numbers relative to advanced waves.
You may not be able to get rid of Many-Worlds so easily. I've seen
it suggested that the TI and other causal/deterministic quantum
interpretations like that of Bohm actually embed Many-Worlds in a
hidden form.
The idea is that in the full MWI, we calculate what happens using the
Schrodinger equation. Now, when there is a measurement, the equation
"decoheres" or splits into two causally discrete parts. Each part can be
thought of as a universe which goes its separate way, unaffected by the
other part. However, measurements are not perfect. Hence decoherence is
never absolute, and therefore one universe does have an effect on another,
albeit an infinitisimal one in practice for macroscopic measurements.
But the interference is still there, in the equations. Any theory which
follows the mathematics of the Schrodinger equations, which I am pretty
sure the TI does, must get the same results as the MWI. The TI denies
the existence of the other universes, but their effects on the one "real"
branch of the wave function are still there mathematically as much as
in the MWI. They have to be there, because these are just different
interpretations of the same mathematics.
Therefore, if you look sufficiently closely at the Transactional
Interpretation, you see other worlds. That is, you have to calculate the
effects of other worlds in order to get a perfect calculation of the One
True Wave Function, which is the one we see in this world. In effect,
the reality of the other worlds is denied by fiat in this interpretation.
We have to calculate as if they were there, but we simply declare that
they do not exist.
This is not very attractive philosophically. If we have to calculate
the effects of other worlds in order to model our own world, what right
have we to deny their existence? That is an arbitrary and unrealistic
step.
Anyway, that's the argument I have heard. I don't actually know the
TI well enough to be certain that it applies to that case, but it makes
pretty good sense to me. The Schrodinger equation strongly predicts
the existence of other worlds, and any interpretation which accepts the
reality of that equation has to be prepared to deal with the reality
of many worlds. Most other interpretations sweep this under the rug
because it is only implicit and not explicit, but look closely enough
and it is there.
Hal
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