Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] Collapse or No Collapse? That is the Question

Bob Zannelli wrote on Sat, February 19, 2011 7:31:58 AM

...in reality the question of no collapse or collapse is not an
interpretational question at all. It is a question of correct or incorrect
physics.
This fact has become very evident with the rise of the theory of
Decoherence. Decoherence at last provides a real physical process to explain
how

quantum measurement works. And many experiments have been done , even
involving the reverse process of re coherence which as will be made clear
shortly, provide significant support for the validly of this idea.

The debate between collapse and no collapse is really the debate on the
fundamental question of time symmetry. All Collapse models violate time
symmetry invariance. The no collapse models on the other hand generate an arrow

of time statistically, just as we see in classic physics.

A look as a thought experiment can make this point clear. Measure the spin
of electrons in the Z axis. This will give us two possible results.

> [ U>_Z = (1/sqrt[2])*( [U>_x + [ d>_x>)

> { d>_Z= 1/sqrt[2])*( [U>_x - [ d>_x>)

We then take every electron in the state [ U>_Z and measure it's spin for
the X axis. We would of course get 50% up and down spins. ( This would of
course be true if we used the [d>_Z state also)

Now however we time reverse the entire measurement process. Of course we
can't really do this but this is a thought experiment. ( There have been
experiments where much simpler systems have been reversed, which does lend in
my view support for the no collapse model. )

Based on the collapse models the quantum states are completely destroyed.
Therefore if we measure the spin of the electron in the Z axis after this
reversal we should get a mix of up and down spin results. However, based on
the no collapse model, the quantum state was never lost, merely submerged
in the environmental states. So the no collapse model predicts that we
would get 100% up spins in the Z axis.
This in the same as if we could time reverse all the trajectories of gas
molecules in a container so that they once again become concentrated in a
smaller volume. Of course this experiment is not possible either.



In my view, all available data, at least up to now, show an overwhelming
evidence that there is an irreversible time arrow in QM associated with
measurement.
I can only illustrate this by considering an example similar to that of Bob,
but my conclusion will be different.
A photon prepared in a diagonal basis (linearly polarized at 45 degrees to
vertical) has 50% chance to pass through a polarizing filter in a rectangular
basis (with vertical transmission axis). If it does pass, it finds itself in the
corresponding eigenstate of rectilinear basis (vertically polarized). If you
illuminate the same filter with a vertically polarized photon, it has 100%
chance to pass and remains vertically polarized after that (which, in this case,
just confirms pre-existing info) and it has 0% chance to pass emerging from the
filter diagonally polarized.
This is merely a summary of experimental evidence which so far showed no
exceptions (within allowed exp. errors) and therefore it forms one of the axioms
in mathematical foundations of today's QM.
Suppose now, we illuminate the same filter with 2 million diagonally
polarized photons. Statistically, 1 million will pass and find themselves all
vertically polarized after that, and 1 million will be absorbed, heating up the
filter. The time-reverse of this would be illumination of the heated filter with
1 million of vertically polarized photons from behind and the emergence of 2
million of diagonally polarized photons from its front side, leaving the filter
cold. This would be a case with huge decrease of entropy - with an exponentially
low probability, but still allowed by laws of statistical mechanics. If such an
outcome would ever be observed, that would be a huge and therefore extremely
improbable, but still physically possible, fluctuation. That would mean that the
process is time reversible but just "hidden" under statistical (quantum)
fluctuations. However, so far I did not hear about at least much smaller
fluctuations of this kind that would exceed experimental error.
There is a very subtle point here if we repeat the whole process from
scratch, but now only with a single diagonally polarized photon (today, it is
within the reach of modern Quantum Optics). Suppose this photon is lucky enough
to pass the filter (no big deal, 50% chance), which would come at the cost of
its polarization changed from diagonal to vertical. Since the photon has passed,
the filter remains cold. Reverse time now. If we require this to be executable,
we must have the vertically polarized photon incident on the cold filter from
its back side and re-emerging from the front side diagonally polarized. With a
cold filter having vertical transmission axis, this is impossible according to
the direct-time QM of a single photon. At least, there are no known observations
of such an effect up to date. And it is difficult to explain such absence of
evidence by equating it to a self-assembly of a broken vase which is possible
but very unlikely. The small probability in the classical case is due to a huge
amount of degrees of freedom involved. In case with a photon or electron spin we
have only a two-state system, and it is not quite clear where and how the
statistical probabilities could, if ever, come in. May be tomorrow some missing
elements in description of photon interaction with the filter on subatomic level
will be found and change this result. That would accordingly change today's
formulation of QM. But this has not happened so far.
BTW, I think that many have read the paper by Aharonov a.o. in Nov. 2010 issue
of Phys. Today. I only now started reading it, very slowly, partially because
their stile is too high-flying for me, and my impression so far is that it is
more in line with Bob's view, - if we accept a smaller reliability of
experimental data.

Moses Fayngold,
NJIT