Re: Is State Vector Reduction a 'Process'?

Souvik wrote:
> Is state-vector reduction (or collapse of the wavefunction) a physical
> process? Or is it really an artefact of our procedure of canonical
> quantisation?
>
> In the Feynman path integral approach to Quantum Theory, state-vector
> reduction doesn't seem to be a process any more than asking the
> question: What is the amplitude of going from a certain initial state
> to a final state?
>

As you see different opinions arise. My take on it is this.
When you view the wave function as a statistical description,
(analogous to a probability distribution) then the assumption
of new information changes your description changes.

Thus even classical probability distributions "collapse".
E.g, your probability of winning the raffle can jump from
a small positive probability to either of certainty or absolute zero
upon the drawing. This occurs "instantaneously for all tickets".
This happens "non-locally" because the
state of "winning the raffle" does not describe the state
of your ticket but the correlation between your ticket and
the state of a ticket drawn in the raffle office.

So I would argue that collapsing wave functions are not physical
processes but rather occur "on paper".


Bill Hobba posted a link:
http://quantum.phys.cmu.edu/quest.html
to another "interpretation" of QM called "Consistent Histories"
which makes this point.

It may help in parsing the various thought experiments to remember
that when you are considering a "one particle experiment" there
is a global non-causal constraint being applied to the system definition
which excludes cases where a second or third particle enters the
experimental domain. So for example when you measure one electron
in position x you are in this act measuring also zero electrons
at every other position. The measurement action is non-local,
and you toss out instances when you get more than one electron total.
This "tossing out of instances" tells you that the collapse
is not occuring "to the particle" but rather "to your description".

This you may relate also to Arnold Neumaier's point about restriction
to a small part of "the whole universe". However I think this point
misleading in that "the wave function of the whole universe"
is not operationally meaningful. There is only one instance of "the
whole universe" so probabilities are meaningless and you cannot
interpret such a "wave function" quantum mechanically,
That is unless you use a 1-dimensional Hilbert space, with one
physical observable "does it exist" which has expectation value 1.


Regards,
James Baugh


.

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