Re: Re: entangled states

Hendrik van Hees <hees@xxxxxxxxxxxxx> wrote:
> Terry Pilling wrote:

> > Suppose two so-called `entangled particles' (whatever that means)
> > are produced at a point and allowed to move freely until separated
> > by some distance (a well known situation).
> > By pre-arrangement, an observable is measured on one of them followed
> > a short time later by a measurement on the second (an experiment
> > easily set up). The two measurment events will be spacelike separated
> > (i.e. _not_ causally connected) and so there will be observers for
> > which either measurement occurs first.
> >
> > A commonly stated point of view is that the first measurement event
> > actually affects the second one and in fact is the physical cause
> > of the result of the second. This is impossible.

> It might be a commonly stated point of view, but it's wrong, at least it
> is not the physics described by quantum theory. You pointed it out
> correctly in your description above: The entangled two-particle state
> was prepared at the very beginning, and this implies the strict
> correlations of the entangled observables (e.g., the polarisation state
> of two entangled photons in Zeilinger's famous "teleportation"

Good! This kind of thing has always bothered me though. It seems
like you can always find two people who will have a philosophical
difference about what is going on here. However I don't see why.
Even in non-relativistic quantum mechanics the equations are completely
deterministic in terms of the amplitude. It is only in the last step,
interpreting the squared amplitude, where statical arguments come into play.
Of course the statistical measurements of two entangled states will
be correlated. It is a consequence of the underlying amplitudes
being determined by the equations of motion along with initial
and boundary conditions. The problem that I was having, and which
you have cleared up for me, is that some people have a different
viewpoint of how things work here and are quite convinced that they
are right and that it is me who is confused.

> Although the single-particle observables are totally
> undetermined, the correlation between the outcome of measurements is
> 100% sure. The measurement on one particle does *not* instantaneously
> act on the other particle, which is far away, but the correlation
> existed the whole time from the preparation of the particle pair.

Exactly how I have always viewed it.

> > But this is not what is claimed.
> > The claim is that nothing is determined until the first measurement
> > is made and this _causes_ the other observable to be affected.

> Where is this claimed? I hope not in any quantum-mechanics textbook?

I hope not also, but I have argued it with several people, seen
posts and discussions claiming this. I have even heard this statement:
"I know it doesn't make sense, but that is quantum mechanics,
you just have to accept it". To which I feel like replying "then you
had better learn some quantum theory, because it makes perfect sense and
it ain't anything like you seem to claim". Where are people being taught
that quantum theory doesn't make sense and has to be taken on faith?
Hopefully not in any physics departments.

I appreciate your reply Hendrik.

--
-Terry

---------------------------------------------------
Terry Pilling
Department of Physics
North Dakota State University
(701) 231-5780

terry[at]member.ams.org
http://www.physics.ndsu.nodak.edu/people/index.html
---------------------------------------------------

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