Re: "spooky action at a distance"

From: Joao Leao <>
Date: Fri, 14 Nov 2003 13:38:27 -0500

scerir wrote:

> Joao Leao:
> > The association between non-locality and "retrocausality"
> > (for lack of a better word) is anything but simple! In any
> > case it has less to do with the flow of time than with its
> > negation! [...]
> Bell's theorem shows that, given the hidden variable lambda,
> the result of the experiment at B is dependent on the angle
> of the measurement at A, *or* the the result of experiment at
> A is dependent on the angle of the measurement at B, *or* both.
> Now, because of symmetry, it must be both. Thus, if there are
> "retrocausations" (or "influences", or "weak signals" as Ian
> Percival calls them) they are in both directions (and with the
> same probabilities).
> So yes, it is difficult to show that the flow of time is
> involved. Antoine Suarez (and the Geneva Group) speaks of
> a-temporal quantum effects.

That is a defensible poin-of-view. The "time symmetric"
approach does not conceive the measurement in these terms
though! It requires the actual symmetrization of the coincidence
measurements, what the Aharonov school calls "pre- and post-selection".
This is a way of symmetrizing the initial with the final conditions...

Your proposal below is not lunatic in the least bit, though!
It has been mentioned in the literature several many times.
I just don't have the references handy.
(but check )
It is a tricky point to
reconcile it with the usual description but it can be done.
You have to bear in mind that the correlations can only
be exacted a-posteriori from the coincidence counts.
A single pairwise detection will not provide you with
any retrodictory inference about an actual value
being set before you choose the basis of observation.
(This is an instance of a delayed choice experiment by
the way.)

> Now let us imagine this set-up.
> I suppose it can be useful also within the MWI, at
> least as a possible answer to the question "If we live
> in all of them can we pick the cheapest one?". So I go
> on trying to describe this gedanken experiment (or
> perhaps lunacy).
> There is the usual SPDC source, two correlated photons,
> mirrors m1 and m2, one human observer, polarization detectors
> (measuring photon-1) and, very close, 4 boxes to collect photon-2.
> Of course the path of photon-1 is shorter that the path of
> photon-2, so there is a time-delay, for photon-2 going
> into one of those boxes (possible delayed choice here?).
> m1
> /----------<-----------<--source-->------>- detectors
> |
> |
> |
> \------------------>----------------->----- boxes (1,2,3,4)
> m2
> Now the observer can measure, with his detectors, or the
> linear polarization of his photon-1, or the circular
> polarization of his photon-1.
> Of course the observer, having measured his photon-1,
> can predict what is the polarization state of photon-2.
> There are 4 possibilities: linear/x, linear/y, circular/+,
> circular/-.
> Being very short the distance between detectors and
> boxes, the observer has time (due to that time-delay)
> to move there and pick up the right box (that one with the
> right label: linear/x, linear/y, circular/+, circular/-)
> and collect, into the right box, the photon-2 which
> is arriving.
> This is possible because he *knows* what was his *choice*
> while measuring, with detectors, the polarization state
> (linear *or* circular) of photon-1. And he also *knows*
> what was the measurement outcome for photon-1: i.e.
> linear/x, or circular/+, or ...
> This is also possible because the observer has *time* to
> move to the other location and pick up the right box,
> to collect photon-2.
> But before observer makes his *choice* the photons
> (and especially photon-2, which is "late") were
> already flying.
> So you could ask: what was the polarization state of
> photon-2, before the observer made his choice measuring,
> with his detectors, the polarization state of photon-1?
> The answer seems to be that photon-2 fits equally well
> in both categories, that is to say: linear polarization
> and circular polarization. Thus neither of these
> properties can be ascribed to it as an objective property.
> Now you can also ask: what if I cut the path lenght
> of photon-2 and I make it equal to the path lenght
> of photon-1? It happens that the observer becomes
> unable to move from the detectors location to
> the boxes location, because there is no time-delay
> now. So, in these conmditions, the observer, loses
> control of the situation. His information remains
> hidden, or useless, ot impossible. But this, imo,
> does not mean that photons gain some objectiveness.
> Or not?

Not really. I mean, what is at stake is not the objectiveness
of the photons but of the value of their polarization in either
base (or both). The fact that you may have stored a value
that you did not know while you waited to "objectify" it
does not particular help you...

> Of course you excluded the possibility of (weak or strong)
> signals traveling FTL, from detectors or from photon-1
> to photon-2. In example making the path lenght of
> photon-2 much much longer than the coherence lenght
> of the photon(s).
> But imagine that your procedure (here above) is not
> enough, and actually there is some FTL effect.
> The interesting point here is that any FTL effect
> from detectors or photon-1 makes actual, objective
> the state of photon-2 *before* its measurement.

An FTL effect is definitely a possibility but somewhat
heavyhanded to deal with the "passion-at-a-distance"
business. FTL would indict QM a lot harder than EPR.
Read Svetlichny's paper and the several sequels...


Joao Pedro Leao  :::
Harvard-Smithsonian Center for Astrophysics
1815 Massachussetts Av. , Cambridge MA 02140
Work Phone: (617)-496-7990 extension 124
Cell-Phone: (617)-817-1800
"All generalizations are abusive (specially this one!)"
Received on Fri Nov 14 2003 - 13:41:26 PST

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