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From: Joao Leao <jleao.domain.name.hidden>

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 http://arXiv.org/abs/quant-ph/9511002 )

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

Date: Fri, 14 Nov 2003 13:38:27 -0500

scerir wrote:

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 http://arXiv.org/abs/quant-ph/9511002 )

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.)

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...

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

-- Joao Pedro Leao ::: jleao.domain.name.hidden 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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