Dear Prof. Jack,
My guess is that we don't require anything exotic (say stuff like breakdown in local-realism etc) to explain the delayed choice experiment. I think the apparent paradoxicality in this case has come from the usual expectation that if particle nature is manifested, then a which-path decision should have occured at slits. Now this is indeed normally the case, as when one directly monitors the slits for path information, so this attitude is understandable.
But in analyzing the delayed choice experiment, I found that the slight novelty here seems to be that the commitment-to-a-path happens at the back-end optics.
http://physicsweb.org/objects/news/11/2/16/Interferometer.jpg
Yes, assuming you mean by "back-end" the "later" light green beam splitter plate at the crossing point before the two gray detectors. In the Bohm interpretation, at least for the interferometer setup for slow neutrons the pilot wave quantum potential Q is reshaped at the (upper right light green plate) "back-end" crossing-point in front of the two detectors.
In contrast, in the Bohr picture definite classical paths of small particles are not allowed and the mirage of retrocausal delayed choice is made at the "earlier" "front-end" light blue (lower-left) beam splitter plate. Therefore, we are forced to the miracle, the immaculate deception of "collapse" of the spread out quantum wave to Newton's hard massy "particle." Thus,there are only tiny-spot irreversible detections at low flux one quantum at a time and we see the statistical pattern slowly build one tiny speck, one momentary "click" at a time. This is mysterious in Bohr's theory but it is completely trivial in the deBroglie-Bohm-Einstein theory (i.e. "interpretation). So Feynman was wrong when he said, when he dissembled, no one understood quantum theory. Bohm did. No one who followed Bohr understands it - at least for slow neutrons. Photons are more difficult for the reason that the classical EM field configuration is infinite dimensional as distinct from the single neutron that has only three center of mass degrees of freedom.
That is, for a single neutron the quantum Q pilot wave is a function of the actual position of the hidden variable tiny neutron. In contrast for the classical electromagnetic (or geometrodynamic, or Yang-Mills electroweak & strong chromodynamic classical fields) the quantum Q pilot wave is a FUNCTIONAL of the classical field function i.e. configuration on a 3D spacelike slice of 4D space-time (Cauchy initial-value problem et-al). The math is much more difficult than for massive fermion leptons and quarks and their bound composites in the non-relativistic limit at least. Even the fermions get harder to deal with in Bohm's picture relativistically with particle creation/annihilation. Note that the Feynman path integrals are the preferred formalism because Feynman was a secret closet Bohmian - one of his paths is the actual one. When you sum over paths that makes the super-Quantum potential! Feynman, Bohm and Vigier spent months together in Rio on the beach ~ 1952.
In the Bohm picture the actual neutron hidden variable has a definite path, but the pilot wave Psi splits to Psi(1) & Psi(2). The effective Q at the "back-end" crossing point is locally reshaped if the light green plate is inserted with relative phases such that the neutron cannot take the path to the vertical detector and must take the path to the horizontal detector. The vertical path is blocked by a Q-barrier. If the light green plate is not there its 50-50 which path the neutron takes. Therefore, for slow neutrons in this particular experiment there is no need for faster than light action and no need for retro-causality in the Bohm picture. Now what happens for photons is actually more complicated and what may happen in a different kind of experiment I don't know. Each case has to be looked at separately since Q can act across a spacelike interval in the case of entanglement (absent in the above case) but so long as the hidden variable is a "test particle" that does not directly react back on its own Q there is no statistical violation of retarded causality i.e. no-cloning et-al is obeyed in Bohm's theory in the "orthodox limit" of what Antony Valentini calls "sub-quantal equilibrium."
It is argued that immense physical resources - for nonlocal communication, espionage, and exponentially-fast computation - are hidden from us by quantum noise, and that this noise is not fundamental but merely a property of an equilibrium state in which the universe happens to be at the present time. It is suggested that 'non-quantum' or nonequilibrium matter might exist today in the form of relic particles from the early universe. We describe how such matter could be detected and put to practical use. Nonequilibrium matter could be used to send instantaneous signals, to violate the uncertainty principle, to distinguish non-orthogonal quantum states without disturbing them, to eavesdrop on quantum key distribution, and to outpace quantum computation (solving NP-complete problems in polynomial time).
http://xxx.lanl.gov/abs/quant-ph/0203049
Thus, at all times quantum information comes through _both_ slits, no matter whether the observer downstream is using a telescope or an interferometer. However, in the former case, the complex amplitude for one of the paths will get filtered out by the optics at a given telescope, so that he perceives an incoherernt mixture, whereas, in the latter, both are allowed to combine interferometrically.
Thus his decision to choose one or the other viewing instrument does not instruct the photon (retrocausally) "to have altered its behaviour", but simply decides whether the rays from the two slits shall interfere or go their separate ways.
A very simple treatment of this attempt at resolution was published in Current Science
"A quantum field theoretic description of the delayed choice
experiment"
R. Srikanth, Current Science 81, 1295 (2001); quant-ph/0106154
The bottomline then seems to be that complementarity is enforced not at the slits plane (in which case it would indeed become mysterious!!), but at the back-end optics, right where the delayed choice happens.
Yes, that is what I am saying - locally right where the delayed choice happens.
Thanking you,
With best regards,
Srikanth
On Mon, 19 Feb 2007, Jack Sarfatti wrote:
No need for retrocausality in Bohm's theory at least for slow moving neutrons. Delayed choice experiment. How does Bohm explain it with the quantum potential Q? In this case the super Q for the EM field. Cramer explains it with advanced waves back from the future. One can do this with Q as well
Q = (R(advanced)R(retarded))^-1/2Grad^2(R(advanced)R(retarded))^1/2
for slowly moving neutrons in a crystal interferometer (Dan Greenberg).
It's the shape of Q that determines how the detectors "click". Therefore, last moment decision to insert or not the second beam splitter locally reshapes Q at the "crossing point" and there is no need for any faster than light/retrocausal effect in Bohm's theory for this particular experiment with slow neutrons (photons more complicated).

http://images.iop.org/objects/physicsweb/news/thumb/11/2/16/ Interferometer.jpg
Above is upper right piece of full apparatus below. Placing the green slab beam splitter clearly locally reshapes Q such that, upper vertical detector will not click at all. There is a factor of i phase shift on each reflection. Look at two beams that go to upper detector. Lower beam reflects only once, upper beam reflects 3 x hence i^2 = - 1 180 deg destructive interference (assuming equal path lengths). For horizontal detector both beams reflect twice and therefore they stay in phase. Note that with real physics one can explain things logically and clearly - that is the goal. If green slab (beam splitter) is not inserted both detectors click at equal rates.
http://physicsweb.org/objects/news/11/2/16/Interferometer.jpg
Choose the right path
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Choose the right path
In Roch's experiment, single photon pulses are emitted one at a time into an interferometer. As they leave a first beam splitter (BS1), they have the option of two 48-metre paths with equal probability, which eventually lead to two separate detectors. Just before the detectors, a second beam splitter (BS2) is randomly inserted or removed by a system that is synchronized with the emitter. With the beam splitter in place, a photon can reach either detector from the same path, preventing its path from being observed. When the beam splitter is removed, however, the detectors can observe a photon's path unambiguously.
Roch's team performed the experiment many times until they could confirm with certainty that unobserved photons behave like waves (i.e. interfere), while observed photons behave like particles (i.e. do not interfere). Crucially, however, they removed the possibility that the photons could somehow be informed of the system's decision, as the decision was only made after the photons had entered the interferometer.
Begin forwarded message:
From: Russell Targ
Date: February 19, 2007 7:55:03 AM PST
To: Jack Sarfatti
Subject: Photons denied a glimpse of their observer (February 2007) - News - PhysicsWeb
http://physicsweb.org/articles/news/11/2/16
Jack Sarfatti
sarfatti@pacbell.net
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