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

"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 , 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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