On Feb 20, 2007, at 9:47 AM, Srikanth R wrote:
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
<< Back to article
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 <radiant@pacbell.net>
Date: February 19, 2007 7:55:03 AM PST
To: Jack Sarfatti <sarfatti@pacbell.net>, Jack Sarfatti <sarfatti@well.com>
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
"If we knew what it was we were doing, it would not be called research,
would it?"
- Albert Einstein
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