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From: Jesse Mazer <lasermazer.domain.name.hidden>

Date: Thu, 12 Aug 2004 01:08:36 -0400

scerir quotes Basil Hiley saying:

*>Sure there is an interference effect simply because Afshar's
*

*>experiments do not 'follow' anything and they do not 'look at' each
*

*>photon as it passes through a pinhole. He is simply collecting and
*

*>counting the distribution of photon arrivals at his two detectors.
*

*>Then he makes inferences about what could possibly be going on and
*

*>concludes, incorrectly that a photon detected in the 'photon detector
*

*>for pinhole 1' came from pinhole 1. However that conclusion is based
*

*>on the assumption that the rays emanating from pinhole 1 arrive at
*

*>the 'photon detector for pinhole 1'. But the ray picture breaks
*

*>down as soon as you enter the region of overlap of the two beams and
*

*>you cannot conclude that the photon entering pinhole 1 arrives at the
*

*>'photon detector for pinhole 1'. You haven't measured which pinhole
*

*>each photon passed through so you have not contradicted Bohr.
*

*>
*

*>Unfortunately Afshar's conclusion, "According to my experiment one of
*

*>the key assumptions about quantum theory is wrong" is incorrect. His
*

*>conclusion is wrong simply because he doesn't understand the physical
*

*>optics that lies behind the experiment he is doing.
*

I think Basil Hiley's analysis here may be incorrect. In the normal

double-slit experiment, the interference pattern in probabilities you get

from quantum physics when you don't know which slit the photon went through

is the same as the interference pattern in light intensities you get from

classical optics when you shine a light through two slits. So, if classical

optics predicts that light from two pinholes shining on a lens will be

focused onto two distinct spots, with no interference between the spots and

with all the light from one pinhole focused on one spot, then it seems

likely that quantum mechanics would predict the same thing.

Also notice that in the analysis of Afshar's experiment by W. Unruh at

http://axion.physics.ubc.ca/rebel.html which scerir linked to, Unruh does

not dispute Afshar's claim that all the photons from the each pinhole end up

in a single detector. In fact, he offers a "simpler version of the

experiment" involving a multiple pass interferometer, depicted in figure 2,

and says that in this experiment you do know which path a photon took by

looking at which detector it hits: "By measuring which detector they

triggered, 5 or 6, one measures which of the beams, 1 or 2, the photon

traveled along". Since the experiment in figure 2 is just supposed to be a

"simpler version" of Afshar's experiment, it's pretty clear that Unruh would

not disagree that the lens insures that knowing which detector absorbed a

photon is enough to tell you which path the photon must have taken through

the pinholes. Unruh is a fairly big-name physicist and his explanation of

what's wrong with Afshar's conclusions about complementarity are pretty

detailed, while I don't know anything about Basil Hiley and his criticisms

are more vague.

Anyway, after thinking more about this experiment it's clear to me that even

if the lens is enough to insure that all photons from the left pinhole end

up in the right detector and vice versa, complementarity should still

predict that wires placed at the interference minima will not register any

hits. Consider modifying Afshar's experiment by adding extra wires at

positions other than the interference minima, and sending the photons

through the pinholes one-by-one. In some cases the photon will be registered

at one of the wires in front of the lens, in others it will be registered at

one of the detectors behind the lens. Now, if you consider *only* the subset

of cases where the photon was absorbed by a wire, in these cases the photon

never passed through the lens, so you have absolutely no information on

which pinhole these photons went through. So if you compare the frequency

that the photons hit different wires, complementarity must predict that

you'll get an interference pattern--wires closer to the interference maxima

will register more hits, wires closer to the interference minima will

register fewer hits, and wires placed exactly at the minima will register

zero. So why should an advocate of complementarity be surprised that, after

removing all the wires *except* those placed exactly at the minima, these

wires continue to register zero hits?

You could also turn this into a "proof-by-contradiction" that

complementarity actually demands that wires exactly at the minima will not

register any photons. Suppose in Afshar's experiment you sent photons

through one-by-one, and found that there was some nonzero number of cases

where the photons hit one of the wires at the minima. Since these photons

did not make it to the lens, you have no information about which slit they

went through, and so complemantarity says that the probability of finding

these photons in any given location is determined by an interference

pattern. But the interference pattern predicts *zero* probability of finding

a photon whose path you don't know at an interference minima, in

contradiction with the initial assumption that you saw a nonzero number of

cases where the photon was detected at one of the wires at these minima.

Thus, the only outcome consistent with complementarity is to have zero cases

where the photons hit one of these wires, just as Afshar found.

Jesse Mazer

Received on Thu Aug 12 2004 - 01:11:00 PDT

Date: Thu, 12 Aug 2004 01:08:36 -0400

scerir quotes Basil Hiley saying:

I think Basil Hiley's analysis here may be incorrect. In the normal

double-slit experiment, the interference pattern in probabilities you get

from quantum physics when you don't know which slit the photon went through

is the same as the interference pattern in light intensities you get from

classical optics when you shine a light through two slits. So, if classical

optics predicts that light from two pinholes shining on a lens will be

focused onto two distinct spots, with no interference between the spots and

with all the light from one pinhole focused on one spot, then it seems

likely that quantum mechanics would predict the same thing.

Also notice that in the analysis of Afshar's experiment by W. Unruh at

http://axion.physics.ubc.ca/rebel.html which scerir linked to, Unruh does

not dispute Afshar's claim that all the photons from the each pinhole end up

in a single detector. In fact, he offers a "simpler version of the

experiment" involving a multiple pass interferometer, depicted in figure 2,

and says that in this experiment you do know which path a photon took by

looking at which detector it hits: "By measuring which detector they

triggered, 5 or 6, one measures which of the beams, 1 or 2, the photon

traveled along". Since the experiment in figure 2 is just supposed to be a

"simpler version" of Afshar's experiment, it's pretty clear that Unruh would

not disagree that the lens insures that knowing which detector absorbed a

photon is enough to tell you which path the photon must have taken through

the pinholes. Unruh is a fairly big-name physicist and his explanation of

what's wrong with Afshar's conclusions about complementarity are pretty

detailed, while I don't know anything about Basil Hiley and his criticisms

are more vague.

Anyway, after thinking more about this experiment it's clear to me that even

if the lens is enough to insure that all photons from the left pinhole end

up in the right detector and vice versa, complementarity should still

predict that wires placed at the interference minima will not register any

hits. Consider modifying Afshar's experiment by adding extra wires at

positions other than the interference minima, and sending the photons

through the pinholes one-by-one. In some cases the photon will be registered

at one of the wires in front of the lens, in others it will be registered at

one of the detectors behind the lens. Now, if you consider *only* the subset

of cases where the photon was absorbed by a wire, in these cases the photon

never passed through the lens, so you have absolutely no information on

which pinhole these photons went through. So if you compare the frequency

that the photons hit different wires, complementarity must predict that

you'll get an interference pattern--wires closer to the interference maxima

will register more hits, wires closer to the interference minima will

register fewer hits, and wires placed exactly at the minima will register

zero. So why should an advocate of complementarity be surprised that, after

removing all the wires *except* those placed exactly at the minima, these

wires continue to register zero hits?

You could also turn this into a "proof-by-contradiction" that

complementarity actually demands that wires exactly at the minima will not

register any photons. Suppose in Afshar's experiment you sent photons

through one-by-one, and found that there was some nonzero number of cases

where the photons hit one of the wires at the minima. Since these photons

did not make it to the lens, you have no information about which slit they

went through, and so complemantarity says that the probability of finding

these photons in any given location is determined by an interference

pattern. But the interference pattern predicts *zero* probability of finding

a photon whose path you don't know at an interference minima, in

contradiction with the initial assumption that you saw a nonzero number of

cases where the photon was detected at one of the wires at these minima.

Thus, the only outcome consistent with complementarity is to have zero cases

where the photons hit one of these wires, just as Afshar found.

Jesse Mazer

Received on Thu Aug 12 2004 - 01:11:00 PDT

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