# Two experiments on the Wave-Particle duality

by Hans de Vries
Tags: duality, experiments, waveparticle
P: 1,136
Two new papers on beautiful experiments were published this month
concerning the Wave Particle duality of the photon:

S.Afshar et. Al. in Foundations of Physics.

Experimental realization of Wheeler’s delayed-choice
GedankenExperiment

J-F. Roch, A. Aspect, P.Grangier in Science.
http://arxiv.org/PS_cache/quant-ph/pdf/0610/0610241.pdf

What these experiments have in common is that they seem to produce
exactly as one would expect from classical optics / EM radiation theory,
and both do so one photon at a time. An effect which continues to
puzzle physicist.

In the above sense these experiments do not bring anything new but
the discussion is all about the interpretation of this effect and more
concrete: The validity of Bohr’s Principle of Complementarity:

 Quote by Niels Bohr A single quantum mechanical entity (= photon, electron...) can either behave as a particle or as wave, but never simultaneously as both. http://en.wikipedia.org/wiki/Complementarity_(physics)
So, Bohr said in 1927 that it's either wave or particle but never both
at the same time. At this time this was a change of mind for Bohr who
had opposed Einstein's idea of the photon as a quantum particle for

http://arxiv.org/PS_cache/physics/pdf/0212/0212090.pdf

The two papers should be seen in the light of this. The authors make
diametrically opposed claims:

The authors of the first experiment claim to show that the photon can
have simultaneous wave and particle properties and disagree with Bohr.

The authors of the second experiment hold on to Bohr and claim that
their experiment demonstrates that "In the present, one can change
something that has already happened in the past" Namely the decision
of the photon to behave as a particle, or, as a wave.

All authors agree that Bohr's principle of complementarity disagrees
with Einstein's ideas and his work on relativity. Both groups quote
Wheeler as if he agrees with their (opposite) claims in the conclusion
of their papers.

Regards, Hans
 Mentor P: 27,565 Note that I have already highlighted the Jacques et al. paper in Noteworthy Physics Papers thread in the GP forum, including a clear overview of it published in PhysicsWeb. Zz
P: 904
 Our realization of Wheeler’s delayedchoice GedankenExperiment demonstrates beyond any doubt that the behavior of the photon in the interferometer depends on the choice of the observable which is measured, even when that choice is made at a position and a time such that it is separated from the entrance of the photon in the interferometer by a space-like interval. http://arxiv.org/PS_cache/quant-ph/pdf/0610/0610241.pdf
Does this mean we can influence the past?

P: 1,136

## Two experiments on the Wave-Particle duality

 Quote by PIT2 Does this mean we can influence the past?
Well, That's a claim the authors make.

 "We, now, by moving the mirror in or out have an unavoidable effect on what we have a right to say about the already past history of that photon". (authors quoting Wheeler)
I would reason that, in theory, one could expand their claim into the extreme
with polarized Cosmic Background Radiation. The same experimental setup
would then be able to change certain events at the time of the Big Bang.

Now do we want to believe this? It's really all about the general validness of
Niels Bohr's 1927 principle of Complementarity. If it's not strictly valid then
the extraordinary claims go away and the universe "returns normal".

Without this principle the photon does not need to make a choice of acting
either like a wave or as a particle, nor does it need to revise this choice
somewhere in the future depending on some observer.

In my opinion, people like Aspect and Grangier may well contribute more to
the diminish of the strict interpretation of the principle of complementarity,
by adhering to these extraordinary claims, than Afshar who's campaigning
directly against it with his experiments.

This, for the people following the field, would be somewhat amusing from
an historical viewpoint.

Regards, Hans
P: 86
 Quote by Hans de Vries Two new papers on beautiful experiments were published this month concerning the Wave Particle duality of the photon: Paradox in Wave-Particle Duality S.Afshar et. Al. in Foundations of Physics. http://www.springerlink.com/content/...7/fulltext.pdf Experimental realization of Wheeler’s delayed-choice GedankenExperiment J-F. Roch, A. Aspect, P.Grangier in Science. http://arxiv.org/PS_cache/quant-ph/pdf/0610/0610241.pdf
and the date of publish is Jan. 2007!
PF Patron
P: 5,055
 Quote by PIT2 Does this mean we can influence the past?
You cannot influence an observation made in the past (or at least you can't prove you have).
PF Patron
P: 5,055
 Quote by Hans de Vries In my opinion, people like Aspect and Grangier may well contribute more to the diminish of the strict interpretation of the principle of complementarity, by adhering to these extraordinary claims, than Afshar who's campaigning directly against it with his experiments.
Extraordinary? The results seem (to me) to be exactly in keeping with what you would expect from QM. Which is weird enough... and of course that is why it is such a cool experiment. Thanks for the reference.
 PF Patron Sci Advisor P: 5,055 1. I think that Afshar is overstepping the bounds of the experimental results in his conclusions. He says: "...the applied technique appears to allow us to circumvent the limitations imposed by Heisenberg’s uncertainty principle..." However, nowhere (that I see) is the HUP shown to be violated. We would need something like the following to make a convincing case: $$\Delta p \Delta q < \hbar$$ 2. Also, as to the principle of complementarity (BPC in the article) itself: nothing says that you cannot measure something "a little" as a particle and "a little" as a wave. Clearly, the referenced experiment purports to show this within error constraints that are not particularly strict. I would have expected a conclusion like "violated by 3 standard deviations" or similar if the point is to be convincing. 3. At this point, I assume the publishing was allowed to go forward because the results themselves are reproducible. I do not doubt that there will be debate on what the results themselves say. I think the quote from Wheeler (". . . for quantum theory to say in one breath ‘through which slit’ and in another ‘through both’ is logically inconsistent...") can be construed many ways. I certainly don't see this experiment as particularly supporting it.
 P: 98 many worlds?
P: 1,136
 Quote by DrChinese 1. I think that Afshar is overstepping the bounds of the experimental results in his conclusions. He says: "...the applied technique appears to allow us to circumvent the limitations imposed by Heisenberg’s uncertainty principle..." However, nowhere (that I see) is the HUP shown to be violated. We would need something like the following to make a convincing case: $$\Delta p \Delta q < \hbar$$
The experimental result is entirely explained by Classical Optics/
EM Radiation, There's no need to use $\hbar$ or Heisenberg's uncertainty
principle.

"Circumventing" Heisenberg’s uncertainty principle here means that
HUP is simply not applicable to the experiment. It doesn't imply that he
claims that he found some way to change $\Delta p \Delta q \geq \hbar$ to $\Delta p \Delta q < \hbar$.

He could (should) have been clearer, certainly.

Regards, Hans
P: 1,136
 Quote by DrChinese 2. Also, as to the principle of complementarity (BPC in the article) itself: nothing says that you cannot measure something "a little" as a particle and "a little" as a wave. Clearly, the referenced experiment purports to show this within error constraints that are not particularly strict. I would have expected a conclusion like "violated by 3 standard deviations" or similar if the point is to be convincing.
Niels Bohr's principle of complementarity is just a subset of the
Copenhagen interpretation of Quantum Mechanics. So, the
probability aspect of the interpretation may turn out to be right
while BPC may turn out not.

BPC says that a single photon can only behave either as a wave
or as a particle, but not both a the same. It also can't be "half"
particle, "half" wave. Other interpretations make other claims.
For example the Bohmian QM interpretation assumes that a photon
is both a particle and a guiding wave at the same time.

So for me we're talking about a QM interpretation issue here.

Regards, Hans.
P: 1,136
 Quote by DrChinese 3. At this point, I assume the publishing was allowed to go forward because the results themselves are reproducible. I do not doubt that there will be debate on what the results themselves say. I think the quote from Wheeler (". . . for quantum theory to say in one breath ‘through which slit’ and in another ‘through both’ is logically inconsistent...") can be construed many ways. I certainly don't see this experiment as particularly supporting it.

A good reviewer looks at the merits of the experiment itself.

I think the reviewer has been convinced that there must be an
interference pattern and that the light through the two holes is
focused on two different locations. I'm convinced as well. It's
just what you expect from classical optics. I would have liked
to see the figure on page 299 earlier. It nicely addresses the
diffraction grating ideas Vanesch and I discussed here:

Now is BPC (Bohr’s principle of Complementarity) in conflict
with Classical Optics?

BPC says that a single photon can only behave either as a wave
or as a particle, but not both a the same time. Classical Optics
says that photons ALWAYS behave as waves, (until they're
finally absorbed) So yes, I think there is conflict indeed.

For me the essential point is that the wave can go anywhere,
follows all possible paths, all over the place, while the absorption
only occurs at one single place.

Now which path was followed by that what caused the absorption?
(that what we call the "particle") Why does the remainder of the
wave lead NOT to an absorption elsewhere? What happens at all
with the remainder of the wave? This is the mystery of Unitariy.

BPC wants to remove the wave from all paths that were unlikely
to be followed by that what caused the absorption. It suggests
a partial remedy for the "Collapse of the wave function":

(1) If a photon is split by a beam splitter and detected Left and
not Right then BPC says that nothing went Right, neither particle,
nor wave. So we don’t need to explain what happened to the
wave at the Right side.

(2) If a photon is split by a beam splitter and both sides are made
to interfere then BPC says that the photon went both ways, So in
this case the wave follows both paths.

Now, In case of the experiment under discussion, BPC would,
when recognizing the detection at Left, want to remove everything
from the other path like in (1) However, this would also remove
the interference and thus the result of the experiment.

So BPC is in conflict with Classical Optics and thus with Bohr’s
other principle: That of Correspondence.

Bohr’s Principle of Correspondence (1923)
Bohr’s Principle of Complementarity (1927)

http://en.wikipedia.org/wiki/Correspondence_principle
http://en.wikipedia.org/wiki/Complementarity_(physics)

Bohr’s Principle of Correspondence states that if the light consist
out of sufficient numbers of photons we should get the Classical
Optics result back. No quantum mechanical interactions took place
other than the final absorption in the detectors.

Both the experiments discussed on this thread produce the results
that would be expected from Classical Optics.

Regards, Hans
PF Patron
P: 5,055
 Quote by Hans de Vries The experimental result is entirely explained by Classical Optics/ EM Radiation, There's no need to use $\hbar$ or Heisenberg's uncertainty principle. "Circumventing" Heisenberg’s uncertainty principle here means that HUP is simply not applicable to the experiment. It doesn't imply that he claims that he found some way to change $\Delta p \Delta q \geq \hbar$ to $\Delta p \Delta q < \hbar$. He could (should) have been clearer, certainly. Regards, Hans
Well, I don't see how one can say "HUP doesn't apply" and "limits of the HUP were exceed" both without demonstrating the violation. I would agree that their language about the HUP could be dropped altogether and it would be more accurate.
PF Patron
P: 5,055
 Quote by Hans de Vries Niels Bohr's principle of complementarity is just a subset of the Copenhagen interpretation of Quantum Mechanics. So, the probability aspect of the interpretation may turn out to be right while BPC may turn out not. BPC says that a single photon can only behave either as a wave or as a particle, but not both a the same. It also can't be "half" particle, "half" wave. Other interpretations make other claims. For example the Bohmian QM interpretation assumes that a photon is both a particle and a guiding wave at the same time. So for me we're talking about a QM interpretation issue here. Regards, Hans.
I don't see the BPC as fundamental anyway. In my view: the HUP is more precise, while BPC is more of an approximation. The HUP says that an observation CAN be both wave-like and particle-like as long as the $$\Delta p \Delta q \geq \hbar$$ relation is satisfied. It is certainly possible to construct a setup in which there is some interference and some which-way information, but not enough of either to be absolutely certain in specific cases. As best I can tell, this is what the experiment is doing.

But I am having a little difficulty understanding their argument anyway. Do you understand it well enough to help me? Specifically, I am trying to understand the 4 diagrams a/b/c/d and how they get to the idea that the wave nature is being demonstrated. I follow that the wire is placed at minima of the expected wave pattern (going from a to b) and it is only slightly destructive. But how does that provide a convincing argument for the wave nature being observed?
P: 1,136
 Quote by DrChinese Specifically, I am trying to understand the 4 diagrams a/b/c/d and how they get to the idea that the wave nature is being demonstrated. I follow that the wire is placed at minima of the expected wave pattern (going from a to b) and it is only slightly destructive. But how does that provide a convincing argument for the wave nature being observed?

One can discuss these 4 diagrams from a Classical Optics point of view:

The bottom two diagram show one hole closed, one hole open. The
wave interacts with the thin wires. These will scatter the wave and
produce a diffraction pattern. (The series of little bumps in the output)

If both holes are open then there will be an interference pattern with
the wires carefully placed in the lows of the pattern. The wires don't
scatter the wave and no diffraction pattern will occur. Most of the
energy will get through unscattered.

So it's the absence of the diffraction pattern in the output which
demonstrates best that there must be interference. To see more how
diffraction patterns from an array of thin lines (or thin slits) should look,
go here:

http://www.msm.cam.ac.uk/doitpoms/tl...onvolution.php

Regards, Hans
Attached Thumbnails

P: 451
 Quote by Hans de Vries Two new papers on beautiful experiments were published this month concerning the Wave Particle duality of the photon: Paradox in Wave-Particle Duality S.Afshar et. Al. in Foundations of Physics. http://www.springerlink.com/content/...7/fulltext.pdf Experimental realization of Wheeler’s delayed-choice GedankenExperiment J-F. Roch, A. Aspect, P.Grangier in Science. http://arxiv.org/PS_cache/quant-ph/pdf/0610/0610241.pdf What these experiments have in common is that they seem to produce exactly as one would expect from classical optics / EM radiation theory, and both do so one photon at a time. In the above sense these experiments do not bring anything new. The authors make diametrically opposed claims: The authors of the first experiment claim to show that the photon can have simultaneous wave and particle properties and disagree with Bohr. The authors of the second experiment hold on to Bohr and claim that their experiment demonstrates that "In the present, one can change something that has already happened in the past". Both the experiments discussed on this thread produce the results that would be expected from Classical Optics.
1.S.Afshar et. al paper is not new. If the presented result is correct, the entire reformulation of QT is required. Since the Classical Optics (Electrodynamics) is in compliance with the standard QT (P.A.M. Dirac), it means that the entire reformulation of classical electrodynamics is required in addition.

2.I consider the A. Aspect et al paper the outstanding experimental achievement. The first time the single photon real life realization of Wheeler’s delayed-choice experiment is performed. However, the “explanation” have nothing to do with the obtained result.

Regards,Dany.

P.S. DrChinese, sorry in advance for the “demons”.
PF Patron
P: 5,055
 Quote by Hans de Vries If both holes are open then there will be an interference pattern with the wires carefully placed in the lows of the pattern. The wires don't scatter the wave and no diffraction pattern will occur. Most of the energy will get through unscattered. So it's the absence of the diffraction pattern in the output which demonstrates best that there must be interference. To see more how diffraction patterns from an array of thin lines (or thin slits) should look, go here:
Thanks, that helps. So here is my next question. I am guessing that they are asserting that the path in the B picture is left to left and right to right, and they base that on the idea that the B is "built up" from the C and D pictures. So that is how they know the "which way" information. Is that correct?
PF Patron