# Two experiments on the Wave-Particle duality

1. Feb 18, 2007

### Hans de Vries

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:

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

Last edited: Feb 18, 2007
2. Feb 18, 2007

### ZapperZ

Staff Emeritus
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

3. Feb 18, 2007

### PIT2

Does this mean we can influence the past?

4. Feb 19, 2007

### Hans de Vries

Well, That's a claim the authors make.

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

Last edited: Feb 19, 2007
5. Feb 19, 2007

### cartuz

and the date of publish is Jan. 2007!

Last edited: Feb 19, 2007
6. Feb 19, 2007

### DrChinese

You cannot influence an observation made in the past (or at least you can't prove you have).

7. Feb 19, 2007

### DrChinese

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.

Last edited: Feb 19, 2007
8. Feb 19, 2007

### 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:

$$\Delta p \Delta q < \hbar$$

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.

Last edited: Feb 19, 2007
9. Feb 19, 2007

### PhilosophyofPhysics

many worlds?

10. Feb 20, 2007

### Hans de Vries

Part I

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

Last edited: Feb 20, 2007
11. Feb 20, 2007

### Hans de Vries

Part II

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.

Last edited: Feb 20, 2007
12. Feb 20, 2007

### Hans de Vries

Part III

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

Last edited: Feb 20, 2007
13. Feb 20, 2007

### DrChinese

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.

14. Feb 20, 2007

### DrChinese

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?

15. Feb 20, 2007

### Hans de Vries

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/tlplib/diffraction/convolution.php

Regards, Hans

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Last edited: Feb 20, 2007
16. Feb 21, 2007

### Anonym

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”.

17. Feb 22, 2007

### DrChinese

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?

18. Feb 22, 2007

### DrChinese

OK, I guess an apology is in order. :uhh: You obviously were not trying to criticize professionals in general, which is what I had thought from my (incorrect) reading of your earlier post.

I don't see how QT needs to be reformulated if the Afshar experiment stands. One would first need to demonstrate that the Heisenberg Uncertainty Principle is violated - I think. How otherwise is the BPC included in the formalism of QT?

19. Feb 22, 2007

### Anonym

I do not identify the difference between your and my arguments. May be only that you use “q-bits” for your arguments and I use “c-bits”. I refer to the QM and Classical Physics formalism without interpretation. I describe the N.Bohr and A. Einstein statements as following:

1. The measurement equipment are the macroscopic devices; therefore, they obey the laws of Classical Physics.
2. The Classical Physics is the theory of certain events.
3. The Classical Electrodynamics require a special relativity space-time geometry.
4. The Quantum Theory is the theory of uncertain events (in general, every object is the extended object: HUR).
5. The results of measurements are given in terms of the eigenvalues (spectrum) of a complete set of mutually commuting self-adjoint operators (observables).

The special relativity require the collapse of wave packet in time (delta t=0, instant) when the measurement performed (A. Einstein, 5-th Solvay conference). BPC require the collapse in space (delta x=0, delta p=0, particle) when the measurement performed. Unitarity require the number of particles to be a conserved quantity. In both experiments above N=1. The operator of number of particles in that case has two eigenvalues 0 or 1 (“which way”). The beam-splitter with the counter is the macroscopic measurement device. The beam-splitter without the counter maintain the HUR delta N>0 (selfinterference).

Check my statements above. If you agree that they provide the adequate verbal description of CED and QED formalism then S.S. Afshar result inconsistent with it.

Regards, Dany.

P.S. Perhaps, our wording lead to confusion: everybody agree that without the measurement the unitary evolution of the QM system is deterministic. Through the unitary evolution the Heisenberg Uncertainty Relation holds with certainty. It is better to say: Heisenberg Dispersion Relation.

Last edited: Feb 23, 2007
20. Feb 23, 2007

### DrChinese

The words are probably OK, and may be a good verbal description. But... I would not agree that the BPC is part of the QM formalism. I did a quick check of a couple of older books (Heisenberg, Dirac) and this was not present as a formal step. I will look some more.

I believe this experiment may be a which way test to a certain degree (let's say 90%), and a wave test otherwise (say 10%). I think this would fit nicely with the HUP. It would also be in the ball park of the observed results, I would guess.

I certainly don't think it is both at the same time - although I could be wrong. It reminds me of good magic... we have been misdirected and don't see the real explanation for the results.

Last edited: Feb 23, 2007