A few of you may have heard all the controversy about the latest issue of
New Scientist. Now I love New Scientist. It, along with Climbing and Fine WoodWorking are the only magazines I read regularly. 
However they do tend to have a penchance for the wild, flashy and sometimes "fringe" in physics. For instance they've covered "faster than light communication," "anti-gravity research" and other stuff that of course, doesn't survive peer review. Indeed occasionally one must question how it ever made it into New Scientist. Generally if something sounds like it will revolutionize science, it won't. A quick trip to sci.physics.research will typically have a discussion showing the errors.
In the latest issue there is a report of an experiment by Shahriar Afshar that purportedly gets rid of the "particle" part of wave-particle duality. Specifically it purports to falsify the Copenhagen interpretation and the Many-Worlds interpretation of Quantum Mechanics. John Cramer says that it would, however, support the Transactional interpreation of QM. As well as being featured in a full article in New Scientist, Afshar is being interviewed today by NPR's Science Friday.
Probably the two best places to read the response to the Afshar experiments are at Kathy Cramer's Blog (here as well) as well as the threads "Afshar experimental refutation of Bohr" and Afshar's Experiment at sci.physics.research. To quote a little bit from New Scientist so as to give an overview of the experiment:
Afshar's trick is to find the location of the minimum points of wave interference, place one or more wires at these minimum points, and observe how much light is intercepted when one is determining the pinhole through which the photons passed. [...] ...the Afshar Experiment falsifies the Copenhagen Interpretation, which requires the absence of interference in a particle-type measurement. It also falsifies the Many-Worlds Interpretation which tells us to expect no interference between 'worlds' that are physically distinguishable, e.g., that correspond to the photon's measured passage through one pinhole or the other.
Now before everyone gets too excited, lets recall that Afshar's paper hasn't yet been accepted for publication in a peer reviewed journal. I'm guessing that before it is, given its controversial nature, there will be some severe fact checking. Further a lot of people suggest that Afshar is making a trivial error regarding quantum mechanics. (See the sci.physics.research threads above) Others say these critics are misunderstanding the experiment. John Cramer said the following (quoting from his daughter Kathryn Cramer's blog):
His reply was to send me some data from single-wire measurements. He has done measurements in which he uses only one wire placed at one of the interference minima and measures the flux everywhere, not just at the image sites. He did such measurements in three situations: (1) wire in & both pinholes open, (2) wire out & both pinholes open, and (3) wire in & only one pinhole open.
Measurements (1) and (2) show very clean images of the pinholes, with essentially nothing detected between them. The two measurement plots were so similar that they were essentially indistinguishable. Measurement (3), on the other hand, is very different and shows considerable flux outside the image positions from light that is scattering from the wire. In other words, many photons are hitting the single wire with one pinhole open, but essentially nothing is hitting the wire with both pinholes open. No light is scattered because the single wire is placed in an interference minimum where there is no flux of light. Further, with only one wire present there should be no diffraction grating effect (which depends on the coherent interference of waves passing through multiple slit openings). Therefore, I find that the single-wire measurement defuses any claim that the Afshar measurement is not purely "which-way" because of grating effects or wire scattering.
Will this all be confirmed? My bet is that it won't. Admittedly it's been a while since college and I've not done much quantum mechanics since, mainly focusing in on philosophy. It does, however, seem like the critics make some good points. I suspect that if we do careful calculations with the wire we'll find that it can all be explained in terms of traditional quantum mechanics. However if it turns out Afshar and Cramer are right, it certainly will revolutionize physics and more importantly the philosophy of physics. However Cramer's transactional interpretation of quantum mechanics is very controversial as it entails backwards causality. Not only at the quantum level but at the macro or regular world level.
Just a quick note that Cramer has a popularized article on all this over at Analog, a science fiction magazine.
Aug 6: A bit more info on this topic that didn't seem deserving of a new thread. The main complaint against Afshar is that diffraction can explain the phenomena. Cramer's comment above addresses this somewhat. Over at New Scientist Afshar himself addresses this issue. In addition Kathy Cramer has put up some comments by John Cramer on the same subject, expanding on his earlier comments. Quoting from her web page:
A number of your readers have pointed out that Afshar's grid wires are placed in just the positions that would form a diffraction grating creating an image of pinhole 1 at the position of the pinhole 2 image. Does this destroy the purity of Afshar's "which-way" measurement?
I raised the same question with Afshar earlier this year, and the answer is no. Reason: the wires intercept no light and so cannot diffract. He has done a variation of his experiment using ONLY A SINGLE WIRE and recorded all the light in the focal plane of the pinholes under three conditions: (1) wire in, one pinhole; (2) wire in, two pinholes; and (3) wire out, two pinholes. Measurement (1) shows lots of scattering from the wire away from the image points, indicating that with only one pinhole open the wire is intercepting and scattering light Measurements (2) and (3) show clear images of the pinholes with nothing in between and are indistinguishable.
Conclusion: no light is scattered or intercepted by the wire in measurement (2) because the interference pattern is present, and the wire is at an intensity-zero position of the pattern. A single wire cannot function as a diffraction grating. Bohr is still wrong.
What are the implications for philosophy if it is confirmed or not? Or merely if it introduces a new era of quantum physics in which interpretations are felt to be experimentally falsifiable?
I'm not at all convinced there will be huge philosophical implications. If only because I think the MWI people have made persuasive arguments for how it doesn't falsify their view. I think the wave/particle view was always a little problematic and a lot of people I've talked to tend to think in terms of "wavicles" or wave packets anyway. But QM hasn't received the close viewing that it really deserves. The old Copenhagen interpretation has been defacto mainly because physicists tend not to be interested in philosophy. There are actually many competing views such as your father's transactional interpretation, Fine's NOMA interpretation, Bohm's waveguides, various forms of MWI, and so forth.
I think the big philosophical issues will be from various views of GUTs. Penrose has recently been promoting his twistor theory again, which he says gets rid of the need for extra dimensions. But it is really in reconciling gravity and QM that things get difficult and it is really there that these issues are more significant. I think many would like a fully relational view ala Leibniz. Einstein tried for that, but as Sklar points out, even GR assumes a substantial space time and not a space time that emerges out of "monads" ala Leibniz or Spinoza. I think that question will, in turn affect how we view particles and waves since it is in the area of GR that I think the ontological issues really appear more robustly. (IMO)
I should explain myself since it seems like my last paragraph in the entry and then my comments contradict one an other. It'll have a big impact if Copenhagen is overturned if only to get people thinking about other alternatives. In that sense in terms of how most people think about QM it will be revolutionary (if real). However in terms of people doing philosophy, I don't think it will affect most of the more interesting conceptions. The real question would be if it has any implications for attempts at GUTs. I don't see how, offhand, it could.
I'm still getting a lot of google searches that bring up this page. So I thought I'd link to an other sci.physics.research thread. There are also a few letters to New Scientish that are of interest as well. The basic consensus is "nothing unusual here -- move along." i.e. the error is in Afshar's interpretation and not some falsification of Bohr.
The problem with these sorts of things is that often they confuse philosophy and physics. Occasionally they cross. But more typically physics doesn't resolve the philosophical or interpretive questions. The most interesting comment arises out of the Bohmian waveguide interpretation. There we basically see the epistemological issues where the question of which slot a photon moves through is lost due to interference. Afshar can be seen as missing the point of Bohr which deals with knowledge of the path of the photon prior to the collapse of the wave function. It's a very good point I should have picked up reading the article at first.
Dear Clark,
In your last post you have a factual error. Bohr's Principle of Complementarity forbids interference and which-way information to coexist in the same experimental setup, the order of observation is not important. In Bohr’s own words: ". . . we are presented with a choice of either tracing the path of the particle, or observing interference effects . . . we have to do with a typical example of how the complementary phenomena appear under mutually exclusive experimental arrangements." [1]
Also, I suggest that it is too early to dismiss my experiment and its implications, and would urge you to wait a bit longer before making categorical statements. The single-photon experiment has been finalized at Rowan University and the results are irrefutable: Bohr was wrong, interference and whic-way information CAN coexist in the same experimental setup.
Best regards.
[1]Neils Bohr, in: Albert Einstein: Philosopher-Scientist, P. A. Schlipp, Ed. (Library of Living Philosophers, Evanston, Illinois, 1949).
I'll definitely keep an open mind. However thus far the only person I've seen expressing confidence is Dr. Cramer. Everyone else has expressed doubts.
In either case it seems that the Copenhagen interpretation hasn't been the most popular interpretation for some time. So it seems the real issue is how other interpretations fair. From what I can see the MWI, wave-guide, and transactional interpretations don't have much to fear.
I must also confess that it has been far too long since I last picked up Hughes' The Structure and Interpretation of Quantum Mechanics. So I'm probably ill equipped at the moment to judge things too much. From my admittedly more passive reading though, some of the comments at New Scientist and elsewhere seem persuasive. So I eagerly await the next chapter in all this.
Let me explain why the claim(s) of Afshar is (are) wrong .
Afshar to interpret his experiment used a old axiom made by wheeler (who was the mentor of Feynman) . Following this axiom when two photon beams 1)intersect and interfere and 2) in a second time separate we know from which beams the photon come from just by detecting the photon after the separation of the two beams .
This is naturally clear if you consider geometrical optic where photon can be considered as a point like particle folliwing Newtonian or Einsteinian dynamic. But here we have interference and the conclusion based on the geometrical optic is not valid.
I send this remark to Afshar just after the publication in new scientist (new scientist refused my letters probably because i am not english or american or australian ... ) and he joined me a email containing the same conclusion pointed out by Cramer. However the essential point is not that the perturbation of the wire is small as claimed by afshar and cramer (visibly afshar reads only one part of my email insisting on the point discussed by you and cramer an other reader of new scientist). Indeed this perturbation is certainly small when we think in term of wave (6 percents ) but in term of particle the solution is not so simple. Bohr says that the reality existing indepently of us is protected by the nature him self because quantum objects are very sensible. bohr and others adds then that the particle traje tory is a metaphysiacal object which can not be considered in science. However some physicists as De Broglie Einstein and Bohm never accepted this point of view. Einstein irronically said that the moon is here even if we never watch it and Schrodinger created his famous cats paradox...
considering a model as the one proposed by de Broglie we see that a photon in general follows a very complicated trajectory. in these models even if the wave obeys to a linear equation as in optic the trajectory of the phton follows a non linear equation. consequences are very important when we consider two beams (associated with the same photon beam) which interfere because now ,in general, the axiom of wheeler and Afshar is not valid any more.
This means that depending of the model the photon could be in the wrong beam after the interaction. I is a little counter intuitive but the fact is here. After it is more mathematical but it is easy to prove following works of scully et al (specialists of these kinds of problem) that any process able to label photon (as cells in the blood) in order to follow the path of the particle create a decoherence which erase fringes. If afshar applies one of this "which path method " he will destroy interference in front of the lens .
My conclusion is that if you create a distinguishability between the two waves you will destroy fringes and dependinging of the model perhaps you will know the trajectory ( it is indeed not obvious as amalysed by scully in the the case of the de Broglie theory that your labelling will be really efficient ). But if you dont do this labelling the indistinguishability is conserved and the fringes will be conserved too . in his experiment afshar put wires at the zero of the fringes. he reveals then the existence of the fringes ( at least a part of the information concerning fringes) but he can say absolutally nothing about the trajectory of the photon.
This means that trajectory is a metaphysical object but nothing of course forbid us to discuss about a metaphysical reality. In fact the de broglie bohm model is not perfect and if one day a complete model can justify all things (including masses and other properties of particles) it will be really a success able to circunvent some paradox discover by schrodinger with his cat example and by enstein with his moon....
i believe personnaly in the existence of such theory
but i dont follow the conclusions of Afshar...
Aurelien Drezet, graz ,Austria
I've noticed that to get published in the letters by New Scientist you have to be brief, pithy and to the point. The letter of mine they published still was edited to about half its length.
You said that Bohr considered trajectory a metaphysical object. Now I confess that beyond reading Hughes (far too many years ago) I've not read much about Bohr's theory. Where does he claim it is a metaphysical object? Not that I doubt you. I'm more after what you see the meaning of that being. Glancing briefly at Hughes' The Structure and Interpretation of Quantum Mechanics I notice that on page 175 in relation to the Bohr atomic theory that they say the following about metaphysics and Bohr:
The theory is interpreted within a particular categorial framework. I borrow the phrase from Korner; a categorial framework is a set of fundamental metaphysical assumptions about what sorts of entities and what sorts of processes lie within the theory's domain. The loci classici for the articulation of the categorial framework of classical mechanics are Kant's Metaphysical Foundations of Natural Science and his Critique of Pure Reason. ...
However in the case of quantum mechanics, a very different situation obtains. The theory uses the mathematical models provided by the Hilbert spaces, but it's not clear what categorial elements we can hope to find represented within them, nor, when we find them, to what extend the quiddities of these representations will impel us to modify the categorial framework within which these elements are organized. (Hughes, 175-6)
Later, quoting Bub, Hughes continues:
...Bohr regards the notion of truth as meaningful only in the context of a Boolean possibility structure, i.e., to ascribe a property to a system only makes sense with respect to a structure of possible properties which form a Boolean algebra. In the case of a quantum mechanical system this possibility structure is non-Boolean. The application of the classical notion of truth, or the attribution of physical properties to such a system, requires reference to a classical measuring system, which fixes a particular Boolean algebra in the non-Boolean possibility structure. (ibid 214-5)
Anyway, I really need to reread the text before opening my mouth further. I was more curious as to what you meant by "metaphysical object."
Ok , I am not a philosopher but a physicist i need more time to read your comment but in my point of view Bohr never discusses the problem of the reality existing indepedently of us just because he was convinced that it will be never possible to test experimentally such reality in a consistent way. For Bohr it was not logical to construct some new quantum concept or a quantum logic as the one proposed by von Neumann or Heisenberg. Bohr just realized that in order to build a coherent model able to explain the wave particle duality you will need something of curious breaking the simple univocal connection betwwen reality and mathematical description. How to explain a wave behavior by a point like object? how to explain the discontinuity by the continuity? the solution is to tell that this moedel arenot completelly pertinent at the atomic level Bohr was very attached to the philosophy associated to his old atomic model (which was just something to solve contradictions with other contradictions). Thats why he was naturally convinced that classical categories will be never able to satisfy the dream of logical coherence of the past (the dream of Einstein i mean ). This idea was too pessimitisc probably but a least give us a chance to mask our ignorance by a principle : nature is resistant in a fundamental way to our scientifical quest. Bohr claimed that because all perturbations and observations modify strongly the system this one has different faces which can not be associated in a unical picture in the sense of the classical ontology.
Hovewer Bohm and de Broglie success partially this challenge to build a dynamical model able to justify results of the quantum mechanic. Bohr never consider these models as pertinent because we can not test them. These models reproduce effectivelly the observations but because of the properties of the schrodinger eqation the impossibility to break down the Heisenberg principle is conserved and accepted . Thats why some physicists as Landau and Heisenberg pretended that the trajectory of a photon doesnt exist at all.
This is too much i think .
You can not say that something that you can not see doesnt exist . Bohr was more prudent he was just convinced that our macroscopical word is not sufficient to see and understand the microscopical world. he told often that the aim of physics is not to describe a mysterious reality but to speak about what we can know about this reality.
The sentence metaphysical object is just for me a way to express the point of view of Bohr as i understood it .
Bohr was irronical and joking all his life
and for him the impossibility to build a coherent model was probably a kind of philosophy of resignation as the one of Kierkegaard. I imagined him laufing with him self when he realized that his complementarity principle could be true.
To accept such philosophy is certainly very couragous but Einstein never accepted his point of view and as you know even when Einstein imagined that he was wrong he was right so for me it was clear since a long time that Bohr philosophy could not be the last world. Personally i like more Einstein than Bohr who has unfortunatelly the virtue of the opium...
but of course it depends of our conception.
perhaps i am not very clear and i appologize in advance for this .
I am in holliday and I have no references to give you. I have a lot of documents concerning Bohr but the best one his certainly the one published for the Birthday of Einstein in 1949 ( the title is Einstein philosopher and scientist i think)
Aurelien
I was fascinated by the recent experiment made recently by S. Afshar (New scientist 24 july, p30 ) and by its controversial interpretation (New scientist 7 august, p 24) . I would like to add that for me there are two fundamental problems in this experiment. The first problem concerns the Bohr complementarity’s. It is important to observe that this concept connects two kinds of different experiments one for the observation or interference and one for the observation of two distinct pinholes images. Both experiments are realized in a statistical way by collecting photons on a screen but one exclude the other because we can only absorb a photon one time. Since the time of Bohr numerous ‘which way’ experiment were made in which the photon (or another particle) is not destroyed and can be used two times two show in principle the interference and the two spots in only one experiment. These experiments confirmed all the impossibility to observe simultaneously the fringes and the original pinholes images.
Now in the experiment made by Afshar photons passing between wires are not absorbed or even detected and we can not observe the fringes pattern. Photons which can be used to draw a interference pattern are the few ones absorbed by the wires and which can not be used to generate the spots images . In counterpart for surviving photon we can observe the two original well separated spots because the wires disturb weakly the wave propagation. In agreement with Bohr complementarity’s we still need then to use two experiments to see the interference and the spots.
The second problem concerns the concept of trajectory followed by the photon between the pinholes and the imaging plane. S. Afshar supposed tacitly (in agreement with Bohr Heisenberg and other) that a particle coming from the hole A ( respectively B) must go on the image spot A’ (respectively B’) of this hole. This is unfortunately a free metaphysical assumption which can not be tested by the experiment of Afshar. The unique non destructive way to record the path information would be to tag or label the photon using entanglement with an other system or with an internal degree of freedom as the polarization. However even such process is not free of interpretation as it was discussed by several physicists as Bohm , Bell or Scully. In any way then the experiment made by Afshar give us information on the photon trajectory.
Aurelien Drezet , Graz University, Austria
Aurelien, was that the letter you wrote to New Scientist?
I'd like to respond, but I really don't want to jump into the topic until I can get my mind back up to par on the philosophical issues of quantum mechanics. Unfortunately I'm in the middle of trying to get up to speed in the free will debate in philosophy as it relates to substantial space-time. So I've spent a lot of the last week studying up on the Hole argument of Einstein and so forth. But I definitely thank you for the comments as I'm still getting about 10 different people a day coming to this page.
Dear Clark,
no it was not my letter but just one of them:
Imediately after reading New scientist I send to them (and to Afshar) the following letter
''
Dear editor,
In his article on the foundations of quantum mechanics (24 July, p30) Marcus Chown reports that the experiment made by Afshar is in contradiction with the principle of Bohr stating that we can’t, in a Young double slit experiment, observe simultaneously an interference pattern and the ‘’which way’’ information concerning the trajectory followed by the particle through the slits.
I would like to point out here that such an assertion taken rigorous-sly is unfortunately wrong and that the position of Bohr is not condemned by the Afshar experiment.
I agree with the fact that the wires in front of the lens give us a way to detect indirectly the presence of the interference pattern in a kind of free interaction measurement.
However, with the wires present, the diffraction pattern on the detector ‘’for pinhole2’’ depends on the status of the aperture1 (open or closed). The principle of superposition of waves tells us that at any point the wave with the two apertures opened is the sum of the 2 waves with only one aperture 1or 2 opened. The wave at detector 2 is then the sum of a contribution coming from slit 2 and of an added one coming necessarily from slit 1. Now the experiment shows that when we open slit 1 interferences on detector 2 disappear. This is clearly a proof that the two contributions interfere and reconstruct the initial pattern existing before the introduction of the wires. This explanation is naturally rigourously equivalent to the one proposed by Afshar (wires at fringes minimum doesn’t disturb wave propagation) but now we see that we can’t be sure that a photon comes from slit 2 or from slit 1.
The problem in the Afshar experiment is that we can’t define univocally a probability for the path information. For an experimentalist the unique way to be sure of the path is to label each photon in order to create a distinguishability. This can be done for example by rotating the polarization just after slit 1 or 2 (or by using entanglement with an other system). But this will be equivalent to close one slit and no interference can be expected in front of the lens. If you don’t label the photons and put wires at zero of the fringes you will be sure that this zero exists but you will not know more about the trajectory. This is the dilemma contained in the complementarity principle, and I conclude that Einstein can keep its Nobel prize.
Aurelien Drezet, University of Graz Austria
''
but i was not please with this letter because I mixed some of my arguments and because I could not speak about Bohm dynamic. Afshar reaction to my letter was very usefull (and comical):
''Dear Dr. Drezet,
Thank you for taking the time to respond to my NS article. I'm afraid you have been a little too hasty, and the issue you raised will be addressed in a peer-reveiwed paper to be published within a month. So I suggest that you wait for that paper, and then reconsider you position. Short of that, I'd like to suggest a simple reanalysis. I used 6 wires, each with a thickness of 127 microns and separations of 1.3 mm. Let's just consider one such wire, in one of the central minima. Please consider the following:
1) If only one hole is open, <2% (Max) of the light would be blocked by the wire, the rest of the wavefunction passes. The part of the wavefunction that touches the edges of the wire produces a diffraction, which leads to some leakage (around 1% to the region on the image plane that is occupied by the image of the other hole).
2) Let's open two holes now, there is no light incident on the wire and no diffraction takes place, so no reduction in which way information ispossible. If you do not like this argument, then
3) Let's assume that we write the final wavefunction as the superposition of the incident 2 wavefucntions + the sum of the 2 scattered wavefuntions by the wire from each hole. The sums of the scattered wavefunctions are zero at all places at all times, simply because they are out of phase by Pi. If you don't like this argument, then
4) Let's (erroneously) assume that the diffraction is
still present even when both holes are open. The wire
does not block any light so that means it is in a
minimum. The greatest amount of light that can leak to
the "wrong" image cannot exceed the the 1% mentioned
in (1). That means the relaibility of which-way
information is reduced to 99% from 100%. But
Complementarity dictates that this reliability should
be reduced to 50%/50% for EACH image; i.e. 50% of the
flux in each image MUST come from the "wrong" hole!
Here is my challenge to you and your fine colleagues:
Write the wavefunction for EACH image as a 50/50
mixture of the wavefucntions originating from the two
pinholes and I will concede! You will also be eligible
for a $1000 prize I have set aside for anyone who can
actually do this.
I must say though, no one (faculty, post docs, grad,
and undergrads) at Harvard, Texas A&M, U Washington,
and other places, where I have given talks, has been
able to win my challenge. I would be delighted if you
or anyone else can do it; using the usual
quantum-mechanical formalism.
Looking forward to hearing from you soon.
Best regards.
Shahriar S. Afshar
Research Professor
Dept. of Physics and Astronomy
Rowan University
''
I then realized that my argument concerning scattering was not correctly presented on this form. For this reason I send to NS the letter contained in my previous post.
Thank you for your time Aurelien
[Note: I moved your comments of four dimensionalism over to the recent post on that subject . I hope you don't mind, but I wanted to keep this thread focused on Afshar since I tend to get a lot of Google hits for that. -- cg ]
Aurelien
I just send this letter to journal to reply to a comment of the Afshar experiment made in the new edition of Newscientist (see the Web):
''
Commenting the interesting experiment made recently by Afshar (24 July, p 30 ) three of your readers (18 September, p’’ ? in Letters’’ ) pointed out that the described results are in perfect agreement with the complementarity principle saying that we can not extract sufficiently of information of one particle to reconstruct in a statistical way all the prediction associated with a given wave function. I agree completely but I would like to add that this is just half of the truth.
Indeed Afshar not only claimed that he can circumvent Bohr’s complementarity but that he can effectively determine the path followed by the particle. Afshar, following here a assumption originally enounced by Bohr , Heisenberg and Wheeler, accepted that even with the two pinholes open a photon trajectory (if trajectory there is) will necessarily connect a pinhole to its optical image as it is if the other pinhole would be closed. However, as is was realized by numerous physicists as de Broglie, Bohm, Bell and Scully, this is a free metaphysical assumption which depends of our model of reality and which can not in general be experimentally tested [read for example “Surrealistic Bohm trajectories”, Englert et al in Z. Naturforsch. Vol 47a , p 1175.].
Effectively nothing in the Afshar’s experiment forbids a photon coming from one pinhole to go in the ‛wrong’ detector for the second pinhole (this is the case for example in the Bohm’s theory). Looking the image of a pinhole recorded in a statistical way by a myriad of photon will not say us from which pinhole an individual photon come from but just how much photon should had crossed this pinhole. In counterpart of course we can not see the fringes and the complementarity principle of Bohr will be, as in every quantum experiment, naturally respected.''
Aurelien Drezet, Graz University, Austria
By the way I like particulary my definition of the principle of Bohr.
It's been a while since there was any news on this topic. But a recent paper was uploaded to the Phil Science repository. Unfortuantely the author for reasons that escape me only uploaded a postscript file rather than a PDF. "The Afshar Two-Slit Experiment and Complementarity", Kastner, Ruth
A modified version of Young's experiment by Shahriar Afshar demonstrates that, prior to a ``which-way'' measurement indicating which slit a particle goes through, an interference pattern exists. It has been claimed that this result constitutes a violation of the Principle of Complementarity. This paper discusses the implications of this experiment and considers how Cramer's Transactional Interpretation accomodates the result. It is shown that the Afshar experiment is isomorphic in key respects to a a spin one-half particle prepared as ``spin up along x'' and post-selected in a specific state of spin along z. It is concluded that Bohr would have had no more problem accounting for the Afshar result than he would in accounting for the aforementioned pre- and post-selection spin experiment, in which the particle's preparation state is confirmed by a nondestructive measurement prior to post-selection.
Just a note that there is a paper up at the philosophy of science repository on this.
The revised version of my Feb. paper on the Afshar experiment is up at the Phil Sci depository as well as at
arxiv.org/abs/quant-ph/0502021
Dear all,
I have ben surprised to find out that prof. Aurelien Drezet, Graz University, Austria already has raised almost identical counter-argument against Afshar. Also it was very funny to see that already there is announced 1000$ prize for what I have partially done.
Well in my paper one can find that the lens produces essentially an suitably scaled Fourier transform of the two pinhole transmission function. Because of the symmetric setup and the crossing of the photon's paths 1 and 2 from slit 1 and 2 to detectors 1 and 2 the which way info is randomised in 50-50 fashion.
Full paper here:
http://philsci-archive.pitt.edu/archive/00002281/
One can easily use some of the theory presented in my paper to write down the proof required by Afshar.
Best,
Danko Georgiev
It was a long time since I didnt consider again the experiment of Afshar : I dont want to waist to much time with this story which is for me completely clear.
However I will just say shortly again that this experiment can not prove anything against Bohr complementarity.
Afshar didn't realized that a photon can not be detected twice in his experiment. If the photon is recorded in the image plane then it can not be seen in the fourier plane or in front of the lens ( where fringes are). the reciprocal is true too . Now with or without wire the result is the same: the fringes are deduced from theory or from observation of photons differentas from the one detected in the image plane.
This is the basis of wave particle duality and Afshar ignoring this is necessarily misleading .
PS : I never prove that I can win 1000 $ and I am 100% sure that nobody can prove the opposite : it will like proving that optics is wrong.
Afshar realized a simple experiment to prove a simple result but we should remember a sentence of Einstein : things in nature must be as simple is possible but not simpler
PS2: I am not yet professor but it 's nice nevertheless
with best regards from austria
Dear Aurelien Drezet,
I have been able to update my paper, and I have found even deeper roots of complementarity. Actually the problem is that when you have quantum superposition this is not equal to statistical mixture. How this problem arises?
Well, if you measure variable A, while in the same time its conjugate variable B is existing in superposition of states psi_1 and psi_2, then the Probability for measuring the quantum system in given state is P = |psi_1 + psi_2|^2. Here we see the manisfestation of the rule - first add, then square.
If we implement this mathematical rule [that is complementarity in action] we will get different results if we have which way info, and if we don't.
Suppose we have "which way" info, and let the two paths are psi_1 and psi_2. Then the final distribution of probability will be P = |psi_1|^2 + |psi_2|^2. Here you don't have interference.
If we don't have "which way" info, then the probability distribution will be P = |psi_1 + psi_2|^2 - i.e. here we have interference.
If there is "which way" info after the lens, then if we put not to mirrors to produce the focused two pinhole image, but if we put off-focus photographic plate, then we expect to find P = |psi_1|^2 + |psi_2|^2 - that gives us "clumping pattern" with central intensity = 2 I_0, where I_0 is the maximal intensity in the single-slit diffraction pattern.
However if Afshar is wrong, as I have claimed, then if we put photographic film off-focus [say 4 meters behind the mirrors, with mirrors removed] then I predict interference and P = |psi_1 + psi_2|^2 distribution. This will give us central maximum with intensity = 4 I_0.
So, Afshar can check that he is wrong.
More detailed paper will be published online soon.
If you are interested in the topic, contact me at dankomed@gmail.com. I have nothing personal against Afshar, except that he is popularizing FRAUD !!!!,
Danko
Here you can find revised paper dealing with complementarity and Afshar's experiment
http://www.geocities.com/dankomed/afshar.pdf
Danko
dear Danko Georgiev ,
You spend a lot of time on this story but I can no personnally do it . I read (not in detail ) your long article but I think you missed the fundamental point. as I understand you say that after the lens we have the choice between the observation of the photon in the image plane (II) or in the focal plane(I) (you can , like Afshar did, record the photon just before the slit the difference is irrelevant for the general result).
You can even like you mention observe the photon after the image plane(plane III) and then observe fringes. As I understand for you this proves that the experiment is not a which path experiment.
This is unfortunatelly wrong.
The essential point is that a photon can not be detected twice (in this experiment) and this imply that your rasonning is wrong
Indeed photons recorded in the image plane (II) or in the focal plane (I) are not the same . Similarly photons recorded in the image plane (II) or in the plane (III) are not the same.
Then since you dont use the same phtotn you can not conclude like you did. In fact dependint of the plane you have or not a which path experiment. You can read for example [L. S. Bartell
Phys. Rev. D 21, 1698-1699 (1980)]
with best regards
AD
Dear Aurelien,
I am glad to hear your reply. Actually in the dialogue we can find the truth. Concerning your points:
1. I know that we cannot detect a photon twice.
2. My thesis is simple: "the lens does not preserve the which way info"
Well some commentary -
from your post as if it follows that the image at the "image plane" contains "which way" info, while the image "out-of-focus" [= not at the image plane as estimated by 1/a + 1/b = 1/f] is "not containing which way" info.
This however is ad hoc claim and I cannot see how you will defend it. Actually my thesis more accurately is - the image produced at the image plane is result of photon wavefunction interference - i.e. Fresnel diffraction + Fourier transform + Fresnel diffraction.
In the limiting case when object is at infinity i.e. a -> oo, then b = f, and the above general scheme of mine is converted into Fraunhofer diffraction [forward Fourier transform] + inverse Fourier transform.
You simply can see that WITHOUT KNOWING THE PHOTON'S TRAJECTORY, YOU CANNOT DETERMINE THE "WHICH WAY".
Please prove me that the photon has followed a straight ray trajectory, because I insist it interfered in the region between the source and the final detector. You can read the section where in my paper where I reply to Unruh.
p.s. Actually the clarification presented here will be added in my paper: from Fresnel diffraction + Fourier transform + Fresnel diffraction, you obtain the Fraunhofer diffraction + Fourier transform when a -> oo, because the image plane is at the focal plane and the second round of Fresnel diffraction is zero.
p.s. 2: please send me the reference that you quote - I don't have it, and I cannot comment on a paper that I have not read.
Thanks for your interest,
Danko
Dear Danko the whole response to your comment is again contained in the basic law 'a photon can not be absorbed twice'.
You can observe a particular photon in the plane that you want but doing this you can not absorb the same photon in an other plane. Now Bohr's principle speaks only about experiment not about metaphysical assumption. you have not the right to say that because you observe fringes out of the image plane that then the which path is not observable with the lens. You can have a which path information in the image plane butto observe this information you need to use different photons that the one producing fringes. This is the message of Bohr and you can not refute that. In other term you want to refute a wrong statement by an other wrong statement . but ''wrong +wrong'' is not equal to ''truth'' .
Now I am completely borred with this Afshar story. I just include here a text used by me for the wikipedia encyclopedie (discussion forum) and this will be my conclusion for ever:
There is one year already the physicist S. Afshar working at Harvard university realized a very controversial experiment in order to refute the famous principle of Niels Bohr called complementarity. This experiment was described in an article of the British journal New scientist [(2457, 24 juillet 2004)]. It was clear for me since the beginning that the result was badly interpreted and that the reasoning of Afshar was misleading.
In order to refute the claim of Afshar I would like, here, to explain briefly my reasoning.
Principle of Afshar's experiment.
The optical experiment represented on the figure included is essentially an extension of a ‘gedanken’ experiment proposed by Wheeler and called the delayed choice experiment. A coherent light impinges on a screen containing two slits or apertures A and B. Light diffracts and produce interference fringes at large distance from the apertures. As well known we can use a lens L to observe these fringes in the focal plane (F). Alternatively we can decide to observe the image A' and B' of the holes in the image plane (I). This is simple and is only a problem of classical optics.
However difficulties arise if we consider the same problem with photon. Indeed a photon is a discrete entity which can produce a local click on a detector. If we suppose that light is made of particle how to explain the existence of the interference? This is the canonical problem called wave particle duality. Einstein and others like de Broglie or Bohm tried to justified this curiosity of nature but Bohr found a simple solution to this problem which is to dismiss the entire issue by avoiding any reference to objective reality. For Bohr and Heisenberg it is a complete non sense to search an explanation for such quantum behaviour because such explanation could not be experimentally tested.
Bohr realized that effectively if you build up a model attributing a trajectory to photons then you should be able to observe experimentally these trajectories. If you can't do that your model is without any physical interest. However Bohr remarked that the observation of the path of the photon always disturbs the coherence of the wave and erase the interferences. In the present context we have the choice between detecting a photon in the image plane or in the focal plane. This means that we have the choice between using each photon to build up the images A' and B' ('which path' information) or the interferogram. But since a photon can not be absorbed twice we can not make the two observations. This is that why Bohr called this property complementarity.
Now let go to the Afshar modification of the preceding experiment. Afshar decided to observe his photons in the image plane (I). Nevertheless he introduced in the focal plane a periodical grid of absorbing wire (in fact he worked just in front of the lens but this doesn't affect my reasoning). The wires are located at the exact minimum of the fringes. Naturally the experiment is not affected by this introduction because the intensity on the wires is close to zero producing consequently no additional disturbances or diffractions on the light propagation. The two spots in A' and B' are then unchanged.
However since no photon are absorbed this give us information on the intensity at the wires location. We know then that the probability for a photon to cross the section of the wire is null and this is already something. Afshar believes that this information is sufficient to prove the existence of fringes in the plane. He is wrong. Naturally he is however right saying that clearly the intensity can not be uniform in the focal plane. But he can not really say what the shape of the fringes in this plane is. Bohr’s principle speaks about physical observation not about metaphysical expectation. To define experimentally the fringes he should use other photons that the ones recorded in the image plane. If you need an analogy you can imagine the following situation. You are living in Paris and you can see from your beautifull flat (with a rent of 8000 $ per month) the apex of the Effel tour. Can you deduce from that the shape of the Effel tour? clearly not... in the Afshar experiment every thing is the same.
This is clearly in disagreement with the conclusion of Afshar and is sufficient to refute all his reasoning.
dr. aurelien drezet university of Graz Austria
Dear Aurelien,
your reasoning about image plane and focal plane is wrong, and I still don't understand what you don't understand.
I have read your commentary about photon "trajectory" so I suppose [I am not sure of that] that maybe you have problem with defining the trajectory of the SINGLE PHOTON IN DOUBLE-SLIT EXPERIMENT. I don't understand whether you fall in his group of people who belive that single-photon double-slit experiment is disproval of complementarity [there are also such type of wrong claims].
In my paper I clearly linked MATHEMATICS to COMPLEMENTARITY.
So let me explain:
if you have XOR-knowledge you have |psi1|^2 + |psi2|^2 distribution [this is the "which way"]
if you have AND-knowledge you have |psi1 + psi2|^2 distribution [this is "no which way" i.e. you have interference].
BUT NOW NOTE THAT "POPULAR MEANING" IS DELUSIVE.
For example if you have double slit and put in a random fashion a shutter on one of the slits, after enough trials when statistically you 50% of the experiments closed the slit1 and in 50% of the experiments you have closed slit2 what you will observe is the "clumping pattern" |psi1|^2 + |psi2|^2. At a certain distance calculated by the Rayleigh formula you will observe also merging of the two peaks into a single peak. HOWEVER THERE WILL BE NO INTERFERENCE MAXIMA OR MINIMA.
Suppose now we have single-photon double-slit experiment. You send photon by photon. What you will see is the accumulation of "interference image" because of the |psi1 + psi2|^2 distribution . But note - IN THE CASE YOU ALSO HAVE INFORMATION ABOUT THE PHOTON TRAJECTORY !!!! You simply know that this is not equivalent to the previous experiment when you closed in a random fashion with a shutter slit1 or slit2.
So you KNOW that it is NOT XOR = XNOR-knowledge. However also you know that if the photon does not pass through any of the slits it will certainly not land on the detector. So from the XNOR you have AND-knowledge. In simple text "THE SINGLE PHOTON PASSED THROUGH BOTH PINHOLES AND INTERFERED WITH ITSELF". This is usually called "no which way", but MATHEMATICALLY has very concrete meaning.
So my advice is that you stop think meta-physically about double registration of photons, here I speak of distributions and QM laws. I show that no matter WHERE you will produce image with a lens, IF the light coming from the double slit is COHERENT then the image that you get [at the "classical image plane" or at any other "out-of-focus plane"] is following the |psi1 + psi2|^2 distribution hence it is product of INTERFERENCE.
So far noone has written so much mathematics, about Afshar's experiment and now I am writing also section on Dirac's bra-ket notation and quantum measurement. The fact that you CANNOT observe both "clumping" and "interference" distribution follows from entanglement between the q-system and the measuring instrument. More on this will be uploaded soon.
Best,
Danko
Dear Aurelien,
Please do not be offended but here I will post refutal of your absurd claim:
" Afshar believes that this information is sufficient to prove the existence of fringes in the plane. He is wrong. Naturally he is however right saying that clearly the intensity can not be uniform in the focal plane. But he can not really say what the shape of the fringes in this plane is."
First of all Afshar KNOWS where the interference minima ARE. If he MOVES the GRID so that it is NOT in the INTERFERENCE MINIMA THEN there will be loss of photons. You simply FORGET to use mathematics. Exactly because photons follow concrete mathematic you can PREDICT THE INTERFERENCE FRINGES ON THE SCREEEN. So it is not necessary to that experiment, I can just take a sheet of paper and a pencil and do the calculations [all of them are presented in my paper in the case of far-field approximation].
So if your WRONG thesis was right THEN THERE WOULDN'T BE ANY DIFFERENCE IF AFSHAR MOVES THE GRID AT LEFT OR RIGHT. But this is absurd claim of yours, and from this will follow that photons do not obey any mathematical principle - i.e. the minimum is everywhere :-) I also will not challenge you with shifting the grid along the optical axis, because in this case the WIDTH of the MAXIMUM changes.
Once again Afshar can simply shift the grid and show you that there is loss of photons when he grid is put at a new position. Thus he immediately will verify his thesis that "gris is measuring the photon's maxima".
Here my thesis is TESTABLE, and it will immediately disprove you. Here I am on Afshar's side, because this follows from the wave mathematics :-)
Best,
Danko
Dear mr. Danko, this will be my last message to you (I dont like to fight against the wind) and like afsahtr your are too nervous for me.
You insist so much on the word mathematics that it seems for you like a religion. Unfortunatelly for you I know math (I think better than you) I did the same calculation that you did one year ago already using Fresnel diffraction theory but this change nothing to the physical discussion and you must realize (but of course you are free ) that when you understand really a physical problem you don't need to make the calculation.
Just briefly :
If you move the grid you can effectivelly measure the absorption i.e the probability of presence in the focal plane for the new wires location. But if you do that you modifiy the shape of the spots in A'and B' (scattering ). Then if math are good you have less which path information. complementarity is still ok and your argument will stay wrong until the end of world.
sincerely
dr. Aurelien drezet Physicist
Dear Aurelien,
From your postings one could derive a lot of wrong conclusions, because you suggest a lot of wrong ideas.
1. Whether the photon will be diffracted by the grid DOES NOT TELL YOU whether the photon interfered or not. Simply photon going along a straight line can be diffracted by the grid, but also photon that passed through both slits and interfered can be diffracted. So difrfaction by the grid tells you nothing about "which way".
2. Existence of grid however will result in notable diffraction if photons go in straight trajectories and conversely the diffraction will be close to zero IF the grid is located in the interference minima.
SO NOW THE PROBLEM IS HOW TO COMPUTE THE PHOTON'S WAVEFUNCTION?
I say that when you have both slits open we should assess |psi1+psi2|^2 i.e. we have interference and "no which way".
If we put color filters on the slits however there WILL BE LOSS OF PHOTONS, because the photon's distribution is assessed by |psi1|^2 + |psi2|^2. That is there is no interference.
IF YOU CLAIM THAT THERE IS "WHICH WAY" INFO THEN IF I PUT DIFFERENT POLARIZATION FILTERS ON THE TWO SLITS YOU SHOULD INSIST THAT THERE WILL BE ALSO NO LOSS OF PHOTONS BY THE GRID.
IF I AM RIGHT HOWEVER THEN THERE WILL BE PHOTON LOSS.
WHAT DO YOU THINK? DO YOU BET THAT PUTTING POLARIZATION FILTERS WILL NOT CHANGE THE AFHAR'S EXPERIMENT OUTCOME. Polarization will not add "new" which way info IF the which way info is already there as you insist.
Best,
Danko
For anyone interested check:
http://www.geocities.com/dankomed/afshar.htm
I have added in my pre-print special reply to A. Drezet, and also I have provided the density matrices for the two main type of experiments. If there is "no which way" the photon's wavefunctions psi1 and psi2 should not be coherent, so the final state is described by mixed state density matrix. In contrast in the double slit experiment and in Afshar's setup the density matrix has off-diagonal terms and exhibit interference effects. I don't see why when the photon is detected by one of the final detectors there is "which way" info?
There is "which way" info if the density matrix is:
rho = |1/2 0|
|0 1/2|
If the density matrix is
rho = |1/2 1/2|
|1/2 1/2|
then there is no which way info.
More info about the difference between these states will be updates soon.
Dear Danko concerning the inclusion of a comment of my point of view in your preprint I have a question: Is your curious work submitted/accepted for a publication somewhere? If yes please tell me the journal in order that I stop to read it (probably I should start to read it before to stop to read it ).
OK without jokes I think that you are completely wrong but since we discuss on a free forum it doesnt really matter. If you try to publish such work I guess that you will have even more problems that Afshar have: so be prudent.
regards
aurelien Drezet
Dear Aurelien,
WHY YOU ARE SO BLIND? If you really are "physicist" as you call yourself then you should KNOW that:
"Cross term of crossed beams is proportinal to 2 cos(kx.sin_theta) where 2 x theta is the angle between the crossed beams". In Afshar's experiment the production of the image is result of crossed beams, so there IS INTERFERENCE.
Please just take a look of the lecture of Rick Trebino:
Rick Trebino (2003) Lecture 18. Coherence and Interference
http://www.physics.gatech.edu/gcuo/UltrafastOptics/3803/OpticsI18CoherenceInterference.ppt
In order to see who is Rick Trebino check:
http://www.physics.gatech.edu/people/faculty/rtrebino.html
And it seems that you need to read a full course in optics:
http://www.physics.gatech.edu/gcuo/UltrafastOptics/index.html
Best,
Danko
dear Danko , I realized that I could offend you (for this I apologize even if your not very polite too).
I can not give you an anwer for your ''questions'' since you missed to many important points (optics, quantum optics ,logic....). Then I have presently no other choice that to stop this senseless discussion with you.
I regreat that we discussed on this page with such agressive style and I apologize too to the creator of this interesting philosophical forum.
with best regards
dr Aurelien Drezet, institut for experimental physik Graz Austria.
Dear Danko , I forgot to say that I realized that you are not a professional physicist . This is not a problem for me and I was stupid to offend you since I am a professional physicist and I learned with time how to find a mistakein a reasonnig very quickly. You seem to be an autodidact and this is very courageous.
but like me you should learn how to be prudent and modest (perhaps I have more to do in this direction).
your argumentation copncerning Afshar is not good but I have no time to explain you again and again why. If you want we can discuss with private email ?
with best regards Aurelien Drezet
Dear Aurelien,
I am glad to continue our conversation by e-mails, so contact me at dankomed@gmail.com
Don't worry about offending me - I NEVER get offended by someone who criticizes any scientific position. What I say is just a scientific statement - that may be true or false, but it says nothing about my personality, so in science I never say statements that I consider "mine" or "expressing my personality". In science there are only ideas that can be true or false, and that can be mine, or yours, or everybody else's. And why to be offended - I consider myself evolving unit - if some statement is proved false, I will immmediately drop it out and will never defend it in the future.
Danko
Dear Drezet,
I thought there is some sense to discuss with you physics, but you ARE SIMPLY IGNORANT IN MATHEMATICS !!! All your non-sense posting just prove that.
I clearly stated above, and you can check in any physics textbook that
Cross term (INTERFERENCE TERM /capital gamma in Afshar setup) of crossed beams is proportinal to 2 cos(kx.sin_theta) where (2 x theta) is the angle between the crossed beams.
Simple mathematics shows that IF theta -> zero, then sin(theta) -> zero, then 2 cos(zero) -> 2, i.e. interference is maximal.
In Afshar setup the image is produced via CROSSING of light beams, yet, the distance between the two pinwholes is several micrometers, the distance to the lens is several meters, hence theta is approching zero. If you are interested do the exact calculations for HOMEWORK taking the values from Afshar paper.
I post this for help of occasional readers, and I will ignore your further non-sensical posts.
Dear Drezet,
I thought there is some sense to discuss with you physics, but you ARE SIMPLY IGNORANT IN MATHEMATICS !!! All your non-sense posting just prove that.
I clearly stated above, and you can check in any physics textbook that
Cross term (INTERFERENCE TERM /capital gamma in Afshar setup) of crossed beams is proportinal to 2 cos(kx.sin_theta) where (2 x theta) is the angle between the crossed beams.
Simple mathematics shows that IF theta -> zero, then sin(theta) -> zero, then 2 cos(zero) -> 2, i.e. interference is maximal.
In Afshar setup the image is produced via CROSSING of light beams, yet, the distance between the two pinwholes is several micrometers, the distance to the lens is several meters, hence theta is approching zero. If you are interested do the exact calculations for HOMEWORK taking the values from Afshar paper.
I post this for help of occasional readers, and I will ignore your further non-sensical posts.
I've closed comments in order to avoid spam since I don't check this older blog as much anymore.
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