www.kathryncramer.com/wblog/archives/000674.html
Quantum Mechanics: Not Just a Matter of Interpretation on April 26th. This post described the experiment and its implications for various interpretations of quantum mechanics.
Transactional Interpretation, cannot be distinguished or falsified by experiment, because the experimental predictions come from the formalism that all such interpretations describe. However, the Afshar Experiment demonstrates in an interaction-free way that there is a loophole in this logic: if the interpretation is inconsistent with the formalism, then it can be falsified. In particular, the Afshar Experiment falsifies the Copenhagen Interpretation, which requires the absence of interference in a particle-type measurement.
The Transactional Interpretation, on the other hand, has no problem in explaining that Afshar results. "Offer waves" from the source pass through both pinholes and interfere, creating a condition in which no transactions to the wires can form. Therefore, no photons are intercepted by wires, as Afshar observes.
Kathryn Cramer at July 23, 2004 08:46 AM You can post the image as the background to an html page and it will be large enough to read. If you need I can make the page for you and send it to you for posting of the two images as html's.
Al at July 24, 2004 12:01 PM Until I get time to post something higher res, here's a suggestion for those wanting bigger type: bump the resolution of your monitor down.
Kathryn Cramer at July 27, 2004 02:55 PM Afshar's claim seems to be that even though they knew which slit the photon went through, they still got interference, violating the principle of complementarity. However, reading various responses online it seems likely that the presence of the wires themselves introduces some new uncertainty about which slit the photon went through, because the wires can scatter photons so that a photon going through the left slit can end up in the right detector and vice versa.
com&rnum=5 Now I haven't done any calculations or read the New Scientist article except looking at the lab setup graphics, but if I would hazard a quick guess, it would be that it will turn out that even if the wires are placed in the interference fields valleys, the finite width of the wires will diffract just enough photons to erase the which-way information that was gained by focusing the detectors at the holes in the wall through the lens. Consider the limiting case with wires placed with their centres in the interference fields valleys as before, but expand their width so much that they almost touch each other. What you have now is yet another wall with a bunch of slits in! Obviously, almost all which-way information is lost after the wavefronts pass these almost infinitesimal slits since they will diffract the photons equally no matter from which hole in the *first* wall they originated, so any detector placed after this obstacle will be like running a new multiple-slit interference setup (although with the lens now severely defocusing the too-closely placed new slits). And since the which-way information from the first wall is erased, interference is free to happen between the first and the second wall. After the secondary wall the detectors can pick up which-way information causing them to behave as if there was little subsequent interference. Conversely, the other limiting case is with no wires (or secondary wall) present. Then all which-way information is present and again the detectors behave as if there was no interference. The experiment shows a case in between these limits and the effect I guessed at above could (and should, according to traditional QM) turn out to always cancel any attempt to find both 100% interference and 100% which-way information. This would be better showed with some calculations of course... Kathryn, would you be willing to ask your father if the finite width of the wires does indeed erase some of the which-path information by causing a nonzero amplitude for paths where the photon goes through one slit but scatters against the wire and thus ends up in the opposite detector? If so, couldn't this show how complementarity is preserved in this experiment?
It might be a moot point though, I just looked at the diagram and accompanying text more carefully and I realized the poster I was quoting may have misunderstood what happened in this experiment--he seemed to be under the impression that the "interference" in this experiment was found at the detectors, and the scattering of photons against the wires could explain this. But the diagram seems to say that no interference was found at the detectors--the interference Afshar is talking about was just in the fact that no photons were scattering against the wires because they were all placed in the interference valleys. So the idea seems to be that interference is the explanation for why no photons scatter against the wires, but the focusing lens behind the wires makes sure that photons from the left slit always go to the left detector and the photons from the right slit always go to the right detector--this is the "violation of complementarity", that the photons behave like a wave in avoiding the wires but behave like particles when arriving at the detectors. I'm not sure that the notion of "complementarity" has ever been sufficiently well-defined to say that this experiment violates it though, and in any case, as long as the results of the experiment match the predictions made by the standard theory of quantum mechanics, it cannot be taken as a falsification of the Everett interpretation, since the basic idea of the Everett interpretation is to keep the standard rules for wavefunction evolution but just to drop the "collapse" idea (the projection postulate).
Jesse at July 28, 2004 04:42 PM Jesse: Here's the response to your question from my dad: Several people have asked me similar questions, usually centered on various effects of the wires in the two-pinhole measurement. Several months ago our UW Physics Department Chair, Prof. David Boulware, pointed out to me that Afshar's wires are placed in just the positions of the opaque stripes in a diffraction grating, which would produce multiple images of the pinhole and, in particular, would put flux from pinhole 1 at image 2, etc. This would compromise the purity of the "which-way" measurement. This point was very troubling, so I immediately asked Shahriar Afshar about grating effects, wire scattering, etc. 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: wire in & both pinholes open, wire out & both pinholes open, and wire in & only one pinhole open. Measurements and 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 , 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.
Kathryn Cramer at July 29, 2004 11:31 AM Just a quick question, as I have only tonight become aware of this work. Are the two mirrors used simply to scan images of the respective pinholes across the detectors to build up an image of each at the (stationary) detectors? Einstein of course proposed at least one experiment to challenge the idea of complimentarity (recoiling screen) but was countered by Bohr, in tha...
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