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A variation of the double-slit experiment-myth or did somone really do it?

  1. Oct 27, 2007 #1
    A variation of the double-slit experiment--myth or did somone really do it?

    I've heard this experiment described at least three times; possibly more. However, none of the sources are terribly reliable--Scott
    Adams in the last chapter of The Dilbert Future (and he freely admits that he gets a lot of stuff wrong), this guy, whom it turns out is apparently a lawyer or something, and various other anonymous person(s) in my all-too-murky memory. The alleged experiment goes like this:

    Classic double-slit experiment, with detectors added at the slits. When activated and recording data, the detectors collapse the waveform and the screen shows particle-like behavior--nothing special here. Then, the data-recording/monitoring aspect of the detectors are deactivated, but the detectors themselves are still functioning as normal. Absolutely nothing has changed from the photon's point of view; it's still getting "detected", but the scientists watching don't know and can NEVER know the results of the "detections." And, after this modification is made, the screen shows wave-like behavior (the interference pattern.)

    This alleged experiment showed that 'detection' in some abstract (some would even say human- or consciousness-centric) sense is what collapses the waveform, not 'detection' in any sort of sense that preserves locality. An alternative to the mystic- and pseudoscientific-sounding "consciousness causes collapse" is perhaps "permanence causes collapse" (meaning that unless the position of the photon has been detected in such a fashion that a lasting record is possible, the waveform remains intact.) It's also been suggested (over at Wikipedia) that thermodynamic irreversibility might have something to do with this--I don't really see the connection myself, especially in light of the statistical (not absolute) nature of thermodynamic law, but it bears mentioning.

    So--does anyone know whether this variation of the double-slit experiment was performed? If so, any thoughts about what it implies?
  2. jcsd
  3. Oct 27, 2007 #2
    Myth; it contradicts the mainstream theory. We can safely bet a Nobel prize that somebody just got the details mixed up along the way.
  4. Oct 27, 2007 #3
    Is there any particular source you can say it contradicts? Not to be picky, but if this really is an urban myth (and apparently a *reasonably* widespread one), I would like to thoroughly debunk it on Wikipedia.
  5. Oct 27, 2007 #4
    One would expect the interaction of the photons with the detector, which is still functioning, to randomly alter their paths, frequency (if there's a collision with a particle involved, which there should be), etc, enough to destroy the interference. I think that it would be easy to debunk it for any particular path measurement apparatus. Maybe there's a general refutation out there also, but that'd be harder to find.
  6. Oct 27, 2007 #5
    This is the cheapest experimental (quantum eraser) example, note that the pattern disappears whenever it is possible-in-principle to measure which-path information of individual photons, despite that no such information is actually recorded. Please, by all means, go forth and debunk. :smile:
  7. Oct 28, 2007 #6


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    The established way to understand what the detectors do in the experiment would be to solve the quantum mechanics of the system "double-slit plus detectors". If it was necessary to solve the system "double-slit plus detectors plus observer" then let me ask another question.

    If we want to find the energy levels of the hydrogen atom, why do we solve the system "proton plus electron" and not "proton plus electron plus observer" ?

    We all know that the latter would be ridiculous. We ought to treat the observer as a negligible perturbation, because if we use the QM prescription that way, we get the energy levels to very high precision.
  8. Oct 28, 2007 #7
    I can, at least personally, say with confidence that this experiment may have been theorized or even performed. Though certainly, the realm of consciousness, as of now, has not been connected with quantum theory, so assuming that I do not believe that the detecting process could simply deactivate. True it would make the particle/wave choose it's destination decisively.

    But so say bluntly, no I do not believe that this experiment was ever performed with the results stated here. It simply violates to many fundamental laws.
  9. Oct 29, 2007 #8
    Duly noted. It would have been interesting if it were true, but I'm not surprised it isn't.
  10. Oct 31, 2007 #9
    David Park, in his book: "Introduction to the Quantum Theory" third edition - Chapter 10 - par. 10.1, computes the wavefunction of the system + detector in the case of a single detector located near one of the slits and notice that in the hypotetical case of "magic" detector, that is, a device who could detect the particle without affecting the particle's wavefunction, the interference pattern would be destroyed as well.

    Then he says:

    <<The mere existence of some dynamical degree of freedom coupled to a particle in a way that distinguishes between two possible places the particle can be or two possible things it can do is enough to eliminate the typically quantum-mechanical phenomena arising from the interference of the particle's two states.>>

    So, which role can have the observer? None, presumably.
  11. Nov 4, 2007 #10
    I looked at the Scientific American virtual demonstration, and did some mental calculations. It seems to me every stage of the experiment can be explained with basic electromagnetism/optics. That demo is probably meant to demonstrate principles of quantum physics, although it might be true as they claim that each photon is interfering with itself exclusively. Either way, the old light beam model works there as far as I can tell, let me know if you think there's something I missed.

    If you want to observe quantum phenomena, I believe you need to conduct far more sensitive experiments than a simple tabletop laser arrangement. Maybe use a crystal to produce diffraction on an atomic scale, things like that.
  12. Nov 4, 2007 #11
    Yes Bork, as you should have noted in the text accompanying the description of that cheap "quantum eraser", their conclusion is technically only valid if you decrease the intensity (to one photon at a time) which in turn requires more sensitive detectors.

    Exactly that has been done in numerous labs, validly confirming the QM results (unable to be explained by basic EM/optics) for this and other experiments. But Sci-Am's typical reader cannot afford such apparatus...
  13. Nov 4, 2007 #12
    Ok, and the only point I wish to make here then is that nothing "strange" is going on, aside from the usual quantum mechanical probability amplitude interference. I was under the impression the demonstration was referred to as a means of proving that the information (what the human eye reads on the output) about a measurement is what counts, not whether a detector actually interfered with the system. Aside from the fact that any detector which interferes with a system is going to leave residual information about that system even if we try to "erase" it, I am merely suggesting that any experiment which can be explained with classical physics couldn't possibly demonstrate such an effect.
    Last edited: Nov 4, 2007
  14. Nov 4, 2007 #13
    Incidentally, I think the whole reasoning behind this "quantum strangeness" is seriously flawed and maybe impossible to ever verify experimentally even if it were true. The claim, if I understand correctly, is that you can measure information about a system using some apparatus, but as long as you have no conscious awareness of whether or not your apparatus was working, it will be as if no measurement ever took place. The problem is, how do you make a measurement on a system, and then alter the output so that the human observer is unaware of it and yet the information detected by the apparatus is still present?

    To prevent a human from being able to observe whether their detectors were functioning, you would somehow have to interfere with the actual particles in your experiment after they have already been measured, because the particles themselves are what contain the information. In the Scientific American tabletop demonstration, the 45 degree polarizer inserted in the final stage is interfering with the incoming photons, destroying information about their original quantum state and setting them up in a new quantum state. So I can't think of any way to separate the human observer from the effects of their apparatus on the system they're trying to observe/ignore. Not that this should be such a surprise- do you really believe the moon isn't there when you're not looking?
    Last edited: Nov 4, 2007
  15. Nov 4, 2007 #14
    Your concern is misplaced. If you actually read carefully the posts in this thread, you should notice two things:
    • This thread is not generally about interpretation issues, but instead is about some particular (and dodgy) descriptions of a specific experiment (which are obviously mistaken, since they describe a result in plain disagreement with all expert-reviewed results actually produced from that specific experiment or similar).
    • Nobody here has indicated any belief that consciousness plays a role in quantum mechanics anyway, quite the opposite.
    There's no point in us arguing about the table-top demonstration. It should serve only as a gateway to the technical literature on proper quantum eraser experiments. I mentioned it to discredit the same viewpoint you seem to be opposing. And the relevant point was when that 45 degree polariser is absent (the pattern fades despite that the human, being unable to see polarisations, is not conscious of which-path measurement records).
    Last edited: Nov 4, 2007
  16. Nov 4, 2007 #15
    Hi cesiumfrog, then it seems we're in agreement. My latest refutations is of course mainly referring to the original post and the associated claims, not to anything you've said.

    Now no offense to the original poster, as you are merely quoting claims from a source and running it by others to see what they think, which is a good idea. However, while on this subject, it's amazing to me how much quackery is found in the popular conceptions of science, especially but not exclusively quantum physics. When I'm in a group and they ask what I study, I usually get reactions like "I hear quantum physics is so strange and bizarre." That it is, at least compared to our daily intuition. However, more often than not I find they are referring to these myths that human consciousness plays a special role in quantum mechanics (if it does, we have never established this to date). I've had friends who took "courses" on spiritual healing where some of the basic jargon of quantum physics was thrown around a lot to make their "teachings" sound like more than just cow manure. I try to explain concepts like the EPR paradox to them (in as simplified a version as I can give it) and why EPR doesn't have anything to do with spritual healing or the "connectedness of the universe." They nod their heads in agreement, and then a week later it's back to the same old nonsense as if I had never even explained it.
    Last edited: Nov 4, 2007
  17. Oct 21, 2009 #16
    Re: A variation of the double-slit experiment--myth or did somone really do it?

    Forgive me for adding to this old thread, but I was wondering if the variation of the two-slit experiment referred to in this physics world article might have some bearing on the discussion here: http://physicsworld.com/cws/article/news/27106
  18. Oct 21, 2009 #17


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    Re: A variation of the double-slit experiment--myth or did somone really do it?

    Welcome to PhysicsForums, moon-watcher!

    Here is a reference to an entire article on a virtually identical experiment (Aspect is one of the authors too):

    Delayed-choice test of complementarity with single photons

    "We report an experimental test of complementarity using clock-triggered single-photon pulses emitted by an individual N-V color center in a diamond nanocrystal. The single photons are sent into a Mach-Zehnder interferometer with an output beamsplitter of adjustable reflection coefficient R. In addition, the choice of introducing or removing this beamsplitter is random and relativistically space-like separated from the entering of the photon inside the interferometer, as required for the Wheeler's delayed-choice regime. Each set value of R allows us to observe interference with visibility V and to obtain incomplete which-path information characterized by the distinguishability D. The measured values of V and D are found to obey the complementarity relation V^2 + D^2 =< 1. "

    Human consciousness plays no part here, which I believe was the original question.
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