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B Watching the double-slit experiment affects the results?

  1. Nov 4, 2015 #1
    Sorry that I haven't done a search here, but I've searched exhaustively on the internet for a definitive answer, but I watched the Dr. Quantum video on the double-slit experiment, and he says near the end that "The observer collapsed the wave function simply by observing" with an eyeball on a tripod watching it as if the mere act of a person standing there watching it or a camera determines whether the light displays on the screen as two lines or an interference pattern.

    To me it's as plain as day what he's saying: that just someone consciously being aware of it changes the results; I even saw a video by Tom Campbell saying that in a decades-old experiment (I can't find it based on his talk), a tape recorder actually recording to tape or not changed the results.

    I'm completely confused; I thought the measurement of the particle by a detector actually interacted with the particle (or wave), determining it's result, not just some conscious being becoming aware of it eventually. Note: I'm not a physicist, just some average joe with the intelligence of a software programmer.
  2. jcsd
  3. Nov 5, 2015 #2
    Has anyone ever actually done this experiment where you shoot electrons through 2 slits and if you observe the electrons, it eliminates the diffraction pattern?

    Considering the technical difficulty of doing this, I don't think it has actually ever been done. This would mean this has only been a "thought" experiment which has never actually been verified by an actual experiment. I think this is the most accepted experiment that has never actually been done. I don't think we should take thought experiments as fact.

    The closest I can find is this experiment:

    Controlled double-slit electron diffraction
    Roger Bach 1,3 , Damian Pope 2 , Sy-Hwang Liou 1
    and Herman Batelaan 1,3
    New Journal of Physics 15 (2013) 033018 (7pp)

    This experiment uses electrons and slits and can detect the diffraction pattern as a real result. But they don't take the next step to measure which electrons go through which slit and watch the diffraction pattern go away. What is even stranger is that when they only have a single slit open, they still see a diffraction pattern. How can that be when there isn't another slit to cause the interfering diffraction?

    So someone prove me wrong that this isn't just some thought experiment that can only be seen on YouTube as a made up fairy tale cartoon. I am looking for a verifiable reference that can found on the web.
  4. Nov 5, 2015 #3


    Staff: Mentor

    You are correct.

    Forget the pop-sci mumbo jumbo.

    QM is a theory about observations, but 'observers' in QM is much more general than in everyday use. Its anything capable of being entangled with what's being observed - conciousness has nothing to do with it at all except in some very fringe interpretations.

  5. Nov 5, 2015 #4


    Staff: Mentor

    I am pretty sure these key experiments have been done - but I personally don't know the details.

    The way QM is presented in some pop-sci accounts is its a logical deduction from some key experiments like the double slit. Actually it was developed from some intuitive leaps of faith. But since then we understand its foundations a lot better. Its now known its a logical consequence of some reasonable assumptions:

    Whats going on with single or double slits is explained here:

  6. Nov 5, 2015 #5


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    Does it have to be electrons?

    First of all, why would there be a problem with the diffraction pattern appearing when only one slit is active. After all, THAT is what you expect to see! A diffraction pattern is different than an interference pattern. The diffraction pattern gives a modulation of the intensity of the interference pattern.

    Secondly, most "double slit" experiments are now done using an interferometer, such as the Mach-Zehnder interferometer, where each branch of the path corresponds to a "slit". So if you were doing a search on just the double slit, it is why you didn't find any. Experiments on the double slit using such interferometer is so common and so well-known, it is now done routinely at the undergraduate level![1]

    And btw, if you still want to hunt for similar demonstration using electrons, look for some early experiments using SQUIDs (superconducting quantum interference device).


    [1] T.L. Dimitrova and A. Weis, Eur. J. Phys. v.31, p.625 (2010); http://departments.colgate.edu/physics/research/Photon/root/ajpbs02.pdf
    Last edited: Nov 5, 2015
  7. Nov 5, 2015 #6


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    And what about this experiment: Decoherence of matter waves by thermal emission of radiation.

    They do the equivalent of the double-slit experiment with C70 (yes, that's a molecule!) and show that by heating up the molecule, the contrast of the interference fringes gets reduced then disappears, because the emission of a photon by the molecule could give which-way information.
  8. Nov 5, 2015 #7


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    You are not confused. You are correct, aside from small nuances that don't really matter in this context (but are still good for many tens or hundreds of passionately argued posts).

    And frankly, I'm not altogether sure that this Dr. Quantum guy is professionally qualified to explain quantum mechanics at all. He claims the title of "Doctor", but I can find no evidence that any reputable degree-granting institution has ever granted anyone by that name a doctoral degree in physics or any related subject. If he were for real, I'd expect to find a LinkedIn profile, and some peer-reviewed and published papers, and a record of his dissertation somewhere... I think you can safely ignore him.

    Seriously, kidding aside, that video is terribly misleading. Some popularizations are better than others, and that one is among my least favorite.
  9. Nov 5, 2015 #8
    Imho, neither the measurement of the detector, nor the "consciousness" of a human being can cause wavefunction collapses - these are merely two variations on the same idea, both containing the same supernatural element. Of course in reality there are no magical collapses at all, it just appears so to us.
  10. Nov 5, 2015 #9

    Some experiments seem to disagree with you and experiments rule out bias and predjudicies

    Reality doesn't exist until it is looked at experimentalists say
  11. Nov 5, 2015 #10


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    Stuff like this is the reason why we have the rule about acceptable sources here, and why scencedaily.com is not one of them.

    That article is terribly misrepresenting (not through malice, but through oversimplification and a desire to make the subject interesting to the casual reader) the experiment and its implications. To get a better view of what the experiment is about, take a look at one of the threads here where it's already been discussed - look for stuff about a recent loophole-free demonstration of Bell's theorem. For the entire backstory, which stretches out over eighty years, you might try our own DrChinese's web site: http://www.drchinese.com/Bells_Theorem.htm
  12. Nov 5, 2015 #11

    Sorry, here is the original peer-reviewed paper the article talks about:


    " Our experiment confirms Bohr’s view that it does not make sense to ascribe the wave or particle behaviour to a massive particle before the measurement takes placehttp://www.nature.com/nphys/journal/v11/n7/full/nphys3343.html#ref1
    Last edited by a moderator: May 7, 2017
  13. Nov 5, 2015 #12


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    Right - and because the word "measurement" (as opposed to, for example, the less precise "looked at") is in there, it does not argue for (or against) a conscious observer.
    Last edited by a moderator: May 7, 2017
  14. Nov 5, 2015 #13

    The 'measurement' in question is availability of which-path information - only conscious observers know and can define what information is, right? I take it for granted that information doesn't exist on its own but is a characteristic of consciousness.
  15. Nov 5, 2015 #14


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    This is a topic that could (and has, to the dismay of the underpaid and overworked PhysicsForums mentors :smile:) generated many hundreds of passionately argued posts delving into the subtleties of the measurement problem. But despite these subtleties, there is a broad mainstream agreement dating from the last decades of the last century, that there are macroscopically irreversible interactions - like moving a needle or putting a spot on a piece of photographic film - that count as "measurements" even before any conscious entity is involved.

    Much of the confusion in this area comes from the way that physicists still use the word "observation", which suggests a conscious observer, even though "interaction" might be more clear. That's a historical accident, dating from the early days of the 20th century before the modern understanding of QM had been formed and "consciousness causes collapse" was more seriously considered. Once a word gets into the language, it's really hard to get rid of it.

    A very good non-technical introduction (although it somewhat glosses over the subtleties I mention above) is David Lindley's book "Where does the weirdness go?".
  16. Nov 5, 2015 #15
    Consciousness can have nothing to do with it, since nowadays measurements are almost always done by computer, with results being analyzed only at a later time. For instance, in a double slit experiment you would still see the disappearance of the interference pattern in real time even though a computer probes the which-path information, where yourself have no idea which path is probed at the time you observe the disappearance of the pattern.
    Last edited: Nov 5, 2015
  17. Nov 5, 2015 #16

    You seem to assume computers are somehow made of classical particles and exist apart of anything quantum but this is demonstrably wrong. The above experiment, if you think carefully, calls into question the existence of so called classical reality in the absence of the faculty of knowing which-path information(aka classicality). Hence the conclusion "Our experiment confirms Bohr’s view that it does not make sense to ascribe the wave or particle behaviour to a massive particle before the measurement takes placehttp://www.nature.com/nphys/journal/v11/n7/full/nphys3343.html#ref1
    Last edited by a moderator: May 7, 2017
  18. Nov 5, 2015 #17
    No, I assume no such thing. I only assume that computers are not conscious in any meaningful sense of the term (at least not yet).
  19. Nov 5, 2015 #18

    You keep referring to something essentially quantum in nature('computers') to explain the very same measurement problem that brings forth the so called computers. This is circular reasoning. Imo you should be able to explain the MP without referring to pretty much unexplained phenomena.
  20. Nov 5, 2015 #19
    But I am not trying to resolve the measurement problem, I mearly point out that the pretty much unexplained phenomenon "conciousness" cannot be the solution.
    Last edited: Nov 5, 2015
  21. Nov 5, 2015 #20
    Doesn't the delayed choice double slit experiment suggest that consciousness is the key to collapsing the wave function?
    As I understand it, a detector records which slit the photon passed through, but the conscious observer decides AFTER the photons have passed the slits whether to look at the detector results or not. The interference pattern depends on the conscious observer's choice, even though the detectors at the slit have been recording all along.
    This was in some youtube video featuring mainstream scientists.
  22. Nov 5, 2015 #21

    You are now contradicting yourself...

    The above means the same as -- 'measurements are almost always done by a collapsed wavefunction(aka 'computer') with results being analyzed only at a later time' .... this is explaining the unexplained by utilizing the unexplained to explain the unexplained. AKA circular reasoning. Hence it can not be an argument against the OP.
  23. Nov 5, 2015 #22
    If you are referring to experiments like this one: http://arxiv.org/abs/quant-ph/0610241 , then conscious observers play no role at all. All decisions are made by random number generators.
  24. Nov 5, 2015 #23
    The video suggested a conscious observer. It even suggested the decision to view the detector results or not could be delayed AFTER the photon has impacted the result screen. In effect, the interference pattern itself was in a state of superimposition until a conscious observer got involved.

    Sorry, I am at work, so can't browse youtube. I will try to find the video later and post the link.
  25. Nov 5, 2015 #24


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    Which youtube videos? Give links so I can downvote them (if appropriate).

    The delayed choice is all done automatically by equipment. The trials happen far too fast for a human to be in the loop making decisions (we can't store photons for long enough for that to be possible, yet). And the experiment would get the same "strange" result independent of someone being in the general area keeping an eye on it.
  26. Nov 5, 2015 #25

    Google which path information in DCQE for clarification why the information about which path the photon took determines whether an interference pattern will be observed.
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