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Recording of data stopping interferrence pattern

  1. Jan 7, 2013 #1

    About mid-way through this video it is explained that the doubleslit experiment was conducted with detectors turned on but simply not recording data, the result of which was an interference pattern. The act of turning the recording component on caused the interference pattern to be destroyed.

    Two questions:
    1. Was such an experiment conducted or is this a simplification of a different experiment

    2. If the experiment was conducted as explained above, is the current explanation that the physical world can somehow determine whether or not data about certain events are being recorded? When the record button is pressed does some kind of force in the universe decide "this course of events will cause the path of the electron to be known, so the electron must go through one detector or the other". It seems this is the consensus, however I want to make sure I'm understanding the situation correctly.
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Jan 7, 2013 #2


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    No, it is not correct.

    It is true that the interference patter disappears if we put a detector next to one slit in a position which allow us to get "which path" information. Moreover. one can make a "hand waving" argument to the effect that this is "caused" by the fact that we gain information.

    However, what matters even in this argument is if we could in principle obtain which path information, i.e. it does not matter if the detector is actually on or not (nor does it matter is someone is watching the signal from the detector etc.)

    Note that this argument needs to be used with some caution, it has nothing to do with "intelligent observers" or anything like that.
  4. Jan 7, 2013 #3
    Is the experiment described in the video non-existent? For the longest time I've always thought the hidden variable theory was still viable - but if the act of pressing a record button can change the outcome of the experiment that would really hurt my head. I don't buy any of the intelligent observer theories and I'm very skeptical of what's stated in the video implying the availability of data determines the distribution of light.
  5. Jan 7, 2013 #4
    Yeah, I'm not sure how much I believe this guy's credibility. It was Max Born, not Schrodinger, who came up with the probability interpretation of Schrodinger's wave function.
  6. Jan 8, 2013 #5
    You are quite right, there is no such experiment. What the guy in the video says from about 2:50 into the video is just plain totally wrong.

    My kind guess is that he heard about the "quantum eraser" experiments, and (mis-)interpreted them too literally. The various quantum eraser experiments are intriguing and easy to over-interpret. In short they show that in some cases it is possible to revert the effect of "detectors" after for example a photon has already passed a polarizer that otherwise would act as a wich-way detector. But fact is this only happens when there is no way even in principle to extract the which-way information, which in effect means that the detectors are turned off (or rather not acting as detectors).

    Actually what destroys the interference pattern is always decoherence. And decoherence depends extremely strongly on how the particle is coupled to its environment. What any detector does is to strongly couple the particle it want to measure to macroscopically different outcomes. For example; current pulse in some cable if a particle is detected, and no current if it is not detected. Otherwise it wouldn't be a detector, right? And once the state of the particle is coupled like that to electrons or atoms in the "detector" (or anywhere!), all interference terms for the single particle are exponentially suppressed.

    What happens in all quantum eraser experiments is that in the cases when no such coupling is generated, the interference pattern is restored. But, as pointed out, this is really equivalent to having the detectors "off". Not having then on and just not recording the data.
  7. Jan 9, 2013 #6


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    Thanks for the explanation . My understanding that the interference pattern in the
    double slit experiment with light is dependent on the light from the two slits to be out of phase and
    be coherent - Well defined and constant phase differences. So your explanation for the disappearance of the interference pattern is the
    interaction of the of the light with the detector where coherence , the well defined phases of the two light sources at the double slits is removed. So there is not any thing mysterious about the interference pattern disappearing with detection. It is a case of cause and effect in the
    realm of classical physics. Could you explain the detector mechanism that causes the
    decoherence in the case of light as a wave and as a photon ?
    Last edited: Jan 10, 2013
  8. Jan 12, 2013 #7
    I was actually trying to explain (very briefly!) the mechanism of decoherence for quantum mechanical systems, i.e. for individual photons or electrons.

    This has to do with the "collapse of the wavefunction" that old school quantum mechanics postulate happens when a measurement is performed. The problem was that for long times there was no clear agreement of what should count as a measurement. And -even worse!- it is easy to show that the linear Schrödinger equation never can take any system starting in a superposition of different possibilities into the single state postulated to be the result of a measurement. This lead (imho) to the confusing interpretations of QM where consciousness supposedly is causing the collaps, or the many worlds interpretation trying to avoid any collapse at all.

    However, our understanding of the measurement process has greatly improved since the work of Zurek and others in the early 1980's on quantum decoherence. It is now well understood how a quantum particle interacting quantum mechanically with an (also quantum) environment effectively drives the state of the particle from a superposition to a classical mixture of definite outcomes. This has the effect that any interference pattern is destroyed, just as in the old wavefunction collapse postulate.

    There is still a philosophical debate whether the fact of quantum decoherence completely solves the full measurement problem in QM or not. But at least it makes perfectly clear that there is no need for any conscious observer to destroy the interference pattern in the two-slit experiment. The only thing needed to collapse the superposition and destroy the interference pattern is that the two paths of the particle leads to different states of any environment. A normal detector is an extreme example of that, where for example one path yields a "click" in a detector whereas the other path does not. The mere presence of such a detector completely kills any interference pattern, regardless if any conscious observer is watching it or not. Or if the signal is recorded or not.

    Quantum decoherence theory goes even further and shows very clearly how fragile any quantum superpositions are. It shows there is even no need for any full macroscopic detector to be present to destroy the interference pattern. Any coupling to anything that evolves differently depending on which path the particle takes is sufficient to suppress (or destroy) the interference pattern. For example, a single photon deflected in different directions depending on if the particle is taking path 1 or path 2 through the slits is enough to exponentially suppress the interference pattern (i.e wash it out in some timescale). The bouncing photon can even fly away completely undetected. The only thing that change if it would interact with anything more (like a full detector setup) is that the destruction of the interference pattern becomes more effective. In this way one can calculate the gradual smearing of the interference pattern from its unmeasured glory to complete wash-out depending on the details of the coupling between the particle and its environment (the bouncing photon, or the full detector).

    So - again - the guy in the video above is simply wrong when he claims that the interference pattern is only destroyed if data was recorded on tape.
  9. Jan 12, 2013 #8


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    I tend to disagree. DCQE experiments rather go the other way around. You start with incoherent light (PDC light is about as first-order incoherent as possible) which naturally does not create interference. Then one needs to perform coincidence counting in order to be able to identify some smaller subset with a higher degree of coherence.

    While I agree that decoherence is very useful and helps emphasizing that conscious observers are not of any importance in quantum mechanics, in this special experiment it plays a minor role. One could argue that it plays some role when one of the photons is detected at the "wrong" detector (the one that gives which-way information) , though. Nevertheless, in DCQE strictly speaking the interference pattern is never destroyed. If one manages to get it in coincidence counts, it is rather reconstructed.
  10. Jan 13, 2013 #9
    I wouldn't take what this guy offers in the way of explanation too seriously.

    Data was recorded, just not via slit obstructions. Slit data requires slit obstructions, which destroys an interference pattern which is the result of unobstructed slits.

    It isn't at all clear what this refers to. In the video he seems to sort of mumble through this.

    The fact of the matter is that obstructing one or both slits will result in no interference pattern being observed. A slit is either obstructed, or it isn't.

    There have been several single particle double slit experiments that show an interference pattern built up through tens of thousands of point-like detection representations.

    It's not even clear what was done. But of course we can determine whether or not data about certain events is being recorded ... by looking at the recorded data.

    This is a totally wrong way of thinking about this stuff.

    unobstructed slits = interference pattern
    obstructed slit(s) = no interference pattern

    Our consciousness has nothing to do with it.

    No, that's not the consensus. It's just some popularized mumbo jumbo that confuses and obfuscates rather than clarifies.

    The results of single particle double slit experiments are not understood. The nature of light and matter remains a mystery.
    Last edited by a moderator: Sep 25, 2014
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