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Double slit: Human vs Machine Observer

  1. Oct 6, 2013 #1
    Would someone help me to understand what would happen in this situation.
    w→ x→ (y)→ z

    Photon emitter is at point w emitting photons one at a time. Double slit is at point x. Screen is at point z.

    There is a detector at point y (after photon has already passed through slits) which is capable of recording which slit the photon passed through. The detector marks this on a piece of paper within a closed box that is inaccessible to any human.

    What would be seen on the screen, an interference pattern, or not? The question has to do with whether it is the actual instrument (detector at point y) that causes collapse of the wave function, or the observation by a sentient being (human). If the human does not see the results, and cannot ever see the result (let's say the paper is burned by the recording instrument after recording before anyone can see the result) would it still result in interference.

    Thanks for your help.
    Last edited: Oct 6, 2013
  2. jcsd
  3. Oct 6, 2013 #2
    No interference pattern.

    The detector (or any other setup/device, for example polarizers, that can provide us which-way) causes the wave-function to collapse.

    Human --- watching or not watching --- does not matter.

    Human/living consciousness has nothing to do with it.

    If the human (eye) can act as a detector -- then it matters.

    In that case the human (eye) caused the collapse of the wave function.

    Whichever device (that can, in principle, provide us which-way) comes first.....collapses the wave function.

    Side note:

    it is possible to have "some/partial" information about which-way and have a "partial" interference pattern

    i.e. it is possible to have say.....80% probability the photon went through slit A and have an interference pattern that is distorted from the normal one.

    as the probability/reliability (of which-way) is increased, the interference pattern gets fuzzier
    Last edited: Oct 6, 2013
  4. Oct 6, 2013 #3
    Different versions have been implemented (look up the delayed choice quantum eraser) and they all show that the detectors have nothing to do with the actual collapse but the availability of the which path information.
  5. Oct 6, 2013 #4
    Definitely no interference pattern. However we cannot assert collapse has occurred yet. What QM predicts is merely entanglement of the photon with the detector by the slit, and the fundamental equation governing this experiment - the Schrodinger equation - predicts no wave function collapse.

    So indeed it may be until a human looks at the detector that it goes through one slit or the other. Or it may be the case, although thought experiments don't support this idea*, that the apparatus causes the photon to go through one slit or the other.

    * e.g. David Albert's book 'Quantum Mechanics & Experience' - chapter on self-measurement; GianCarlo Ghirardi's book 'Sneaking a Look at God's Cards' and thought experiment contained within.
  6. Oct 6, 2013 #5
    So if the "detector" is a human QM predicts that that person would be entangled with the photon? Just like Schrödinger's cat?
  7. Oct 6, 2013 #6

    This is taking it to a street level and it thus can become misleading. What the double slit shows is the uncertainty principle in action - you can not know(not determine, but know) to a high precision both the position(which of the two slits has been traversed) and direction(speed) of a particle(seen as interference pattern on the screen) at the same time.


    So the experimentally verified cornerstone of quantum mechanics(the HUP) already poses quite a challenge for theories positing naïve realism.
    Last edited: Oct 6, 2013
  8. Oct 6, 2013 #7
    You won't get an interference pattern, because in principle you can use the smoke from the burning of the paper to work it out. No joke. Although, I'd be the first to admit that this logic is somewhat backwards.

    If you're not already familiar with them, it's worth looking into quantum erasers though, which are capable of performing the function that you're looking to achieve.

    To entertain the fantasy, as far as I'm aware, it is still unproven and arguably unprovable, whether a human observer is actually required at the end of the chain as the final stage in the detection process to bring the world into reality. Furthermore, it may actually be a very individual thing, where reality is entirely subjective. But you're not going to be able to prove either of those with an experiment like that.
    Last edited: Oct 6, 2013
  9. Oct 6, 2013 #8
  10. Oct 6, 2013 #9


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    When you ask someone else to open the cat's box, that person is "Wigner's friend", I believe.
  11. Oct 6, 2013 #10
    Entanglement equating to measurement isn't the only interpretation. In fact, I don't believe it's even a particularly promiment viewpoint.

    Traditionally, entanglement and measurement are completely different concepts. You'll certaintly find that the two terms are not used interchangably and a description of entanglement is very likely to rely on the concept of measurement.
    Last edited: Oct 6, 2013
  12. Oct 6, 2013 #11


    Staff: Mentor

    Of course. But that's not really of deep import to this situation.

    And as everyone else mentioned you get no interference pattern.

    But I sense perhaps a deeper issue - that you may be thinking because the word observation is often used it needs a conscious observer. It doesn't. In QM an observation is anything that leaves a 'mark' here in the commonsense macro world that exits independent of us regardless of if anyone is around to observe it or not - in the majority of interpretations anyway - this consciousness causes collapse thing is very much a minority view that's even less held than it once was. The high priest of it, the great mathematical physicist, Eugene Wigner, when he heard of some early work of Zurec on decoherence realized it was no longer required and abandoned it. Its still a valid interpretation, but its weirdness leaves many like me cold.

    If you want to get to the bottom of this stuff I suggest Susskinds lectures:

    Last edited: Oct 6, 2013
  13. Oct 6, 2013 #12


    Staff: Mentor

    Not quite so sure about that. It most certainly is not the only interpretation, but most modern writers on the subject seem to take that view - ie most modern interpretations seem to incorporate decoherence in them somewhere - Decoherent Histories, MWI, Ensemble Ignorance Interpretation.

    The following gives a good overview on its effect on interpretations:

    Last edited: Oct 6, 2013
  14. Oct 6, 2013 #13
  15. Oct 7, 2013 #14


    Staff: Mentor

    Sorry - couldn't get past the first couple of minutes.

    QM is NOT magic, it does not break all the rules, and is not behind, well to be blunt, gutter trash, like The Secret.

    From what I can gather she is a history professor. I think Susskind is a better source of what QM REALLY says.

    Had a bit more of a look at it - she has a dirty big slide entitled Quantum Mysticism with reference books like the Tao of Physics - sorry - she is way off the mark.

    The REAL issue with QM is not this QM is magic stuff etc etc. The REAL, the FUNDAMENTAL issue, is we have all these different interpretations that are equally as valid and no way experimentally to tell them apart. Some like De-Brogle-Bohm are most definitely NOT strange in the kind of world view it engenders - they have other issues - but the world view it gives us is not that different to the everyday commonsense view we have. We have other interpretations that fix up those issues but create others like a weird conscious observer created reality. What we don't have is an interpretation that satisfies everyone - that's it - that's all.

    Last edited: Oct 7, 2013
  16. Oct 7, 2013 #15
    I watched a couple of hours of that. She's an excellent speaker and very entertaining to watch, but I think it's clear that she'd be very concerned to be considered in any way, an authority on quantum physics or its interpretations.
  17. Oct 7, 2013 #16
    Thanks to everyone for all of your replies. The delayed choice quantum eraser experiments make me think even more that some "human knowledge" is required for wavefunction collapse. http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

    In this case we have two different instruments acting on a photon and the act of "measurement" does not cause the collapse since when both splitters are present there is no collapse. This leads to a couple of conclusions in my understanding.

    1. The act of measurement does not matter since the presence of two splitters cancels out the wavefunction collapse
    2. Only the potential knowability matters (i.e. having the potential to look and see for sure which pathway was taken)

    This importance of knowability for the collapse to occur makes me think that a sentient observer must be present since the quantum eraser experiment shows that an instrument alone cannot cause the collapse if the information is later erased.
  18. Oct 7, 2013 #17
    If you want to incorporate sentience into your interpretation, first you're going to need to be able to define sentience. It's not as easy as it first appears.

    In the process of doing so, I expect that you'll realise that the only reliable sentience, in that respect is oneself, à la Decartes, if you pardon my French or that an arbitrary macroscopic object performs the role equally well. In which case you abandon the role of sentience and look to decoherence for a solution to the measurement problem.
    Last edited: Oct 7, 2013
  19. Oct 7, 2013 #18


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    This is incorrect. You do not cancel out collapse. The beam splitters never actually perform a measurement. The measurement always happens at the detectors as does the collapse. There is no irreversible interaction prior to the detectors and irreversible interactions are the things that count and constitute measurements. You cannot erase a measurement that has been performed, only the possibility to do a measurement giving you some certain kind of information. These are very different things.

    This is correct, but it is evidence for the opposite of what you make out of it. "Potential knowability" is just a measurement (irreversible interaction) taking place.
  20. Oct 7, 2013 #19
    That statement comes across as if the measurement problem is solved.
  21. Oct 7, 2013 #20


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    It was not intended to. I was just aiming at contrasting reversible interactions (like using a half wave plate, shifting the phase of a light beam and such stuff) to irreversible interactions (absorption, incoherent scattering and so on). Leaving all the questions of the measurement problem aside, it is pretty well agreed upon, that the latter actually constitute a measurement, while the former do not.
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