Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Double Slit detectors question

  1. Oct 26, 2016 #1
    My question about the double slit experiment is this: why is it that nobody suspects that the detectors used to detect particles as they pass through the slits in the double slit experiment aren't causing some interference with the experiment which makes it seem as though they are acting like particles? Why can't physicists give a good mathematical explanation for what constitutes "observation"?
  2. jcsd
  3. Oct 26, 2016 #2


    User Avatar
    Gold Member
    2017 Award

    Hi jacobassett!
    If you have not heard about quantum decoherence, you could have a look at it: Quantum decoherence (as a starter).
  4. Oct 26, 2016 #3
    Yeah so why do people think it's so phenomenal that these waves act like particles then?
  5. Oct 26, 2016 #4


    User Avatar
    Gold Member

    Suppose when I was not looking at you, you were an elephant. But every time I look at you, you turn into a human. Wouldn't you think that was weird?
  6. Oct 26, 2016 #5


    User Avatar
    Gold Member
    2017 Award

    I can not really speak for others, only for myself, but I will give it a try... One thing people seem to get hung up on is the so-called wave-particle duality, but this is an outdated concept (see the forum FAQ). One counterintuitive thing is that quantum objects interfere, yet when they are detected they are only detected as discrete units, i.e. as whole, and each object is detected in only one place at a time. Quantum objects neither behave like classical* particles nor classical* waves (* and with "classical" I mean like we are normally used to in our macroscopic world, i.e. they do not behave like billiard balls, marbles or sound waves and water waves). They behave like quantum objects as described by quantum mechanics. A more modern/advanced view is that they behave like excitations of an underlying quantum field in quantum field theory.
    Last edited: Oct 26, 2016
  7. Oct 26, 2016 #6
    Yes I understand that part of it that's not what i was trying to clarify.
  8. Oct 26, 2016 #7
    Well yes but under just the right circumstances there is a situation where that could be true no matter how improbable.
  9. Oct 26, 2016 #8


    User Avatar
    Gold Member
    2017 Award

    Ok. Please be more specific... :smile:
  10. Oct 26, 2016 #9


    Staff: Mentor

    The detectors do "interfere with the experiment"; that's why you get different results when the detectors are present than you do when they are absent. So it certainly isn't true that "nobody suspects" that the detectors have an effect; their effect is the standard explanation for the change in the results.

    Because, first, "observation" is too vague a term, and second, we don't fully understand how the "classical" measurement results we observe are produced from the underlying quantum laws. Decoherence is a key part of the picture, though, and there is certainly plenty of mathematical theory connected with that.
  11. Oct 26, 2016 #10


    User Avatar
    Science Advisor
    Gold Member

    I would certainly say that has been considered. However, there is plenty of convincing evidence to the contrary.

    An example may assist: if you place 2 polarizers, one each, behind the 2 slits, and there is no camera to otherwise record the path taken: If they are parallel, there WILL be interference. If they are perpendicular, there will be NO interference. The reason is that it would be possible to observe the polarization of photons as they hit the screen and determine the path when the polarizers are perpendicular. You don't actually need to take that step though, to eliminate the interference.

    So the moral is: if a photon were going through *only* 1 slit or the other, the relative orientation of the polarizers - parallel or perpendicular - shouldn't matter. But in fact it makes all the difference. And that is because the polarizer orientation affects whether the photon is "wave-like" or "particle-like" (in simple terms).

    Note that there are no detectors. But the quantum rule still applies: if you could have known, in principle, which slit the particle passed through, there will be no interference.
  12. Nov 15, 2016 #11
    The quantum eraser experiments show that it is not due to interference, but to information.
  13. Nov 15, 2016 #12
    "Observation" can be a confusing term. Usually people think of it as detection or measurement. However, an "observer" can also pre-select the boundaries and possibilities of what can be measured by the way they set up an experiment. On top of that, what an "observer" sees is conditioned by their frame of reference. So different "observers" see things differently. As scientists we don't like all that subjectivity so we try to interpret what they see as pertaining to an objective "reality" by involving theoretical concepts. Do you see why it makes sense to break the whole idea down and look at each aspect separately?
  14. Nov 15, 2016 #13


    User Avatar
    Gold Member

    The laws of physics are the same in all inertial frames.
    Observables in GTR are defined as scalar contractions of tensors, which are invariant.
  15. Nov 15, 2016 #14
    Yes, of course. That's a fundamental assumption and our job is to discover what they are. This involves theory as well as experiment and requires us to relate the differing observations of differing observers.

    Which is one of the theoretical tools by which we make those relations.
  16. Nov 16, 2016 #15
    "The quantum eraser experiments show that it is not due to interference, but to information"

    Reference https://www.physicsforums.com/threads/double-slit-detectors-question.890825/

    Yes this can be deduced from the experiment, but how does one gather information without observation, and how does one observe
    (detect) without affecting the observed quantum object.
    The next question would then be. Was the information there(or the same) even though we don't make an observation?

    Back to the same debate , "is the moon still the same as before I observed it".

    Are there any other references in the forum where this has been covered?
    The subject remains unanswered in my view?
  17. Nov 16, 2016 #16
    The fact that we get an interference pattern without detectors is enough ..isn't it?
  18. Nov 16, 2016 #17


    User Avatar
    Science Advisor
    Gold Member

    This is not the case, however. (I mean regarding whether or not there are detectors.) Note the comment about information below.

    Which is not quite correct either. So let me clarify the double slit rules:

    1. You can have detectors on both slits (polarizers work for photons). If they are oriented one way, there IS interference. Another way, NO interference. If the configuration gives you which path information, there is NO interference.
    2. You do not actually need to receive the information. If there is merely the possibility (potential) that the information is "out there", there will be NO interference.

    It is NOT the physical detector disturbing the photon as it goes through one side or the other that leads to NO interference. It is the possibility of learning which slit a particle goes through that is critical. This may not be the only way to state the rules, but it does get some essential ideas across.
  19. Nov 16, 2016 #18
    oh, my last comment is very confusing for this thread, sorry. I merely meant when the double slit experiment is conducted without path information ..we get interference. It's telling use that the particle acts like a wave even if we are observing it from our (real world) scale.
  20. Nov 16, 2016 #19


    User Avatar

    Staff: Mentor

    I have deleted a bunch of posts that were off topic. Please keep this thread to a discussion directly related to the OP's question.
  21. Nov 18, 2016 #20
    The word possibility is for me a grey area, it leads me to a paradox.
    If it is possible, then the information exists somewhere , not so? Whether it was recorded or not.
    So why should this be the deciding factor as to whether the interference pattern was formed or not.
    Is it not more logical to say rather that.

    "The 'process' of making the information available to us , has some influence on whether we measure the interference or not?"
  22. Nov 18, 2016 #21


    User Avatar
    Science Advisor
    Gold Member

    This is a case where the information exists and is theoretically available, but no conscious observer sees it. So there is NO interference, and no paradox I am aware of.

    By *why* is nature that way? I don't think there are good answers for questions like this.
  23. Nov 19, 2016 #22

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    The detectors only ever detect the particle as a particle though.
    Unlike similar experiments with mechanical waves in a fluid (air, water, whatever) the quantum interference at slits experiment never has anyone observe any wave between the source and the detector. You realise you can put the detector anywhere you like, even over one of the slits.

    People conversant with the modern formalism of quantum mechanics do not usually do that - that would imply wave-particle duality, which you have indicated you understood is no longer a thing.

    A lot of the confusion around this sort of thing comes from being a bit glib about what the quantum interference at slits experiment actually does.
    My goto B-level explanation thing is the Feynman lecture on this stuff - as a good starting point. It has aged well and quite accessible.
    ... see all the videos. The entire description uses only the particle model fro a "shut up and calculate" perspective.

    For a more formal "A" description that invokes "measurement" ideas see
    Rothman and Boughn (2010) Quantum interference at slits revisited (including how you get wave behaviour from particles)
    ... but also see the Marcella (2007) paper cited in the abstract for more detail on what is measured (idealized) and where.
    ... they are both very mathy, but it should provide an idea of how the "B" level description is not gonna get you there.
  24. Nov 22, 2016 #23
    Agreed. My point is that. It is in the process of making the information available that causes the interference to disappear.
    Whether that is unavoidable seems to be the million dollar question. Personally I think not.
    And I believe , QM is silent/unclear on this process.

    Totally agreed! The "why" comes from not accepting current interpretations as to "how" this happens.
    Although I don't have one myself..

    Thanks for the insights.
  25. Nov 22, 2016 #24
    Regarding the famous experiment? https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser#The_experiment_of_Kim_et_al._.282000.29

    Would the following be an accurate paragraph.

    If the signal photons are local and 'Alice' records the results at D0.
    But the idler photons need to travel 1 light year to 'Bobs' detectors D1-D4.

    Then the pattern type Alice records is totally dependant on 'Bobs' setup. For example

    1. If Bob decides to remove the Detectors ( D4,D3) and mirrors (Sa ,Sb) during the idler photons flight before the first idler photon arrives
    . Then Alice MUST(have) observe(d) interference.

    2. If Bob decides to replace the mirrors (Sa, Sb) with 100 % reflecting mirrors. I.e. all photons are directed to D4 and D3 , during the idler photons flight.
    The Alice MUST(have) observe(d) NO interference.

    So Alice's result today will depend on Bob's decision in a years time.( Even though Alice will only receive the results 2 years later).
    But the effect on Alice's results depends on Bob's decision in Alice's future??

    Is this correct?
  26. Nov 22, 2016 #25
    So Alice's result today will depend on Bob's decision in a years time.( Even though Alice will only receive Bobs DATA 2 years later).
    But the effect on Alice's results depends on Bob's decision , in Alice's future??

    Just clarifying my statement better.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted