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Double slit experiment with multiple sources of particles

  1. Jul 25, 2013 #1
    Im somewhat new to modern physics, had one introductory class in my university and because it fascinated me so much, i've watched all the leonard susskind lectures on quantum mechanics, entanglement, and particle physics on youtube on the Stanford channel.

    Today I noticed something strange about the double slit experiment. As far as anything I can find, it has only been performed by one individual source of particles.

    We know that you can slow down a source enough to make individual particles go through the slits and if you do not put a detector near the double slit to entangle the particle that you want to check to see which slit it goes through, you observe an interference pattern as if it was a wave that passed through.
    In the case you do put a detector there, you observe two regular bands, as if it was just a bunch of particles that pass through.

    Now what would happen if you have lets say 500 different sources of electrons/photons that you direct at the double slit. The moment you observe a blip on the screen behind it, you switch that source for another source.
    Would you still observe the interference pattern, or again 2 bands as if individual particles passed through?

    I cannot find an experiment like that anywhere on the internet, nor can I find HOW the particles in the experiments with individual "independent" particles are generated.
    This leads me to consider the real possibility that the particles of the single source are in an entangled state and maybe owe their interference pattern to that entanglement instead of to the fact that they have a supposed individual wave-particle duality.
    And thus if you would perform the experiment I just posed, you could potentially observe 2 bands instead of an interference pattern on screen?
  2. jcsd
  3. Jul 25, 2013 #2


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    Welcome to PhysicsForums, Angelus!

    There are a number of issues here. The detector only changes the outcome IF it allows the possibility of learning which-slit information. There are detectors which can be placed in front of each slit - specifically, polarizers - that do NOT change the outcome unless their relative orientation is not parallel. So the physical detector is not doing anything by itself.

    Second, I am not sure what you are referring to when you mention the "entangled" element. Entangled photons do not yield an interference pattern through the double slit setup. I can supply a reference on that if needed.

    But to answer your main question: changing from a single source to multiple sources will not affect the pattern outcome. A light bulb would really be considered multiple sources anyway.
  4. Jul 25, 2013 #3
    I see, thank you!
  5. Aug 18, 2014 #4


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    Hi DrChinese,

    I am new here. I have been studying QM on and off for a few years as a hobby. I have no education in the field. Thank you for answering neophyte questions. My question is 2 part and hopefully not annoying.

    1. If the individually fired photons are not entangled how do the photons know to create the wave interference pattern?
    2. I thought that when the first photons were fired they looked like particles and that only upon aggregation of a significant number did the interference pattern emerge, is this false? Are there example where the first photon lands in a position not in keeping with the slit geometry?

    Thanks Dr C!
  6. Aug 18, 2014 #5


    Staff: Mentor

    Well first a correct analysis with photons really needs QED (Quantum Electrodynamics) since the photons are actually excitations of an underlying EM field - I will let those more conversant with QED answer it from that view. The issue with the usual explanation is photons, because they travel at the speed of light do not have a rest frame, hence not an actual position.

    I will instead confine myself to if they were actually quantum particles in the usual sense, which is the usual way its treated in beginning treatments - its wrong - but we all must start somewhere.

    In that view each photon interferes with itself. However the actual analysis of that is usually the handwavey wave-particle duality - which in itself isn't correct either:

    Instead I will give a link to a better explanation that's sort of halfway between the wave particle duality and QED:

    Notice its not in terms of position, but rather of an uncertain direction of momentum, which is a bit closer to what's going on with photons.

    It is regrettable that right from the start its hard to give the real explanation, and even though I have read a lot of books on QM I don't understand QED well enough to give the answer in terms of that.

    Hopefully however the link will help you in understanding at the beginning level.

  7. Aug 18, 2014 #6
    I disagree. If you shine a laser beam into the double slit then where the pattern appears on the screen will depend on the angle that the beam makes to the normal of the plane in which the double slit is in. In effect the pattern depends on the nature of the details of the beam, i.e. the source of wave packets of light.
  8. Aug 20, 2014 #7


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    Thanks very much Bill. I will read these threads. May I ask you to comment more specifically on question #2? Thanks again! RJP
  9. Aug 20, 2014 #8


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    Okay I take my last question back Bill. I have looked at the progressive wave interference images on Google images and the answer to question #2 is a resounding yes. Sans detection, or dare I say it information creation, single photons land outside classical geometric limits. Hence to wave of wave particle duality. And this is not entanglement because it is a SINGLE photon doing the magic trick. I can't believe how long it is taking me to fully understand this one experiment.

    Can you comment on the reported experiments where detectors on / recorder off = wave and dectectors on / recorder on = particle? and the implications that some draw on this re consciousness or digital philosophy?

    I know it appears I am headed into what is currently the philosophical area, but my question is not... is this apparatus setup/result correct?
  10. Aug 20, 2014 #9


    Staff: Mentor

    That's OK - I will steer clear of the philosophy.

    I don't know the experiments you are talking about but I can confirm that QM does NOT require conciousness of any form.

    For what's really going on at the foundations of QM check out a post I did a while ago - see post 137:
    https://www.physicsforums.com/showthr...=763139&page=8 [Broken]

    A few comments:
    1. Observations occur in a common-sense classical world that exists independent of us - conciousness not required.
    2. The state is simply a mathematical aid, required by the foundational axiom, to help in calculating the probabilities.

    Last edited by a moderator: May 6, 2017
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