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Photons interfering with themselves-double slit experiment

  1. Jun 16, 2010 #1
    How can a photon, or any other particle, interfere with itself? What does the uncertainty principle have to do with it? Why can't a device be used to track particles/waves? Please help with real answers! Thanks!
  2. jcsd
  3. Jun 16, 2010 #2
    Question 1. We do not know, perhaps they are not particles after all.

    Question 2. The uncertainty principle is a consequence of the probability wave function, or wave mechanics in general. Anything that exhibits wave like behavior has an associated uncertainty between some observables.

    Question 3. A measurement device can track particles.
  4. Jun 16, 2010 #3
    I ment, when we measure a electron it collapses the wave function. How does that work?
  5. Jun 16, 2010 #4
    Answer that and you'll win a Nobel Prize.
  6. Jun 17, 2010 #5
    haha, I'll work on it.
  7. Jun 17, 2010 #6


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    Don't trust Nisse. You don't get Nobel prize for trivial things.

    And indeed it is trivial. You reduce ensemble with your measurement.
    You separate photons (I would like to use photons instead of electrons) onto different paths for later detection or you do detect some and do not detect other photons.
    If you detect all photons from original ensemble then it's unitary evolution, nothing happens with wavefunction.
  8. Jun 17, 2010 #7
    The wave function is a set of probabilities of finding a particle in a specific location. If we find the particle in a specific location it is no longer logical to specify the probabilities in other space locations but rather set the location we know it to be to 1 and the rest to 0.

    The function collapses, out of definition.
  9. Jun 18, 2010 #8
    ok from what i can intefere from the various topics regarding the doyuble slit exp. is that

    1) Photon causes inference pattern cool

    2) If I fire just a single photon ok just one the It also produces inference :S now that will mean that a photon is passing through the two slits simultaneously ?? I.e that it is existing at two points at the same time :S can some one elaborate ?
  10. Jun 18, 2010 #9
    This has been answered many times (including by me recently)

    Any particle has a wave property associated with it (we don't know how/why) which passes through both slits and is responsible for the interference pattern.

    If you attempt to detect which slit the "particle" passes through you must either collapse the wave to another wave localised near one of the slits (otherwise your measurement doesn't tell you which slit) or you cause the slits' positions to become uncertain (eg by attempting to measure a momentum change of a slit as the particle passes near it) so as to destroy the inteference.

    There are clever alternative attempts to detect "which-way" information such as using an asymmetric slit arrangement and measuring the time of transit to determine which slit, but in all cases the interference gets destroyed.

    It's a puzzle, but it happens, no one can really explain why, the solution is looking like it may emerge from the extra dimensional structure of space suggested by string theory, but no one really knows, and the phenomena is best accepted as a "fact of nature" for now.
  11. Jun 18, 2010 #10
    ^^ every thread has the same answer, as far as what I can gather,

    collapse = 100% probabilty of finding the particle. If we do that then the inference is gone isnt it ?

    So, basically measurement causes the dissapearence of the inference, the seems implausibl :(

    now still even if we dont measure, then the inference means the particle is passing both the slits at the same time IS IT SO OR NOT ? plz answer this specific point as this creates doubt.
  12. Jun 18, 2010 #11
    What is a particle? Plz answer specific point as this creates doubt.
  13. Jun 18, 2010 #12
    ^ photon.
  14. Jun 18, 2010 #13
    ok, well explain what a photon is then :smile:

    You probably won't be able to, since no one knows.

    However you can still say a lot about what photons do, including producing interference effects (over time) in single emission double slit experiments (single emission confirmed by single detection in photomultiplier)

    If you want a state-of-the-art explanation then you need to study quantum field theory, but you still won't get an answer to the question "does the photon go through both slits", you will however get an excellent mathematical model to calculate in more detail what the photon does.
  15. Jun 18, 2010 #14
    Ok so there have been experiments where only ONE photon was fired and it produced an inference OR we cant define what is " ONE photon " so there is no possibilty of conducting an experiment with a single photon ?

    afaik photons r packets of lights isnt it ? I believed that the same single packet of light may somehow maybe split a good way to check this would have been to check the brightness of the inference wrt to the orignal source :)

    as for the QFT i dunno if I have that much time :) I am a law student hence I am really bad with numbers but good with logic so what is do is skip the mathematical part and jump to the logical part I know this is faulty but numbers seems to go over head for me, w/o the I dont know If study of such advanced concepts are possibl.

    Anyways thanx for explaining things to a layman like me :)
  16. Jun 18, 2010 #15
    Single photon experiments have been done for years, here's a slightly modified version where a single pair of entangled photons is emitted (but only one of them travels to the slits) in a clever attempt to get "which way" information (ie. detect which slit a single photon passed through)

    http://grad.physics.sunysb.edu/~amarch/ [Broken]

    of course, this fails (the interference is not seen if "which way" information is obtained) :smile:
    Last edited by a moderator: May 4, 2017
  17. Jun 19, 2010 #16
    ^ Thank you for the link, I clarified some misconceptions and is forcing me to believe in non locality but still I hold the the LHV theory supported by Eientien

    The paper clearly stated that a photon if fired singly and then wont be able to produce the inference but over some time it will i.e the photon is passing one slit but we cant know which one. This is rather strange, I hope a quantum level extra dimention maybe able to solve th rpoblem rather than complex and implausibl theory like the MWI or copehangen.
  18. Jun 19, 2010 #17
    Obviously a single photon doesn't produce an interference pattern, but if you send many single photons one after the other a spatial pattern of dots emerge at the detector, which shows characteristic interference bands. There wlll be bands where no photons ever hit, which is not explainable without single-photon interference. EDIT: In practice the forbidden bands will more likely have reduced intensity rather than zero hits

    The scientific consensus is that Einstein was (clearly) wrong about QM, in that he hoped a local hidden statistical mechanism would explain quantum behaviour.

    The only reason people are still speculating is that the real mechanism is still unknown and is in fact looking like it may be something quite incredible, probably explained by projections from higher dimensional spaces (branes) onto our naive assumption of a 3-dimensional world.

    So that the locality is restored in the higher dimensions but is broken when projected onto our observable 3-D space.
    Last edited: Jun 19, 2010
  19. Jun 21, 2010 #18


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    Single photon is single "click" in photon detector.
    But there does not seem to be practical way to conduct experiment with single photon because you will have difficulties first to fire one single photon and then to detect this one single photon with considerable efficiency (it can end up at many different places on screen and it can hit the barrier not making trough any of the two slits).
    So in any practical experiment you just lower the rate of photons coming from the source.

    If you consider light to be quantized then single photon can't be split.
    Even if you consider that it splits in two parts then why should both parts end up at the same spot on screen?

    Anyways there are performed double-slit experiments with fullerene molecules and in that case you would have to stretch you imagination quite far to consider that the molecule itself somehow splits.
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