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Double slit interference destroyed by observer or by photons

  1. Jul 26, 2010 #1


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    When the electron double slit interference pattern is destroyed during measurement detection, to determine which slit the electron passed through, the explanation is that the observation is responsible.
    Given that detection is done with photons that can interact and alter the path of the electrons.
    Why is this not the primary explanation ?
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  3. Jul 27, 2010 #2


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    This is the explanation, primary or not. The observation of which-way information unavoidably destroys the interference pattern in the doubl-slit-experiment and a setup such that one can observe the interference pattern necessarily implies less knowledge about the which-way information.
  4. Jul 29, 2010 #3
    Because an observer, whether physics likes it or not, is the sole decider how the particles behave. If there was no observation, there can be no universe.
  5. Jul 29, 2010 #4


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    This experiment can be done in a variety of different ways. The rule that determines whether there is interference or not is:

    Is it possible, in principle, to know which slit the particle passed through? There is never interference if the answer to this is YES.

    It is possible for the information to be obtained and then completely "erased". If this is done, then interference is possible. This is sometimes done in setups in which light is sent through slits with polarizers behind them (some settings provide which path information). More information can be supplied on this, if that will help. But the point is that when you look at all of the double slit experiments, the above rule is what jumps out.
  6. Jul 29, 2010 #5
    Now I know you have a healthy and justified respect for Mermin, and focus your primary attentions on the phonomena of entanglement. Yet you fail to see the relationship between entanglement and diffraction.
    Pretend all you wish, but the properties of electromagnetic radiation are all too simple; so simple, in fact, that spooky action at a distance is intrinsically related with "wave/particle" structure--properties of quanta.
    You've been avoiding this relation between the two for too long.
  7. Jul 30, 2010 #6
    That is so vague as to be meaningless, please clarify.
  8. Jul 30, 2010 #7
    Okay this is my first post here so if I'm way off then just tell me. :blushing: Can't you time the measurement so that it's being executed (or not) after the electron must have crossed the slit-barrier? To me that kinda suggests it can't be explained by 'common sense' notions of physical interaction, because it would appear as if the speed of light laws had been violated. Conversly, thinking about it as a quantum phenomenon related to the concept of Information makes better sense, and fits more snugly into the framework of our modern scientific understanding.

    Would you? I'd be interested in hearing more about such experiments.

  9. Jul 30, 2010 #8


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    I happen to think you are correct about this. Entanglement and wave/particle duality are related by the Heisenberg Uncertainty Principle - which I hold as fundamental to Quantum Theory. You could, with some justification, say that a single particle is everywhere when its momentum is precisely known. That's spooky to me.
  10. Jul 30, 2010 #9


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    Welcome to PhysicsForums, eloheim!
  11. Jul 30, 2010 #10
    Hah, thanks. I'm basically a lay-person here, but the double-slit experiment has a special place in my heart. I remember reading about it for the first time (in Brian Green's Elegant Universe I believe), maybe my freshman year of high school, and just having my mind blown. Especially the idea that even if you released the photons one at a time, even with periods of years in between, they would still build up that famous interference pattern. To a 'realist' like myself, it felt like discovering that magic really existed. I suspect that's as close as I'll ever come to having something like what people call a 'religious experience.'
  12. Jul 30, 2010 #11
    Do you believe that it is possible to know the momentum of a particle precisely?
  13. Jul 30, 2010 #12


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    The following is an exact quote from Halliday-Resnick from the chapter on
    Waves and Particles , the section on the uncertainty principle with electrons passing through a slit:
    "Here too we can hope to "see" the electrons only if we reflect light or another particle
    from it. In this case the recoil that the electron experiences when the light (photons)
    bounces from it completely alters the electron's motion in a way that cannot be
    avoided or corrected for "
    It follows for me that since the above type of measurement/detection is done during the double slit experiment ( before the electron/particle/wave) reaches the screen that it is photons that destroy the interference pattern.
  14. Aug 14, 2010 #13
    By emitting a photon the electron is leaking information about the wavefunction into the em field which measures and records the electrons path. It is whether enough information is recorded or not that causes the wavefunction to collapse (or appear to collapse). High energy electrons will be less likely to form an interference pattern as more photons are being lost to the field. So consciousness is not required but that doesn't mean it has no role to play.

    If we imagine a macroscopic object which is yellow in colour, say a melon. The melon exists as all it's quantum particles are being constantly measured by it's environment. However, Our conscious observation is capable of putting the last piece of the jigsaw in place as it is our consciousness that made it look 'yellow'.

    In other words, if there is a conscious observation of the system then it will be entangled with the system and you will have to include it as part of the system you are measuring, but with no conscious observer then you just get a colourless and tasteless melon.
  15. Aug 15, 2010 #14
    That's a long way of saying that the moon is absent when it isn't viewed, and no less wrong for that.
  16. Aug 15, 2010 #15
  17. Aug 15, 2010 #16
    The Einstein-Bohr argument about "wlegher weg' (Which Way, or WW) lead Feynman and Hibbs write (p. 9 of teh newly awvailable Dover edition):
    "We shall state the uncertainty principle as follows. Any determination of the alternative taken by a process capable of followingmore than one alternative destroys the interference between alternatives."
    This can be written
    [UP]<=> [WWDetermination => no interfeerence]

    As DrC tells us, WWK (K=Knowledge)is not WWD (D=Determination). WWD is subjective while WWK is objective (no one has to know but the determination exists somehow: perhaps one "could know" but anyway D is not K)

    Now this statement of Feynman and Hibbs and others is very false a I will show later
    if no one takes the charge of that (but some people know that,although Einstein, Bohr, and otehrs missed the point, while Dirac quite probably knew at least about the WWK
    case but WWD is not really harder: see Dirac book editions of 1968 and later: in 1930, no way to deduce that from his manual, at least I cannot make this deduction,

    Now, erasure is another matter: in general, after erasure, one does not get usual interferences but interferences "in coincidence" with another signal: very impoertant istinction since otherwise, super-luminal signaling would be possible. Such matters have been discussed for one of the main experiments of delayed erasure in thread but I think we should have a thread where we discuss all the examples of erasure beside what is discussed here. Perhaps this can be the that thread, but I would like to bring back teh participation of some contributors to the former erasure thread.

    I have to stop but will watch this thread.
  18. Aug 15, 2010 #17
    This is essentially the Bohr 1927 argument that serves as basis to the statement of Feynman-Hibbs p.9 Dover edition that I have quoted in my previous post, This misses the point that for "general nonsense" reasons, path determination prevents interference. Dirac new that with a single path, only one phase get involved in the summation, thus no interference is possible. Neither Einstein, nor Bohr, nor many of the followers (there is an abundant literature of WWD , with Scully et al. realizing that the UP was not the key to interference suppression, but then invoking another specific reason using another method (in fact several of them) to mark the path(s)). I will upload some of that.

    Notice that the UP type computation is correct but irrelevant and even misleading: there is no blurring of the interference patterns, but since single phase means no summation, there is only the superposition of the distributions for the two holes.

    This is only part of the legends and false tales associated to interferences, with or without delay, with or without erasure. An interesting example of experiment misunderstood by the author and most writers on that matter was done by Afshar et al: they thought that they had an example where one has WWD and interferences at once. The number of different arguments given against them is almost funny. Notice that, according to Bohr in the Schilpp book, and some comments from Heisenberg on the debate at Solvay 1927, Einstein also thought that with the recoil story he had an example where interferences and WWD coexist.

    I'll detail any part of that than would be requested (with the time needed depending on how many question arise, if any).
  19. Aug 16, 2010 #18
    Does this imply that a single slit diffraction is actually a superposition of two 'half' slit distributions? By that I mean two 'abstract' half slits as opposed to just two slits half the size of a single slit????
  20. Aug 22, 2010 #19
    I wrote:
    Notice that the UP type computation is correct but irrelevant and even misleading: there is no blurring of the interference patterns, but since single phase means no summation, there is only the superposition of the distributions for the two holes.

    Then you quote part of that and ask a question that is absolutely obscure to me. Truncating a statement to use that as a quote is not a scholarly very acceptable behavior; since this is a main problem for contemporary advanced physics (the physics most cited by other physicists, I cannot say that this is characteristic of Physics Forum (I guess that many people like me do not yet know how to get a part of a post as quote), bur here, I am at lsot when confronted to a question that supposedly originates from a quote of of one of my posts.
  21. Aug 25, 2010 #20
    As Dirac understood: one path means no summation, hence no interference. The details may change depending on the way you mark the path, but there is here a triviality that has been missed by Einstein, Bohr, Heisenberg (at least ) at Solvay 1927, and later missed by many and in particular by Feynman-Hibbs (somewhat repeated in Feynman Sand Lectures)
    and in 6 easy Pieces of Feynman (I think) as they write something false there about the UP becasue they missed the triviality of:
    one way=> no interferences.
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