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Observing a photon passing by

  1. Jul 17, 2014 #1
    In a real double-slit experiment, the kind of pattern obtained will be different as an observer (which may or may not be human) is or is not present, transversely placed in relation to the path of photons.
    My questions:
    1. How can we make this observation? This is: How can that observer (human or not) to "make sure" that he has "saw" a photon to pass?
    2. How can a non-human detector register the passage of a photon by a pathway orthogonal to the direction of observation?
    ( I am assuming that we operate in a vacuum)
    (It is clear for me that the screen on which the pattern appears cannot be regarded as being an observer. Am I wrong?)
    Last edited: Jul 17, 2014
  2. jcsd
  3. Jul 17, 2014 #2


    Staff: Mentor

    The issue is much deeper than you probably think.

    The double slit experiment with photons isn't really what basic textbooks make out.

    For example since photons travel at the speed of light there is no frame where it is at rest so it cant have a position which means the screen is not observing the position of the photon.

    There is one and only one way to analyse the situation and that is via QED.

    That said why you would regard the screen as not being an 'observer' in the general sense of observation I am not sure. If you mean in the sense it cant be observing position then yes you are correct. But I am not sure that's what you mean.

    Because of this I think is probably better discussing the double slit experiment with electrons if its issues with it you want to chew the fat about.

    Last edited: Jul 17, 2014
  4. Jul 17, 2014 #3
    That's exactly what I mean.

    No, because electrons can be "observed" using light (photons).
    Please focus on the experiment done with photons.
  5. Jul 17, 2014 #4
    Moreover, the screen is part of the experiment. The observer is not (or it is?).
  6. Jul 17, 2014 #5


    Staff: Mentor

    Electrons can't be observed with photons.

    The interaction of electrons and photons is very complex requiring QED, but you can say 100% for sure in no sense do photons observe electrons. In fact according to QED the electric field around an electron is made of photons (they are the excitations of the EM field - just like electrons are the excitations of the electron field in QED) so your idea makes no sense at all.

    I think you may have been reading too many pop sci accounts. In the double slit experiment the screen is the observer (in the sense of the photon interacting with the screen). Its got nothing to with the introduction of any other kind of 'observer', whatever you mean by such.

    But like I said the photon double slit experiment is not amenable to a simple CORRECT analysis. It requires QED and a discussion of that is way beyond anything at the lay level. But, purely for completeness so you can see there is an answer the gory detail has been written up:

    But from my perspective I have to say that paper is right on the limit of my understanding.

    QFT is HARD, really really hard and my progress in it proceeds slowly.

    The guy Strangerep that wrote the comment in my signature said it well - one finds humility in field theory - I know I certainly have.

    Last edited: Jul 17, 2014
  7. Jul 17, 2014 #6
    What I want to mean is: (I think that) to measure something (position, level of energy, speed...) about an electron we must use photons to interact with.

    My question is about photons, not electrons.
  8. Jul 17, 2014 #7


    Staff: Mentor

    Why you think that has me beat - eg you can see electron trails in cloud chambers.

    As to the other part of you question I know of no way to detect a photon passing by, or even if such a concept as passing by is applicable to a correct treatment of photons.

    They are excitations of the EM field and as such are described in what as known as a Fock space which is something quite abstract.

    Last edited: Jul 17, 2014
  9. Jul 17, 2014 #8
    Sorry, I had no intention of rudeness. Please excuse me.
    Going back to my question: when you do the double-slit exp. with photons with and without observer, do you get the same pattern?
    If not, What kind of interaction occurs when the observer is present?
  10. Jul 17, 2014 #9


    User Avatar

    Staff: Mentor

    Yes. the presence of a conscious observer is completely irrelevant here.

    If the experiment is set up in such a way that it is in principle possible to know which slit the photon passed through, then there will be no interference pattern.
  11. Jul 17, 2014 #10


    Staff: Mentor

    You were not being rude, and I too apologise if I somehow suggested you were. My bad :redface::redface::redface::redface:

    Yes. Observation in QM in no way ever depends on the presence of an observer. That conciousness is involved is a very old interpretation championed by the great mathematical physicist Wigner and equally great mathematician Von Neumann. It was for reasons no need to go into here - if it interests you start another thread and we can chat about it. Von Neumann died early and wasnt around when a lot of work on the phenomena of decoherence was done. But Wigner was and when he found out about some early work on it by Zurek realised it was no longer required and quickly abandoned it.

    However it still seems to have a grip in the pop-sci press and many people have the mistaken view somehow the observer is involved. Even those who read serious textbooks sometimes have it because they do not make clear that semantically in QM observation does not mean an observer. I was in that group for a while until I nutted out it was a fallacy. An observation in QM is, in general, something that leaves 'marks' here in an assumed common-sense macro world. This of course raises issues of exactly what a mark is and how a theory that assumes the existence of such a world explains it. These are legitimate questions, and a lot of progress in sorting them out has been made, but some issues do remain, although of course research is ongoing and it is hoped even they will eventually be resolved, but science often has the habit of surprising, so really who knows what future progress will bring.

    The presence or absence of a concious observer makes no difference.

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