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Quantum delay

  1. Feb 5, 2009 #1
    Just thought I would get a couple of opinions from you guys on something I find truly amazing. In Brian Greene's Fabric of the Cosmos he explains how the delayed quantum measurement experiment works.
    A single photon has already passed through a beam splitter and made the 'choice' of whether to go down both left and right routes or to just act as a particle and use only one route. If we turn on the detector a certain amount of time after the photon has passed through the splitter we find that the particle had chosen to take only one route as one individual photon (or electron). But if we did not turn on the detector then the photon would evidently have 'chosen' both paths, and contributed to an interference pattern. This is strange enough, but what I find more amazing is this....
    When the photon actually passes through the splitter and is not being observed or measured, it must decide to choose both paths at that point. Then, a certain time after it has chosen this we turn on the detector and suddenly find the photon has chosen only one route. So how can the photon choose both routes, then when it is measured long after it has made this choice suddenly be only in one place? And surely if it is measured and found to only take one route, then it must have only took one route from the initial emission. Even though it could not do this! Could somebody elaborate or help me in some way with this, as it really has baffled me. Greene mentioned something along the lines of the future affecting the past, and to be honest I myself see no other option. I apologize if my terminology is incorrect, but hope people can see where I am coming from.
    Thanks for reading ,
  2. jcsd
  3. Feb 9, 2009 #2
    Yes, all observings are done at screen end so not matter what happening before. Look at it thinking what I say you.
  4. Feb 9, 2009 #3
    I don't remember that but I also have that book. Could you direct me to the pages in question?
  5. Feb 9, 2009 #4
    No, I not saying book. This one point telescope at slit or look at screen all the time. Means choosing state and measuring. No matter what happen before - particle no place and wave no places. ONLY when measures.
  6. Feb 10, 2009 #5
    Thanks for the replies, and the place in the book is under the chapter, The Contingency of History. Starting on page 186.
  7. Mar 2, 2009 #6
    This bizzare phenoma is a result of the linear superposabilty of solutions of the Schrofinger equation, i.e. if you have two (or more) solutions of the Schrodinger equation, multiply them all by arbitrary constants, add all of them together, and the resulting function is still a sol'n of Schrodinger's equation. Translated into 'real world' terms, before we make a measurement and collapse the wavefunction, it is a linear superposition of all possible states. When we make the measurement or observation, we collapse the wavefunction, and the only state that remains is the one we measured.

    Hope this helps!
    Karl G.
  8. Mar 3, 2009 #7
    Thanks for the reply Karl. Basically then, the photon or electron takes an infinite amount of routes to its destination and when we decide to measure the outcome only one of the 'choices' becomes reality and the others become non existent. So to simplify things the photon took routes one and two, and as we measured it to have took route two, then the route one choice somehow is cancelled out. So our observation determines the actual outcome, and also erases certain histories that could have been. In fact histories that must have actually happened. The photon took route one, but our recording of it having taken route two somehow erases the route one event. This amazes me, and I find it incredibly difficult to comprehend. It just goes to show me that reality is not what it seems.
    Thanks again,
  9. Mar 3, 2009 #8
    To say this behavior is bizarely counterintuitive is a drastic understatement. Coincidentally, I was just reading about the photon double-slit experiment today. To qoute from Claude Cohen-Tannoudji's QM text (which I highly recomend!) : "It is impossible to observe the interference pattern and know at the same time through which slit each photon has passed."
  10. Mar 4, 2009 #9


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    The reality is strange, as you say. There are a couple of interpretations of QM that lend themselves to making some sense out of this (of course as always with a cost):

    New Insights on Time-Symmetry in Quantum Mechanics
    http://www.npl.washington.edu/npl/int_rep/gat_80/ [Broken]

    Both of these posit that influences from the future affect the past. So in a delayed choice setup, you are seeing just that. Of course, there is still no possibility of FTL influences or causal loop problems in these interpretations as they still follow standard QM in their predictions.
    Last edited by a moderator: May 4, 2017
  11. Mar 5, 2009 #10
    And then, of course, there is Bohm's 'hidden variables' to resolve the Bell Paradox, which is as strange as (or stranger than!) the photon's trajectory mentioned above
  12. Mar 5, 2009 #11
    No, no it wavefunction travel NOT photon. You think wrong of this.
  13. Mar 5, 2009 #12
    DrChinese, thanks for the links. Although to be honest I was a little overwhelmed by the mathematics. Could I ask your opinion as to whether the future measurement actually does affect the past in such a case? Could it be possible that our conscious and sub conscious minds actually give us a false impression of the flow of time, so when we get down to the quantum world things appear very odd?
  14. Mar 6, 2009 #13
    True! True! My mind is still accustomed to thinking of particles or waves, not wavefunctions!
  15. Mar 6, 2009 #14


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    There may be a good reason why we perceive the flow of time as being in one direction only. But if so, we don't really know yet. The laws of physics are essentially symmetric as to the time direction, just as they are to left-right or up-down transformations. So a time symmetric interpretation would have a certain appeal even though it would not explain why we cannot travel (or signal) back in time.

    So to answer your question: I believe that a future observational state has some influence on the present (or past). This could be deduced from the results of Delayed Choice Quantum Eraser experiments (although I acknowledge that other explanations are possible). I am not aware of any experiment that would tend to take away from that conclusion, and it certainly explains a lot while respecting relativity. That is just my opinion, and I would not say that it is generally accepted (on the other hand, there is no generally accepted interpretation).
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