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Other (wave) state still there after collapse?

  1. Jun 22, 2011 #1
    since erasure of which-way can "restore" the entangled photons back to the "wave-state"

    can we conclude that the information (of the wave state stays) even during collapse?

    or let me explain via double slit

    if we get which-way....we can expect disappearance of interference pattern

    however if we erase which-way we can expect appearance of interference pattern....

    now if we again get which-way.....we can put the photon back in the "which-way path"

    thus we have to choice to ability to keep switching the photon between "which-way path" to

    "no-which-way" path......it we retain the ablity to change the photon path and "striking position" on the detector (anytime prior to strike/detection)

    this would seem to suggest that the photon carriers both path (blob, int pattern) info (wave/no-wave forces) at all times....

    in other words (and this is just imaginary since we don't know exactly what happens)

    we can move/shift the photon's path....such that it would fall inside the fringes (int pattern) or make a blob (which-way)
    Last edited: Jun 22, 2011
  2. jcsd
  3. Jun 22, 2011 #2
    You could say that something fundmantal and some inherent information about the system is being sent back in time. Is this the quantum eraser experiment by any chance? The transactional interpretation has a lot to say on it, and Wheelers Delayed Choice experiment also shows how the present state can alter the past by observations we make today.
  4. Jun 22, 2011 #3
    1. One interpretation could be (as you said above) - ability to go back in time
    2. another one could be (the photon always kept/carried the information, of all paths......)

    I am looking at both

    1. Double slit without entanglement/DCQE...just a single photon....in this case we have to imagine/assume that a way to erase which-way info has been developed (without use of entanglement)

    2. Double slit with entanglement/DCQE
  5. Jun 22, 2011 #4
    Yes, you can say something instrinsic information remains, even after transactions. It would ultimately make us understand how the universe not only knows what path it will take, but it ''remembers'' it's own environment. But then the universe is something quite different to a particle like a photon.... but it gives you an idea, information can always be retained in QM. Information never disappears.
  6. Jun 22, 2011 #5
    yes....good post goldstone ... information is never "really" erased perhaps only suppressed/embedded......however we can get only one of them at a time (int or no-int pattern) ......and the choice remains with us....till the moment the photon strikes/registers on the detector....

    (side note: we do have the choice of getting mixture too...for example partial int and partial which-way)

    int and no-int are complementary
    position and momentum are complementary
    two sides of the same coin.....

    perhaps they are the same thing......however in different....states/dimensions?
    Last edited: Jun 22, 2011
  7. Jun 22, 2011 #6


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    Really, you should think of there being a context which includes a variety of points in time. Essentially the initial state and the final state. What happens in between is not "real" and will lead to paradox.
  8. Jun 22, 2011 #7
    A paradox? That's quite a stern analysis of the situation, but I don't think it's correct. Thinking about events ''in between'' is like searching for a theory where information travels faster than light somehow. Maybe information can tunnel under the conditions of an experiment, and travel vast distances? That to me isn't a paradox, and only relies on some modifications of how to apply field theory to relativity.

    What I would say is that there is not even any reason to discuss the seperation of the particles, and there is no instantaneous interaction because they where connected before that as well. There is no exchange of information, just events happening according to some determinism within the theory.
  9. Jun 22, 2011 #8


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    Actually, the delayed choice doesn't unambiguously show that .. what it shows is that the results of an experiment on photons depends on the entire context of the experiment between the source and detector. How you interpret the results depends somewhat on your interpretation, but there is an analysis in terms of standard QM that doesn't require or imply any sort of retrocausality.

    An interesting side-point is whether or not the delayed choice can be experimentally achieved for a single electron, as it was for a single photon in 2007 by Aspect. I am interested in this because, as I understand it, massive particles like electrons "experience time" in a fundamentally different way than photons, which apparently can be said not to experience the passage of time at all (I am not familiar with the details of the theory). Anyway, I see no fundamental reason (admittedly I only took a few minutes to think about it) why delayed choice should not work for massive particles, for example in an appropriately designed SG-based atom interferometer. Wouldn't this address the question of whether information "travels back in time" a little more directly? Or would it still be open to the same interpretational ambiguity as the photon experiment? I think it is the latter, but I can't decide if I agree with myself about that ...
  10. Jun 22, 2011 #9

    Well, decide for yourself the photon-thought experiment. A real object existing in our past cone taking all possible paths, it isn't until it hits a detector in present time, does it's wave function collapse, and a single history is made. In all fairness, to me this says a weak measurement was made and created a past a real event!
  11. Jun 23, 2011 #10
    great thought, well put.

    however if we make a measurement in-between, does that then become the starting/end point?
  12. Jun 23, 2011 #11
    Why should it? Isn't the middle something different to the end result? And likewise, is not the beginnning a seperation of ''it'' and an ''end''?
  13. Jun 23, 2011 #12


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    A measurement in the middle (say B) will change the context. In other words: If we have a context consisting of starting point A and end point C, then the context is AC. What happens at B is considered counterfactual and is not definite if we have no way of ascertaining the outcome at B. On the other hand, if we can in principle know the outcome, the context is now AB. So we have AB & BC. Which is different than AC alone.
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