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Wheeler's delayed choice gedanken experiment

  1. Jun 23, 2007 #1
    Hello !

    here is a link towards an article describing an experiment that is a realisation of Wheeler's delayed choice gedanken experiment :


    the conclusion is quite interesting :

    "Our realization of Wheeler’s delayed choice
    GedankenExperiment demonstrates
    beyond any doubt that the behavior
    of the photon in the interferometer
    depends on the choice of the observable
    which is measured, even when that
    choice is made at a position and a time
    such that it is separated from the entrance
    of the photon in the interferometer by a
    space-like interval. In Wheeler’s words,
    since no signal traveling at a velocity less
    than that of light can connect these two
    events, “we have a strange inversion of
    the normal order of time. We, now, by
    moving the mirror in or out have an unavoidable
    effect on what we have a right
    to say about the already past history of
    that photon” (7). Once more, we find
    that Nature behaves in agreement with
    the predictions of Quantum Mechanics
    even in surprising situations where a ten-
    sion with Relativity seems to appear (27).

    Some of the authors claim that this experiment sets some evidence for some kind of "temporal non locality", just like EPR experiments show some "spatial non-locality".

    they also claim that this experiment definitively rules out all interpretations of Quantum Mechanics, except the Copenhagen and the Bohm interpretation.

    Namely, it rules out all "realistic" interpretations of the nature of the wave function, like Everett's or Zurek's.

    Maybe be this subject has already been discussed somewhere else in this forum ?

    Best regards,

  2. jcsd
  3. Jun 25, 2007 #2
    Being new to this forum, this may indeed have been discussed elsewhere and I apologise in advance if this is so.

    I do not really think that the mathematics of quantum physics is a genuine description of the reality underlying our experiments. It is fantastic as a calculational tool but when, as we have here, information is 'transmitted' between space-like separated points I think we have to admit that a reformulation of quantum theory is needed to fully make sense of what is going on. Wheeler's experiment begs the question 'does the wavefunction really exist?' I don't believe it does. I think we need a more fundamental description of quantum phenomena before we can make progress understanding the mechanism by which Wheeler's experiment seems so miraculous.
  4. Jun 26, 2007 #3

    Hans de Vries

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    It was discussed here along with another test involving Bohr’s Principle of
    Complementarity (BPC) which is a subset of the Copenhagen interpretation
    of QM.

    http://en.wikipedia.org/wiki/Complementarity_(physics [Broken])

    Note that we are talking about an interpretation of QM. Bohr stated in this
    interpretation that the photon should either be a particle or a wave, but
    not both at the same time. So it has to make a choice somewhere.

    This is what these test are about. The claim is made that if we hold on
    to BPC then we are able to change the choice the photon made in the
    past: to be a particle or a wave, even if that past is billions of years ago.....
    Well, here you have the original gedanken experiment:


    Now, studying the experiment which you mention one finds that the results
    are conform with what one would expect from classical optics. Now, look
    at Figure 2 on page 4:

    1) A beam splitter generates 2 parallel beams. path 1 and path 2

    2) One beam is horizontally polarized, the other vertically polarized.

    At the end, the beams are brought together and they don't interfere,
    logically, H and V polarized beams do not interfere, neither classically nor
    quantum mechanically.

    Then, An extra section is inserted at the end of the paths. This is the box
    outlined by the blue stippled rectangle.

    What this section does is shown by Figure 5 on page 7: It can randomly
    choose to rotate the H and V beams by 45 degrees. The upper three
    images show what happens without rotation. The bottom three images
    are with a 45 degrees rotation.

    Now, at the very end, There is a so called Wollaston Prism. This is a HV
    filter for the beams. It lets go through photons either at one side which
    are then H polarized, or at the other side, V polarized.


    Case 1, Without 45 degrees rotation

    Without the 45 degrees rotation the H polarized beam follows one path
    of the Wollaston Prism and the V polarized beam follows the other path.
    Again there is no interference as we would expect.

    Case 2, With 45 degrees rotation

    Now, the (original) H beam goes half through the H path of the Wollaston
    Prism and half of it goes through the V path. Exactly the same happens
    with the original V beam.

    Of course, we now do see interference. The V path as well as the H path
    has photons coming from both beams.


    Case 1, Without 45 degrees rotation

    Because we see no interference the authors claim that according to BPC
    the photons (in the past) choose to be particles and not waves.

    Case 2, With 45 degrees rotation

    Because of the interference the authors claim that according to BPC
    the photons (in the past) choose to be waves instead. Now, because
    the choice to rotate the beams by 45 degrees was made at the end
    of the trajectory, the claim is made that they changed the choice the
    photons made in the past.... and thus changed the past.

    My personal comment here would be, first and for all, that in case 1 we
    would not expect interference, even in the case that the photons are
    waves since H and V polarized waves do not interfere. The induction
    that the photons must have behaved as particles would therefor be

    And a second personal comment who be, one I already made, is that we
    are talking about a particular subset (BPC) of the Copenhagen interpre-
    tation of QM. This particular interpretation is for some physicist, including
    the authors of the paper, the only interpretation of QM. While other
    physicist are exploring different interpretations instead.

    There have been many discussions on the interpretation of QM right here,
    and there will be, without doubt, many in the future.

    Regards, Hans
    Last edited by a moderator: May 2, 2017
  5. Jun 26, 2007 #4


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    Here is my personal interpretation of these things:

    I believe that the main lesson from the delayed choice quantum erasers is the following:
    The wave-function collapse is not objective. Instead, other branches of the wave function are still there even after the collapse. The collapse is only an effective description of our subjective knowledge.

    The second lesson (with which not everyone needs to agree) is the following:
    The photon (or electron) is not wave OR particle, depending on the context.
    Instead, it is both wave AND particle all the time, while only our knowledge about it may depend on the context.
    But how it may be logically consistent if it is both wave and particle at the same time? The only logical possibility is that it consists of TWO SEPARATE entities: one is the wave, and the other is the particle. And this is nothing but the main idea of the Bohmian interpretation.
  6. Jun 26, 2007 #5
    (For reference, I'll take the nice description of the http://www.bottomlayer.com/bottom/basic_delayed_choice.htm" [Broken] linked in another message of this thread.)

    In the two-slit experiment, even if no path information is measured, i.e. the photon behaves like a wave, doesn't a single photon still leave a single dot on the screen? Only after a lot of photons hit the screen, a distribution like from interference emerges.

    The interference pattern tells us that a wave is propagating through space. If the wave interacts with our measurement device, it leaves a single tiny spot --- this statement is true for both types of measurement, with and without path information.

    So the question does not seem to be whether the photon is a wave or a particle. It is a wave, but when it interacts with something, the whole energy of the wave is "realized" or "put to work" in one place, either on the screen or in one of the telescopes (see link above). If I
    1. use different measurement devices (screen vs. telescope) or if I
    2. close one slit and thereby mess up the whole setup or if I
    3. try other tricks to measure the "path" of the photon
    I should not be surprised that the wave, when finally interacting, leaves its energy in different places.

    To me it looks like the goal must be to explain how the wave interacts. My certainly naive idea is that if the wavefront hits the screen, there are so many electrons the photon can interact with, that the chosen one is nearly random, except that the wave's interference modulates the uniform distribution to result in the observed interference distribution. If we provide two telescopes instead of the screen, the possibilities for interaction are quite reduced for the wave and we should not be surprised, that it chooses one of them. If we close one slit, well, the result is even less surprising.

    The reasoning is certainly to simple. May I nevertheless ask someone in the know to provide a realistic solution for Maxwells equation that describes a typical photon used in a two-slit experiment. Lets say, before it passes through the slits.

    Last edited by a moderator: May 2, 2017
  7. Jun 27, 2007 #6
    The experiment can be easily understood in a local, realistic and deterministic way if one agrees with the following assumptions:

    1. A photon is emitted only when a suitable absorber exists (transactional interpretation)
    2. The system is deterministic (each particle follows a deterministic trajectory which can be calculated if the past state of the system is known)

    The experiment takes place as follows:

    Step 1: all charged particles in the system (by system I define everything with relevance for the experiment: source, screen, random number generator, etc.) send a message towards the source, at the speed of light, of the sort: “I’m at the position x, y, z and momentum px, py, pz”

    Step 2: The potential emitters in the source “calculate” where each absorber would be at the absorption time (based on the information received at step 1), choose the most suitable one, and send a photon towards the calculated absorber position.

    Step 3: The absorption takes place.


    The random number generator and any other “tricks” one may employ in this experiment are irrelevant. They are all deterministic mechanisms and their behavior is taken into consideration when the source “computes” the absorbers’ trajectory.


    I see no obvious reason to believe that something strange takes place during this experiment. The source of the so-called paradox is a pair of logically incompatible assumptions (causal determinism and “free” or “random” choice) that almost every paper on this topic insists making.
  8. Jul 3, 2007 #7

    Hans de Vries

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    I do tend to agree here, at least with the wave nature.

    The old two slit interference experiments in air or vacuum have evolved into
    ones where both optical pathways can contain lenses, glass fiber, diffraction
    gratings et-cetera.

    One can imagine a particle, without a wave, to follow a straight path in air
    or vacuum, but how would a particle, without any wave behavior, bend itself
    through glass fiber, diffraction gratings and lenses in exactly the same way
    as a wave of a certain frequency would? In these cases we just seem to need
    the wave nature.

    This is a mayor point, which one finds often ignored in discussions on
    "which way" experiments involving Bohr's principle of complementarity.

    Regards, Hans
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