Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Copenhagen & Many Worlds Falsified?

  1. Apr 28, 2004 #1


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Professor John Cramer, author of the transactional interpretation of QM, recently gave a talk at Boskone, the famous science fiction con, that sugggests a new experiment may be about to falsify the predictions of the Copenhagen and Many Worlds interpretations(and also decoherence and Consistent Histories). Not surprisingly, since Cramer is giving the talk, the transactional interpretation is NOT falsified by the experiment.

    Here's the Power Point presentation from the talk
  2. jcsd
  3. Apr 28, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    It would have been nice if he had included an exact reference to the Afshar experiment, especially since his whole argument rests solely on the validity of its result.

    It is also strange that he argued that his theory of TI of QM and the formalism of QM both are consistent with the Afshar experiment, but CI and MW aren't. I find it strange because both CI and MW are BASED on the formalism of QM, i.e. they are not separate from it. So how he came up with the conclusion that they offer a result that differ from the QM formalism itself is puzzling. This is because if that is true, then one can LOGICALLY falsify both interpretations without having to do any experiment.

  4. Apr 28, 2004 #3


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Read the presentation again, note his distinction between an interpretation and a model, and also the different responses to Afshar's wire grid. It seems that in spite of the mantra about the equivalence of the interpretations there is indeed a difference, and that the difference is testable. At any rate, if this is not so, the negation needs to be supported.
  5. Apr 28, 2004 #4


    User Avatar
    Science Advisor
    Gold Member

    Apparently, Afshar gave a talk at Texas A&M yesterday on a forthcoming paper: Violation of Bohr's principle of complementarity in an optical "which-way" experiment. I could not locate much on him.

    Unless the HUP is shown to be violated, I am not sure how an experiment is going to detect differences between competing interpretations. Maybe the answer is in the powerpoint from Cramer. But I would object to drawing a distinction between a model and an interpretation. Either way, we are dealing with "scientific theory" regardless of what you call it.
  6. Apr 28, 2004 #5
    I am not familiar with Ashfar's experiment, but there is certainly a difference between the predictions of Many-Worlds and the orthodox von-Neumann approach to quantum mechanics. I will not discuss Copenhagen here, because it probably has enough variants to evade any experimental test you might propose.

    In the orthodox approach, wavefunction collapse is postulated to happen when you make a measurement. Whether this collapse is a real physical process, or just an updating of the information represented by the wavefunction is deliberately not addressed. However, if we can agree that some particular physical process constitutes a measurement, then we should not be able to recover the original superposition after the measurement has taken place.

    In contrast, Many-Worlds postulates no-collapse, so we should be able to undo a measurement if we can control a sufficiently large number of degrees of freedom of the quantum system, apparatus, environment, etc.

    The difficulty of testing this is to actually agree on what constitutes a measurement. Different interpretations can evade the predictions of many proposed experimental tests can by defining the notion of measurement differently. I suspect that a similar issues might arise in the analysis of Ashfar's experiment, but I have to look at it in more detail before I make any further comment.

    Proponents of different interpretations have got very good at making it look like their point of view is the only reasonable one to take. Therefore, I would advise people to take Cramer's view with a pinch of salt until the implications of the experiment have been fully debated. It is worth noting that Copenhagen, Many-Worlds and the transactional interpretation are not the only options on the market and I don't think that the dividing line between theory and interpretation can be made as clear as he thinks it is.
  7. Apr 28, 2004 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Obviously, I'm not the only one who has never heard of this experiment till now. Considering that I troll most of the major physics journals regularly (daily?), I'm quite curious as to where this result was published. Considering the implication of the experiment, such measurement would qualify to be considered for publication in "The Big 3" of physics journals. If it wasn't, why? [.. and all the usual warning bells typically would go off here - can we say "cold fusion"?]

    If anyone can find the citation to this experiment, pass it along. Thanks!

  8. Apr 29, 2004 #7
    It appears that this result has not been published yet - nor is it available as a preprint. I imagine that it is in the process of being written up. Ashfar has just been giving a few seminars about the result to selected audiences and this is apparently how Cramer knows about it. We will have to wait until the paper appears to make any sensible comments. I hope he posts it to arXiv/quant-ph soon.
  9. May 1, 2004 #8
    I was just about to post a topic about this, but lucky for me I searched the forum first. Anyways....

    I'll quote from my source:
    "It has been widely accepted that the rival interpretations of quantum mechanics, e.g., the Copenhagen Interpretation, the Many-Worlds Interpretation, and my father John Cramer's Transactional Interpretation, cannot be distinguished or falsified by experiment, because the experimental predictions come from the formalism that all such interpretations describe. However, the Afshar Experiment demonstrates in an interaction-free way that there is a loophole in this logic: if the interpretation is inconsistent with the formalism, then it can be falsified. In particular, the Afshar Experiment falsifies the Copenhagen Interpretation, which requires the absence of interference in a particle-type measurement. It also falsifies the Many-Worlds Interpretation which tells us to expect no interference between "worlds" that are physically distinguishable, e.g., that correspond to the photon's measured passage through one pinhole or the other." http://www.kathryncramer.com/wblog/archives/000530.html [Broken]
    Last edited by a moderator: May 1, 2017
  10. May 2, 2004 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    The blog did not indicate two things: (1) that there's no references to the Afshar experiment and (2) that it is having problems in the refereeing stage. In fact, unconfirmed reports have indicated that the Afshar experiment report that was uploaded to the e-print archive was removed, something that is unheard of for arXiv.


    Last edited by a moderator: May 1, 2017
  11. Jun 11, 2004 #10
    I see that this thread hasn't been moving for quite some time. Nor has it been read much, lately, either. So, I'll post what I have to say only in brief.

    This is how I see the situation:

    1) The reported results of the Afshar experiment are just what the quantum formalism predicts.

    2) The type of thinking that Cramer employs - in the name of "Copenhagen" - in order to arrive at a conclusion which contradicts the Afshar results is, in fact, a manner of thought which the Copenhagen Interpretation itself deems invalid.

    3) This manner of thought results from taking a rather extreme point of view regarding a notion which must be invoked in any typical discussion of a Wheeler-Delayed-Choice scenario. This type of thinking is inconsistent with the quantum formalism.
    Last edited: Jun 13, 2004
  12. Jun 13, 2004 #11
    Afshar Setup

    The following summary of the Afshar experiment may prove helpful.


    Afshar experimental arrangement:

    A beam of light of fixed wavelength from a coherent source is incident upon the following sequential arrangement:

    - double-slit [1, 2]
    - wire gird [WG]
    - lens [L]
    - particle detectors [1' ,2']

    See diagram.


    Some details:

    1) The wire grid WG consists of wires placed at the positions of the (would-be) interference minima (for a screen placed at that distance from the double-slit).

    2) The lens L is such that rays from slit 1 (2), in the absence of any obstacles, will be focused at detector 1' (2').

    3) With only one slit open, and WG in place, the detector experiences a 6% loss relative to the setup with no WG. WG is therefore said to have 6% opacity.


    Summary of reported results:

    a) No WG + both slits open --> no loss;

    b) WG + only one slit open --> 6% loss;

    c) WG + both slits open --> < 0.1% loss.



    Since the results of c) are very nearly the same as those of a) and, on the other hand, appreciably different from those of b), the situation in c) must be that of "wave-like" behavior (i.e. the quantum system interacts with both slits, and not just one slit). That is to say, two waves are propagating, one from each slit, and in the vicinity of WG these waves overlap and interfere to produce minima at the locations of the wires in WG. For this reason, WG is essentially transparent to the incident beam.

    Attached Files:

    Last edited: Aug 1, 2004
  13. Jun 13, 2004 #12


    User Avatar
    Science Advisor
    Gold Member

    Thanks for supplying this information.

    It doesn't seen so weird to me. There must be a lot of experimental setups which demonstrate the wave nature of light. For example, you can insert a polarizer lens between 2 other polarizer lenses, and at the proper angles get more light from 3 than from 2. This loosely compares to the Afshar setup, which also gets more light with 2 slits than with 1.

    Do you have any idea how Afshar concludes this separates one QM interpretation from another? I cannot picture how the formalisms would yield anything but identical predictions.
  14. Jun 13, 2004 #13
    I think it's fair to say that, in the literal sense, by definition, an "interpretation of a formalism" is something necessarily consistent with that formalism. Of course, in the process of formulating such an interpretation, one could err in some way, and consequently, end up with something which does, in fact, contradict the formalism. Regarding this, I would use the expression "the interpretation fails" in order to indicate that the said interpretation is inconsistent with the formalism.

    With the above in mind, what Afshar and Cramer have claimed[*] regarding "Copenhagen" and "Many Worlds" amounts to the following:

    These interpretations fail on account of certain unwitting implications which contradict the quantum formalism.

    That is to say, Afshar and Cramer claim that the authors of these interpretations have unintentionally "stepped out of line" with the quantum formalism, making assertions which, when given the proper consideration, turn out to have implications which contradict the formalism itself.[**]

    [*] This is only one of two parts of their claim. The other part is that these interpretations fail in relation to what the experimental facts happen to be (whence the significance of the Afshar results).

    [**] Does the content of this post serve to dispel the strangeness about which ZapperZ wrote (in the second post of this thread)?

    Last edited: Jun 16, 2004
  15. Jun 14, 2004 #14
    Something is bothering me in what I wrote above ... in things like:

    How can one calculate what the formalism predicts, unless, at least at some level and to some degree, one employs an interpretation to "guide" that calculation?


    I think it would be better to skip all of the generalizations and go right to the unambiguous particulars of what Cramer purports to be implied by "Copenhagen" and "Many Worlds". Doing that will certainly clear things up.
  16. Jun 22, 2004 #15
    Cramer, in the name of "Copenhagen"

    Let us now establish the particulars of Cramer's argument in relation to the Copenhagen Interpretation.


    Two Measurement Options:

    Take a look at the diagram. It shows a modification of the Afshar setup in which the wire grid WG has been removed. For this setup, consider the following two measurement options:

    (a) Before light can reach the lens, at a screen labeled
    sigma1, measure an interference pattern;

    (b) After light has passed through the lens, at a screen
    labeled sigma2, measure two distinct, separate images,
    situated at 1' and 2'.


    Cramer's Rendition:

    Here is what Cramer has to say, in the name of "Copenhagen", regarding these two options (see diagram):

    (C-a) Measure at sigma1, the interference pattern, giving the
    wavelength and the momentum of the photon;

    (C-b) Measure at sigma2, which slit the particle passed
    through, giving its position.



    By statements (C-a) and (C-b), Cramer purports that, according to the Copenhagen Interpretation, the following is true:

    P1: Options (a) and (b) are experiments which measure complementary attributes of the quantum system.


    Argument for (C-b):

    Cramer's statement (C-b) regarding option (b) is built upon the following type of argument:

    P2: A photon arriving at 1' (2') must have passed through slit 1 (2) and not 2 (1). Therefore, a measurement at sigma2 corresponds to a "determination" of which slit the photon went through.

    Since this type of argument serves as a basis for Cramer's statement (C-b) concerning (b), and (C-b) itself is said in the name of "Copenhagen", it must be that the precise sense in which an argument like P2 eventually leads to Cramer's (C-b) is seen by Cramer to be deemed valid by the Copenhagen Interpretation.


    Cramer's Conclusion:

    In a final step, the wire grid WG is put back into the arrangement of (b) to give, once again, the Afshar setup. With all of the above in mind, Cramer then concludes - in the name of "Copenhagen" - regarding the Afshar experiment (see diagram):

    The measurement-type forces particle-like
    behavior, so there should be no interference, and
    no minima ...

    This statement makes the intended meaning of Cramer's (C-b) absolutely clear. That intended meaning is equivalent to the following proposition:

    P3: In option (b), the measurement performed at sigma2 is physically equivalent to a nondemolition measurement which takes place at the site of the slits and determines which slit the photon passes through.

    To repeat, according to Cramer, P3 (like the propositions P1 and P2 above) is deemed correct by "Copenhagen".

    Attached Files:

    Last edited: Jul 30, 2004
  17. Jun 22, 2004 #16
    So did this experiment ever get published?
  18. Jul 26, 2004 #17
    I can accept this, but how does Afshar's setup determine that photon's momentum when, excluding those points barred by the wires, it could have passed through the lens at any other point with varying probability?
  19. Jul 26, 2004 #18


    User Avatar

    This has gotten into Newscientist as well. I can't claim to be an expert, but I am not convinced.

    Why must a photon arriving at 1' (2') have passed through slit 1 (2)?
  20. Jul 26, 2004 #19
    The Afshar setup doesn't determine the photon's momentum (... except for the magnitude, in that a lens with such-and-so index of refraction focuses the light as it does (... but this information, like that of the alleged "position measurement", is only retrodictive)).

    However, in order for Cramer to make his point, there is no need for a momentum determination.

    And what is Cramer's point? His point (erroneously made!) is:

    According to the Copenhagen Interpretation, proposition P3 is true.

    The Copenhagenist is not convinced either. Here is his response:

    Proposition P2 is a "valid" statement in the following sense: Given that a photon has arrived at 1' (2'), then if a nondemolition measurement had been performed at the site of the slits, the photon would have been found to pass through slit 1 (2). However, in the absence of the actual performance of such a measurement, it is in general incorrect to say that, on account of our "knowing" what the result of such a measurement would have been, the quantum phenomena will be the same as those obtained when the measurement is in fact performed. That it is incorrect to say so follows from Bohr's "Principle of Nonseparability".

    In fact, with a bit of reflection on the two "measurement options", we see that they are not at all "complementary". They were:

    Option (a) is consistent with a "wave picture" but not a "particle picture", whereas, option (b), on the other hand, is consistent with both a "wave picture" and a "particle picture". The two options are therefore not complementary.

    All of this P2-like thinking is due to a "playing" of "Wheel-er's Game of Delayed Choice of Fortune".
    Last edited: Jul 27, 2004
  21. Jul 27, 2004 #20
    Is that really what the MWI tells us? I thought it wasn't.

    I think the claim that the MWI is inconsistent with the formalism of QM is rather strange, because essentially MWI is what happens when you take the formalism literally (it says there's a wave function, and nothing else).

    It looks to me like what has actually been falsified, if anything, is the standard strawman version of MWI that has world-splitting as a fundamental physical process. Am I correct?
    Last edited: Jul 27, 2004
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook