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I What is the value of "delayed choice" experiments?

  1. Jan 1, 2018 #21


    Staff: Mentor

    Well here we face an issue. I want a precise concise statement of the principle. I found one and show it falls to pieces. You say it was Bohr's way of saying what it is cant be expressed in English and this was an attempt to try and convey something of its mystery in English. But the math tells us exactly what it is. Just use that - forget wishy washy 'slogans' that fall to pieces when looked at closely. Maybe that's why Bohr and Dirac clashed.

  2. Jan 1, 2018 #22
    In case somebody doesn't understand something doesn't mean that this something is some esoterical philosophy.
  3. Jan 1, 2018 #23


    Staff: Mentor

    That's true. It is well known Bohr spoke softly, thought on his feet, retracted statements as he was thinking, and tried to be very careful and subtle about what he said. Without doubt Einstein, his good friend understood what Bohr meant but he never did express his thinking as clearly as Einstein. BTW Einstein wasn't always right - but he was clear. Bohr tried to be right but clarity was not his strong suit.

    I have no doubt Bohr knew what he was trying to say, it just wasn't expressed in the way more mathematically oriented physicists would like. In fact he sometimes clashed with that type - he clashed with Feynman on one occasion (at Shelter Island where Feynman was explaining his methods) because he couldn't follow Feynman mathematically and didn't understand what he was saying. He totally clashed with Dirac.

  4. Jan 1, 2018 #24


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    Science Advisor
    Gold Member

    My viewpoint probably isn't *that* much different than yours.

    I would guess definitely more than 100 actively performing theoretical work on MWI, Bohmian Theories, RBW, etc.; but admittedly a very small group in total. And a much larger group nipping away experimentally at the fringes of various interpretations. And by that I mean *top* teams doing work on delayed choice, erasers, entanglement of particles that have not interacted in the past, and similar. Precisely the kinds of things that might eventually lead to ruling out an interpretation**. I would certainly say those teams, while not specifically favoring one interpretation over another, are quite interested in locating potential areas of subtle differences between interpretations. So in that sense, you're right, the group actively working on this is relatively a small proportion of physicists in total. That is no surprise in and of itself.

    And I would certainly agree (with your basic idea) that past the above, few working physicists lose sleep at night over interpretations. Nonetheless, I stand by my comment that "most scientists believe that a better interpretation will eventually lead to better predictive/explanatory power." Simply for the fact that I think most scientists believe a better interpretation will one day arise. And in fact I think that has already happened, and continues to happen.

    EPR's entanglement (1935) brought us to a stalemate on whether QM was a complete theory. Bell (1964) and Aspect (1981) broke that stalemate. And recent experiments (last 20 years) have created a new kind of EPR: Entanglement of particles that have never been in a common light cone (no causal contact). If only EPR/Einstein could have witnessed that! I'm sure that would have been inconceivable to both Einstein and Bohr.

    Has the "standard" interpretation of QM changed in the last 80 years? I'd say so, and thus we really do have a better interpretation. And I would also say that some of the interpretations might be better classed as full-on theories anyway. (Although obviously the ability to predict consistent with "standard" theory is a critical factor.) I think better interpretations are around the corner, and there will be interpretations falling by the wayside going forward. Of course, that part is just my opinion. But I don't think it's really much different than the belief of many others either.

    **There are those, for example, that believe most Bohmian-type theories have been ruled out - but that is not universally accepted. And certainly not by Bohmians. :smile:
  5. Jan 1, 2018 #25


    Staff: Mentor

    I have a book - Conceptual Developments in 20th Century Field Theories. It says progress comes from asking the right question - that's the key - finding the right question to ask. String theory certainly proved an interesting question to ask - and its still going - but I think its now generally believed its not the right one. It may eventually suggest the right one - but so far we are all waiting.

  6. Jan 1, 2018 #26
    That's the point! As Carl Friedrich von Weizsäcker says in “The Structure of Physics”:

    Bohr’s thinking was never based on mathematical structures, on what physicists somewhat condescendingly call the formalism, but rather on the classical description of experience and an unrelenting examination of the meaning of concepts.
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