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    24 Nov '18 15:47
    The late Richard Feynman mentioned in his lectures (QED) that positrons move backwards in time. Is that merely a popular theory or is there some compelling reason to believe that is what is really happening?

    Does anyone know who first proposed this theory?
  2. Standard memberDeepThought
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    24 Nov '18 18:16
    @metal-brain said
    The late Richard Feynman mentioned in his lectures (QED) that positrons move backwards in time. Is that merely a popular theory or is there some compelling reason to believe that is what is really happening?

    Does anyone know who first proposed this theory?
    This is Feynman's interpretation. Essentially that a normal electron propagating backwards in time would appear as a particle with all its quantum numbers reversed propagating forwards in time. It depends on how one interprets the phase. It's one of the standard interpretations.
  3. Standard memberwolfgang59
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    24 Nov '18 22:02
    @deepthought said
    This is Feynman's interpretation. Essentially that a normal electron propagating backwards in time would appear as a particle with all its quantum numbers reversed propagating forwards in time. It depends on how one interprets the phase. It's one of the standard interpretations.
    Really?
    I thought that anti-particles were represented on Feynman diagrams
    as if they were travelling backwards in time. I didn't think that it was
    proposed that anything actually did go back in time. Am I wrong?
  4. Standard memberDeepThought
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    25 Nov '18 07:29
    @wolfgang59 said
    Really?
    I thought that anti-particles were represented on Feynman diagrams
    as if they were travelling backwards in time. I didn't think that it was
    proposed that anything actually did go back in time. Am I wrong?
    Well, it depends on how you look at it. Our notion of "forwards in time" has to do with entropy rather than anything an individual particle is doing. "It's as if ..." isn't really distinguishable from "It is the case that ..." here - essentially it depends one how one interprets a minus sign in a formula, it's more of a philosophy question than a physics one.
  5. Standard memberwolfgang59
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    25 Nov '18 08:46
    @deepthought said
    Well, it depends on how you look at it. Our notion of "forwards in time" has to do with entropy rather than anything an individual particle is doing. "It's as if ..." isn't really distinguishable from "It is the case that ..." here - essentially it depends one how one interprets a minus sign in a formula, it's more of a philosophy question than a physics one.
    It's physics.
    What is you background? I assumed you were a physicist.
  6. Standard memberDeepThought
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    25 Nov '18 13:21
    @wolfgang59

    Physics is an empirical subject, if no experiment can be performed to answer a question then as far as physics is concerned the question is unanswerable, philosophers might propose an answer though.
  7. Standard memberXYYZ
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    25 Nov '18 15:47
    @deepthought said
    @wolfgang59

    Physics is an empirical subject, if no experiment can be performed to answer a question then as far as physics is concerned the question is unanswerable, philosophers might propose an answer though.
    What if the math behind the thought experiment is solid? Doesn't that answer the question? The physical experiment verifies the answer. Einstein's gravitational lensing is an example. It started with a thought experiment and he showed, with math, what would happen. And was later proven that gravity has a lensing effect on light.
  8. Standard memberDeepThought
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    25 Nov '18 17:00
    @xyyz said
    What if the math behind the thought experiment is solid? Doesn't that answer the question? The physical experiment verifies the answer. Einstein's gravitational lensing is an example. It started with a thought experiment and he showed, with math, what would happen. And was later proven that gravity has a lensing effect on light.
    But that was an in principle measurable effect at the time Einstein thought of it. What we are talking about isn't even in principle measurable. It's an interpretation of the theory, and one which I like, but it isn't the only one, for example Dirac's interpretation (look up Dirac Sea on Wikipedia). Gravitational waves are a prediction of GR and are a necessary part of the theory, backwards in time propagating electrons are an interpretation of the formalism of Quantum Electro-dynamics and not a necessary part of the theory.
  9. Standard memberXYYZ
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    25 Nov '18 17:06
    @deepthought said
    Dirac's interpretation (look up Dirac Sea on Wikipedia). Gravitational waves are a prediction of GR and are a necessary part of the theory, backwards in time propagating electrons are an interpretation of the formalism of Quantum Electro-dynamics and not a necessary part of the theory.
    Interesting read.
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