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Particle & Anti-Particle Question

  1. Sep 25, 2008 #1
    hey PF! I've been reading on some time travel on the reality of it.I know that "forward" time travel is possible by Time Dialation.But i was going over something that an anti-particle is a particle "going Back in time" vice versa and that reverse causality is taken into place.I don't understand this is it the particle or anti particle that is going back in time? it dealt something with something called T-symmetry and Feynman's Diagram and on Feyman's diagram i just saw the word "back in time" and though ''Hmm,time travel?"
     
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  3. Sep 25, 2008 #2

    vanesch

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    Look at a similar, old, thread:
    https://www.physicsforums.com/showthread.php?p=1052918#post1052918
     
  4. Sep 25, 2008 #3

    clem

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    The "back in time" is just a Feynman quip about lines in his FDs. It does not happen in the physical world.
     
  5. Sep 25, 2008 #4

    HallsofIvy

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    Frankly, I have never been clear on how much Feynman said was serious and how much was not!

    If that is physically true, it would explain why all electrons are identical- there are all really the same electron!
     
  6. Sep 25, 2008 #5
    Which of the two particles in reality "go back in Time"?
     
  7. Sep 25, 2008 #6

    vanesch

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    You pick your preferred view. There are 3 equivalent views:

    - there are particles and there are anti-particles. Both go forward in time (this is the "normal" view let's say).

    - there are particles ; some of them go backward in time, and appear to us, who go forward in time, as anti-particles.

    - same, but with the identities of particles and anti-particles swapped.

    The point is that these three views have identical mathematical representations. Naturally, one falls on the second one, but it doesn't change anything by "seeing it" as the first, which is what most people do.
     
  8. Sep 26, 2008 #7
    so it's only mathematical, and they don't violate causality or are retrocausal?
     
  9. Sep 26, 2008 #8

    vanesch

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    In order to answer that, one would have to know *exactly* what definition you use for causality and for retrocausal. But if you stick to the ordinary sense of these words, then, no, anti-particles don't do anything with grandpa :smile:, don't violate the second law of thermodynamics, and don't allow you to see into the future.
     
  10. Sep 26, 2008 #9
    As I read Feynman I believe he was willing to treat backwards causality in Feynman Diagram collisions as physically real within strict limitations. That is all Feynman Diagram interactions must resolve themselves within microscopic times and distances smaller than our macroscopic measurement uncertainty. That means no matter how real backwards causality might in fact be; it would remain beyond our ability to observe it as real. (IMO very much like a Copenhagen assumption)

    That requires that electrons, photons etc. remain independent entities along with the virtual particles that are involved with or a part of them.
    Not that all electrons are part of the same one electron
     
  11. Sep 26, 2008 #10
    ok the causality im talking is for example,cause: i jump off a building,Effect:i die falling of the building. retrocausality is causality run backwords for me.is there different definitions of causality and retrocausality?
     
  12. Sep 26, 2008 #11

    vanesch

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    Would you consider that you dying down under retrocaused you to jump off the building ?
     
  13. Sep 26, 2008 #12

    nrqed

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    Wheeler was the first to point that out, when Feynman was still his student
     
  14. Sep 26, 2008 #13
    retro-causality

    NO; That does not describe retro-causality.
    Retro-causality is your secretly jumping off the building – you cannot advise your wife or anyone.
    And after the impact kills you, your wife upon seeing the event decides to buy some extra insurance on you the day before you make the jump.

    It is the insurance purchase that happens retro-causally based on your impact.

    Nothing in our Macro experience indicts that this would happen.
    It may seem like a sensible think for your wife to do – but I’m having a little trouble figuring out how you would confirm the result after making the jump.

    Although Feynman may have believed it true in the micro case, I’m confident he would recommend against this Macro case test.
     
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