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Do antiparticles really go back in time?

  1. Jul 31, 2006 #1
    do antiparticles really go back in time?
     
  2. jcsd
  3. Jul 31, 2006 #2

    vanesch

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    I think it is fair to say that we don't know and that this is a question, related to the ontology (the interpretation/the picture) one gives to a physical theory. The mathematics suggests it at first sight. It is fun to think about it that way. But that doesn't mean that things really are that way. It's maybe a wrong interpretation of a formula.
    It is not because a formula is suggestive of something, that things have to be that way. After all, physical theories are models of reality, and we like to think of them as "toy universes", but nothing tells us that these models are reality.

    That said, it is often good thinking practice to "live oneself" into the toy universe given by the mathematical model, in order to devellop an intuition for its formalism.

    At least that's how I look upon these things.
     
  4. Jul 31, 2006 #3
    I agree with the "toy universe"-vision but if models give correct predictions then we know they are right (within a certain physical regime). So, in that case, if the mathematics suggest that "particles go back in time" we can/must conclude that they actually do this, no ? At least that is how i look at this. Of course, i agree that "going back in time" has a somewhat mysterious aura to it but the mathematics involved clearly show what "going back in time" is supposed to mean.

    regards
    marlon
     
  5. Jul 31, 2006 #4

    vanesch

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    Yes, but "going back in time" then simply means what the mathematics says, so that both concepts ("going back in time", and the formula with the inversed sign of Exp(i E t) ) become identical.

    However, in natural language, when talking about "going back in time" you get all kinds of "back to the future" visions, and maybe the wrong idea that if you made a car out of anti matter you might drive back in time to say hi to your great grand pa when he was a kid and so on... this is then of course delusional.

    So it is a matter of abstraction.
     
  6. Aug 2, 2006 #5
    Richard P. Feynman told this in Feynman Diagrammes didn't he? He says a particle which is going forward in time is equal to an antiparticle which is going back in time. Am i right?

    Note: I don't speak english very good, i am a turk so forgive my mistakes
     
  7. Aug 2, 2006 #6
    Yes, he did say that. But all you can really infer is that the mathematics of each situation will give the same predictions. The best you can ever do is show that a theory models reality within experimental precision. Whatever interpretation one dreams up and claims to be "really" happening can never be verified or falisified.

    edit: One could try and determine a method of propagating information this way. If it were feasible, and I feel quite sure that it is not, then one could send a message to the past. This would add a lot of credence to the idea that something travels backwards in time.
     
  8. Aug 2, 2006 #7
    Will a letter written in anti-Ink on Anti-paper arive yesterday by first class anti-royal mail?
     
  9. Aug 2, 2006 #8

    vanesch

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    No, that's the point. The anti-letter will arrive tomorrow, destroying a normal envellope, which will not be there anymore the day after tomorrow. So you should rather say that it is the envellope that decided to turn back in time tomorrow, to arrive today onto your desk as an anti-letter you are writing with anti-ink.

    uh...

    In Feynman speak, fermion lines are un-broken in a diagram, but can go back and fro in time. The pieces that go back are then re-interpreted as "anti particles going forward", and the "annihilation" between matter and antimatter is the interpretation of "the turning point where the fermion line went from forward in time to backward in time".
     
  10. Aug 3, 2006 #9
    I think he is being facetious, pointing out the illogicity of the situation.
     
  11. Aug 3, 2006 #10

    vanesch

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    Maybe, but then it misses the point. It is not the anti-matter that goes back in time, it is matter going back in time that appears to us as anti-matter (if you follow the mathematical formulae).

    The way in which this makes sense is what I tried to illustrate:

    A particle (fermion) that goes forward in time, looks to us (who are bound to go forward in time), well, as a particle that goes forward in time.

    A particle that goes forward in time, stops, and turns around, going backwards in time, looks, to an observer going forward in time, as:

    TWO things that go forward in time, come together, and disappear. One "leg" is the particle, normally going forward in time. The other leg is the same particle, after it turned around, going backwards in time, but now seen in the opposite way. That's what we call an "anti-particle". It is a normal particle, going backwards in time, that we look upon as another thing, going forward in time. This is the mathematical manipulation. In how much one has to think of this as "real" is a matter of taste of course (see the beginning of this thread).
     
  12. Aug 3, 2006 #11
    Yes i understand you, thanks for that.

    So to correct that statement:

    A letter written in on paper that arives yesterday by 0th class royal mail (this is the new class stamp that will be developed in 2142) will be seen by mail workers as an anti-letter?
     
  13. Aug 3, 2006 #12

    vanesch

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    Yes (although this is of course a crazy example :-).

    Today, the mail workers will see it come by as an anti-letter, which, in one way or another, avoids annihilation with a normal letter until some day in 2142. We can then say that the normal letter with which it annihilated that day, is in fact the "anti-letter", and the point of annihilation was where this letter "decided" to go back in time. So at an earlier time slice, like, today, we see two things go forward in time: the letter, and the anti-letter.

    It might be that the letter, at some point, would decide to go forward in time again. Say, yesterday. It would be observed as a letter-anti-letter creation.

    In the case that the forward-going letter is the same as the one the anti-letter is going to annihilate with in 2142, we have a vacuum bubble :-) (or better, a 1-loop correction to some radiation: the one that created the pair, and the one that resulted from the annihilation).

    With letters, it is of course totally crazy, but that's exactly how Feynman diagrams are working. Most of the time one doesn't see this however, because usually, Feynman diagrams are worked out in momentum space and not in position space. (one is the fourier transform of the other).

    In how much this is "real" and in how much we're simply over-interpreting a formal technique is not evident of course.
     
  14. Aug 3, 2006 #13
    Of course its crazy, science is crazy! :surprised

    One day it may be possible to talk to me today.....erm...:bugeye:
     
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