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Weak interactions: I know why they're weak, but why are they slow?

  1. May 28, 2012 #1
    Compared to, for example, electromagnetic interactions.

    Are the "weakness" and "slowness" perhaps somehow related? Perhaps both "weakness" and "slowness" can both be reformulated as "having a low probability of occuring", making the "slowness" apparent, and making the "weakness" a time-averaged result.
  2. jcsd
  3. May 29, 2012 #2


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    See e.g. the Fermi rule: The probability PER UNIT TIME of decay is proportional to the square of the coupling constant.

    By the way, it is quite frustrating to see that most textbooks on quantum mechanics or quantum field theory do not contain a convincing derivation of the Fermi rule and exponential decay law. One of the rare textbooks that does contain a convincing derivation is the old textbook by E. Merzbacher (1961).
    Last edited: May 29, 2012
  4. May 29, 2012 #3

    Simon Bridge

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    We know Weak interactions are weak because of their low probability of happening at all but when they do happen they also take their time about it - about 10-6s.

    It is my understanding that these are different properties and the fundamental forces are like that because that is how they are.
    Now watch someone correct me :)
  5. May 29, 2012 #4


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    Messiah Chap XVII beats it to death. But actually Schiff I think has the clearest treatment. He points out that Fermi originally called it "Golden Rule #2". (Rule #1 was the second order perturbation formula.)
    Ok, sure! :smile: Weak interactions are not intrinsically slow. Neutrino events in OPERA, for example, are rare but not slow. Also the weak coupling constant g is not that much smaller. The reason weak decays have a longer lifetime is kinematical: the heavy mass of the W.
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