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Issue about CPT

  1. Sep 26, 2008 #1
    What is wrong in this reasoning about CPT symmetries?

    Spatial inversion P preserves spin and inverses velocity. (See wikipedia,
    at http://en.wikipedia.org/wiki/P-symmetry )

    Motion or time inversion T inverses both spin and velocity (obvious, see also wikipedia,
    at http://en.wikipedia.org/wiki/T-symmetry ).

    Now, charge conjugation C preserves chirality. ( http://en.wikipedia.org/wiki/Charge_conjugation )
    That means that either spin and velocity are both inverted or they are both preserved.

    But: In either of these two cases, C cannot be equal to TP.
    Now, C=TP is a deep theorem in physics!
    What is wrong in this argument chain?


    François
     
  2. jcsd
  3. Sep 26, 2008 #2
    Nobody really says that C=PT, not in the sense that you get identical results from applying these two symmetries.

    In a classical world, C, P, and T are all symmetries by themselves. Meaning, you can take a physical process or an area of spacetime and apply C or P or T and you get a new valid process.

    In a quantum world, P is not a symmetry, because it takes valid weak-interaction processes into invalid processes. It is maximally violated by the weak interaction. CP is a lot better. The CPT theorem essentially states that you can take any valid process, apply C, P, and T (reverse spins and replace all particles with antiparticles), and you ALWAYS get a valid process.
     
  4. Sep 27, 2008 #3
    Oh, I understand. I have read so often that CP and T are either both broken or both
    conserved that I thought that this means that CP equals T. Ok, that is wrong.
    Thanks!

    François
     
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