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Change of energy under charge conjugation

  1. May 20, 2009 #1
    I was going through an article on antiparticles:
    http://www.statemaster.com/encyclopedia/Antiparticles

    The article mentions that energy is unchanged under charge conjugation among the CPT operations.
    I do not understand this. Shouldn't a charged particle in an electric field have a change in energy under charge conjugation?
     
  2. jcsd
  3. May 20, 2009 #2
    The electric field would also change under charge conjugation, and the energy would stay the same.
     
  4. May 20, 2009 #3
    Well..yes..that is fine..
    so..you mean to say that when I apply charge conjugation operation, I should act it on everything in the system, including the source of the electric field?

    But what about parity? Unless we have the special case where the electric field is invariant under parity, how can one say that the energy will be invariant under parity?
     
  5. May 20, 2009 #4
    The strength of the electric field won't change under parity, just its direction.
     
  6. May 21, 2009 #5
    Thanks....now I understand how energy is unchanged under charge conjugation when we consider electric charge...but charge conjugation here means flipping all the associated quantum numbers...how can we say that none of these changes will affect the energy??
     
  7. May 21, 2009 #6

    Vanadium 50

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    Let's turn it around. Try and write down some operator that does change the energy.
     
  8. May 22, 2009 #7
    Do you mean to say that there are no operators that change energy?? Can that be proved??

    What about, say, a scale change operator, which scales all distances in the system by a constant factor? Assuming that the electromagnetic interactions are between point charges, in which case it seems somewhat logical to assume that the charge will not be changed, we will have a change in the electromagnetic energy of the system.
     
  9. May 22, 2009 #8

    Vanadium 50

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    A second ago you were talking about the charge conjugation operator.
     
  10. May 23, 2009 #9

    malawi_glenn

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    charge conjugation flips the charge quantum number, no other.

    where did you learn to do charge conjugation? :P
     
  11. May 23, 2009 #10
    I have thought of this operator because I thought that is what you instructed me to do..or did I misinterpret your post?

    I read about charge conjugation in Griffiths Elementary Particles: sec 4.7...
    You can also find the definition at http://hyperphysics.phy-astr.gsu.edu/HBASE/Particles/cpt.html

    The definition says you have to flip all the internal quantum numbers..
     
  12. May 23, 2009 #11

    malawi_glenn

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    Read carefully:

    In quantum mechanical systems, charge conjugation has some further implications. It also involves reversing all the internal quantum numbers like those for lepton number, baryon number and strangeness. It does not affect mass, energy, momentum or spin.

    Lepton number, baryon number and strangeness etc are related to charge. The others are not.
     
  13. May 23, 2009 #12
    Yes..I have read that..but what I wanted was an explanation for the statement that the energy does not change..Now can I say this..that out of lepton no,baryon no, charge etc..only charge couples with a field to give contribution to the hamiltonian..and through this thread, I have come to understand how flipping the charge does not change the energy..but am confused by the last line..

    What do you mean when you say lepton no etc are related to charge?
     
  14. May 23, 2009 #13

    malawi_glenn

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    postive electron lepton number is just a fancy way to say "negative electrical charged"
     
  15. May 24, 2009 #14
    Hmm..not really ..consider the neutrino..it has positive lepton no...an antineutrino has negative lepton no..but both of them are chargeless...
     
  16. May 24, 2009 #15
    Clarification: In the last post no stands for number..
     
  17. May 24, 2009 #16

    malawi_glenn

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    Well their lepton number is then related to another charge (like weak charge)
     
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