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I'm wondering why the 'C' in CPT - charge conjugation - refers specifically to electric charge. Of course you could say that C is just defined as $e^+ \leftrightarrow e^-$... but there has to be something that makes electric charge special. For example, why doesn't e.g. weak hypercharge or any other quantum number take it's role? After all, in the derivation of the CPT theorem you use Lorentz invariance and the fact that the Hamiltonian is bounded from below, but not specific knowledge about QED.

This should be closely related to the question why we choose the particle with flipped electric charge as the antiparticle, and not ones with other flipped quantum numbers. Again, besides historical reasons, there must be a reason electric charge is special.

I guess this is somehow related to the fact that (electrically) charged Fermions are represented by spinors, but I can't really make the connection explicit.

(The reason that I came to this question is that I was thinking about some of the implications of CP variance. If CPT is conserved, T must be violated. While certainly not at odds with observation - I mean we do have an arrow of time - it seems at least to me unintuitive that the laws of nature are not time reversal invariant. I was wondering if there was a loophole - maybe the measured CP violation is really a CPX violation, or the fundamental symmetry is CPTY (X and Y being some other transformations). This would probably warrant a question of its own, but I thought I'd provide some context.)

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    $\begingroup$ You have misunderstood. Charge refers to all gauge couplings, not just the electromagnetic ones. $\endgroup$
    – genneth
    Commented Aug 18, 2012 at 13:45
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    $\begingroup$ Interesting note, here, BarBar has reported an observation of explicit T-symmetry breaking. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Aug 18, 2012 at 15:28
  • $\begingroup$ Good question, even though it has a simple answer (btw @genneth, I'd encourage you to post that as an answer - it'd be fine with the addition of a sentence or two of detail) $\endgroup$
    – David Z
    Commented Aug 18, 2012 at 15:43
  • $\begingroup$ @genneth: Ah, that indeed would make more sense. I was aware that quarks and gluons can have anti-color (e.g. blue and anti-blue), but was not sure if that was just a sloppy figure of speach or refered to an actual charge conjugation analogous to the EM case. I agree with David, if you'd post a simple answer I could accept it. $\endgroup$
    – jdm
    Commented Aug 18, 2012 at 20:26

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Electric charge is not special. Every charge is replaced by its opposite under the C conjugation.

For instance, electric charge goes from positive to negative and vice versa. Color charge goes from blue to antiblue and vice versa. Mutatis mutandis with green/red color charge.

Every charge is sent to its anti-charge.

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