The background assumption is that “complete reversal” should have effect of reversing the sign of 4-vectors in spacetime. David Malament, for example, has suggested that time reversal in classical electrodynamics should have this effect on timelike vectors. The proposal here is that “complete” motion reversal to have this effect on all vectors (timelike, spacelike, and null).
What about PT? After all, flipping about two axes is equivalent to a rotation. Shouldn’t that be enough to reverse all four vectors? As it turns out, it’s not enough, at least when it comes to 4-current ja. Since both P and T fix charge density and reverse current, we have:
To reverse current, we need an operator C that sends particles to antiparticles, and thus sending ja to -ja. Thus, to get “total” motion reversal in a world with current, we need the CPT operator.
What I like about this thinking is that it depends crucially on the kind of matter fields in play. It’s only in the presence of 4-currents that PT is not enough to completely reverse motion. But similarly, the discovery of additional exotic matter fields might someday imply that CPT is not enough to reverse motion, either.
Update: Wolfgang reports news about evidence for CPT-violation in a recent Fermilab experiment.
Soul Physics is authored by Bryan W. Roberts. Thanks for subscribing.
Want more Soul Physics? Try the Soul Physics Tweet.
- Even more philosophy of physics conferences, Summer 2010
- The three-way duel