MadSci Network: Physics

Re: Is positron equivalent to an electron of opposite parity moving back in time

Date: Thu Jun 12 11:12:34 2003
Posted By: Benjamin Monreal, Grad student, Physics, MIT
Area of science: Physics
ID: 1055410655.Ph

Hi Jagmeet,

That's right!

I would point out that when you say "opposite parity", you have to
think about both the particle's direction-of-travel, and also its spin
direction.   In a parity transformation, you have to reverse the direction
but NOT the spin.  OK, so let us start with an electron traveling
north-west, with its angular momentum vector pointing up.  When you flip
the parity, you'll get an electron traveling south-east, with its spin
still pointing up.  If you now reverse the sense of time, the particle
will be going north-west again, and now its spin will be pointing down.  A
charge-congugation turns the - charge into a + charge, and now you've got
a spin-down positron traveling northwest; exactly the CPT "mirror" of the
original situation.  (Due to the properties of the weak interaction, we
expect electrons and positrons to have opposite spins, or more
correctly opposite "handedness", in many situations.  Indeed the CPT
reversal generates electrons and positrons with the correct handedness.)

So you're right, we can't tell the difference between a CPT-reversed
electron, and an "ordinary" positron.  


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