Incorrect explanation of parity violation?

[bcrowell]

This APS page http://focus.aps.org/story/v22/st19 describes the classic experiment by Wu et al. in the 50's that demonstrated parity violation. It contains the following explanation: "[1] The magnetism of the nuclei can be thought of as resulting from their spin. With the nuclei aligned with their north poles pointing up, the mirror image would reverse their spin and cause the north poles to point down. But upward-emitted electrons will still move upward in the mirror. So if the decay respected mirror symmetry, electrons should be equally likely to be emitted upwards as downwards."

Isn't this exactly backwards? The magnetic field and the angular momentum are both pseudovectors, which you can see because $L=r \times p$ is invariant under parity (both r and p flip). The velocity vector of the emitted betas is a real vector, not a pseudovector. So under parity, the spin *doesn't* reverse, but the direction of emission *does*.

Or am I just on drugs?

[EDIT] OK, I think I have it. I was assuming that when they said "mirror," they meant total parity inversion, not just inversion in a plane. Inversion in a vertical plane would reverse a vertical L vector but not a vertical v vector.

 

[AndyW]

Hello,

Thank you for bringing up this interesting point. You are correct in your understanding that the spin of the nuclei does not actually reverse under parity inversion, but the direction of emission does. This is because parity inversion only affects spatial coordinates, not spin or angular momentum.

The explanation given in the APS page is simplified and may be confusing for those with a deeper understanding of the concept. As you mentioned, the mirror symmetry in this case refers to inversion in a plane, not total parity inversion. This means that the direction of the emitted electrons will be reversed, while their spin will remain the same.

I hope this clarifies any confusion you may have had. It is always important to critically evaluate and question scientific explanations, so thank you for bringing up this point.