1. Sep 26, 2008

### franoisbelfor

What is wrong in this reasoning about CPT symmetries?

Spatial inversion P preserves spin and inverses velocity. (See wikipedia,
at http://en.wikipedia.org/wiki/P-symmetry )

Motion or time inversion T inverses both spin and velocity (obvious, see also wikipedia,
at http://en.wikipedia.org/wiki/T-symmetry ).

Now, charge conjugation C preserves chirality. ( http://en.wikipedia.org/wiki/Charge_conjugation )
That means that either spin and velocity are both inverted or they are both preserved.

But: In either of these two cases, C cannot be equal to TP.
Now, C=TP is a deep theorem in physics!
What is wrong in this argument chain?

François

2. Sep 26, 2008

### hamster143

Nobody really says that C=PT, not in the sense that you get identical results from applying these two symmetries.

In a classical world, C, P, and T are all symmetries by themselves. Meaning, you can take a physical process or an area of spacetime and apply C or P or T and you get a new valid process.

In a quantum world, P is not a symmetry, because it takes valid weak-interaction processes into invalid processes. It is maximally violated by the weak interaction. CP is a lot better. The CPT theorem essentially states that you can take any valid process, apply C, P, and T (reverse spins and replace all particles with antiparticles), and you ALWAYS get a valid process.

3. Sep 27, 2008

### franoisbelfor

Oh, I understand. I have read so often that CP and T are either both broken or both
conserved that I thought that this means that CP equals T. Ok, that is wrong.
Thanks!

François