Building better Mirrors
The fall of Parity triggered the question: If our Universe does not obey simple mirror-symmetry, then what kind of symmetry does it obey? In other words, perhaps we were looking in the wrong mirror.
P-symmetry is a mirror of parity, reversing left and right. We can think of it also as a transformation: It is an exchange of every Left-handed interaction with a Right-handed interaction.
Let's consider other types of mirrors. Physicists also examined another mirror called C-symmetry, or Charge symmetry. C Symmetry is a mirror which exchanges every positively charged particle with a negatively charged particle. In the world of physics this essentially means that the C mirror transforms particles into anti-particles and vice-versa. A proton and electron become an anti-proton and a positron.
So, what happens when we use the C Mirror, when we swap particles for anti-particles? Do the anti-particles obey the same laws of physics? The answer is No. C-symmetry is not a property of this Universe.
Ok, let's consider another abstract mirror. What if we take all particles and interactions in the Universe and move them backwards? Shown a film of a particle interaction, can we tell whether the film had been reversed? (Note to all clever folks: This is not the same as the question addressed by the Second Law since that applies only to macroscopic systems.)
Is our Universe symmetric in Time? This question is known as that of T-symmetry. The answer is...mostly yes. That is, we cannot detect a reversed film unless it contains some particular time-asymmetric interactions. This all comes down to the interactions of one particle: The Kaon. I summarize here from a physics abstract:
Most fundamental physical processes are symmetric in time. The motion of the planets in the gravity field of the sun is reversable- a film of the motion of a planet around its sun can be shown backwards without anyone being able to tell. Similarly to gravity, the strong nuclear and electromagnetic forces are also time-symmetric. Only the weak nuclear force appears to violate this symmetry, and this so far only in the behavior of the neutral kaon.
We'll return to discuss the Kaon, later.
If I reflect a mirror image of a mirror image, I get the original. Mirrors can be combined to produce more mirrors.
The failure of P and C and T symmetry in our Universe is confounding. Recall that P symmetry is closest to what we consider a classic mirror, reversing left and right. What if we combine two mirrors (or more), combining the C mirror with the P mirror?
What this means in practice is that the mirror on the wall does show a valid Universe if we also exchange particles for anti-particles. The resulting symmetry is known as CP symmetry.
In 1980, James Cronin and Val Fitch earned the Nobel prize in physics for demonstrating that CP symmetry is not conserved. It was mostly conserved but violated in some interactions of the Kaon.
CPT: the Ultimate Mirror
The CP violation of the Kaon and the T violation of the Kaon are in fact related. They can be canceled out by throwing togther CP and T to make CPT: antiparticles in a mirror going backwards in time.
There is every reason to believe that CPT symmetry holds for our Universe. Even the Kaon (so far) obeys CPT symmetry. But what exactly does CPT symmetry imply? And what makes the Kaon such an anomaly?