C, P, and T.
- (Charge) The symmetry C means that the laws are the same for particles and antiparticles.
- (Parity) The symmetry P means that the laws are the same for any situation and its mirror image (the mirror image of a particle spinning in a right-handed direction is one spinning in a left-handed direction).
- (Time) The symmetry T means that if you reverse the direction of motion of all particles and antiparticles, the system should go back to what it was at earlier times; in other words, the laws are the same in the forward and backward directions of time.
In 1956 two American physicists, Tsung-Dao Lee and Chen Ning Yang, found out that the weak force does not obey the symmetry P.
In other words, the weak force would make the universe develop in a different way from the way in which the mirror image of the universe would develop.
They also found that the weak force did not obey the symmetry C.
In other words, it would cause a universe composed of antiparticles to behave differently from our universe.
Nevertheless, it seemed that the weak force did obey the combined symmetry CP.
That is, the universe would develop in the same way as its mirror image if, in addition, every particle was swapped with its antiparticle!
However, in 1964 two more Americans, J. W. Cronin and Val Fitch, discovered that even the CP symmetry was not obeyed in the decay of certain particles called K-mesons.
There is a mathematical theorem that says that any theory that obeys quantum mechanics and relativity must always obey the combined symmetry CPT. In other words, the universe would have to behave the same if one replaced particles by antiparticles, took the mirror image, and also reversed the direction of time.
But Cronin and Fitch showed that if one replaces particles by antiparticles and takes the mirror image, but does not reverse the direction of time, then the universe does not behave the same. The laws of physics, therefore, must change if one reverses the direction of time – they do not obey the symmetry T.
Certainly the early universe does not obey the symmetry T: as time runs forward the universe expands – if it ran backward, the universe would be contracting.
And since there are forces that do not obey the symmetry T, it follows that as the universe expands, these forces could cause more antielectrons to turn into quarks than electrons into antiquarks.
Then, as the universe expanded and cooled, the antiquarks would annihilate with the quarks, but since there would be more quarks than antiquarks, a small excess of quarks would remain.
It is these that make up the matter we see today and out of which we ourselves are made.
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"päätä nyrjäyttävä analogia on se, että universumissa saattaa olla galakseja, jotka meidän näkökulmasta katsottuna koostuu antimateriasta, jossa pariteetti on väärinpäin ("vasen on oikea"), ja jossa AIKA kulkee eri suuntaan"