M.C. Escher and CP Violation

I’ve had these pictures for quite a while and can’t remember where I got them from, but I used to show them in my lectures on Theoretical Particle Physics when I was in Nottingham to illustrate CP-violation and used them in this morning’s lecture at Maynooth. The following picture by M.C. Escher is called Day and Night: If you look at it you can see two kinds of symmetry emerging. One is a kind of reflection symmetry about a vertical axis drawn through the centre of the picture that applies to shapes but not to colour. The other is between black and white. But it is obvious that the picture doesn’t display these symmetries separately: to get a picture unchanged from the original you would have to do the mirror reflection and change black to white (and vice-versa). The mirror reflection in the image can be taken to represent parity (P). Strictly speaking parity refers to a reflection through the origin in 3D rather than a mirror reflection, but it’s just for illustration. We know that…