Why does nature bother having a universe?

Nature is symmetrical in many ways. Take for example your typical atom. You have negatively charged electrons orbiting positively charged protons. Could the opposite happen? Could you have positively charged electrons orbiting negatively charged protons? Yes, you could. A positively charged electron is called a positron and a negatively charged proton is called an antiproton. Put these two together and you get antimatter.

Symmetry would lead us to believe that matter and antimatter should be produced at equal rates in the universe, but that can't be the case or we wouldn't be here. When matter and antimatter meet, they destroy each other in a blinding flash of energy and there'd be no coffee tables, no penguins and no you. In fact, there'd be no universe at all.

So, whilst symmetry is a common — and often fundamental — part of the laws of nature, it must be violated somewhere in order to get the matter-dominated universe we live in.

Physicists attach properties to particles. We met one above: charge, which can either be positive or negative (or neutral, but don't worry about that). There is another symmetry called parity, which is a sort of 'handedness'. If you look at yourself in a mirror, your left arm becomes your right arm and the mirror could be said to be showing you with the opposite parity.

For the most part, nature doesn't care too much about charge or parity and it operates quite contentedly whether you swap the charge or the parity. Swap the charge and you get antimatter, and imagine holding a mirror next to a science experiment — if you recreated the experiment to look like its mirror image, you'd expect it to work fine.

Mousetrap game and its mirror image.
Mousetrap works the same whether its left-handed or right-handed.

There is however a rebel force at work. One of the four forces of nature — the so-called weak force, which is responsible for radioactive decay — is like an unruly teenager and it rejects the symmetries of nature. It respects neither charge symmetry nor parity symmetry. It is a habitual violator.

One of the particles associated with the weak force is the neutrino. These are itsy-bitsy little particles in the same class as electrons. If parity symmetry was followed, we'd see a similar number of left-handed neutrinos and right-handed neutrinos, but the thing is we never see right-handed neutrinos. If charge symmetry was followed, we'd be able to flip the charge of a left-handed neutrino and get a left-handed antineutrino, but we never see those either.

Order was restored for a while when scientists decided not to consider charge symmetry and parity symmetry separately but to consider them together. If you flip both the charge and the parity together, your left-handed neutrino becomes a right-handed antineutrino, and they do exist. Phew, thought scientists, that was a close one. They had come up with what they call CP symmetry and everything worked as long as they considered both symmetries together.

CP symmetry drawing showing neutrinos.
CP symmetry. We have to flip both charge and parity.

This made everything neat and tidy at a theoretical level but we're back to having a universe that shouldn't exist. Symmetries must be broken somewhere in order to get more matter than antimatter and scientists have been trying to hunt down these dirty CP violators.

They found it with a class of particles called kaons, which are certain combinations of quarks, which are themselves another class of subatomic particle. Quarks combine to form protons, which sit in the nucleus of atoms. They also combine in another way to make the aforementioned kaons.

When kaons decay in a particular way, they ever so slightly prefer to decay along a path that involves a neutrino rather than an antineutrino. In other words they are CP symmetry violators and nature appears to have a preference for matter over antimatter. This particular violation is significant but it's not enough on its own to account for our matter-dominated universe. Scientists started to look for asymmetries in neutrinos themselves and they may just have found it.

So that's what CP symmetry violation is all about and why it's so exciting. And when I say exciting, I mean for nerdy weirdos like me who like this sort of thing. Think about it, though, it literally tells us why there's something rather than nothing and there aren't many bigger questions than that.