Symmetry and CP violation

A symmetrical world?

Physics describes symmetry in terms of conservation laws that occur both in ordinary space and time and also in abstract mathematical spaces. The relatively “ordinary” symmetries are familiar because they are also features of our natural world on a larger, easily recognizable scale. Snowflakes and crystals, for instance, have a symmetry we can see under a simple microscope or even with the naked eye. And for living things, symmetry is par for the course.

When I was a budding (heh) biology student, we learned in particular about radial and bilateral symmetry in living things. Radial symmetry is the more primitive, evolutionarily speaking—consider simple animals like the jellyfish and simple flowers like the magnolia. Physicists would call this a “rotational” symmetry. As living things evolved, a new kind of symmetry started to appear—bilateral symmetry. Examples are everywhere, including humans and butterflies and flowers like orchids.

None of this is mind-blowing, but what is appealing to me is to consider that sometimes, the behavior of quantum particles (in this case, symmetry) appears in the world on another scale—the one we live in and can readily observe. Much of what we’re learning about conceptual physics tells us that the very tiny (quantum) world follows different rules than those laid out in classical physics, which are based on easily observable things like the trajectory of a rocket or an apple falling on Isaac Newton’s head.  But at least in these examples of symmetry, we can relate to it as we experience it in the beauty of our natural world.

Just what does CP violation really mean?

Still flummoxed by this question, I turned to the internet and found a very helpful explanation in the surprisingly readable (to me) article “Charge-parity violation” in Symmetry, the journal published by Fermilab and SLAC National Accelerator Laboratory.  The article explains that instances of CP violations that physicists have found in experiments dating back to the 1950s are at heart about why we exist, in a universe that is dominated by matter.

The big bang, which happened 14 billion years ago, should have generated matter and antimatter in equal amounts. And while conservation laws of particle physics tell us that matter and antimatter should behave similarly, we know this can’t be the case, because we live in a universe with much more matter than antimatter. If this were not the case—if matter and antimatter behaved exactly the same—the equal amounts of matter and antimatter in the universe created by the big bang “would have annihilated each other, leaving nothing behind but energy.” CP violation is necessary to explain why the matter/antimatter imbalance exists, and thus why we are here with our homework and all of this other stuff made of matter.

What puzzles physicists is that, while we know CP violation is critical to understanding this imbalance, it is rarely observed in their experiments. We know CP violation must be happening or have happened on a large scale to explain the existence of our universe, so why can’t we find it? It’s possible conditions of the early universe involved particles we can’t recreate on Earth. Meanwhile physicists continue to look for CP violation in the decay of all different kinds of quantum particles to try to account for our universe full of matter.