What is “Symmetry” in Physics?

Question

I understand the concept of symmetry in geometry, but what do theoretical physicists mean when they speak of symmetry?

In: 15

Answer 1

Well that depends on the context because it can mean many things. A big issue with physics, though, is a specific kind of symmetry known as CPT Symmetry.

C stands for charge, P stands for parity, and T stands for time.

For C-symmetry, if you were to take every particle and swap it with its antiparticle (electrons for positrons, protons for anti-protons, etc.) then nothing changes.

For P-symmetry, if you were to physically mirror the universe along any of the three spatial axes, then nothing changes.

For T-symmetry, if you were to reverse the direction of time, then nothing changes.

You can consider these symmetries individually, or in combination with each other.

For example, some system might not be C-symmetric or P-symmetric but might be CP-symmetric. That is, if you *just* swap every particle for its antiparticle *or* mirror it along some spatial axis, then it is different, but if you do both, then nothing changes. This would be called CP-symmetry.

Currently, the universe, on the whole, isn’t C, P, or T symmetric. And it isn’t symmetric for any combination of two of them. But it is believed to be symmetric for all three, known as CPT-symmetry. So if you were to replace every particle in the universe for its anti-particle *and* mirror it about a spatial axis *and* reverse the direction of time, then it would appear the same as this universe.