system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is "Symmetry" in Physics?

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