I understand the concept of symmetry in geometry, but what do theoretical physicists mean when they speak of symmetry?
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It means that if you perform an experiment, that experiment should work exactly the same way if you mirror everything. In fact, that applies to time, too. The experiment should work the same way forwards and backwards in time. Now, doing things “backwards” in time requires you to do things like drive south instead of north. But as long as you’re careful, you shouldn’t be able to tell if a video of the experiment is running forward or backwards.
But here’s the thing. Sometimes, very rarely, it doesn’t work the same way backwards! That’s called “symmetry breaking” and tells us a little bit about why time flies in one direction and not the other.
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