# Time symmetry

Rodney Flores
I recently posted a question on energy and matter and I got a very good answer to that one so I thought another one wouldn't hurt. I understand that time symmetry is what makes all conservation laws possible as is stated in Noether's theorem. Time symmetry is what explains the conservation of matter and energy and in part also unifies the two. Because time symmetry doesn't change the way it works, and energy and matter do, would the symmetry of space not change as well?

## Answers and Replies

Staff Emeritus
I recently posted a question on energy and matter and I got a very good answer to that one so I thought another one wouldn't hurt. I understand that time symmetry is what makes all conservation laws possible as is stated in Noether's theorem. Time symmetry is what explains the conservation of matter and energy and in part also unifies the two. Because time symmetry doesn't change the way it works, and energy and matter do, would the symmetry of space not change as well?

Er.. correction. The time symmetry is relevant to NOT ALL conservation laws, only the conservation of energy+mass!

The spatial symmetry is responsible for the conservation laws we see for momentum (and angular momentum if you include space anisotropy).

Noether theorem applies to not just energy conservation. It simply states that in the most general form, every conservation laws implies an underlying symmetry. So each conservation law has its own symmetry principle. You should not use one symmetry principle for all conservation laws that might not be appropriate.

Zz.

Staff Emeritus