# Does a symmetry need to leave the whole action invariant?

1. Jan 27, 2014

### nonequilibrium

Typically a symmetry is taken to be something that leaves the action invariant. However, on a classical level, isn't that asking way too much? To match what we conceptually mean by symmetry, we only need something that maps solutions to solutions, so something which leaves the action invariant in the neighbourhood of extremal solutions(?) (After all who cares what a symmetry operation does to unphysical solutions?). Is it because of Noether's theorem?

Last edited: Jan 27, 2014
2. Jan 28, 2014

### nonequilibrium

Alright I solved my own problem. You need it to get Noether's theorem to work.