[Physics] Invariance of Lagrangian in Noether’s theorem

classical-mechanicslagrangian-formalismnoethers-theoremsymmetry

Often in textbooks Noether's theorem is stated with the assumption that the Lagrangian needs to be invariant $\delta L=0$.

However, given a lagrangian $L$, we know that the Lagrangians $\alpha L$ (where $\alpha$ is any constant) and $L + \frac{df}{dt}$ (where $f$ is any function) lead to the same equations of motion.

Can we then consider that the Lagrangian is invariant under a transformation if we find $\delta L=\alpha L$ or $\delta L=\frac{df}{dt}$ instead of $\delta L=0$?

Best Answer

Here I would like to mention the notion of quasi-symmetry. In general, if the Lagrangian (resp. Lagrangian density resp. action) is only invariant up to a total time derivative (resp. space-time divergence resp. boundary term) when performing a certain off-shell$^1$ variation, one speaks of a quasi-symmetry, see e.g. Ref. 1.

Noether's first Theorem does also hold for quasi-symmetries. For examples of non-trivial conservation laws associated with quasi-symmetries, see examples 1, 2 & 3 in the Wikipedia article for Noether's theorem.

References:

  1. J.V. Jose & E.J. Saletan, Classical Dynamics: A Contemporary Approach, 1998; p. 565.

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$^1$ Here the word off-shell means that the Euler-Lagrange (EL) eqs. of motion are not assumed to hold under the specific variation. If we assume the EL eqs. of motion to hold, any variation of the Lagrangian is trivially a total derivative.