Physvillain

이 글은 2020.10.14에 작성됨. This post was written on Oct 14, 2020. Sometimes it is the case that $\delta S$ vanishes for certain limited variations of the path without imposing any condition at all. When this happens, we say that $S$ has a symmetry. A symmetry of an action functional $S[x]$ is an infinitesimal transformation of the path, $x^i(t) \rightarrow x^i+\epsilon^i(x^j)$ that leaves the action ..