Feynman's Principle Quantum amplitude is sum over all possible paths: $$ \langle x_f|e^{-iHt/\hbar}|x_i\rangle = \int \mathcal{D}[x(t)] e^{iS[x]/\hbar} $$ Where $S[x]$ is the classical action: $$ S[x] = \int_0^t L(x, \dot{x}, t') dt' $$ Key Idea Classical mechanics: One path (least action) Quantum mechanics: All paths …