Ch 7.3, Ch 31.2 The amplitude $K(x_f, t_f; x_i, t_i) = \langle x_f|\hat{U}(t_f - t_i)|x_i\rangle$ for a particle to travel from $(x_i, t_i)$ to $(x_f, t_f)$. In the path integral formulation, the propagator is the sum over all paths weighted by $e^{iS/\hbar}$. *See also:* Time evolution operator, Pa