16.1 Path integrals and classical physics
Since Richard Feynman invented them over 60 years ago, path integrals have been used with increasing frequency in high-energy and condensed-matter physics, in finance, and in biophysics (Kleinert, 2009). Feynman used them to express matrix elements of the time-evolution operator exp(−itH/ħ) in terms of the classical action. Others have used them to compute matrix elements of the Boltzmann operator exp(−H/kT), which in the limit of zero temperature projects out the ground state |E0〉 of the system
a trick used in lattice gauge theory.
Path integrals magically express ...