What alternative formulation of quantum mechanics did Feynman develop?

Richard Feynman developed the path integral formulation of quantum mechanics. It describes a quantum process as a sum over all possible paths between an initial and final state.

In the older classical picture, a particle follows one trajectory determined by the laws of motion. Feynman’s formulation instead assigns an amplitude to every conceivable path, with each contribution weighted according to the action along that path. The paths interfere with one another, and in the classical limit the paths near the stationary-action trajectory dominate, helping classical mechanics emerge from quantum theory.

Feynman completed the formulation in 1948, building on earlier ideas from Paul Dirac and mathematical work related to functional integration. The approach was especially powerful because it uses the Lagrangian and action rather than starting with operators and a Hamiltonian. It later became foundational in quantum field theory, where the summed objects are possible histories of fields. “Path integral” is sometimes confused with a single Feynman path, but the central idea is the weighted sum over infinitely many alternatives.