A formulation of quantum mechanics that represents transition amplitudes as a functional integral over all possible classical-like histories (paths), each weighted by the phase factor exp(iS/ħ) or by exp(-S/ħ) in imaginary time.

Principle

Quantum amplitudes arise from summing (integrating) contributions of all histories with weights determined by the action; semiclassical and stationary-phase approximations select dominant paths, and proper regularization defines the measure.

Demonstration

Compute the propagator of the harmonic oscillator by integrating Gaussian fluctuations around the classical path; derive instanton contributions to tunneling rates via Euclidean path integrals.

Misapplication

Treating the path integral as a literal probability measure on differentiable trajectories or ignoring the need for regularization/renormalization in field-theoretic applications, leading to ill-defined manipulations.

Consequence

Provides a unified route to perturbation theory, semiclassical approximations, and nonperturbative phenomena (instantons, topological effects); connects quantum and classical variational principles.

Reversal

Use operator (canonical) quantization and the Schrödinger picture where states and operators evolve differently and path-sum intuition is replaced by spectral decompositions of operators.

Boundary

Formal path integrals require discretization, boundary condition specification, and measure definition; for interacting quantum field theories the continuum limit may require renormalization and leaves nonrigorous constructions in many cases.

Semantic Tension

Tension between path integral intuition (sum over histories) and operator-formalism rigor (Hilbert-space operators); between formal manipulations and the necessity of regularization in field theory.

Synthesis

The path integral is an alternative representation of quantum dynamics: a formal functional integral that encodes amplitudes as a superposition of histories, practical for semiclassical analysis and nonperturbative effects but reliant on discretization and regularization to make sense in interacting field theories.