In the Schrödinger picture, the wave function or field is the solution to the Schrödinger equation ; i ℏ ℏ ∂ ∂ ∂ ∂ t ψ ψ = H ^ ^ ψ ψ one of the postulates of quantum mechanics. All relativistic wave equations can be constructed by specifying various forms of the Hamiltonian operator Ĥ describing the quantum system. Alternatively, Feynman 's path integral formulation uses a Lagrangian rather than a Hamiltonian operator.