This article relates the Schrödinger equation with the path integral formulation of quantum mechanics using a simple nonrelativistic one-dimensional single-particle Hamiltonian composed of kinetic and potential energy.
Contents |
Schrödinger's equation, in bra-ket notation, is
where is the Hamiltonian operator. We have assumed for simplicity that there is only one spatial dimension.
The Hamiltonian operator can be written
where is the potential energy, m is the mass and we have assumed for simplicity that there is only one spatial dimension q.
The formal solution of the equation is
where we have assumed the initial state is a free-particle spatial state .
The transition probability amplitude for a transition from an initial state to a final free-particle spatial state at time T is
The path integral formulation states that the transition amplitude is simply the integral of the quantity
over all possible paths from the initial state to the final state. Here S is the classical action.
The reformulation of this transition amplitude, originally due to Dirac[1] and conceptualized by Feynman,[2] forms the basis of the path integral formulation.[3]
Note: the following derivation is heuristic (it is valid in cases in which the potential, , commutes with the momentum, ). Following Feynman, this derivation can be made rigorous by writing the momentum, , as the product of mass, , and a difference in position at two points, and , separated by a time difference, , thus quantizing distance.
Note 2: There are two errata on page 11 in Zee, both of which are corrected here.
We can divide the time interval from 0 to T into N segments of length
The transition amplitude can then be written
We can insert the identity
matrix N-1 times between the exponentials to yield
Each individual transition probability can be written
We can insert the identity
into the amplitude to yield
where we have used the fact that the free particle wave function is
The integral over p can be performed (see Common integrals in quantum field theory) to obtain
The transition amplitude for the entire time period is
If we take the limit of large N the transition amplitude reduces to
where S is the classical action given by
and L is the classical Lagrangian given by
The integral
is an integral over all possible paths the particle may take in going from its initial state to its final state. This expression actually defines the manner in which the path integrals are to be taken. The coefficient of the integral is a normalization factor and has no significance.
This recovers the path integral formulation from Schrödinger's equation.