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Method of Images and Green functions in Quantum Field Theory

Physics Asked on September 1, 2021

Traditional Laplace equation solutions techniques for a system of conductors rely on the fact that the Green function for solving Laplace’s equation represents the potential due to an image charge distribution ([1] Sections 1.10 and 2.1-2.6, for example). I would like to know whether the connection between the method of images and Green functions holds up similarly in the context of QFT.

For example, for the Schrödinger equation,
$$left[hat{H} – ifrac{d}{dt}right]phi(x,t)=0$$
we suppose there is a Green function $G(x,t,x’,t’)$ that satisfies

$$left[hat{H} – ifrac{d}{dt}right]G(x,t,x’,t’)=-idelta(x-x’)delta(t-t’)$$

In [2] (and many more), this Green function is identified as the propagator that evolves the quantum state in spacetime from $phi(x,t)rightarrow phi(x’, t’)$. But this diverges from the E&M technique since $G$ does not directly recover the solution for Schrödinger’s equation (but is instead used to propagate an initial state).

My question is, is there an interpretation that can explain Green functions and propagators in QFT in terms of method of images?


[1] Jackson, J.D. Classical Electrodynamics

[2] Lancaster and Blundel, Quantum field theory for the gifted amateur

One Answer

Method of images is based on the uniqueness theorem: the solution to a differential equation with specified boundary conditions is unique (up to the integration constants, which for PDEs may be functions). That is, if you guess a solution that satisfies both the equation and the boundary conditions, then this is the correct solution.

Method of images is a method for guessing solutions in certain classes of problems, where (e.g., due to the symmetry of the problem) guessing turns out to be easier than solving the differential equation with straightforward mathematical techniques.

Thus, the method of images applies equally well to quantum mechanical problem, but the class of problems is very limited.

Note that the method of images is also used for the diffusion equation with either reflecting or absorbing boundary conditions.

Answered by Roger Vadim on September 1, 2021

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