Feynman's Path-Integral Formulation (QED)

A Couple of Relevant YouTube Videos

YouTube video: Richard P. Feynman's 1979 Lectures at the University of Auckland, NZ	YouTube video: Physics With Elliot

Book resulting from Auckland lectures QED: The Strange Theory of Light and Matter

Figures Similar to Feynmann's Fig. 24

Here are a few figures that we generated (in Excel) to explain Fig. 24 of Feynmann's Book

Following pp. 38-40 of Feynmann, the upper portion of our set of figures displays a mirror that, as labeled, extends from point "A" to point "M"; it is divided into 12 segments. Above the mirror is a "source" "that emits light of one color (red) at very low intensity"; a "detector is identified as well. "… there are millions of ways a photon could go" as it travels from the source to the detector: "it could go down to the left-hand part of the mirror at "A" — as indicated by the green arrow — or "B" (for example)

Figure 1

Slit Aperture Instead of Mirror

Figure 2

Panel a: Same as Figure 1, but with fewer photon trajectories displayed.
Panel b: Positions of detector and source modified from Panel a.	Panel c: Mirror replaced by slit; position of source flipped to position such that light passes through the slit rather than bouncing off of mirror.	Panel d: If the source is a laser, then photon trajectories between the source and the slit are all in phase with, and parallel to one another.