List of Footnotes

1 Dirac’s original expression actually involved the rate of change of the acceleration vector on the right-hand side. The resulting equation gives rise to the well-known problem of runaway solutions. To avoid such unphysical behaviour I have submitted Dirac’s equation to a reduction-of-order procedure whereby daν∕dτ is replaced with m −1dfνext∕dτ. This procedure is explained and justified, for example, in [47Jump To The Next Citation Point, 26Jump To The Next Citation Point].
2 His analysis was restricted to a minimally-coupled scalar field, so that ξ = 0 in his expressions. The extension to an arbitrary coupling constant was carried out by myself for this review.
3 Note that I use sans-serif symbols for the frame indices. This is to distinguish them from another set of frame indices that will appear below. The frame indices introduced here run from 0 to 3; those to be introduced later will run from 1 to 3.