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 is replaced with . This procedure is explained and justified, for example, in [47, 26]. | |

2 | His analysis was restricted to a minimally-coupled scalar field, so that 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. |

http://www.livingreviews.org/lrr-2004-6 |
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