### 1.7 Motion of an electric charge in curved spacetime

I have argued in Section 1.4 that the self-force acting on a point electric charge is produced by the
radiative field, and that the charge’s equations of motion should take the form of ,
where is an external force also acting on the particle. Substituting Equation (32) gives
in which all tensors are evaluated at , the current position of the particle on the world line. The
primed indices in the tail integral refer to a point which represents a prior position; the integration
is cut short at to avoid the singular behaviour of the retarded Green’s function
at coincidence. To get Equation (33) I have reduced the order of the differential equation by
replacing with on the right-hand side; this procedure was explained at the end of
Section 1.2.
Equation (33) is the result that was first derived by DeWitt and Brehme [24] and later
corrected by Hobbs [29]. (The original equation did not include the Ricci-tensor term.) In flat
spacetime the Ricci tensor is zero, the tail integral disappears (because the Green’s function
vanishes everywhere within the domain of integration), and Equation (33) reduces to Dirac’s
result of Equation (5). In curved spacetime the self-force does not vanish even when the electric
charge is moving freely, in the absence of an external force: It is then given by the tail integral,
which represents radiation emitted earlier and coming back to the particle after interacting
with the spacetime curvature. This delayed action implies that, in general, the self-force is
nonlocal in time: It depends not only on the current state of motion of the particle, but also on its
past history. Lest this behaviour should seem mysterious, it may help to keep in mind that the
physical process that leads to Equation (33) is simply an interaction between the charge and a
free electromagnetic field ; it is this field that carries the information about the charge’s
past.