### 1.8 Motion of a scalar charge in curved spacetime

The dynamics of a point scalar charge can be formulated in a way that stays fairly close to the electromagnetic theory. The particle’s charge produces a scalar field , which satisfies a wave equation
that is very similar to Equation (13). Here, is the spacetime’s Ricci scalar, and is an arbitrary coupling constant; the scalar charge density is given by a four-dimensional Dirac functional supported on the particle’s world line . The retarded solution to the wave equation is
where is the retarded Green’s function associated with Equation (34). The field exerts a force on the particle, whose equations of motion are
where is the particle’s mass; this equation is very similar to the Lorentz-force law. But the dynamics of a scalar charge comes with a twist: If Equations (34) and (36) are to follow from a variational principle, the particle’s mass should not be expected to be a constant of the motion. It is found instead to satisfy the differential equation
and in general will vary with proper time. This phenomenon is linked to the fact that a scalar field has zero spin: The particle can radiate monopole waves and the radiated energy can come at the expense of the rest mass.

The scalar field of Equation (35) diverges on the world line, and its singular part must be removed before Equations (36) and (37) can be evaluated. This procedure produces the radiative field , and it is this field (which satisfies the homogeneous wave equation) that gives rise to a self-force. The gradient of the radiative field takes the form of

when it is evaluated of the world line. The last term is the tail integral
and this brings the dependence on the particle’s past.

Substitution of Equation (38) into Equations (36) and (37) gives the equations of motion of a point scalar charge. (At this stage I introduce an external force and reduce the order of the differential equation.) The acceleration is given by

and the mass changes according to
These equations were first derived by Quinn [48]2.

In flat spacetime the Ricci-tensor term and the tail integral disappear, and Equation (40) takes the form of Equation (5) with replacing the factor of . In this simple case Equation (41) reduces to and the mass is in fact a constant. This property remains true in a conformally-flat spacetime when the wave equation is conformally invariant (): In this case the Green’s function possesses only a light-cone part, and the right-hand side of Equation (41) vanishes. In generic situations the mass of a point scalar charge will vary with proper time.