### 1.3 Green’s functions in flat spacetime

To see how Equation (5) can eventually be generalized to curved spacetimes, I introduce a new layer of mathematical formalism and show that the decomposition of the retarded potential into symmetric-singular and regular-radiative pieces can be performed at the level of the Green’s functions associated with Equation (1). The retarded solution to the wave equation can be expressed as
in terms of the retarded Green’s function . Here is an arbitrary field point, is a source point, and ; tensors at are identified with unprimed indices, while primed indices refer to tensors at . Similarly, the advanced solution can be expressed as
in terms of the advanced Green’s function . The retarded Green’s function is zero whenever lies outside of the future light cone of , and is infinite at these points. On the other hand, the advanced Green’s function is zero whenever lies outside of the past light cone of , and is infinite at these points. The retarded and advanced Green’s functions satisfy the reciprocity relation
this states that the retarded Green’s function becomes the advanced Green’s function (and vice versa) when and are interchanged.

From the retarded and advanced Green’s functions we can define a singular Green’s function by

and a radiative Green’s function by
By virtue of Equation (8) the singular Green’s function is symmetric in its indices and arguments: . The radiative Green’s function, on the other hand, is antisymmetric. The potential
satisfies the wave equation of Equation (1) and is singular on the world line, while
satisfies the homogeneous equation and is well behaved on the world line.

Equation (6) implies that the retarded potential at is generated by a single event in spacetime: the intersection of the world line and the past light cone of ’ (see Figure 1). I shall call this the retarded point associated with and denote it ; is the retarded time, the value of the proper-time parameter at the retarded point. Similarly we find that the advanced potential of Equation (7) is generated by the intersection of the world line and the future light cone of the field point . I shall call this the advanced point associated with and denote it ; is the advanced time, the value of the proper-time parameter at the advanced point.

Figure 1: | In flat spacetime, the retarded potential at depends on the particle’s state of motion at the retarded point on the world line; the advanced potential depends on the state of motion at the advanced point .