We wish to represent the motion of the body through the external background spacetime , rather than through the exact spacetime . In order to achieve this, we begin by surrounding the body with a (hollow, three-dimensional) world tube embedded in the buffer region. We define the tube to be a surface of constant radius in Fermi normal coordinates centered on a world line , though the exact definition of the tube is immaterial. Since there exists a diffeomorphism between and in the buffer region, this defines a tube . Now, the problem is the following: what equation of motion must satisfy in order for to be “centered” about the body?
How shall we determine if the body lies at the centre of the tube’s interior? Since the tube is close to the small body (relative to all external length scales), the metric on the tube is primarily determined by the small body’s structure. Recall that the buffer region corresponds to an asymptotically large spatial distance in the inner expansion. Hence, on the tube, we can construct a multipole expansion of the body’s field, with the form (or – we will assume in the buffer region). Although alternative definitions could be used, we define the tube to be centered about the body if the mass dipole moment vanishes in this expansion. Note that this is the typical approach in general relativity: Whereas in Newtonian mechanics one directly finds the equation of motion for the centre of mass of a body, in general relativity one typically seeks a world line about which the mass dipole of the body vanishes (or an equation of motion for the mass dipole relative to a given nearby world line) [66, 152, 83, 144]. This definition of the world line is sufficiently general to apply to a black hole. If the body is material, one could instead imagine a centre-of-mass world line that lies in the interior of the body in the exact spacetime. This world line would then be the basis of our self-consistent expansion. We use our more general definition to cover both cases. See Ref.  and references therein for discussion of multipole expansions in general relativity, see Refs. [173, 174] for discussions of mass-centered coordinates in the buffer region, and see, e.g., Refs. [160, 65] for alternative definitions of centre of mass in general relativity.
As in the point-particle case, in order to determine the equation of motion of the world line, we consider a family of metrics, now denoted , parametrized by , such that when is given by the correct equation of motion for a given value of , we have . The metric in the outer limit is, thus, taken to be the general expansion
In the remainder of this section, we present a sequence of perturbation equations that arise in this expansion scheme, along with a complementary sequence for the inner expansion.
In the outer expansion, we seek a solution in a vacuum region outside of . We specify to be an open set consisting of the future domain of dependence of the spacelike initial-data surface , excluding the interior of the world tube . This implies that the future boundary of is a null surface . Refer to Figure 11 for an illustration. The boundary of the domain is . The spatial surface is chosen to intersect at the initial time .
Historically, in derivations of the self-force, solutions to the perturbative field equations were taken to be global in time, with tail integrals extended to negative infinity, as we wrote them in the preceding sections. But as was first noted in Ref. , because the self-force drives long-term, cumulative changes, any approximation truncated at a given order will be accurate to that order only for a finite time; and this necessites working in a finite region such as . This is also true in the case of point charges and masses. For simplicity, we neglected this detail in the preceding sections, but for completeness, we account for it here.
Within this region, we follow the methods presented in the case of a point mass. We begin by reformulating the Einstein equation such that it can be solved for an arbitrary world line. To accomplish this, we assume that the Lorenz gauge can be imposed on the whole of , everywhere in , such that . Here
Just as in the case of a point mass, this choice of gauge reduces the vacuum Einstein equation to a weakly nonlinear wave equation that can be expanded and solved at fixed . However, we now seek a solution only in the region , where the energy-momentum tensor vanishes, so the resulting sequence of wave equations reads
Again as in the case of a point particle, we can easily write down formal solutions to the wave equations, for arbitrary . Using the same methods as were used to derive the Kirchoff representation in Section 16.3, we find
One should note several important properties of these integral representations: First, must lie in the interior of ; an alternative expression must be derived if lies on the boundary . Second, the integral over the boundary is, in each case, a homogeneous solution to the wave equation, while the integral over the volume is an inhomogeneous solution. Third, if the field at the boundary satisfies the Lorenz gauge condition, then by virtue of the wave equation, it satisfies the gauge condition everywhere; hence, imposing the gauge condition to some order in the buffer region ensures that it is imposed to the same order everywhere.
While the integral representation is satisfied by any solution to the associated wave equation, it does not provide a solution. That is, one cannot prescribe arbitrary boundary values on and then arrive at a solution. The reason is that the tube is a timelike boundary, which means that field data on it can propagate forward in time and interfere with the data at a later time. However, by applying the wave operator onto Eq. (21.8), we see that the integral representation of is guaranteed to satisfy the wave equation at each point . In other words, the problem arises not in satisfying the wave equation in a pointwise sense, but in simultaneously satisfying the boundary conditions. But since the tube is chosen to lie in the buffer region, these boundary conditions can be supplied by the buffer-region expansion. And as we will discuss in Section 23, because of the asymptotic smallness of the tube, the pieces of the buffer-region expansion diverging as are sufficient boundary data to fully determine the global solution.
Finally, just as in the point-particle case, in order to split the gauge condition into a set of exactly solvable equations, we assume that the acceleration of possesses an expansion
The outer expansion is defined not only by holding fixed, but also by demanding that the mass dipole of the body vanishes when calculated in coordinates centered on . If we perform a gauge transformation generated by a vector , then the mass dipole will no longer vanish in those coordinates. Hence, a new world line must be constructed, such that the mass dipole vanishes when calculated in coordinates centered on that new world line. In other words, in the outer expansion we have the usual gauge freedom of regular perturbation theory, so long as the world line is appropriately transformed as well: . The transformation law for the world line was first derived by Barack and Ori ; it was displayed in Eq. (1.49), and it will be worked out again in Section 22.6.
Using this gauge freedom, we now justify, to some extent, the assumption that the Lorenz gauge condition can be imposed on the entirety of . If we begin with the metric in an arbitrary gauge, then the gauge vectors , , etc., induce the transformation.
For the inner expansion, we assume the existence of some local polar coordinates , such that the metric can be expanded for while holding fixed , , and ; to relate the inner and outer expansions, we assume , but otherwise leave the inner expansion completely general.
This leads to the ansatz
Since we are interested in the inner expansion only insofar as it informs the outer expansion, we shall not seek to explicitly solve the perturbative Einstein equation in the inner expansion. See Ref.  for the forms of the equations and an example of an explicit solution in the case of a perturbed black hole.
Living Rev. Relativity 14, (2011), 7
This work is licensed under a Creative Commons License.