An important point to bear in mind is that all the methods covered here mainly compute the self-force on a particle moving on a fixed world line of the background spacetime. A few numerical codes based on the radiative approximation have allowed orbits to evolve according to energy and angular-momentum balance. As will be emphasized below, however, these calculations miss out on important conservative effects that are only accounted for by the full self-force. Work is currently underway to develop methods to let the self-force alter the motion of the particle in a self-consistent manner.
The first evaluation of the electromagnetic self-force in curved spacetime was carried out by DeWitt and
DeWitt [132] for a charge moving freely in a weakly curved spacetime characterized by a Newtonian
potential
. In this context the right-hand side of Eq. (1.33
) reduces to the tail integral,
because the particle moves in a vacuum region of the spacetime, and there is no external force
acting on the charge. They found that the spatial components of the self-force are given by
A similar expression was obtained by Pfenning and Poisson [141] for the case of a scalar charge. Here
The force required to hold an electric charge in place in a Schwarzschild spacetime was computed, without approximations, by Smith and Will [163]. As we reviewed previously in Section 1.10, the self-force contribution to the total force is given by
where
Self-force calculations involving a sum over modes were pioneered by Barack and Ori [16, 7], and the
method was further developed by Barack, Ori, Mino, Nakano, and Sasaki [15, 8, 18
, 20
, 19
, 127
]; a
somewhat related approach was also considered by Lousto [117]. It has now emerged as the method of
choice for self-force calculations in spacetimes such as Schwarzschild and Kerr. Our understanding of the
method was greatly improved by the Detweiler–Whiting decomposition [53
] of the retarded field into
singular and regular pieces, as outlined in Sections 1.4 and 1.8, and subsequent work by Detweiler, Whiting,
and their collaborators [51
].
For simplicity we consider the problem of computing the self-force acting on a particle with a scalar charge
moving on a world line
. (The electromagnetic and gravitational problems are conceptually similar,
and they will be discussed below.) The potential
produced by the particle satisfies Eq. (1.34
), which we
rewrite schematically as
From the point of view of Eq. (2.5), the task of computing the self-force appears conceptually
straightforward: Either (i) compute the retarded and singular potentials, subtract them, and take a
gradient of the difference; or (ii) compute the gradients of the retarded and singular potentials,
and then subtract the gradients. Indeed, this is the basic idea for most methods of self-force
computations. However, the apparent simplicity of this sequence of steps is complicated by the following
facts: (i) except for a very limited number of cases, the retarded potential of a point particle
cannot be computed analytically and must therefore be obtained by numerical means; and (ii)
both the retarded and singular potential diverge at the particle’s position. Thus, any sort of
subtraction will generally have to be performed numerically, and for this to be possible, one requires
representations of the retarded and singular potentials (and/or their gradients) in terms of finite
quantities.
In a mode-sum method, these difficulties are overcome with a decomposition of the potential in spherical-harmonic functions:
When the background spacetime is spherically symmetric, Eq. (2.4 Fortunately, there is a piece of each -mode that does not contribute to the self-force, and that
can be subtracted out; this piece is the corresponding
-mode of the singular field
.
Because the retarded and singular fields share the same singularity structure near the particle’s
world line (as described in Section 1.6), the subtraction produces a mode decomposition of the
regular field
. And because this field is regular at the particle’s position, the sum over
all modes
is guaranteed to converge to the correct value for the self-force. The
key to the mode-sum method, therefore, is the ability to express the singular field as a mode
decomposition.
This can be done because the singular field, unlike the retarded field, can always be expressed as a local
expansion in powers of the distance to the particle; such an expansion was displayed in Eqs. (1.28) and
(1.29
). (In a few special cases the singular field is actually known exactly [43, 114, 33
, 86
, 162].) This local
expansion can then be turned into a multipole decomposition. Barack and Ori [18, 15, 20, 19
, 9], and
then Mino, Nakano, and Sasaki [127], were the first to show that this produces the following generic
structure:
With these elements in place, the self-force is finally computed by implementing the mode-sum formula
where the infinite sum over
The practical use of the mode-sum method can be illustrated with the help of a specific example that can
be worked out fully and exactly. We consider, as in Section 1.10, an electric charge held in place at
position
in the spacetime of an extreme Reissner–Nordström black hole of mass
and charge
. The reason for selecting this spacetime resides in the resulting simplicity of the
spherical-harmonic modes for the electromagnetic field.
The metric of the extreme Reissner–Nordström spacetime is given by
whereThe charge density of a static particle can also be decomposed in spherical harmonics, and the mode coefficients are given by
where For the general solution to the homogeneous equation is
, where
and
are
constants and
. The solution for
must be regular at
, and we select
. The solution for
must produce a field that decays as
at large
, and we
again select
. Since each constant is proportional to the total charge enclosed within a
sphere of radius
, we arrive at
According to the foregoing results, and recalling the definition of Eq. (2.13), the multipole coefficients of
the electromagnetic field at
are given by
The self-force acting on the static charge is then
This expression agrees with the Smith-Will force of Eq. (1.50
The mode-sum method was successfully implemented in Schwarzschild spacetime to compute the scalar and
electromagnetic self-forces on a static particle [31, 36] . It was used to calculate the scalar self-force on a
particle moving on a radial trajectory [10], circular orbit [30, 51, 87, 37], and a generic bound orbit [84
]. It
was also developed to compute the electromagnetic self-force on a particle moving on a generic bound
orbit [85
], as well as the gravitational self-force on a point mass moving on circular [21
, 1] and eccentric
orbits [22
]. The mode-sum method was also used to compute unambiguous physical effects associated with
the gravitational self-force [50
, 157, 11
], and these results were involved in detailed comparisons with
post-Newtonian theory [50
, 29
, 28
, 44
, 11
]. These achievements will be described in more detail in
Section 2.6.
An issue that arises in computations of the electromagnetic and gravitational self-forces is the choice of
gauge. While the self-force formalism is solidly grounded in the Lorenz gauge (which allows the formulation
of a wave equation for the potentials, the decomposition of the retarded field into singular and regular
pieces, and the computation of regularization parameters), it is often convenient to carry out the numerical
computations in other gauges, such as the popular Regge–Wheeler gauge and the Chrzanowski radiation
gauge described below. Compatibility of calculations carried out in different gauges has been debated in the
literature. It is clear that the singular field is gauge invariant when the transformation between the
Lorenz gauge and the adopted gauge is smooth on the particle’s world line; in such cases the
regularization parameters also are gauge invariant [17], the transformation affects the regular
field only, and the self-force changes according to Eq. (1.49
). The transformations between the
Lorenz gauge and the Regge–Wheeler and radiation gauges are not regular on the world line,
however, and in such cases the regularization of the retarded field must be handled with extreme
care.
The reliance of the mode-sum method on a spherical-harmonic decomposition makes it generally impractical to apply to self-force computations in Kerr spacetime. Wave equations in this spacetime are better analyzed in terms of a spheroidal-harmonic decomposition, which simultaneously requires a Fourier decomposition of the field’s time dependence. (The eigenvalue equation for the angular functions depends on the mode’s frequency.) For a static particle, however, the situation simplifies, and Burko and Liu [35] were able to apply the method to calculate the self-force on a static scalar charge in Kerr spacetime. More recently, Warburton and Barack [181] carried out a mode-sum calculations of the scalar self-force on a particle moving on equatorial orbits of a Kerr black hole. They first solve for the spheroidal multipoles of the retarded potential, and then re-express them in terms of spherical-harmonic multipoles. Fortunately, they find that a spheroidal multipole is well represented by summing over a limited number of spherical multipoles. The Warburton–Barack work represents the first successful computations of the self-force in Kerr spacetime, and it reveals the interesting effect of the black hole’s spin on the behaviour of the self-force.
The analysis of the scalar wave equation in terms of spheroidal functions and a Fourier decomposition permits a complete separation of the variables. For decoupling and separation to occur in the case of a gravitational perturbation, it is necessary to formulate the perturbation equations in terms of Newman–Penrose (NP) quantities [172], and to work with the Teukolsky equation that governs their behaviour. Several computer codes are now available that are capable of integrating the Teukolsky equation when the source is a point mass moving on an arbitrary geodesic of the Kerr spacetime. (A survey of these codes is given below.) Once a solution to the Teukolsky equation is at hand, however, there still remains the additional task of recovering the metric perturbation from this solution, a problem referred to as metric reconstruction.
Reconstruction of the metric perturbation from solutions to the Teukolsky equation was tackled in the past in the pioneering efforts of Chrzanowski [41], Cohen and Kegeles [42, 105], Stewart [166], and Wald [179]. These works have established a procedure, typically attributed to Chrzanowski, that returns the metric perturbation in a so-called radiation gauge. An important limitation of this method, however, is that it applies only to vacuum solutions to the Teukolsky equation. This makes the standard Chrzanowski procedure inapplicable in the self-force context, because a point particle must necessarily act as a source of the perturbation. Some methods were devised to extend the Chrzanowski procedure to accommodate point sources in specific circumstances [121, 134], but these were not developed sufficiently to permit the computation of a self-force. See Ref. [184] for a review of metric reconstruction from the perspective of self-force calculations.
A remarkable breakthrough in the application of metric-reconstruction methods in self-force calculations
was achieved by Keidl, Wiseman, and Friedman [107, 106, 108
], who were able to compute
a self-force starting from a Teukolsky equation sourced by a point particle. They did it first
for the case of an electric charge and a point mass held at a fixed position in a Schwarzschild
spacetime [107
], and then for the case of a point mass moving on a circular orbit around a
Schwarzschild black hole [108
]. The key conceptual advance is the realization that, according to the
Detweiler–Whiting perspective, the self-force is produced by a regularized field that satisfies vacuum field
equations in a neighbourhood of the particle. The regular field can therefore be submitted to
the Chrzanowski procedure and reconstructed from a source-free solution to the Teukolsky
equation.
More concretely, suppose that we have access to the Weyl scalar produced by a point mass moving
on a geodesic of a Kerr spacetime. To compute the self-force from this, one first calculates the singular Weyl
scalar
from the Detweiler–Whiting singular field
, and subtracts it from
. The result is a
regularized Weyl scalar
, which is a solution to the homogeneous Teukolsky equation. This sets
the stage for the metric-reconstruction procedure, which returns (a piece of) the regular field
in the radiation gauge selected by Chrzanowski. The computation must be completed by
adding the pieces of the metric perturbation that are not contained in
; these are referred to
either as the nonradiative degrees of freedom (since
is purely radiative), or as the
and
field multipoles (because the sum over multipoles that make up
begins at
). A method to complete the Chrzanowski reconstruction of
was devised by Keidl et
al. [107, 108], and the end result leads directly to the gravitational self-force. The relevance of the
and
modes to the gravitational self-force was emphasized by Detweiler and
Poisson [52].
When calculating the spherical-harmonic components of the retarded potential
– refer back
to Eq. (2.6
) – one can choose to work either directly in the time domain, or perform a Fourier
decomposition of the time dependence and work instead in the frequency domain. While the time-domain
method requires the integration of a partial differential equation in
and
, the frequency-domain
method gives rise to set of ordinary differential equations in
, one for each frequency
. For particles
moving on circular or slightly eccentric orbits in Schwarzschild spacetime, the frequency spectrum is limited
to a small number of discrete frequencies, and a frequency-domain method is easy to implement and yields
highly accurate results. As the orbital eccentricity increases, however, the frequency spectrum
broadens, and the computational burden of summing over all frequency components becomes
more significant. Frequency-domain methods are less efficient for large eccentricities, the case
of most relevance for extreme-mass-ratio inspirals, and it becomes advantageous to replace
them with time-domain methods. (See Ref. [25] for a quantitative study of this claim.) This
observation has motivated the development of accurate evolution codes for wave equations in 1+1
dimensions.
Such codes must be able to accommodate point-particle sources, and various strategies have been
pursued to represent a Dirac distribution on a numerical grid, including the use of very narrow Gaussian
pulses [116, 110, 34] and of “finite impulse representations” [168]. These methods do a good job with
waveform and radiative flux calculations far away from the particle, but are of very limited accuracy when
computing the potential in a neighborhood of the particle. A numerical method designed to provide
an exact representation of a Dirac distribution in a time-domain computation was devised by
Lousto and Price [120] (see also Ref. [123]). It was implemented by Haas [84, 85] for the specific
purpose of evaluating at the position of particle and computing the self-force. Similar
codes were developed by other workers for scalar [176
] and gravitational [21, 22
] self-force
calculations.
Most extant time-domain codes are based on finite-difference techniques, but codes based on pseudo-spectral methods have also been developed [67, 68, 37, 38]. Spectral codes are a powerful alternative to finite-difference codes, especially when dealing with smooth functions, because they produce much faster convergence. The fact that self-force calculations deal with point sources and field modes that are not differentiable might suggest that spectral convergence should not be expected in this case. This objection can be countered, however, by placing the particle at the boundary between two spectral domains. Functions are then smooth in each domain, and discontinuities are handled by formulating appropriate boundary conditions; spectral convergence is thereby achieved.
The mode-sum methods reviewed in the preceding subsection have been developed and applied
extensively, but they do not exhaust the range of approaches that may be exploited to compute a
self-force. Another set of methods, devised by Barack and his collaborators [12, 13
, 60
] as well as
Vega and his collaborators [176
, 177
, 175], begin by recognizing that an approximation to
the exact singular potential can be used to regularize the delta-function source term of the
original field equation. We shall explain this idea in the simple context of a scalar potential
.
We continue to write the wave equation for the retarded potential in the schematic form
To see this, we write the effective source more specifically as
With the window functionThe effective-source method therefore consists of integrating the wave equation
for the approximated regular potential The method is also well suited to a self-consistent implementation of the self-force, in which the motion
of the particle is not fixed in advance, but determined by the action of the computed self-force. This
amounts to combining Eq. (2.33) with the self-force equation
The work of Barack and collaborators [12, 13] is a particular implementation of the effective-source
approach in a 2+1 numerical calculation of the scalar self-force in Kerr spacetime. (See also the independent
implementation by Lousto and Nakano [119].) Instead of starting with Eq. (2.27), they first decompose
according to
As was seen in Eqs. (1.33), (1.40
), and (1.47
), the self-force can be expressed as an integral over the past
world line of the particle, the integrand involving the Green’s function for the appropriate wave equation.
Attempts have been made to compute the Green’s function directly [132
, 141
, 33
, 86
], and to
evaluate the world-line integral. The quasilocal approach, first introduced by Anderson and his
collaborators [4, 3, 6, 5], is based on the expectation that the world-line integral might be dominated by
the particle’s recent past, so that the Green’s function can be represented by its Hadamard expansion,
which is restricted to the normal convex neighbourhood of the particle’s current position. To
help with this enterprise, Ottewill and his collaborators [136, 182, 137, 39] have pushed the
Hadamard expansion to a very high order of accuracy, building on earlier work by Décanini and
Folacci [48].
The weak-field calculations performed by DeWitt and DeWitt [132] and Pfenning and Poisson [141] suggest that the world-line integral is not, in fact, dominated by the recent past. Instead, most of the self-force is produced by signals that leave the particle at some time in the past, scatter off the central mass, and reconnect with the particle at the current time; such signals mark the boundary of the normal convex neighbourhood. The quasilocal evaluation of the world-line integral must therefore be supplemented with contributions from the distant past, and this requires a representation of the Green’s function that is not limited to the normal convex neighbourhood. In some spacetimes it is possible to express the Green’s function as an expansion in quasi-normal modes, as was demonstrated by Casals and his collaborators for a static scalar charge in the Nariai spacetime [40]. Their study provided significant insights into the geometrical structure of Green’s functions in curved spacetime, and increased our understanding of the non-local character of the self-force.
The accurate computation of long-term waveforms from extreme-mass-ratio inspirals (EMRIs) involves a lengthy sequence of calculations that include the calculation of the self-force. One can already imagine the difficulty of numerically integrating the coupled linearized Einstein equation for durations much longer than has ever been attempted by existing numerical codes. While doing so, the code would also have to evaluate the self-force reasonably often (if not at each time step) in order to remain close to the true dynamics of the point mass. Moreover, gravitational-wave data analysis via matched filtering require full evolutions of the sort just described for a large sample of systems parameters. All these considerations underlie the desire for simplified approximations to fully self-consistent self-force EMRI models.
The adiabatic approximation refers to one such class of potentially useful approximations. The basic
assumption is that the secular effects of the self-force occur on a timescale that is much longer than the
orbital period. In an extreme-mass-ratio binary, this assumption is valid during the early stage of inspiral; it
breaks down in the final moments, when the orbit transitions to a quasi-radial infall called the plunge. From
the adiabaticity assumption, numerous approximations have been formulated: For example, (i) since the
particle’s orbit deviates only slowly from geodesic motion, the self-force can be calculated from a field
sourced by a geodesic; (ii) since the radiation-reaction timescale , over which a significant shrinking of
the orbit occurs due to the self-force, is much longer than the orbital period, periodic effects of the self-force
can be neglected; and (iii) conservative effects of the self-force can be neglected (the radiative
approximation).
A seminal example of an adiabatic approximation is the Peters–Mathews formalism [140, 139], which
determines the long-term evolution of a binary orbit by equating the time-averaged rate of change of the
orbital energy and angular momentum
to, respectively, the flux of gravitational-wave
energy and angular momentum at infinity. This formalism was used to successfully predict the
decreasing orbital period of the Hulse–Taylor pulsar, before more sophisticated methods, based on
post-Newtonian equations of motion expanded to 2.5pn order, were incorporated in times-of-arrival
formulae.
In the hope of achieving similar success in the context of the self-force, considerable work
has been done to formulate a similar approximation for the case of an extreme-mass-ratio
inspiral [124, 125, 126, 98, 61
, 62
, 159
, 158
, 78
, 128, 94
]. Bound geodesics in Kerr spacetime are
specified by three constants of motion – the energy
, angular momentum
, and Carter constant
.
If one could easily calculate the rates of change of these quantities, using a method analogous to the
Peters–Mathews formalism, then one could determine an approximation to the long-term orbital
evolution of the small body in an EMRI, avoiding the lengthy process of regularization involved in
the direct integration of the self-forced equation of motion. In the early 1980s, Gal’tsov [77]
showed that the average rates of change of
and
, as calculated from balance equations
that assume geodesic source motion, agree with the averaged rates of change induced by a
self-force constructed from a radiative Green’s function defined as
. As
discussed in Section 1.4, this is equal to the regular two-point function
in flat spacetime, but
in curved spacetime; because of its time-asymmetry, it is purely dissipative. Mino [124],
building on the work of Gal’tsov, was able to show that the true self-force and the dissipative
force constructed from
cause the same averaged rates of change of all three constants of
motion, lending credence to the radiative approximation. Since then, the radiative Green’s
function was used to derive explicit expressions for the rates of change of
,
, and
in terms of the particle’s orbit and wave amplitudes at infinity [159, 158, 78], and radiative
approximations based on such expressions have been concretely implemented by Drasco, Hughes and their
collaborators [99, 61, 62].
The relevance of the conservative part of the self-force – the part left out when using – was
analyzed in numerous recent publications [32, 148
, 146
, 147
, 94
, 97
]. As was shown by Pound et
al. [148, 146, 147], neglect of the conservative effects of the self-force generically leads to long-term errors
in the phase of an orbit and the gravitational wave it produces. These phasing errors are due
to orbital precession and a direct shift in orbital frequency. This shift can be understood by
considering a conservative force acting on a circular orbit: the force is radial, it alters the centripetal
acceleration, and the frequency associated with a given orbital radius is affected. Despite these errors,
a radiative approximation may still suffice for gravitational-wave detection [94
]; for circular
orbits, which have minimal conservative effects, radiative approximations may suffice even for
parameter-estimation [97]. However, at this point in time, these analyses remain inconclusive
because they all rely on extrapolations from post-Newtonian results for the conservative part of
the self-force. For a more comprehensive discussion of these issues, the reader is referred to
Ref. [94
, 143
].
Hinderer and Flanagan performed the most comprehensive study of these issues [69], utilizing a
two-timescale expansion [109
, 145
] of the field equations and self-forced equations of motion in an EMRI. In
this method, all dynamical variables are written in terms of two time coordinates: a fast time
and
a slow time
. In the case of an EMRI, the dynamical variables are the metric
and the phase-space variables of the world line. The fast-time dependence captures evolution
on the orbital timescale
, while the slow-time dependence captures evolution on the
radiation-reaction timescale
. One obtains a sequence of fast-time and slow-time equations by
expanding the full equations in the limit of small
while treating the two time coordinates as
independent. Solving the leading-order fast-time equation, in which
is held fixed, yields a metric
perturbation sourced by a geodesic, as one would expect from the linearized field equations for a point
particle. The leading-order effects of the self-force are incorporated by solving the slow-time
equation and letting
vary. (Solving the next-higher-order slow-time equation determines similar
effects, but also the backreaction that causes the parameters of the large black hole to change
slowly.)
Using this method, Hinderer and Flanagan identified the effects of the various pieces of the self-force. To describe this we write the self-force as
where ‘rr’ denotes a radiation-reaction, or dissipative, piece of the force, and ‘c’ denotes a conservative piece. Hinderer and Flanagan’s principal result is a formula for the orbital phase (which directly determines the phase of the emitted gravitational waves) in terms of these quantities: where To understand this result, consider the following naive analysis of a quasicircular EMRI — that is, an
orbit that would be circular were it not for the action of the self-force, and which is slowly spiraling into the
large central body. We write the orbital frequency as , where
is the frequency as a function of energy on a circular geodesic, and
is the correction to this due to the conservative part of the first-order self-force (part of the
correction also arises due to oscillatory zeroth-order effects combining with oscillatory first-order
effects, but for simplicity we ignore this contribution). Neglecting oscillatory effects, we write the
energy in terms only of its slow-time dependence:
. The
leading-order term
is determined by the dissipative part of first-order self-force, while
is
determined by both the dissipative part of the second-order force and a combination of conservative
and dissipative parts of the first-order force. Substituting this into the frequency, we arrive at
To be of relevance to gravitational-wave astronomy, the paramount goal of the self-force community remains the computation of waveforms that properly encode the long-term dynamical evolution of an extreme-mass-ratio binary. This requires a fully consistent orbital evolution fed to a wave-generation formalism, and to this day the completion of this program remains as a future challenge. In the meantime, a somewhat less ambitious, though no less compelling, undertaking is that of probing the physical consequences of the self-force on the motion of point particles.
The intriguing phenomenon of a scalar charge changing its rest mass because of an interaction with its
self-field was studied by Burko, Harte, and Poisson [33] and Haas and Poisson [86
] in the simple context of
a particle at rest in an expanding universe. The scalar Green’s function could be computed explicitly for a
wide class of cosmological spacetimes, and the action of the field on the particle determined without
approximations. It is found that for certain cosmological models, the mass decreases and then increases back
to its original value. For other models, the mass is restored only to a fraction of its original
value. For de Sitter spacetime, the particle radiates all of its rest mass into monopole scalar
waves.
The earliest calculation of a gravitational self-force was performed by Barack and Lousto for the case of a point mass plunging radially into a Schwarzschild black hole [14]. The calculation, however, depended on a specific choice of gauge and did not identify unambiguous physical consequences of the self-force. To obtain such consequences, it is necessary to combine the self-force (computed in whatever gauge) with the metric perturbation (computed in the same gauge) in the calculation of a well-defined observable that could in principle be measured. For example, the conservative pieces of the self-force and metric perturbation can be combined to calculate the shifts in orbital frequencies that originate from the gravitational effects of the small body; an application of such a calculation would be to determine the shift (as measured by frequency) in the innermost stable circular orbit of an extreme-mass-ratio binary, or the shift in the rate of periastron advance for eccentric orbits. Such calculations, however, must exclude all dissipative aspects of the self-force, because these introduce an inherent ambiguity in the determination of orbital frequencies.
A calculation of this kind was recently achieved by Barack and Sago [22, 23], who computed the shift in
the innermost stable circular orbit of a Schwarzschild black hole caused by the conservative piece of the
gravitational self-force. The shift in orbital radius is gauge dependent (and was reported in the
Lorenz gauge by Barack and Sago), but the shift in orbital frequency is measurable and therefore
gauge invariant. Their main result – a genuine milestone in self-force computations – is that the
fractional shift in frequency is equal to ; the frequency is driven upward by the
gravitational self-force. Barack and Sago compare this shift to the ambiguity created by the
dissipative piece of the self-force, which was previously investigated by Ori and Thorne [135]
and Sundararajan [167]; they find that the conservative shift is very small compared with the
dissipative ambiguity. In a follow-up analysis, Barack, Damour, and Sago [11
] computed the
conservative shift in the rate of periastron advance of slightly eccentric orbits in Schwarzschild
spacetime.
Conservative shifts in the innermost stable circular orbit of a Schwarzschild black hole were first
obtained in the context of the scalar self-force by Diaz-Rivera et al. [55]; in this case they obtain a
fractional shift of , and here also the frequency is driven upward.
In another effort to extract physical consequences from the gravitational self-force on a particle
in circular motion in Schwarzschild spacetime, Detweiler discovered [50] that
, the time
component of the velocity vector in Schwarzschild coordinates, is invariant with respect to a
class of gauge transformations that preserve the helical symmetry of the perturbed spacetime.
Detweiler further showed that
is an observable: it is the redshift that a photon suffers
when it propagates from the orbiting body to an observer situated at a large distance on the
orbital axis. This gauge-invariant quantity can be calculated together with the orbital frequency
, which is a second gauge-invariant quantity that can be constructed for circular orbits in
Schwarzschild spacetime. Both
and
acquire corrections of fractional order
from the
self-force and the metric perturbation. While the functions
and
are still gauge
dependent, because of the dependence on the radial coordinate
, elimination of
from these
relations permits the construction of
, which is gauge invariant. A plot of
as a
function of
therefore contains physically unambiguous information about the gravitational
self-force.
The computation of the gauge-invariant relation opened the door to a detailed
comparison between the predictions of the self-force formalism to those of post-Newtonian
theory. This was first pursued by Detweiler [50], who compared
as determined accurately
through second post-Newtonian order, to self-force results obtained numerically; he reported
full consistency at the expected level of accuracy. This comparison was pushed to the third
post-Newtonian order [29, 28, 44, 11]. Agreement is remarkable, and it conveys a rather deep
point about the methods of calculation. The computation of
, in the context of both the
self-force and post-Newtonian theory, requires regularization of the metric perturbation created by
the point mass. In the self-force calculation, removal of the singular field is achieved with the
Detweiler–Whiting prescription, while in post-Newtonian theory it is performed with a very
different prescription based on dimensional regularization. Each prescription could have returned a
different regularized field, and therefore a different expression for
. This, remarkably, does
not happen; the singular fields are “physically the same” in the self-force and post-Newtonian
calculations.
A generalization of Detweiler’s redshift invariant to eccentric orbits was recently proposed and computed by Barack and Sago [24], who report consistency with corresponding post-Newtonian results in the weak-field regime. They also computed the influence of the conservative gravitational self-force on the periastron advance of slightly eccentric orbits, and compared their results with full numerical relativity simulations for modest mass-ratio binaries. Thus, in spite of the unavailability of self-consistent waveforms, it is becoming clear that self-force calculations are already proving to be of value: they inform post-Newtonian calculations and serve as benchmarks for numerical relativity.
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