THE NEW DYNAMICS 291
on the TV-body problem of motion [E59, E60]. In these papers, the gravitational
field is no longer treated as external. Instead, it and the motion of its (singular)
sources are treated simultaneously. A new approximation scheme is introduced in
which the fields are no longer necessarily weak but in which the source velocities
are small compared with the light velocity. Their results are not new; the same or
nearly the same results were obtained much earlier by Lorentz and Droste, de
Sitter, Fock, and Levi-Civita (P. Havas, private communication). The equations
obtained have found use in situations where Newtonian interaction must be
included. '[These equations] are widely used in analyses of planetary orbits in the
solar system. For example, the Gal Tech Jet Propulsion Laboratory uses them,
in modified form, to calculate ephemerides for high-precision tracking of planets
and spacecraft' [Ml3].
In his report to GR9 on the problem of motion, Ehlers stressed the difficulties
of defining isolated systems in general relativity and the need not to treat the prob-
lem of motion as an isolated question. Rather, the problem should be linked with
other issues, such as the description of extended bodies and gravitational radiation
(see also [E61 ]).* A particle physicist might like to add that the problem of motion
should perhaps not be dissociated from the fact that a body has a Compton wave-
length, a parameter of little interest for big things—and vice versa.
15g. What Else Was New at GR9?
The program of GR9 showed that all the topics discussed in the preceding sections
continue to be of intense interest. I conclude by listing other subjects discussed at
that meeting. Exact solutions are now examined by new analytic methods as well
as by computer studies. Other classical interests include the important Cauchy
problem.** Current experimental results (notably the huge precession of the per-
iastron of PSR 1913 + 16) and future terrestrial and planetary experiments were
discussed, with refined tests of general relativity in mind. There was a debate on
relativistic thermodynamics, a controversial subject to this day. There were reports
on the fundamental advances of our understanding regarding the general structure
of relativity theory, with special reference to singularity theorems, black holes, and
cosmic censorship. We were told that the best of all possible universes is still the
Friedmann universe, not only in our epoch but since time began. These beginnings
(especially the earliest fraction of a second) were reviewed with reference to bary-
*For example, it so happens that in the approximation defined by Eqs. 15.11-15.14, sources move
with constant velocity (!) [E57].
**Mme Y. Choquet-Bruhat told me that Einstein did not show much interest in this problem when
she once discussed it with him.