THE NEW DYNAMICS 269tinued. There was also further research on the problem of motion (which had
interested Einstein since 1927), the question of if and how the equations of motion
of a distribution of matter can be obtained as a consequence of the gravitational
field equations. By and large, throughout this period the advances due to general
relativity are perceived to be the 'three successes'—the precession of the perihelion
of Mercury, the bending of light, and the red shift—and a rationale for an
expanding universe.
However, in the 1930s a new element was injected which briefly attracted
attention, then stayed more or less quiescent for a quarter of a century, after which
time it became one of general relativity's main themes. Principally as an exercise
in nuclear physics, J. Robert Oppenheimer and his research associate Robert Ser-
ber decided to study the relative influence of nuclear and gravitational forces in
neutron stars [Ol].* One of their aims was to improve the estimate made by Lev
Davidovich Landau for the limiting mass above which an ordinary star becomes
a neutron star. (Landau discussed a model in which this mass is ~ 0.001 O. He
also suggested that every star has an interior neutron core [L2a].) Their work
attracted the attention of Richard Chase Tolman. As a result of discussions
between Tolman and Oppenheimer and his co-workers, there appeared in 1939,
a pair of papers, one by Tolman on static solutions of Einstein's field equations
for fluid spheres [Tl] and one, directly following it, by Oppenheimer and George
Volkoff entitled 'On massive neutron cores' [O2]. In this paper, the foundations
are laid for a general relativistic theory of stellar structure. The model discussed
is a static spherical star consisting of an ideal Fermi gas of neutrons. The authors
found that the star is stable as long as its mass < % O. (The present best value
for a free-neutron gas is — 0.7 O and is called the Oppenheimer-Volkoff limit.)**
Half a year later, the paper 'On continued gravitational attraction' by Oppenhei-
mer and Hartland Snyder came out [O3]. The first line of its abstract reads,
'When all thermonuclear sources of energy are exhausted, a sufficiently heavy star
will collapse; [a contraction follows which] will continue indefinitely.' Thus began
the physics of black holes, the name for the ultimate collapsed state proposed by
John Archibald Wheeler at a conference held in the fall of 1967 at the Goddard
Institute of Space Studies in New York [W5]. At that time, pulsars had just been
discovered and neutron stars and black holes were no longer considered 'exotic
objects [which] remained a textbook curiosity.... Cooperative efforts of radio and
optical astronomers [had begun] to reveal a great many strange new things in the
sky' [W6].
Which brings us to the change in style of general relativity after Einstein's
death.
During Einstein's lifetime, there was not one major international conference
*I am indebted to Robert Serber for a discussion of the papers on neutron stars by Oppenheimer
and his collaborators.
**For further details, see [M2], p. 627.