THE NEW DYNAMICS 271ulus for the vastly increased activity and the new directions in general relativity.
The few dozen practitioners in Einstein's days are followed by a new generation
about a hundred times more numerous.
Now, in 1982, the beginning of a new era described by Sciama has already been
followed by further important developments. In June 1980 I attended the GR9
conference in order to find out more about the status of the field. Some of my
impressions are found in what follows. Each of the next five sections is devoted to
a topic in general relativity in which Einstein himself was active after 1915. In
each section I shall indicate what he did and sketch ever so briefly how that subject
developed in later years. In the final section, I list those topics which in their
entirety belong to the post-Einsteinian era.15b. The Three Successes
In 1933 Einstein, speaking in Glasgow on the origins of the general theory of
relativity [E2], recalled some of his struggles, the 'errors in thinking which caused
me two years of hard work before at last, in 1915,1 recognized them as such and
returned penitently to the Riemann curvature, which enabled me to find the rela-
tion to the empirical facts of astronomy.'
The period 1914-15 had been a confusing two years, not only for Einstein but
also for those of his colleagues who had tried to follow his gyrations. For example,
when in December 1915 Ehrenfest wrote to Lorentz, he referred to what we call
the theory of general relativity as 'the theory of November 25, 1915.' He asked if
Lorentz agreed with his own understanding that Einstein had now abandoned his
arguments of 1914 for the impossibility of writing the gravitational field equations
in covariant form [E3]. All through December 1915 and January 1916, the cor-
respondence between Lorentz and Ehrenfest is intense and reveals much about
their personalities. Lorentz, aged 62, is calculating away in Haarlem, making
mistakes, correcting them, finally understanding what Einstein has in mind. In a
letter to Ehrenfest he writes, 'I have congratulated Einstein on his brilliant result'
[L3]. Ehrenfest, aged 35, in Leiden, ten miles down the road, is also hard at work
on relativity. His reply to Lorentz's letter shows a glimpse of the despair that
would ultimately overwhelm him: 'Your remark "I have congratulated Einstein
on his brilliant results" has a similar meaning for me as when one Freemason
recognizes another by a secret sign' [E4].
Meanwhile Lorentz had received a letter from Einstein in which the latter
expressed his happiness with Lorentz's praise. Einstein added, 'The series of my
papers about gravitation is a chain of false steps [Irrwegen] which nevertheless by
and by led to the goal. Thus the basic equations are finally all right but the der-
ivations are atrocious; this shortcoming remains to be eliminated' [E5]. He went
on to suggest that Lorentz might be the right man for this task. 'I could do it
myself, since all is clear to me. However, nature has unfortunately denied me the
gift of being able to communicate, so that what I write is correct, to be sure, but