22 INTRODUCTORY
light touch and deceptive ease so typical of all his work published in 1905. The
first steps are made in 1907, as he discovers a simple version of the equivalence
principle and understands that matter will bend light and that the spectral lines
reaching us from the sun should show a tiny shift toward the red relative to the
same spectral lines produced on earth (9). During the next three and a half years,
his attention focuses on that crisis phenomenon, the quantum theory, rather than
on the less urgent problems of relativity (10). His serious concentration on general
relativity begins after his arrival in Prague in 1911, where he teaches himself a
great deal with the help of a model theory. He gives a calculation of the bending
of light by the sun. His result is imperfect, since at that time he still believes that
space is flat (11). In the summer of 1912, at the time of his return to Ziirich, he
makes a fundamental discovery: space is not flat; the geometry of the world is not
Euclidean. It is Riemannian. Ably helped by an old friend, the mathematician
Marcel Grossmann, he establishes the first links between geometry and gravity.
With his habitual optimism he believes he has solved the fifty-year-old problem
(13) of finding a field theory of gravitation. Not until late in 1915 does he fully
realize how flawed his theory actually is. At that very same time, Hilbert starts
his important work on gravitation (14d). After a few months of extremely intense
work, Einstein presents the final revised version of his theory on November 25,
1915 (14c).
One week earlier he had obtained two extraordinary results. Fulfilling an
aspiration he had had since 1907, he found the correct explanation of the long-
known precession of the perihelion of the planet Mercury. That was the high
point in his scientific life He was so excited that for three days he could not work.
In addition he found that his earlier result on the bending of light was too small
by a factor of 2. Einstein was canonized in 1919 when this second prediction also
proved to be correct (16b).
After 1915 Einstein continued to examine problems in general relativity. He
was the first to give a theory of gravitational waves (15d). He was also the founder
of general relativistic cosmology, the modern theory of the universe at large (15e).
Hubble's discovery that the universe is expanding was made in Einstein's lifetime.
Radio galaxies, quasars, neutron stars, and, perhaps, black holes were found after
his death. These post-Einsteinian observational developments in astronomy
largely account for the great resurgence of interest in general relativity in more
recent times. A sketchy account of the developments in general relativity after 1915
up to the present appears in (15).
I return to earlier days. After 1915 Einstein's activities in the domain of rela-
tivity became progressively less concerned with the applications of general relativ-
ity than with the search for generalization of that theory. During the early years
following the discovery of general relativity, the aim of that search appeared to be
highly plausible: according to general relativity the very existence of the gravita-
tional field is inalienably woven into the geometry of the physical world. There
was nothing equally compelling about the existence of the electromagnetic field,