

1.1 Preface
This historical review of classical unified field theories consists
of two parts. In the first, the development of unified field theory
between 1914 and 1933, i.e., during
the years Einstein lived and worked in Berlin,
will be covered. In the second, the very active period after 1933
until the 1960s to 1970s will be reviewed. In the first version of
Part I presented here, in view of the immense amount of
material, neither all shades of unified field theory nor all the
contributions from the various scientific schools will be discussed
with the same intensity; I apologise
for the shortcoming and promise to improve on it with the next
version. At least, even if I do not discuss them all in detail, as
many references as are necessary for a first acquaintance with the
field are listed here; completeness may be reached only (if at all)
by later updates. Although I also tried to take into account the
published correspondence between the main figures, my presentation,
again, is far from exhaustive in this context. Eventually,
unpublished correspondence will have to be worked in, and this may
change some of the conclusions. Purposely I included mathematicians
and also theoretical physicists of lesser rank than those who are
known to be responsible for big advances. My aim is to describe the
field in its full variety as it presented itself to the reader at
the time. The review is written such that physicists should be able
to follow the technical aspects of the papers
(cf. Section 2),
while historians of science without
prior knowledge of the mathematics of general relativity at least
might gain an insight into the development of concepts, methods,
and scientific communities involved. I should hope that readers
find more than one opportunity for further in-depth studies
concerning the many questions left open.
I profited from earlier reviews of the field, or
of parts of it, by Pauli
([242
], Section V);
Ludwig [211]; Whittaker
([415],
pp. 188-196); Lichnerowicz [208];
Tonnelat ([356
], pp. 1-14);
Jordan ([175],
Section III); Schmutzer ([289],
Section X); Treder ([182
], pp. 30-43);
Bergmann ([12],
pp. 62-73); Straumann [334, 335
]; Vizgin [384, 385
];
Bergia [11]; Goldstein
and Ritter [146]; Straumann and
O’Raifeartaigh [239
]; Scholz [291
], and
Stachel [330
]. The section on Einstein’s unified field theories
in Pais’ otherwise superb book presents the matter neither with the
needed historical correctness nor with enough technical
precision [240
]. A recent
contribution of van Dongen, focussing on Einstein’s methodology, was also
helpful [371
]. As will be seen,
with regard to interpretations and conclusions, my views are
different in some instances. In Einstein biographies, the subject
of “unified field theories” - although keeping Einstein busy for the second half
of his life - has been dealt with only in passing, e.g., in the book of Jordan [176], and in an
unsatisfying way in excellent books by Fölsing [136] and by
Hermann [159]. This
situation is understandable; for to describe a genius stubbornly
clinging to a set of ideas, sterile for physics in comparison with
quantum mechanics, over a period of more than 30 years, is not very
rewarding. For the short biographical notes, various editions of
J. C. Poggendorff’s
Biographisch-Literarischem Handwörterbuch and
internet sources have been used (in particular [1]). If not
indicated otherwise, all non-English quotations have been
translated by the author; the original text of quotations is given
in footnotes.

