224 RELATIVITY, THE GENERAL THEORYOn September 9, 1913, first Einstein then Grossmann read papers before the
annual meeting of the Swiss Physical Society [E36, G2]. These papers are sim-
plified versions of EG and contain nothing substantially new. Einstein had already
moved to Berlin when their next and last joint paper came out [E37]. In this work,
they returned to the gravitational equations to ask, What are the most general
transformations admissible under the assumption that the g^ are completely
determined by the field equations? In EG they had shown that the demand of
linearity was sufficient for this purpose. Now they found that some nonlinear
transformations are admissible as well (including accelerations of various kinds).
Actually, they were getting closer to the correct answers: their unjustified criterion
of uniquely determined g^, is expressed by a set of four not generally covariant
constraints. As is now well known, four constraints with this property (the so-
called coordinate conditions) are indeed required in the correct general theory in
order to eliminate the ambiguities in the gm by means of the choice of some par-
ticular coordinate system [W10].
All publications by Grossmann during the next seven years deal with pedagog-
ical and political subjects. Among social issues to which he devoted himself during
the First World War was aid to students of all nations who had been taken pris-
oners of war. Between 1922 and 1930 he wrote another five papers on his favorite
subject: descriptive geometry.
By 1920, the first signs of the disease that would fell him, multiple sclerosis,
had already appeared. By 1926 the symptoms were severe. His daughter Elsbeth
Grossmann told me that from then on he had difficulties speaking. In 1927 he had
to resign his professorship at the ETH.
In 1931 Grossmann wrote his last paper [G3]. It is a polemic, without formu-
lae, against the concepts of parallel displacement (Levi-Civita), absolute parallel-
ism (Cartan), and distant parallelism (Einstein). The paper originated as a reac-
tion to what Grossmann was told by a friend about a lecture by Einstein on unified
field theory. Grossmann asserts that there are logical objections to all the concepts
just mentioned. One cannot but feel sad upon reading this paper. Its contents were
discussed in a correspondence between Einstein and Grossmann that is friendly
yet strained. Einstein also wrote to Cartan, urging him not to answer Grossmann
publicly [E38]; Cartan agreed [C2].*
After Grossmann's death in 1936, Einstein wrote a moving and deeply respect-
ful letter to his widow [E39] about Grossmann's 'gruesome fate after early years
rich in work and aspiration.' He writes of Grossmann 'the exemplary student
. .. having good relations with the teachers .... I, separate and dissatisfied, not
very popular.' He writes of Grossmann's helping him to obtain a job, 'without
which I would not have died but might have spiritually wasted away.' He writes
of 'the joint feverish work a decade later.' And adds, 'Aber eines ist doch schon.
Wir waren und blieben Freunde durchs Leben hindurch.'**
* These two letters are contained in the published Cartan-Einstein correspondence [C3].
**But one thing is really beautiful. We were and remained friends throughout life.