344 THE LATER JOURNEYin this new version. Thus there are eighty fundamental fields, all of which are to
be varied independently in his variational principlewhere /?„, is once again the Ricci tensor (still a tensor, as was noted earlier).
Equation 17.50 looks, of course, very much like the variational principle in gen-
eral relativity. Indeed, Eq. 17.21 is recovered in the symmetric limit (not surpris-
ing since in that case the procedure reduces to the Palatini method [PI]). In the
general case, relations between 1^, and g^ can be obtained only up to the intro-
duction of an arbitrary 4-vector.
Einstein attempted to identify the symmetric part of gm with gravitation, the
antisymmetric part 0^ with the electromagnetic field. However, </>„, is in general
not a curl. The closest he could come to the first set of Maxwell equations was to
show that in the weak-field limitThere the paper ends. Einstein himself realized soon after the publication of
this work that the results were not impressive. He expressed this in three letters
to Ehrenfest. In the first one, he wrote, 'I have once again a theory of gravitation-
electricity; very beautiful but dubious' [E45]. In the second one, 'This summer I
wrote a very beguiling paper about gravitation-electricity ... but now I doubt
again very much whether it is true' [E46]. Two days later, 'My work of last
summer is no good' [E47]. In a paper written in 1927 he remarked, 'As a result
of numerous failures, I have now arrived at the conviction that this road [ Weyl
—» Eddington -* Einstein] does not bring us closer to the truth' [E48].
[Remark. Einstein's work was done independently of Cartan, who was the first
to introduce nonsymmetric connections (the antisymmetric parts of the Fj, are now
commonly known as Cartan torsion coefficients). There is considerable interest by
general relativists in theories of this kind, called Einstein-Cartan theories [H3].
Their main purpose is to link torsion to spin. This development has, of course,
nothing to do with unification, nor was Einstein ever active in this direction].
1921.\ Einstein returns to the Kaluza theory. His improved treatment turns
out to be identical with the work of Klein. In January 1928 he writes to Ehrenfest
that this is the right way to make progress. 'Long live the fifth dimension' [E49].
Half a year later, he was back at the connections.
- All attempts at unification mentioned thus far have in common that one
could imagine or hope for standard general relativity to reappear somehow,
embedded in a wider framework. Einstein's next try is particularly unusual, since
the most essential feature of the 'old' theory is lost from the very outset: the exis-
tence of a nonvanishing curvature tensor expressed in terms of the connection by
Eq. 17.26.
It began with a purely mathematical paper [E50], a rarity in Einstein's oeuvre,
in which he invented distant parallelism (also called absolute parallelism or tele-