“The idea of achieving [a unified field theory] by means of a five-dimensional cylinder world never dawned on me. [...] At first glance I like your idea enormously.” (letter of Einstein to Kaluza of 21 April 1919)

This remark is surprising because Nordström had suggested a five-dimensional unification of his scalar
gravitational theory with electromagnetism five years earlier [238], by embedding space-time into a
five-dimensional world in quite the same way as Kaluza did. In principle, Einstein could have known
Nordström’s work. In the same year 1914, he and Fokker had given a covariant formulation of Nordström’s
pure (scalar) theory of gravitation [104]. In a subsequent letter to Kaluza of 5 May 1919 Einstein still was
impressed: “The formal unity of your theory is startling.” However, on 29 May 1919, Einstein became somewhat
reserved^{83}:

“I respect greatly the beauty and boldness of your idea. But you understand that, in view
of the existing factual concerns, I cannot take sides as planned originally.”^{84}

Kaluza’s paper was communicated by Einstein to the Academy, but for reasons unknown was published only in
1921 [181]. Kaluza’s idea was to write down the Einstein field equations for empty space in a
five-dimensional Riemannian manifold with metric , i.e., , , where
is the Ricci tensor of , and to look at small deviations from Minkowski space:
.^{85}.
In order to obtain a theory in space-time, he assumed the so-called “cylinder condition”

Kaluza also showed that the geodesics of the five-dimensional space reduce to the equations of motion for a charged point particle in space-time, if a weakness assumption is made for the components of the 5-velocity : , . The Lorentz force appears augmented by an additional term containing of the order which thus may be neglected. From the fifth equation of motion Kaluza concluded that the fifth component of momentum , with being the particles’ electric charge (up to a constant of proportionality). From the equations of motion, charge conservation also followed in Kaluza’s linear approximation. Kaluza was well aware that his theory broke down if applied to elementary particles like electrons or protons, and speculated about an escape in which gravitation had to be considered as some “difference effect”, and the gravitational constant given “a statistical meaning”. For him, any theory claiming universal validity was endangered by quantum theory, anyway.

From the cylinder condition, a grave objection toward Kaluza’s approach results: Covariance with regard to the diffeomorphism group of is destroyed. The remaining covariance group is given by

The objects transforming properly under (112) are: the scalar , the vector-potential , and the projected metric Klein identified the group; however, he did not comment on the fact that now further invariants are available for a Lagrangian, but started right away from the Ricci scalar of [185]. The group is isomorphic to the group of transformations for five homogeneous coordinates with homogeneous functions of degree 1. Here, contact is made to the projective formulation of Kaluza’s theory (cf. “projective geometry” in Sections 2.1.3 and 6.3.2).While towards the end of May 1919 Einstein had not yet fully supported the publication of Kaluza’s manuscript, on 14 October 1921 he thought differently:

“I am having second thoughts about having kept you from the publication of your idea
on the unification of gravitation and electricity two years ago. I value your approach more
than the one followed by H. Weyl. If you wish, I will present your paper to the Academy
after all.”^{86}
(letter from Einstein to Kaluza reprinted in [49], p. 454)

It seems that at some point Einstein had set his calculational aide Grommer to work on regular spherically symmetric solutions of Kaluza’s theory. This led to a joint publication which was submitted just one month after Einstein had finally presented a rewritten manuscript of Kaluza’s to the Berlin Academy [105]. The negative result of his own paper, i.e., that no non-singular, statical, spherically symmetric exact solution exists, did not please Einstein. He also thought that Kaluza’s assumption of general covariance in the five-dimensional manifold had no support from physics; he disliked the preference of the fifth coordinate due to Equation (109) which seemed to contradict the equivalence of all five coordinates used by Kaluza in the construction of the field equations [105]. In any case, apart from an encouraging letter to Kaluza in 1925 in which he called Kaluza’s idea the only serious attempt at unified field theory besides the Weyl–Eddington approach, Einstein kept silent on the five-dimensional theory until 1926.

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