While Einstein pursued his research in UFT along the lines of mixed geometry at the Institute for Advanced Studies of Princeton, NJ, his colleague in the mathematics department of Princeton University, L. P. Eisenhart, kept quiet until the beginning of the 1950s. He had written a book on Non-Riemannian Geometry in 1927 [182*], and since the twenties had had a long-standing interest in teleparallelism and UFT (cf. Section 6.4 of Part I and ). Being three years older than Einstein he had retired in 1945. Nevertheless, in the 1950s he wrote three further papers about UFT. He first introduced a non-symmetric metric and a non-symmetric connection  in 1951. Unlike Einstein, whose papers he did not refer to, Eisenhart did not take the connection as an independent variable but built it entirely from the metric tensor and its first derivatives such as the Japanese physicist K. Hattori248 had done in the 1920s [240*]:, Eq. (1.6), p. 540). The tensor to be added was chosen by Eisenhart to be (indices are moved with the symmetric part of the metric): 433*) we notice that 249 Eisenhart’s paper dealt only with differential geometry; no physical motivation or interpretation were given. This applies also to a subsequent publication in which, after formal manipulations, several expressions for possible curvature tensors and Einstein’s Hermitian-symmetrized Ricci tensor (196*) were derived . The 3rd edition of Einstein’s Meaning of Relativity  now was referred to.
Eisenhart’s second attempt, after the death of Einstein, presented a new unified theory of gravitation and electromagnetism within metric-affine geometry [186*, 187*, 188*], and [189*]. Although in a different geometrical setting, eventually the theory formally led to the Einstein–Maxwell field equations in Riemannian geometry. The main difference to Einstein’s approach was that Eisenhart kept the metric tensor symmetric while embedding the electromagnetic field tensor into the connection by an ad-hoc ansatz :250 Thus, vector torsion does vanish. In the first paper, Eisenhart’s field equations were: 437*), the equation for the auto-parallels of the connection read as: : 441*):
Again two months later, in his 3rd installment, Eisenhart finally arrived at the Einstein–Maxwell equation of general relativity [188*]. This time, the ansatz (440*) with (441*) was changed substantially into:, p. 881.) This quotation was repeated in the last of this sequence of papers by Eisenhart, in which “the final result of my third paper” were derived “in a somewhat different manner.” (, p. 333.)
In his last contribution to UFT, when he was 83 years old, Eisenhart returned to mixed geometry . Starting from Einstein’s condition (200*) on the metric, he aimed at solving it for the connection as a function of the metric and its first derivatives. Unaware of the solutions given previously (cf. Section 10.2.3), he achieved this only with the help of the additional constraint for torsion:444*) with the notation of (3*), (4*), Eisenhart’s solution is given by: [285*] in Section 15.1.
Hlavatý is the fourth of the main figures in UFT besides Einstein, Schrödinger, and Tonnelat. His research was published first in a sizeable number of articles in the Journal of Rational Mechanics and Analysis of Indiana University251 and in other journals; they were then transformed into a book [269*]. According to its preface, his main intent was “to provide a detailed geometrical background for physical application of the theory”. As he was very optimistic with regard to its relation to physics, he went on: “It so happens that the detailed investigation of Einstein’s geometrical postulates opens an easy way to a physical interpretation”([269*], p. X). We have noticed in Section 9.7 that this possibly could not have been the case. In the preface of his book, Hlavatý became more explicit; his program was to encompass: (1) an investigation of the structure of the curvature and torsion imposed on space-time by the field equations, equations which he claimed to be “of a purely geometrical nature” without physical interpretation being “involved in them a priori”. The two further points of his program, i.e., (2) an attempt to identify the gravitational field and the electromagnetic field by means of the field equations, and (3) an investigation of the physical consequences of his theory, were treated only in “an outline of the basic ideas” ([269*], p. XVIII). In comparison with Einstein, Schrödinger and Tonnelat who followed their physical and mathematical intuition, Hlavatý’s investigations were much more systematical and directed first to what could be proven by mathematics; whether a relation to physics could be established became secondary to him. Although mostly working and publishing alone, he corresponded with about 40 scientists working on UFT. He also was a frequent reviewer for Mathematical Reviews (cf. Section 18.1). Hlavatý began by introducing a systematical classification of the non-symmetric metric according to the non-vanishing eigenvalues of its skew-symmetric part (remember . He distinguished three classes:
Throughout his research, the symmetric part is used for raising and lowering indices. From Eq. (30*) he concluded that there are metrics for which this “metric compatibility” equation does not admit any solution”, and cases in which (30*) admits more than one solution [261*]. According to him, the condition for uniqueness of the solution is if (class 1), and if (class 2, 3). The gravitational potential is identified with while the electromagnetic field is taken to be252 [261*]:9.7.
According to Hlavatý, the first two classes cannot be handled simultaneously with the third class
([266*], p. 421). This makes it more involved to read his papers, because the results proven by him must now
be distinguished according to the special class of .
i) Fields of third class.
In the course of his investigations when he tried to interpret geometrical quantities in terms of physical variables, Hlavatý replaced the four equations following from (449*) by four complicated looking equations:
The tensor received its meaning from what Hlavatý called “the gravitational field equations”, i.e., Einstein’s equations with a geometrical energy-momentum tensor of matter:446*); mass density by with a scalar function and the unit vector ; is the mass of a particle ([269*], p. 175–176). In Hlavatý’s theory, Maxwell’s equations were taken to be: 452*) is equivalent to (449*) plus , we also have . Of course, the electromagnetic field in (446*), for the third class, reduces to .
For incoherent matter, , and from (451*) . A somewhat disappointing consequence is that, in a manageable approximation, charged particles remain unaffected by the electromagnetic field: they move along geodesics in the gravitational field (, p. 329; [269*], p. 174, 187). Upon neglect of the cubic terms in , i.e., for , the equation of motion coincides with the geodesic equation:
This is the more strange as Hlavatý claimed:
“In the unified theory the electromagnetic field is always present; hence we might look upon it as a primary field which […] creates the gravitational field. However, there is at least one known electromagnetic field which does not create a gravitational field (i.e., the field of the plane wave in the electromagnetic theory of light).” ([266*], p. 420.)
ii) Fields of class 1 and 2.
Here, and two new quantities were introduced:451*) and (452*) remain the same except for an exchange of by and a different complicated expression for if is kept as a field equation; cf. [269*], p. 204, Eq. (20.3a,b), p. 203 Eqs. (20.1), (20.2b). Hlavatý did present an exact spherically symmetric solution with , constants which is obtained from Papapetrou’s solution (cf. Section 8.3) by setting there . The electromagnetic field in Hlavatý’s solution is . The gravitational function replacing the gravitational constant is . But in this case, according to Hlavatý “we are unable to derive the second set of Maxwell’s equations from our field equations” ([269*], p. 208). Therefore, as for classes 1, 2, this field equation again is replaced by (450*). In consequence, for the motion of a particle Hlavatý obtained an improved result: A (massive) charged particle moving freely in the unified field describes an auto-parallel of the unified connection (, p. 211). Thus, two of the three effects in the planetary system were the same as in general relativity; the third (Perihelion shift) in his theory depended on the electrical field of the sun. Hlavatý did not get as far as to clearly show the experimental physicist how this electric field enters the formula for the perihelion shift.
For paths of photons Eq. (453*) still holds. If gravitation is neglected, i.e., , Hlavatý found a discrepancy with the special relativistic explanation of the Michelson-Morley experiment. Although he referred to the judgment of Shankland et al. that Miller’s result is erroneous , he concluded: “From the point of view of the unified field theory Miller’s result, properly interpreted, is not necessarily at variance with the assumption of the constant velocity of light.” (, p. 471).
Hlavatý’s research will be appealing to some by its logical guideline concerning mathematical structures. His many special cases and set up “agreements” in proving results are somewhat bemusing for a physicist. An example is given by his publications dealing with the special case when the symmetric part of the metric is degenerated [267, 268]. It is a purely mathematical exercise meant to fill a gap, but is without physical meaning. For the cases in which the theory could be applied to physical systems, in principle, Hlavatý was also forced to alter the original field equations in order to avoid objections against the unphysical results following from them. It is not unfair to conclude that he did not succeed in making a break-through in the sense of his physical interpretations being more convincing than those suggested by others.
The investigations of his doctoral student R. Wrede were directed to the mathematical structure of the theory: He partially extended Hlavatý’s theory to an n-dimensional space by adhering to the two principles: A.) The algebraic structure of the theory is imposed on the space by a general real tensor ; B.) The differential geometrical structure is imposed on the space by the tensor by means of a connection defined by (30*). Hlavatý’s third principle, i.e., the existence of the constraints with an arbitrary vector field is left out . The paper solves (30*) in dimensions for the various possible cases.
A theoretician of the younger generation and assistant at Princeton University, R. L. Arnowitt , tried to look at UFT from the point of view of the electromagnetic field forming a link between the description of microscopic charges by quantum field theory and macroscopic ray optics [5*]. He introduced four postulates:
- Any unified field theory should reduce to Einstein–Maxwell theory in a first approximation for weak electromagnetic fields.
- First-order corrections to the Coulomb field of the electron should not become appreciable for .
- The affine connection has the form , where is related to the vector potential of the Maxwell field. Also is assumed.
- The Lagrangian must be invariant under the combined gauge transformation and (for the metric) . The metric tensor is also symmetric.
The appearance of a microscopic length-parameter (and the cosmological constant) in his Lagrangian and the occurrence of two “metrical” tensors turned out to be a consequence of the postulates. The symmetrical first one is supposed to “be measured by rods and clocks” and used to set up the Lagrangian; the second asymmetric one is derived from the Lagrangian: . It is an auxiliary device for the introduction of the electromagnetic field. Arnowitt chose the Lagrangian:Buchdahl’s gauge-invariant UFT published in the same year (cf. Section 13.1) but is different. Immediately,
In order to obtain the field equations, the quantities and are to be varied. From the first two variations and resulted where was introduced by and interpreted to be the “gravitational” metric tensor. The first of these equations was rewritten as sourceless Maxwell equation such that is the “electromagnetic” metric tensor:
After some manipulation, variation with respect to led to:459*) formally became Einstein’s equations. Again, in a weak field approximation introduced by with small , the free parameters were fixed to be and with the cosmological constant . Thus, is the microscopic length parameter mentioned above. In linear approximation, the author also has obtained a static, spherically symmetric Schwarzschild-like solution with an event horizon and finite electrical field (and field energy) for .
A further contribution came from B. Kursunŏglu, whom we have met before in Section 9.3.3, now situated in Coral Gables, Florida. He continued to alter and study his variant of the Einstein–Schrödinger field equations . In place of (300*) – (302*), he postulated the system:253
The denominator is related to .
The auxiliary field satisfies the vacuum Maxwell equations.