20 Concluding Comment

A leafing through the pages of this review of the history of classical unified field theory in the years between the mid 1930s and mid 1960s will suggest that some historians of physics, and almost all journalists writing about the evolution of classical unified field theories or “Theories of Everything” adjust their published views: their focus on Einstein’s work in this field is not only excessive but unrealistic. This is due to at least two counts:

1. Others like Schrödinger, Hlavatý and the totally neglected French groups (Tonnelat, Lichnerowicz) – not to point to the many further names given in this review – have contributed equal shares;
2. In this field, Einstein has not produced novel ideas from which physics and mathematics could have benefited conceptually (cf. Section 18.1.2). Certainly, he was the most prominent player; it is fair to say that without the influence of the three eminent Nobel prize winners Einstein, Schrödinger and de Broglie (who backed M.-A. Tonnelat), the active period of research in classical UFT would have been shorter-lived, and its history much easier to write.

It remains a riddle why the signs pointing to a dead end of the Einstein–Schrödinger research-line of unified field theory, already highly visible before the 1950s, were overlooked or pushed aside by Einstein and others for so long: the flood of geometrical structures drowning a small number of physical concepts, the ambiguity in the dynamics for the “total field” (Lagrange density), the almost total lack of empirically testable output. Kaluza–Klein theory as the other type of unitary field theory fared much better. In combination with Weyl’s second attempt toward a gauge principle, it paved the way to Yang–Mills theory able to describe the fundamental interactions with the exception of the gravitational. The final form of a generally accepted gauge theory of gravitation is still being discussed. Unfortunately, since the late 1930s, Einstein had written off Kaluza–Klein theory (cf. Section 3.2) and thus cannot be held in esteem for the successes of gauge theory.

In the wording of this review, the avowal, encountered occasionally, that Einstein’s papers were read “sympathetically”, will not be found. Like the papers of all other researchers, I tried to read his publications as neutrally as possible. While valuating research according to its place inside the body of unified field theory, I strove to judge past research from the physics at the time, not from the angle of the up-to-date state of the art. I am aware of the limits to this: the increase of knowledge since the period looked at, cannot be blanked out completely. Vice versa, no lesson from failed classical unified field theories is drawn here with regard to actual speculative theories with their claim to unify all four fundamental interactions. We just hope that current scientific practitioners are prepared to learn from the history of physics.