EINSTEIN, Albert. “Zu Kaluzas Theorie des Zusammenhanges von Gravitation und Elektrizität - Erste Mittelung”, pp 23-25; with in the same issue, “Zu Kaluzas Theorie des Zusammenhanges von Gravitation und Elektrizität - Zweite Mitteilung”, pp 26-30, in *Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften*, Physikalisch-mathematische Klasse VI, Berlin 1927, pp. 17–30, February 17, 1927). Individual issue in original wrappers.

- Wrappers are a bit sunned along the edges, otherwise fine. $500.00

Theodor Franz Eduard Kaluza (1885-1954) was a German mathematician and physicist known for the Kaluza–Klein theory (“the two pioneers of unified field theory”, A Pais,* Subtle is the Lord,* pp 329). Einstein thought very highly of Kaluza (according to MacTutor) and in 1921 encouraged him to publish his paper in which he solved Einstein's equations of general relativity by using field equations in five-dimensional space-time. (Nordstrom had proposed “to use a five dimensional space for the unification of em with a scalar gravitational field” (A.Pais) publishing in *Phys Zeit* 15, p 504, 1914.) Of Kaluza's work Einstein writes to Lorentz: ”It appears that the union of gravitation and Maxwell's theory is achieved in a completely satisfactory way by the five-dimensional theory (Kaluza-Klein-Foch)”--Pais, *Subtle..*., p 333.

Again according to Pais, Einstein wrote to Kaluza in 1921 saying “the formal unity of your theory is startling” (*Subtle*..., p330) after which he communicated the K paper to the Prussian Academy. It was in these two papers presented above that Einstein wrote on Kaluza's work—this some four years after an earlier paper Einstein wrote with Grommer. (Einstein would return to Kaluza in a paper with Peter Bergmann, “On a generalization of Kaluza's theory of electricity” in the *Annals of Mathematics*, Vol. 39, no. 3, in July 1938 and which he would address again in *Science* in 1931 with Meyer and in the Prussian Academy the same year.)

“[Kaluaza] was teaching at Königsberg in April 1919 when he wrote to Einstein and told him about his ideas to unify Einstein's theory of gravity and Maxwell's theory of light. Einstein encouraged him to publish his highly original ideas which he did in 1921 in his paper on the unity problem of physics...Kaluza's ideas involved the introduction of a fifth dimension and, although he has been criticised for introducing this as a purely mathematical idea, his work is important and was explored by others...Kaluza is remembered for this in Kaluza-Klein (named after the mathematician Oskar Klein) field theory, which involved field equations in five-dimensional space. The theory, initially a popular topic of research, quickly lost favour with the introduction of quantum mechanics.”--MacTutor, St. Andrew's Math History.

“As is so often the case, string theory arose from a collection of discredited ideas... When string theory became mainstream, physicists realised that these early insights were extraordinarily prescient. The story begins in 1919 with a little known Polish mathematician, Theodor Kaluza. Inspired by Einstein’s revolutionary ideas, he attempted to overthrow a central tenet of physics. “What if there are extra dimensions we just can’t see?” he asked. Working alone, he attempted to incorporate a hidden dimension into Einstein’s model for gravity. Unsurprisingly, his five-dimensional theory had more equations than the usual four-dimensional approach. Looking closely at the extra equations he had found, Kaluza spotted something remarkable. They were precisely Maxwell’s equations governing the electromagnetic field...”--”Whystringtheory” (supported by Oxford and the Royal Society)