D O C U M E N T 2 7 1 O C T O B E R 1 9 2 1 3 1 7
AKS (Ingola Kaluza, Hannover). Sabbata and Schmutzer 1983, pp. 454–455. [65 729]. The postcard
is addressed “Herrn Dr. Kaluza (Universität) Steinmetzstr. 34 Königsberg,” and postmarked “Berlin–
W 15.10 21. 7–8 N[achmittags].”
[1]Theodor Kaluza (1885–1954) was Privatdozent in Mathematics at the University of Königsberg.
[2]Kaluza had proposed a unification of electromagnetism with gravity by introducing a fifth
dimension, His theory extended the structure of general relativity to five dimensions, but all deriv-
atives with respect to were put to zero, or taken to be of small order to ensure consistency with the
observed four-dimensional world. Four of the additional components of Kaluza’s metric, (Greek
indices indicating the usual four spacetime components, with the timelike coordinate), were inter-
preted as proportional to the electromagnetic potential; he found that components of the Christoffel
symbols could be interpreted as the electromagnetic field: , with a constant, while
the retained their gravitational interpretation (for his theory, see Kaluza 1921).
After initially expressing willingness to submit a paper by Kaluza for publication in the Prussian
Academy’s Proceedings (see Einstein to Theodor Kaluza, 21 April 1919 [Vol. 9, Doc. 26]), Einstein
later withdrew his offer because he found that interactions due to the physically unknown com-
ponent of the metric would dominate in the geodesic equation for an electron in five-dimensional
spacetime (Einstein to Theodor Kaluza, 14 and 29 May 1919 [Vol. 9, Docs. 40, 48]). Subsequently,
Kaluza did not publish his ideas at all. On Kaluza and Einstein’s response to his work, see Pais 1982,
pp. 329–336; O’Raifeartaigh and Straumann 2000, pp. 8–11; Dongen 2002; Goenner 2004, sec. 4.2;
and Wünsch 2005.
[3]Hermann Weyl; for his theory, see Weyl 1918a, and for Einstein’s objections against it, see Ein-
stein 1918g (Vol. 7, Doc. 8). Einstein also compared Kaluza’s theory favorably with Weyl’s theory in
his letter to Kaluza of 21 April 1919 (Vol. 9, Doc. 26).
[4]Einstein left Berlin on 15 October (Doc. 260) for travels to Italy, Switzerland, and the Nether-
lands, and would return on or before 29 November 1921 (Doc. 306).
271. To Ralph de Laer Kronig[1]
Berlin, den 14. X. 21.
Sehr geehrter Herr!
Beim Durchfressen durch einen grossen Berg von Korrespondenz finde ich Ih-
ren interessanten Brief vom September letzten
Jahres.[2]
Wir wissen, dass der Weltradius grösser sein muss als Lichtjahre. Es muss
in der Natur irgend einen Grund geben, der bewirkt, dass das ausgestrahlte Licht
irgend wie verschwindet; sonst müsste der Himmel hell
sein.[3]
Hier steckt eines
von vielen ungelösten Rätseln. Jedenfalls ist nach meiner Meinung eine Prüfung
der Relativitäts-Theorie auf dem von Ihnen skizzierten Wege nicht möglich. Ent-
schuldigen Sie bitte die Kürze, in Eile!
Mit vorzüglicher Hochachtung
A. Einstein.
TLS. [71 571]. The letter is addressed “Herrn Ralph de Laer Kronig New York City.”
[1]De Laer Kronig (1904–1995) was a student at Columbia University in New York.
[2]See Ralph de Laer Kronig to Einstein, 26 September 1920 (Vol. 10, Calendar).
[3]This is the so-called Olbers’s paradox, solved later by dropping the static model of the universe.
x0.
x0
g0
x4
0
F =
g00
107