D O C U M E N T S 4 0 8 , 4 0 9 N O V E M B E R 1 9 2 6 3 8 7 408. To Gerhard Kowalewski[1] [Berlin,] 7 November 1926 Dear Colleague, Mr. Kaluza[2] has developed an idea that certainly deserves some attention, and has clearly expressed this idea, to which Klein[3] (Cambridge) recently returned. It involves an attempt to allow the gravitational field and the electromagnetic field to emerge as a unity. This goal is pursued by assuming the world to be a continuum in five dimensions whose metric tensor does not, however, depend on the fifth coordinate.[4] This constraining condition compels a factual four-dimensionality but has the disadvantage of hardly looking natural when considered from the point of view of general covariance.[5] But since all the other endeavors to bring gravita- tion and electricity under a unifying perspective have failed, Kaluza’s idea is the only one among those which have surfaced that does not seem to be entirely with- out promise. Irrespective of what the truth may ultimately be, Kaluza’s idea demonstrates cre- ative talent and imagination. This achievement is the more notable as Kaluza works under difficult external conditions.[6] It would please me very much if he were to attain a suitable sphere of activity. Kind regards, your A. Einstein 409. To Chaim Weizmann Berlin, 8 November 1926 Dear Weizmann, I hereby declare that I will not accept the selection as chair of the Academic Council since I do not wish to assume the responsibility under the prevailing circumstances.[1] I already indicated the reasons in my letter to the Board of Governors.[2] I ask you to also bring this to the attention of the other members of the Board of Governors. Best regards, your (signed) A. Einstein