D O C . 3 1 I D E A S A N D M E T H O D S 2 8 1

p. 355, but had already been used in Einstein 1907j (Vol. 2, Doc. 47), sec. V. The definition given here

is the original definition of the equivalence hypothesis (or principle) rather than the definition given

in Einstein 1918e (Doc. 4); see note 4 therein for a discussion. The latter definition, however, is given

in passing on [p. 24] below.

[39]Eötvös 1890.

[40]The following considerations follow those in Einstein 1916e (Vol. 6, Doc. 30), sec. 2, and Ein-

stein 1917a (Vol. 6, Doc. 42), sec. 21.

[41]This is the definition of the equivalence principle given in Einstein 1918e (Doc. 4); see note 38.

[42]The definition of the “principle of general relativity” given here is the definition given in Ein-

stein 1916e (Vol. 6, Doc. 30), p. 776, rather than the one given in Einstein 1918e (Doc. 4); see note 3

therein for a discussion.

[43]The original result, derived directly from the equivalence principle in Einstein 1911h (Vol. 3,

Doc. 23), sec. 4, was corrected in Einstein 1915h (Vol. 6, Doc. 24), p. 834.

[44]For a discussion of the role of the Gaussian theory of surfaces in the development of general

relativity, see Vol. 4, the editorial note, “Einstein’s Research Notes on a Generalized Theory of Rela-

tivity,” pp. 192–199.

[45]In Einstein 1907j (Vol. 2, Doc. 47), sec. 19, and Einstein 1911h (Vol. 3, Doc. 23), sec. 3, Ein-

stein derived the redshift formula from the equivalence principle by considering rectilinear accelera-

tion rather than rotation in Minkowski space-time.

[46]In his lectures on general relativity at the University of Berlin in 1918, Einstein pointed out that

the gravitational potential needs to be represented by the metric tensor rather than by a scalar field if

one wants to take into account Coriolis forces as well as centrifugal forces (see Doc. 19, note 6).

[47]At this point in the original text Einstein indicates a note he planned to append at the foot of the

page, presumably for a reference. Both Leonhard Ch. Grebe (1883–1967) and Albert J. Bachem

(1888–1957) were Privatdozenten at the University of Bonn. In a series of controversial papers

(Grebe and Bachem 1919, 1920a, 1920b), they claimed to have found strong evidence for the gravi-

tational redshift of lines in the solar spectrum, as predicted by general relativity. They also claimed to

be able to explain why earlier experiments had failed to detect this effect. Einstein took a strong inter-

est in the work of Grebe and Bachem and even secured some financial support for it (see Einstein to

Board of Trustees of the Kaiser Wilhelm Institute of Physics, 25 April 1919 [GyBP, Abt. I, Rep. 1A,

Mappe Nr. 1656, Dok. Nr. 55–56]). He found their results very convincing (see Einstein to Robert W.

Lawson, 26 December 1919), but others were skeptical (see, e.g., Arthur S. Eddington to Einstein, 15

March 1920, and Willem H. Julius to Einstein, 8 May 1920). For a detailed discussion of Grebe and

Bachem’s work and its reception, see Hentschel 1992.

[48]Einstein first discussed the problem of the rotating disk in print in Einstein 1912c (Vol. 4, Doc.

3), p. 356. See Stachel 1980 for an analysis of the role of these considerations in the development of

general relativity.

[49]Einstein first published the views expressed below on the nature of geometry in Einstein 1917a

(Vol. 6, Doc. 42), sec. 1. He elaborated on these views in Einstein 1921c (Doc. 52).

[50]The first two equations on [p. 33] have empty parentheses instead of equation numbers. The

empty parentheses in the penultimate paragraph on [p. 33] refer to the second equation.

[51]Ricci and Levi-Civita 1901.

[52]This new ether concept is discussed at greater length in Einstein 1920j (Doc. 38).

[53]At this point in the original text Einstein indicates a note he has appended at the foot of the page:

“Die vom mathematischen Standpunkte aus denkbar einfachste Möglichkeit.”

[54]For a discussion of Einstein’s struggles in the period 1912–1915 to find satisfactory field equa-

tions for the metric field, see Norton 1984, Stachel 1989, and Renn and Sauer 1999.