1 0 V O L U M E 8 , D O C U M E N T 2 2 7 a

Raum für den “Wahren” zu halten, und ihn “Galileischer Raum”, oder vielleicht

sogar “Aether” zu nennen. In dieser Weise verstehe ich ihr

“Nachtrag”.[14]

Habe ich

darin Recht? Wenn nicht, so bitte ich Sie mich zu verzeihen und zu belehren.

Die Amsterdamer Akademie druckt sehr langsam, und es werden noch wohl ein

paar Monate vergehen ehe meine Arbeit über die Mondbewegung gedruckt sein

wird.[15]

Ihr ergebener

W. de Sitter

ALS (NeLU). [87 953].

[1]De Sitter’s handwritten date, corrected on the assumption that it is a reply to Einstein’s letter of

22 June (Vol. 8, Doc. 227) (see also notes 12 and 14 below), and that De Sitter thanks Einstein for

sending him proofs of Einstein 1916g (Vol. 6, Doc. 32), submitted 22 June. It is possible that De Sitter

simply wrote down the incorrect month when dating this letter, which was found tucked between

pages of a research notebook he kept in 1916 (NeLU, notebook S12). Twice in that notebook De Sitter

entered dates, for 4 June and 6 June, which he later corrected in pencil to July (pp. 192 and 193). This

suggests that he made a similar error in the present case. If so, the date of this letter would be 7 July,

congruent with the other evidence above.

[2]Einstein 1916g (Vol. 6, Doc. 32), p. 692. In this paper, Einstein uses harmonic coordinates in

linearized approximation to study weak field wave solutions of the gravitational field equations.

[3]Equations (8) and (9) of Einstein 1916g (Vol. 6, Doc. 32) concern the definition, in equation (8),

of the quantities related to the linearized components of the gravita-

tional field, , specified by the ansatz . These quantities obey

a wave equation, analogous to the equation for electromagnetic waves, which can be solved by a

retarded Green’s function, given in equation (9). Einstein subsequently acknowledged an error in his

calculation arising from a conflation of the quantitites and (see Einstein 1918a [Vol 8,

Doc. 1], p. 157).

[4]Einstein 1915h (Vol. 6, Doc. 24).

[5]The components of the stress-energy tensor , are given in terms of their order in an expan-

sion in the small parameter , where is the speed of the matter in the system. If

is of order unity, this assumes geometrized units, in which

[6]Droste 1916b.

[7]Square brackets are in the original.

[8]Equation (4) in Einstein 1916g (Vol. 6, Doc. 32) specializes to harmonic coordinates in the form

(see note 3 above for the meaning of ).

[9]Hilbert 1915, theorem I (p. 397).

[10]Presumably, De Sitter intended to delete the remainder of the entire paragraph which continues

on the next page of the letter.

[11]Einstein 1915h (Vol. 6, Doc 24), p. 833.

[12]In his letter of 15 July (Vol. 8, Doc. 235) Einstein wrote, apparently in reply to this letter: “It is

not good if in my letter I called the system ‘Galilean.’ Only a space in which all the ’s

are constant can be called that.” Einstein referred to his earlier letter of 22 June 1916 (Vol. 8,

Doc. 227), in which he had written about a set of coordinates which conducted “wavelike motions

1

2

-- -

–=

g – += +=

T

v c

2

8 G c4 = G c 1. = =

x

----------- 0=

g – 1 = g