D O C . 3 8 E T H E R A N D R E L AT I V I T Y 3 2 1
Published by Julius Springer (Berlin, 1920). Completed before 7 April 1920, presented 27 October
1920. An autograph manuscript [80 818], [80 819], [70 964] is also available (SzGBod). Significant
variations between manuscript and published text are noted. A manuscript with Einstein’s unpub-
lished introductory and closing remarks is also available [1 006] (see note 30 for a transcription).
This document presents the text of Einstein’s inaugural lecture at the University of Leyden
where he had been appointed Extraordinary Professor (Bijzonder Hoogleeraar). During Einstein’s
visit to Leyden in October 1919, Lorentz urged him to present his current views on the ether to the
public (see Einstein to Paul Ehrenfest, 12 January 1920). In Einstein to Hendrik A. Lorentz, 15
November 1919, NeHR, Archief H. A. Lorentz, Einstein promised to do so as soon as an opportunity
would arise. In an article in the Nieuwe Rotterdamsche Courant, 19 November 1919, published in
English in the New York Times, 21 December 1919, p. 20, Lorentz expressed his conviction that a
search for “mutations and movements that might take place in it [the ether,] . . . abandoned at present,
will once more be followed with good results . . . Einstein’s theory need not keep us from so doing;
only the ideas about the ether must accord with it.”
When Lorentz wrote to Einstein concerning the Leyden appointment the following month, he
mentioned that an inaugural lecture, though not absolutely required, would be much appreciated
(Hendrik A. Lorentz to Einstein, 21 December 1919). Einstein wrote back that he would hold an in-
augural lecture on the topic of the ether (Einstein to Hendrik A. Lorentz, 12 January 1920, NeHR,
Archief H. A. Lorentz). As he was writing the lecture, Einstein characterized it as a “more or less col-
ored retrospective look at the development of our opinions about the physical properties of space”
(“ein mehr oder weniger gefärbter Rückblick auf die Entwicklung unserer Meinungen von den phys-
ikalischen Eigenschaften des Raums”; Einstein to Hendrik A. Lorentz, 18 March 1920, NeHR, Ar-
chief H. A. Lorentz). The final version of the lecture was finished in early April 1920 (see Einstein
to Paul Ehrenfest, 7 April 1920).
The lecture, originally scheduled for 5 May 1920 (see title page of the document), had to be post-
poned because of delays both in the approval of the appointment and in issuing visa documents, and
because of Einstein’s other commitments. The position was officially created only on 24 June 1920
(see Proclamation, 24 June 1920), and Einstein was appointed shortly thereafter (see Einstein to Paul
Ehrenfest, 19 July 1920). Einstein finally delivered the lecture on 27 October 1920.
The salutation follows the formulation suggested in Paul Ehrenfest to Einstein, 10 March 1920.
For a discussion of the development of Einstein’s views on the ether, see Kostro 2000.
The following discussion of the role of the ether concept in nineteenth-century optics and elec-
trodynamics is similar to the discussion in secs. 2–3 of the unpublished manuscript presented as Doc.
31. For historical accounts, see Whittaker 195153 (Vol. 1) and Schaffner 1972.
For more detailed explanations of why Fizeau’s experiment and the phenomenon of stellar aber-
ration were seen as strong evidence for the hypothesis of an immobile ether, see Doc. 31, [p. 2], and
Doc. 64, [pp. 8–9], respectively.
In Doc. 31, [p. 3], Einstein also mentioned in this context Henri Poincaré’s observation that an
infinite number of mechanical models is possible whenever there is one mechanical model, something
that had already been recognized byJames Clerk Maxwell (see Doc. 31, note 10, for further discus-
Hertz’s views on the relation between ether and matter are expounded in Hertz 1890 (for histor-
ical discussion, see Darrigol 1993). In Einstein to Mileva Mari , 10? August 1899 (Vol. 1, Doc. 52),
Einstein mentioned that he was carefully rereading Hertz 1892, in which Hertz 1890 was reprinted.
For a discussion of Einstein’s remarks on Hertz in this letter, see Vol. 1, the editorial note, “Einstein
on the Electrodynamics of Moving Bodies,” pp. 223–225.
See McCormmach 1970 and Miller 1981 for historical discussion of attempts around 1900 by
Wilhelm Wien, Max Abraham (1875–1922), and others to reduce mechanics to electrodynamics and
of the importance in this context of measurements of the velocity-dependence of the electron mass.
In Hertz’s theory, the earth drags along the ether in its vicinity. The theory is therefore incom-
patible with the result of Fizeau’s experiment (cf. note 5 above).
The next two paragraphs were written on a sheet interleaved with the manuscript. The passage
replaces an earlier draft, which is deleted in the manuscript: “So standen die Dinge, als H. A. Lorentz
eingriff. Er beseitigte mit kühnem Griffe den genannten Dualismus. Für ihn war nur der alles durch-
dringende Aether Träger von elektromagnetischen Feldern, nicht aber die ponderable Materie.