Albert Einstein | Autograph letter signed, to Alfred Bucherer, on the nature of mass, 29 October 1917

Albert Einstein

Autograph letter signed ("A. Einstein"), to Alfred Bucherer, confirming the accuracy of an equation and discussing the nature of mass, suggesting that Bucherer made a mistake in a letter to Einstein (“…indem sie weniger betonen willten, dass side schwere Masse von der Geschwindigkeit, als dass Sie vom Gravitationspotential abhänge…” [you did not so much want to emphasise that heavy mass depends on velocity as that it depends on gravitational potential]), explaining, with formulae, the meaning of his claim that mass is variable (the only constant being an object's "rest mass") and warning of the care with which the term "mass" should be used, in German, 2 pages, 8vo (220 x 142mm), Haberlandstr. 5, Berlin, 29 October 1917, splitting at folds, small tear in upper corner, some creasing, remains of mount

“ … Es ist aber klar, dass ein solcher Massenbegriff nur zu einer solchen Veranschaubarlichung der Trägheitsbegriff auszuknüpfen gestattet. Am richtigesten ist es, nur von der 'Ruhemasse' m zu reden, welche konstant bleibt, solange dem Massenpunkte nicht Wärme oder sonstige 'Materie' zugeführt wird… [such a concept of mass can only serve to make the properties of inertia more comprehensible and is only useful insofar as it allows the common concept of inertia to be used. It is most correct to speak only of the ‘rest mass’ m, which remains constant as long as the mass point is not supplied with heat or other ‘matter’]…”

A LETTER WITH SIGNIFICANT SCIENTIFIC CONTENT DISCUSSING THE DIFFERENCE BETWEEN GRAVITATIONAL MASS AND INERTIAL MASS IN GENERAL RELATIVITY. This letter to the German physicist Alfred Bucherer (1863-1927) evidently responds to a critical letter by Bucherer on work by Einstein (see previous lot). Einstein here makes clear that Bucherer had misunderstood Einstein’s argument, and discusses the representation of gravitational mass by Riemannian tensors. Bucherer’s experimental work on the behaviour of electrons had been key early evidence for Einstein’s theories (see previous lots) and he had also been the first to coin the term “Einsteinsche Relativitätstheorie” [Einstein’s theory of relativity] as early as 1906. THIS LETTER IS UNPUBLISHED.