Albert Einstein (1879-1955)
Autograph letter signed ('A. Einstein') to Cornelius Lanczos, Princeton, 4 December 1949
In German. Half page, 280 x 216mm, including three scientific equations. Provenance: Sotheby's, 26 November 1980, lot 166.

'The only natural generalisation of the gravitational equations'. Einstein thanks Lanczos for sending him a copy of his book The Variational Principles of Mechanics (published that year), and for having dedicated it to him: he praises the book in the warmest terms ('You know that I am a passionate non-reader. But your book will free me for a time from this burden'), and hopes that it will gradually find a general distribution in US universities. Returning to their shared obsession with gravitational theory, Einstein provides three equations which he now feels certain constitute the 'overdetermined system' which is 'the only natural generalisation of the gravitational equations'. The reasoning that has brought him to this conviction appears as an appendix in 'the new edition of my old relativity book', which he will send Lanczos as soon as he has a copy.