Albert Einstein (1879-1955)
Autograph letter signed ('A. Einstein') to Cornelius Lanczos, n.p., 29 September 1937
In German. One page, 288 x 215mm, including six scientific formulae, on lightweight paper. Provenance: Sotheby's, 26-27 November 1980, lot 159.

'A reasonable interpretation for the electric field must exist somehow'. Einstein plunges immediately into a scientific idea (on gravitational field theory) which Lanczos has sent him: 'Your tensor B_ikn has according to your presentation the identity / R_iklm = B_iklm + L(R_ik), / whereby L( ) means a linear formation using g_ik'. He follows through the consequences of applying the Bianchi operation to this equation, producing a new form 'whereby L again means a linear-homogenous formation. / From this can immediately be seen that your equations contain as solutions all solutions of the old gravitational equations. So this is a generalisation of the old gravitational equations through unique differentiation'. Lanczos's equations will form 'an overdetermined equation-system if they do not possess sufficiently numerous differential identities ... If independent identities do not exist in a sufficient number, this system is essentially no more general than the system R_ik=0'. Einstein concludes by expressing his pleasure at Lanczos sending him these ideas: 'A reasonable interpretation for the electric field must exist somehow'.