Princeton, August 1942

Einstein's 'hobby horse': the unified field theory.

'My epistolary debts to you stink to high heaven. But this time I shall write straightaway, as you have stroked my hobby horse. It concerns the old question of the unitary conception of the total field (gravitation and electricity). You know of course that up till now all attempts (which came principally from Weyl and Kaluza) have failed. My new attempt has this in common with the earlier ones, that it tries to put in place something as equations for the total field which is formally analogous to the gravitational equations for empty space, but which applies itself to the total field'. He goes on to explain his approach in considerable detail, illustrating it with a series of equations, beginning with Kaluza's conception of a five-dimensional space and submitting it to Riemann's geometrical approach: this however still leaves one with two independent fields, rather than a unified one. The next step is to envisage a space with four complex coordinates – in reality an 8-dimensional space. The question is then which second-order differential equations are to be applied to this. He concludes 'The integration is however difficult, and it will be far from simple to determine whether my castle in the clouds has anything to do with the work of the good lord'. He concludes this dense scientific letter with the extended signature 'A. Einstein', noting humorously 'I think one must write the name out in full'.