Zurich, 31 December 1909

Energy distributions compatible with Maxwell's equations.

'I have not discovered much. The most interesting thing is that one can generate an endless variety of distributions of energy which are compatible with Maxwell's equations. Perhaps the solution to the question of quanta lies in this'. Einstein sets out equations representing the intensity of energy and demonstrating this compatibility, concluding 'Hopefully this is not a blind window'. The letter also sends New Year's greetings, and notes Einstein's enjoyment of his teaching work (at the University of Zurich, where he had begun teaching his first course on 15 October): 'I am getting more used to schoolmastering and it brings me much pleasure'.

Maxwell's equations are the four fundamental equations of electromagnetism: Einstein's work with them was fundamental to the Special Theory of Relativity (1905). The integration of quantum theory into the theory of relativity remains one of the enduring unanswered questions in Einstein's work.