Einstein on married life in the weeks after his wedding – and the finalisation of one of his first important papers.

Einstein's letter, written shortly after his wedding to Mileva Maric on 6 January 1903, mixes the pleasures of married life with a flurry of scientific projects: 'I am now a married man and lead a most agreeable existence with my wife. She looks after everything excellently, is a good cook and is always in good humour'. Einstein has just sent off a paper on thermodynamics ['Eine Theorie der Grundlagen der Thermodynamik', published in Annalen der Physik, II, 1903]: 'I sent off my work on Monday in the end, after many modifications and corrections. But it is now all completely clear and straightforward, so that I am completely happy with it. Working from the principle of energy and of atomic theory, the notions of temperature and entropy emerge...'.

The letter goes on to relate a curious incident involving Einstein's sister, who has been convinced by the accountant of their deceased father to pay him a large part of her salary, a matter which amuses Einstein greatly – though he asks his friend to intervene and prevent Maja wasting her money (Maja was employed as a tutor to Besso's sister Bice, in Trieste). He concludes by saying that he has decided to become a Privatdocent (non-stipendiary university lecturer) again: 'But I will not work for a doctorate, as it is not much use to me, and that whole comedy has become tedious. Next I want to study molecular forces in gases, and then do some extended research on electron theory'. The letter ends with excuses for his bad handwriting – he is in bed with the flu (which he has caught from Mileva).

The paper Einstein refers to is one of only five scientific papers he published (all in Annalen der Physik) prior to his annus mirabilis of 1905. Although not ground-breaking in itself, it shows a remarkable grasp of the molecular foundations of thermodynamics, and already demonstrates his ability to test classical mechanics to its limits. In spite of his disclaimer, Einstein was to receive a doctorate from the University of Zurich two years after this letter.