Leiden University
Einstein in Leiden

A. Einstein - Collection Academic Historic Museum, Leiden

P. Ehrenfest - Collection Instituut-Lorentz, Leiden

Paul Ehrenfest (1880-1933) became professor of theoretical physics at Leiden University in 1912. He was a close personal friend of Albert Einstein.

All of us are now in complete agreement that we must undertake to get you to Leiden. The business is extremely simple: if you just say yes to me, it will be possible - at least according to all human expectations - to arrange things extremely quickly according to your wishes... You can spend as much time as you want in Switzerland, or elsewhere, working, giving lectures, traveling, etc., provided only that one can say "Einstein is in Leiden - in Leiden is Einstein"... Bear in mind that you would be taken in here by a group of people who are really fond of you personally, and not just of the brain drippings that ooze out of you!
Ehrenfest in a letter to Einstein, 1919. Translated by M.J. Klein, in "Paul Ehrenfest, The Making of a Theoretical Physicist", North-Holland, Amsterdam, 1970.

Ehrenfest invited Einstein to become "Bijzonder Hoogleraar" (special professor) at Leiden University. This would bring him to Leiden regularly for a few weeks a year. Einstein liked the idea of such a "comet-like existence in Leiden". He began his official duties on October 27, 1920, with an inaugural lecture on "Ether and Relativity Theory" (where "ether" refers to the gravitational field, not the abandonded concept of the electromagnetic ether).

Einstein & Ehrenfest with his son Paul Jr., June 1920 --- collection Museum Boerhaave

"It's so remarkably good for both of us to be together more often, because it's just as though Nature had made us for each other... Each of us feels less of a stranger in this world because of the other." (Einstein in letters to Ehrenfest.)

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