Ehrenfest letters surface

Paul Ehrenfest's letter

The Austrian-born theoretical physicist Paul Ehrenfest wrote that to some of his former PhD students on 15 August 1932, about a year before his suicide at age 53. In the letter, he tells them that "each of you has been, during some stretch of your life, something like my own child" and "I have you much more to thank than you realize. Your affection, your consistent wish to give me confidence in myself made it possible until just recently for me to maintain my enthusiasm. Forgive me that it is now over."

That letter is one of four given in 1992 to the Museum Boerhaave in Leiden, the Netherlands, by the descendants of Ehrenfest's first student, Johannes Burgers, who is perhaps best known for the eponymous equation for nonlinear diffusion. The letters went unnoticed until the museum's new director, Dirk van Delft, happened on them recently. "The most important one is the last one in the set," says van Delft. "Very probably it was Ehrenfest's last letter. It was written on the 24th of September 1933. The next day he committed suicide."

One of the letters, dated 21 August 1918, is to Burgers's fiancée, Jeannette Roosenschoon. In it, Ehrenfest writes: "Maybe it's wrong, but you know that my wife and I are convinced that the key to a lasting marriage is common love for something other. Usually the others are the children, but for a man like Jan [Johannes] for whom intellectual work is so much a source of happiness, it is very, very good, in my view, if his wife is a true [waschechte] physicist." In that letter, he also mentions the birth of his son Vassily. On 21 October 1918, he tells Burgers—complete with sketches—how to label the envelopes with his dissertation in preparation for his defense. In the 1932 letter to his students he writes: "My belief in the absolute (unanalyzable) worth of the natural and mathematical sciences grows unabated! That I myself have completely lost contact to it, THAT is the crux of my collapse, my 'surrender' ['Lebens-Muedigkeit']." And he asks them to keep an eye on and mentor his older son: "From you in particular, I am not asking for any material support for the children. But I do ask you, through a loose organization of all of you, to promote especially Pawlik's healthy, strong development." (Pawlik became a cosmic-ray physicist and was later killed in an avalanche.)

In his final letter, Ehrenfest writes that Pawlik will give Burgers correspondence about Jewish German intellectuals, and he makes a request: "Please arrange, together with [Adriaan] Fokker, that at least some of the cases are handled." From the time Hitler came to power, explains Martin Klein, who wrote a biography of the first part of Ehrenfest's life, "Ehrenfest was very active in trying to rescue German Jews—in particular he used his influence to find jobs for physicists." Ehrenfest, who was Jewish, felt guilty about "occupying a principal chair in Europe and being unproductive, especially with all the Jewish physicists being dismissed from their jobs," Klein adds.

Ehrenfest wrote a letter similar to the one foreshadowing his suicide, but with a different tone, to Albert Einstein, Niels Bohr, Abram Ioffe, and other contemporaries, according to Klein, who refers to page 408 of Abraham Pais's book, Niels Bohr's Times (Oxford Clarendon Press, 1991). "I don't think [those letters] were sent," says Klein. "I can't imagine that if [they] had been sent, that there wouldn't have been a recorded and visible reaction from some if not all of the recipients." The group letter is the only typed one among the four newly found letters.

In autobiographical notes published posthumously last year by the J. M. Burgers Centre in the Netherlands and the University of Maryland at College Park, where Burgers was on the physics faculty, Burgers wrote,

Sometimes it looked . . . as if he [Ehrenfest] gave away everything he had found or observed, without building up a reserve, a kind of stronghold, within himself. . . . His analytical mind stirred up everything. . . . On the long run this pushed his students somewhat away from him and I have also experienced this effect. There were things which we did not like to have analyzed. It may look as if this betrays a lack of intellectual interest, but in several cases it was an instinctive protective reaction from our side.

About a 1918 meeting for physics teachers that Ehrenfest arranged, Burgers wrote,

This meeting naturally gave him great pleasure, but what was strange to us was that he said it had given him more pleasure than the birth of his youngest child in that same year. . . . I spoke with [physicist Hendrik] Lorentz and asked him whether he could talk with Ehrenfest, and help him find a way back to feelings which looked more normal to us. . . . While we perceived that Ehrenfest's self-analysis could take dangerous forms and lead to utter despair, we could not help him.