Autograph postcard signed by Albert Einstein to Ludwig Hopf. 18 lines written verso and recto, address also in Einstein's handwriting. Postmarked June 21, 1910.
Published in The Collected Papers of Albert Einstein, Volume 5: The Swiss Years: Correspondence, 1902-1914, Princeton University Press, 1993, n°218, p. 242.

Exceptional and very aesthetic card from Albert Einstein to “the friend of the greatest geniuses of his time” – according to Schrödinger – the mathematician and physicist Ludwig Hopf, who allowed Einstein to meet another genius of the 20th century: Carl Jung . The master invites his student to a dinner including among the guests the scientist Max Abraham, future great rival of the Zurich years and fervent opponent of Einstein's theory of relativity.

The recipient Ludwig Hopf joined Einstein in 1910 as an assistant and student at his physics and kinetic theory seminars at the University of Zürich. They signed two fundamental papers on the statistical aspects of radiation and gave their names to the "Einstein-Hopf" velocity-dependent drag force. Their letter exchanges retrace the complex path of Einstein's work on relativity and gravitation, bearing witness to their great complicity and Hopf's invaluable contribution to the Master's research. A few months after writing the postcard, Hopf even found an error in Einstein's calculations of the derivatives of certain velocity components which Einstein corrected in a paper the following year. They also formed a musical duo – Hopf accompanied on the piano the Master's violin, performing pieces by great musical geniuses like Bach and Mozart.
With this card, Einstein invited his pupil and friend Hopf to dinner with Max Abraham, at the dawn of a major scientific controversy that would pit them against each other from 1911 onwards. Abraham's theory of special relativity failed to convince Einstein, who criticized its lack of observational verification and its failure to predict the gravitational curvature of light. In 1912, their dispute became public through scientific articles. Abraham never acknowledged the validity of Einstein's theory.
During their brilliant artistic and intellectual exchanges, Hopf undoubtedly succeeded where Freud had failed, as he declared to him in a letter: "I shall break with you if you boast of having converted Einstein to psychoanalysis. A long conversation I had with him a few years ago showed me that analysis was as hermetic to him as the theory of relativity is to me" (Vienna, September 27, 1931). As a fervent supporter of psychoanalysis, Hopf is known to have introduced the famous psychoanalyst Carl Jung to Einstein. Hopf and his teacher both left for Prague's Karl-Ferdinand University in 1911, where they met writer Franz Kafka and his friend Max Brod in Madame Fanta's salon.

 Kafka and Max Brod

With the rise of the Nazi regime, the fates of the two theoreticians were plagued by persecution and exile. Einstein first took refuge in Belgium, Hopf in Great Britain after his dismissal in 1934 from the University of Aachen because of his Jewish origins. They continued their prolific correspondence in the midst of the turmoil, Einstein suggesting to Hopf the opening of a university abroad for exiled German students. Hopf died shortly after his appointment as chair of Mathematics studies at Trinity College Dublin in July 1939.
A precious invitation from the great physicist to one of the final dinner gatherings of the "old school" of science embodied by Max Abraham, on the eve of the publication of the theory of general relativity which would overturn classical conceptions of space and time and propel Science into the 20th century.

"Lieber Herr Hopf, ich hatte bei unserer Verabredung vergessen, dass ich morgen 6 Uhr Fakultätssitzung habe. Deshalb habe ich Herrn Abraham für morgen nach der [biffé :Vorlesung] Sitzung zum Abendessen gebeten (halb 8 Uhr). Ich bitte Sie, auch zu kommen. Herrn Rusch werde ich auch einladen und wahrscheinlich Herrn Prof[essor] Zermelo. Mit besten Grüssen / Ihr Einstein"

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