The Solvay conference of 1911 was the first international conference in the history of science. Under the patronage of the Belgian industrialist Ernest Solvay, and chaired by Hendrik Lorentz, it brought together in Brussels the leading physicists of the time for a week-long discussion on the quantum theory of radiation. Lorentz would remain the scientific organizer until his death -- the 1927 Solvay conference on quantum mechanics was the last he chaired. (See a film shot by Irwing Langmuir at that conference, with commentary by Nancy Thorndike Greenspan.) The legendary Bohr-Einstein debate on the probabilistic nature of quantum mechanics started at this 1927 Solvay conference. Seventeen of the 29 attendees would become Nobel laureates.
Throughout the first decade, Lorentz remained the energetic and dedicated organizer of the Solvay conferences. He presented Ernest Solvay with a clear program of action: to encourage the study of fundamental questions regarding natural phenomena, work that could be done primarily by individuals but for which "the exchange of ideas among those who practice research is very fruitful" (Lorentz to Solvay, 4 January 1912).
Further reading: Diana Kormos Barkan, The Witches' Sabbath: The First International Solvay Congress in Physics, Science in Context 6 (1993), pp. 59-82.
Albert Einstein and Niels Bohr at the 1930 Solvay conference, photographed by Paul Ehrenfest.
"H. A. Lorentz chaired the meeting with incomparable tact and unbelievable virtuosity. He speaks all three languages equally well and has a unique scientific acumen." (Albert Einstein in a letter to H. Zangger, 7 November 1911.)
"The discussions were most interesting but the result is that we seem to be getting deeper into the mire than ever. On every side there seem to be contradictions. [..] I got on very well with Einstein, who made the most impression on me here except perhaps Lorentz. [..] However, he does not care much for appearances and goes to dinner in a frock coat. He says he knows very little mathematics and can only set up general considerations, but he seems to have had a great success with them..." (F. A. Lindemann to his father, 4 November 1911.)
The Solvay conferences continue. I had the opportunity to attend the 2008 Solvay conference on the "Quantum Theory of Condensed Matter". It was held in the same Hotel Metropole as the first Solvay conference (although the room from the 1911 photograph no longer exists).