I N T R O D U C T I O N T O V O L U M E 1 6 x l i i i I. The Fifth Solvay Conference The Fifth Solvay Congress, which convened in Brussels 24–29 October 1927, was one the most significant events in the history of quantum theory,[1] famous, among other things, for the beginning of the Einstein-Bohr debate. The documentary sources for this debate consist of mostly after-the-fact recollections that neverthe- less have had influential repercussions. In this volume, we do not include these later recollections but rather concentrate on the firsthand primary documents, of which there are precious few. The first three days of the Solvay congress on “Electrons and Photons” were taken up with reports by William Bragg, Arthur Compton, Louis de Broglie, Max Born and Werner Heisenberg (jointly), and Erwin Schrödinger. They were followed by general discussion periods interspersed with several rec- reational activities, including lunch with the king and queen of Belgium.[2] Those who later saw themselves as subscribing to the “Copenhagen interpretation” of quantum mechanics, such as Heisenberg and Niels Bohr, credited the meeting with solidifying the tenets of their interpretation of the theory. More recent scholarship has revealed that, far from settling the interpretive issues, many of the controversies that still survive today were given a sophisticated articulation by one or another of the conference participants at much later dates.[3] The First Solvay Congress had taken place in the fall of 1911, when the German chemist Walther Nernst garnered financial support from Belgian industrial chemist and philanthropist Ernest Solvay (see Introduction to Vol. 3, sec. IV). Einstein played a prominent role in this first meeting, both in the delivery of his report on specific heats and quanta (see Vol. 3, Doc. 26) and during the discussion periods (see Vol. 3, Doc. 25). The following year, Solvay founded the Institut International de Physique Solvay that would oversee the organization of future conferences, the sec- ond of which took place in October 1913.[4] Plans for further meetings were interrupted by the outbreak of World War I. When the Solvay conferences eventually resumed, the bitterness felt toward Ger- many by other European nations was not easily dispelled. The two German mem- bers of the Institut Solvay’s Scientific Committee—Walther Nernst and Emil Warburg—were both removed (see Lorentz to Einstein, 26 July 1919 [Vol. 9, Doc. 76]), and German scientists were excluded from the third and fourth confer- ences held in 1921 and 1924.In this respect, however, Einstein was regarded as a special case, thanks to his outspoken pacifism, his criticism of German national- ism, and by virtue of his Swiss citizenship, as H. A. Lorentz explained when he was attempting to persuade Einstein to rejoin the conferences after the war (see Lorentz to Einstein, 9 June 1920 [Vol. 10, Doc. 49]). With cautious ambivalence, Einstein accepted an invitation to present a report at the third congress, though