The term refers to the city of Copenhagen in Denmark, and was apparently coined during the 1950s. Earlier, during the mid-1920s, Heisenberg had been an assistant to Bohr at his institute in Copenhagen, where they helped originate quantum mechanical theory. At the 1927 Solvay Conference, a dual talk by Max Born and Heisenberg declared "we consider quantum mechanics to be a closed theory, whose fundamental physical and mathematical assumptions are no longer susceptible of any modification." (This book contains a translation of the entire authorized proceedings of the 1927 Solvay conference from the original transcripts.) In 1929, Heisenberg gave a series of invited lectures at the University of Chicago explaining the new field of quantum mechanics. The lectures then served as the basis for his textbook, The Physical Principles of the Quantum Theory, published in 1930. In the book's preface, Heisenberg wrote:
