On March 17, 1938, Bethe attended the Carnegie Institute and George Washington University 's fourth annual Washington Conference of Theoretical Physics. There were only 34 invited attendees, but they included Gregory Breit, Subrahmanyan Chandrasekhar, George Gamow, Donald Menzel, John von Neumann, Bengt Strömgren, Edward Teller, and Merle Tuve. Bethe initially declined the invitation to attend, because the conference's topic, stellar energy generation, did not interest him, but Teller persuaded him to go. At the conference, Strömgren detailed what was known about the temperature, density, and chemical composition of the Sun, and challenged the physicists to come up with an explanation. Gamow and Carl Friedrich von Weizsäcker had proposed in a 1937 paper that the Sun's energy was the result of a proton–proton chain reaction :