Salzburg was where the world first heard Einstein speak. In September 1909, a 30-year-old patent clerk stood up before an audience of Europe's leading scientists and delivered his first major public lecture on relativity. Max Planck, the most powerful physicist in Germany, was in the room.
The Story
On 21 September 1909, Albert Einstein addressed the 81st Assembly of German Natural Scientists and Physicians in Salzburg. The lecture, titled "Uber die Entwicklung unserer Anschauungen uber das Wesen und die Konstitution der Strahlung," was his first sustained public argument for the theory he had published four years earlier. He was still employed as a patent clerk in Bern. Almost no one outside specialist circles knew his name.
The content was extraordinary. Einstein argued that radiation behaves like discrete particles carrying momentum, weaving together his 1905 photoelectric paper and Planck's black-body law into a case for wave-particle duality that most physicists had not yet accepted. He showed how the energy of a light quantum is proportional to its frequency, a claim that would later earn him the Nobel Prize in Physics in 1921.
Max Planck sat in the audience. As the most eminent physicist in the German-speaking world, Planck had been quietly championing Einstein's work and helping it gain acceptance in the scientific community. His presence in Salzburg, and his later praise of the lecture's boldness, accelerated the wider recognition of relativity. The two men at that congress represents one of the great encounters in the history of science.
Einstein went on to develop general relativity, receive the Nobel Prize, and eventually flee Nazi Germany for Princeton. But the lecture he gave in Salzburg in 1909 was the public beginning of everything.