When Albert Einstein gave his first public lecture on relativity in Salzburg in 1909, the man sitting in the audience who mattered most was Max Planck: the founder of quantum physics and the most powerful voice in German science. Planck had already staked his reputation on Einstein's work. Salzburg was the moment that endorsement became public.
The Story
Max Planck arrived at the 81st Assembly of German Natural Scientists and Physicians in Salzburg in September 1909 already carrying the weight of one of the century's greatest discoveries. Nine years earlier, in December 1900, he had introduced the concept of the energy quantum, proposing that energy is emitted not continuously but in discrete packets. He had been unsettled by his own idea and had spent years trying to walk it back. Physics would not let him.
By 1909, Planck had become the most respected physicist in Germany and one of the most influential voices in European science. He had also done something that required real intellectual courage: he had recognized and championed Albert Einstein's 1905 papers on relativity and the photoelectric effect at a time when almost no one else had. He helped ensure that Einstein's work received serious attention in the German-speaking physics community rather than being ignored or dismissed.
In Salzburg, Planck sat in the audience while Albert Einstein, then still a patent clerk, delivered his first major public lecture arguing the case for special relativity and the quantum nature of radiation. The lecture was addressed to the community of which Planck was the acknowledged senior figure. His presence and his endorsement mattered enormously.
The encounter between the two men in Salzburg represents one of the cleaner moments in the history of science: an established master in the audience for the public debut of the mind that would eventually surpass him, both working on different faces of the same revolution.