German physicist who originated and developed the Quantum Theory. Awarded the Nobel Peace Prize in 1918. ALS in German, signed 'M. Planck,' one page, 3.5 x 5.5, September 6, 1920. Letter to a friend on a picture postcard, with an image of Rattvik on the reverse. Planck has also addressed the card in his hand. In fine condition, with pencil notation above address area. COA John Reznikoff/University Archives and RRAuction COA.
