J. Robert Oppenheimer's letter to his brother Frank Oppenheimer (October 14, 1929), as quoted in "Robert Oppenheimer: Letters and Recollections" edited by Alice Kimball Smith (p. 135), 1995.
Walter Lewin (2012). “For the Love of Physics: From the End of the Rainbow to the Edge of Time - A Journey Through the Wonders of Physics”, p.2, Simon and Schuster