Source:
(1,2) The Feynman Lectures on Physics, Vol. 1: Mainly Mechanics, Radiation, and Heat (Hardcover), (CH 5) by Richard P. Feynman (Author), Robert B. Leighton (Author), Matthew Sands (Author)
Vol. 1 Richard Feynman
(3) Source: Roger Penrose's "The Road to Reality: A Complete Guide to the Laws of the Universe (Hardcover)", CH 17