He gave the popular lectures that became "The Character of Physical Law." He gave lectures to Caltech undergrads that became "The Feynman Lectures on Physics." He gave lectures to Caltech graduate students that became "The Feynman Lectures on Gravitation." In every one of these books, every one of these sets of lectures, he emphasized this puzzle: Why did the early universe have such a small entropy?
Dio clases a los estudiantes de pregrado de Caltech que luego se llamaron "Clases de física de Feynman". Dictó clases a los estudiantes graduados de Caltech que se volvieron "Clases de gravitación de Feynman". En todos sus libros, en todas esas series, él hacía hincapié en el enigma: ¿por qué el Universo temprano tenía tan baja entropía?