President Thompson, members of the Society, ladies, and gentlemen: Thank you, Eric and John, for your very flattering introduction. The Penrose Medal is a great honor, never expected, but which I accept with gratitude and with a very strong sense of humility—with gratitude to teachers, colleagues, and students who have helped me along the way and to the Society for having selected me; humility as I think of the extraordinary contributions of past Penrose medalists.

Following an undergraduate degree at Cornell in 1961, I arrived at Harvard in 1963 and my timing could not have been better. Ulrich Petersen had...

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