St. Augustine used to say: if you don’t ask me, I know what time is; but, as soon as you ask me, I no longer know. Time is no doubt one of the deepest secrets of existence, and physics is struggling with this mystery no less than philosophy or theology are. In this lecture we explore two notions of time intertwined in all of modern physics. On one hand we have a kind of time which partakes of the idea of ‘being’ or ‘eternity’, a ‘timeless time’ which is merely a geometrical property of reality – essentially all simultaneously present and blind to the distinction between past and future. This is microscopic time, time at the most fundamental level of physics, at the level of atoms and below. But we encounter quite a different notion of time when we look at things on a larger scale, the scale of trees and humans and rocks and stars. Here the distinction between past and future is a basic, primal fact of existence: this is irreversible time, the time of ‘becoming’, the time of the ‘arrow of time’. The difficult coexistence of these two notions is a fascinating story that takes us to the heart of the mystery of time.

A good collection of essays about time is Raymond Flood and Michael Lockwood, eds., The Nature of Time, Basil Blackwell, Oxford and New York, 1986

For a taste of contemporary discussions about time in physics, see e.g.:

Julian B. Barbour, The End of Time, New York: Oxford University Press, 2000

Brian Greene, The Hidden Reality, New York, Knopf, 2011

Stephen Hawking and Leonard Mlodinow, The Grand Design, New York: Bantam, 2010

Roger Penrose, Cycles of Time, New York, Knopf, 2011

Carlo Rovelli, La realtà non è come ci appare, Milano: Raffaello Cortina, 2014 (in Italian)

Lee Smolin, Time Reborn,Houghton Mifflin Harcourt, New York, 2013

Leonard Susskind, The Cosmic Landscape, New York: Little, Brown, 2005