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Unformatted text preview: arXiv:0903.3489v1 [gr-qc] 20 Mar 2009 THE NATURE OF TIME (This article won the first prize awarded on March 7th 2009 by the jury in the essay competition of the Foundational Questions Institute (fqxi.org) on The Nature of Time.) Julian Barbour email: [email protected] Abstract. A review of some basic facts of classical dynamics shows that time, or precisely duration , is redundant as a fundamental concept. Duration and the behaviour of clocks emerge from a timeless law that governs change. 1 Introduction My library contains four books on mechanics, the science of change in time. Three of them – all modern classics – fail to define either time or clocks! Relativity textbooks do discuss time and clocks but concentrate on only one of the two fundamental problems of time that Poincar´ e identified in 1898 : the definitions of duration and of simultaneity at spatially separated points . Since then the first problem has been remarkably neglected, probably because Einstein’s solution of the second in 1905 created such excitement. The failure to discuss duration at a foundational level largely explains the unease many feel when confronted with the idea that the quantum universe is static. This suggestion emerged in 1967 from a rather high-level attempt by Bryce DeWitt to meld Einstein’s classical general theory of relativity  with quantum theory and has given rise to decades of agonizing over ‘the problem of time’. In my view, had duration been properly studied in classical physics, its disappearance in the conjectured quantum universe would have appeared natural. In this essay I will not discuss quantum theory at all but instead question the standard assumptions made about duration in classical physics. I shall develop from scratch a theory of time and clocks, linking it to work that astronomers began in antiquity. The best guide to the nature of time is the practice of astronomers. They cannot afford mistakes; a missed eclipse is all too obvious. Moreover, they work directly with concrete facts, their observations, not obscure metaphysical notions of time. My discussion begins with Newton’s comments on astronomical practice in his great Mathe- matical Principles of Natural Philosophy (1687), my only book on mechanics that does discuss duration. Newton’s discussion leads directly to two key intimately related questions: How can we say that a second today is the same as a second yesterday? What is a clock? The answers to these questions, which are seldom addressed at a sufficiently foundational level, will tell us much about time and the way the world works. We shall find answers to them by examining successive important discoveries made over two millennia....
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