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The Janus Point

A New Theory of Time’s Arrows and the Big Bang.

The discovery of the 2nd Law of Thermodynamics in the 1850s created a puzzle that has not yet been resolved. All the known laws of physics make no distinction with regard to the direction of time. They are time-reversal symmetric. However, the 2nd Law says that in a dynamically closed system entropy will in general increase or at least not decrease. In fact, virtually all natural processes exhibit an ‘arrow of time’ in that they all unfold in a common direction. For example, all animals, like all stars throughout the universe, get older in the same direction. We never meet anyone getting younger. The discrepancy between facts like this and the time-reversal symmetry of the laws of nature is very striking. I will argue that a resolution can be found by noting that the 2nd Law presupposes confinement of the system whose entropy is being studied. However, the universe can hardly be said to be confined; this changes the way we need to think about entropy. I will show that at least for simple plausible model universes which satisfy time-reversal symmetric laws all possible histories they can have divide into two qualitatively symmetric halves at a ‘Janus point’. On either side of this point, the ‘arrows of experienced time’ point in opposite directions. This observation has the potential to resolve the conflict between time-reversal symmetry of the laws of nature and the 2nd Law.


Julian Barbour

After completing a PhD in theoretical physics, I became an independent researcher to avoid the publish-or-perish syndrome. For 50 years I have worked on the nature of time, motion, and the quantum theory of the universe. I am the author of about 50 papers and two books: The Discovery of Dynamics and The End of Time, in which I argue that time is an illusion. I am currently writing a further book with the provisional title The Janus Point: A new theory of time’s arrows and the big bang. Details of my research work and publications are given at my website www.platonia.com. Between 2008 and August 2017 I was a Visiting Professor in Physics at the University of Oxford.