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Abstract
For over 150 years theoretical physicists have been faced with a conundrum: with one exception that is far too small to affect the issue, the known laws of nature give no explanation of why all macroscopic phenomena in the universe unfold in a common temporal direction. Like the stars, we all get older together; we never meet anyone getting younger. This issue first came to prominence with the discovery of entropy in the 1850s and the demonstration that in a closed system it can never decrease and, in accordance with the second law of thermodynamics, has a general tendency to increase. It is widely claimed that, in agreement with the second law, the entropy of the whole universe is increasing and that the only explanation for this is some very special – and inexplicable – state in the past, probably at the big bang. I will argue that the failure to find a more satisfactory explanation arises from an inappropriate approach to the problem, which goes back to the discovery of thermodynamics through the study of steam engines. Critical for the working of those machines, which had such a great impact on the development of the industrial revolution, was confinement of the steam in a cylinder. This led to the development of the beautiful theory of the statistical mechanics of systems confined to a ‘conceptual box.’ But it seems hardly plausible that the universe is in a box. By a simple example I will show that the theoretical consideration of unconfined systems has the potential to remove all the puzzles surrounding the existence of time’s arrows.