We live life forward as time flows from past to future, and believe that any closed system, like a box of particles, follows an “arrow of time” into the future as the system’s entropy grows. This seems paradoxical, since a video of a single particle in motion looks equally valid running forwards or backwards. The video does not distinguish past from future because Newtonian mechanics is unchanged under reversed time. But statistical mechanics shows that a large group of particles is highly likely to irreversibly tend towards greater disorder. As a measure of disorder, entropy is considered to show the universal forward march of time. In contrast, The Janus Point: a New Theory of Time, by physicist and science writer Julian Barbour, presents a different approach that leads to a time-symmetric universe.
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