Few facts of experience are as obvious and pervasive as the distinction between past and future. We remember one, but anticipate the other. If you run a movie backwards, it doesn’t look realistic. We say there is an arrow of time, which points from past to future.
One might expect that a fact as basic as the existence of time’s arrow would be embedded in the fundamental laws of physics. But the opposite is true. If you could take a movie of subatomic events, you’d find that the backward-in-time version looks perfectly reasonable. Or, put more precisely: The fundamental laws of physics — up to some tiny, esoteric exceptions, as we’ll soon discuss — will look to be obeyed, whether we follow the flow of time forward or backward. In the fundamental laws, time’s arrow is reversible.
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