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.
Logically speaking, the transformation that reverses the direction of time might have changed the fundamental laws. Common sense would suggest that it should. But it does not. Physicists use convenient shorthand—also called jargon—to describe that fact. They call the transformation that reverses the arrow of time “time reversal,” or simply T. And they refer to the (approximate) fact that T does not change the fundamental laws as “T invariance,” or “T symmetry.”
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