The particles of which the universe is made don’t much care which way time goes. But we do, and so do the stars and the planets.

At what we might call the most “fundamental” level, the laws of nature do not much care in which direction time flows. Yet from our point of view, as participants in the physical universe, the arrow time is an inescapable and supremely important fact. Put briefly, some things cause other things, and we get old.

A snooker, or pool, game provides a good image to help understand the problem¹. Film the moment of impact of any shot, or any collision between two balls, and run the video forwards and backwards. Assuming the cue and the player are out of frame, the video looks just as realistic backwards as forwards. With one exception. The break, the opening shot, will look ridiculous when played backwards. In the correct time direction, an orderly triangular array of balls is shattered. In the other direction, it spontaneously assembles out of nowhere. This never happens, and so the correct time direction is determined. Even though the fundamental physical laws that cover the collisions run the same backwards as forwards.

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