The Arrow of Time

One of the deepest puzzles in physics is also one of the most obvious: time only ever runs forwards. We remember the past, not the future; a cup falls and shatters but never reassembles. Yet the fundamental laws of physics work equally well in both directions. So where does the one-way flow of time come from? This question, the arrow of time, was one Hawking returned to often.

Three arrows

Hawking, following others, distinguished several "arrows" that all point the same way. The thermodynamic arrow is set by entropy, the measure of disorder, which the second law of thermodynamics says always increases: disorder grows, which is why the shattered cup is the future and the whole cup is the past. The psychological arrow is the direction in which we feel time passing and remember events, and Hawking argued it is really just the thermodynamic arrow experienced from the inside, since recording a memory itself increases disorder. The cosmological arrow is the direction in which the universe expands.

Why the arrows agree

The striking thing is that these arrows all point the same way, and Hawking wanted to know why. The answer traces back to the beginning. For entropy to increase steadily over the whole history of the universe, the universe must have started in a state of extraordinarily low entropy, a very smooth, ordered early cosmos. Why it began so ordered is among the great unsolved questions, and Hawking hoped his no-boundary proposal might help explain it.

A mistake honestly admitted

The arrow of time is also the setting for one of the more revealing episodes of Hawking's career. He had once argued that if the expanding universe were to stop and recollapse, the thermodynamic arrow would reverse, with disorder decreasing and, in effect, time running backwards. Younger colleagues showed him the argument was flawed. Rather than defend his position, Hawking publicly admitted he had been wrong, and revised his views in later editions of A Brief History of Time. It is a small example of the intellectual honesty explored on the page about how his mind worked: a willingness to follow the argument even when it overturned his own earlier claims.