Why Time Seems to Have a Direction

Film a pendulum swinging in a vacuum and play it in reverse, and no one could tell which version is real. The equations that describe its motion work just as well in either direction. The same is true, at a fundamental level, for most of the laws of physics we know.

Now film a cup of coffee cooling on a table, or an egg falling and breaking, and play those backwards. Everyone can tell immediately. Something about the everyday world clearly has a direction, even though the microscopic rules underneath it mostly do not.

Counting arrangements

The resolution lies in a quantity called entropy. Roughly speaking, entropy measures how many microscopic arrangements are consistent with what we see at large scales.

A hot cup in a cool room is a special, rare kind of arrangement: lots of energy concentrated in one small place. There are vastly more ways for that energy to be spread out evenly between the cup and the room. As molecules jostle and collide, the system wanders from rare arrangements into common ones, simply because common ones are overwhelmingly more numerous.

Things do not tend toward disorder because disorder is preferred. They drift there because there is so much more of it.

The puzzle moves back

This explains why entropy increases, but it raises a deeper question. If systems drift from low entropy to high entropy, then the past must have had lower entropy than the present. Why?

Following that question backwards leads, eventually, to the early universe. For the arrow of time to point the way it does, the universe must have begun in an extraordinarily low-entropy state.¹ Why it did remains one of the open questions in physics.

The arrow in your memory

There is a quiet consequence of all this. We remember the past and not the future, and we can affect the future but not the past. Many physicists suspect these psychological and causal arrows are also rooted in the same thermodynamic asymmetry.

Every time you notice a cup of coffee cooling, you are watching the universe's initial conditions, still unwinding.