Why does time only move forwards? Possibly just because we’re ignorant

The one-way flow of time is one of the great mysteries of physics. It might be that we see causes and effects just because our information about reality is incomplete


17 November 2021

WHY does time only move forward?

Kitti Suwanekkasit/Shutterstock

BY THE time you have finished reading this, you will be a couple of minutes older. Hopefully you won’t regret those minutes, because you can’t get them back. Time, as we all know, only moves in one direction for us. The question of why, however, doesn’t come with a simple answer.

Searching for time’s arrow in the underlying, microscopic laws of physics certainly draws a blank. They give us no reason to think atoms, molecules and so on can’t move backwards as well as forwards in time, much as they (and we) move freely in three dimensions of space. The laws don’t differentiate between past, present and future, or between cause and effect.

“That distinction only becomes relevant in the macroscopic world, where our incomplete information about the precise physical configuration of a system leads us to perceive an arrow of time, and to put causes first and effects after,” says physicist Sean Carroll at the California Institute of Technology.

At issue here is the concept of entropy. This is a measure of the amount of disorder in a system, defined as the number of different microscopic configurations it can have without changing its macroscopic, or overall, appearance. A box of hot gas has high entropy, for example, with a vast number of equivalent configurations with different positions and velocities for each atom or molecule. A human, though, has low entropy – try to reconfigure us too much and things rapidly start to fall apart.

The crucial point is that there are more ways for any …

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