Two Perspectives on the Nature of Time

Dear Reader, through this essay I aim to present two perspectives on the nature of time. First I will discuss the concepts of disorder and irreversibility. Second I will describe our place in the history of the universe. Developed over the past 200 years by generations of scientists, these insights are founded on layers of experimental and observational evidence.

Disorder and Irreversibility

Scenario 1: A room is filled with air at standard temperature and pressure. The number of air molecules in the room (roughly 10²⁴ or a trillion-trillion) is large beyond intuition. The molecules move randomly, scattering off each other and the walls. A movie of this commotion would look qualitatively the same whether it was played forward or backward. A physicist would say the motions of the particles are reversible.

Scenario 2: A door is opened from the room in Scenario 1 to an evacuated room. The molecules originally in the first room diffuse into the second until the densities of molecules in the two rooms are equal. Unlike in Scenario 1, a movie of this process would not look qualitatively the same when played in reverse (showing all the molecules coalescing into one room). The motions of the particles in Scenario 2 are in some sense irreversible.

Common sense dictates this difference between Scenarios 1 and 2. However, physics provides further insight. The microscopic laws of physics do not rule out the "rewound" version of either scenario. Whether played forward or backward, the forces between colliding particles are equal and opposite and of a magnitude required to produce the reverse motion.

Intuition may be gained by likening Scenario 2 to the start of a game of billiards in which an orderly racked triangle of balls is broken by the cue. The balls then scatter every which way on random paths. Nothing in the microscopic laws of physics prevents the reverse of this play from happening: balls following random paths could converge, exerting the precise forces on one another required to stop each in its track, leaving the whole in a perfectly racked triangle. However, while a randomly chosen set of trajectories could beget an orderly triangular state, this situation is simply very unlikely: most randomly chosen sets of trajectories will simply beget a different disordered set of trajectories.