The Order of Time

by Carlo Rovelli

In our experience, the notion of cause is thus asymmetrical in time: cause precedes effect. When we recognize in particular that two events “have the same cause,” we find this common cause102 in the past, not in the future. If two waves of a tsunami arrive together at two neighboring islands, we think that there has been an event in the past that has caused both. We do not look for it in the future. But this does not happen because there is a magical force of “causality” going from the past to the future. It happens because the improbability of a correlation between two events requires something improbable, and it is only the low entropy of the past that provides such improbability. What else could? In other words, the existence of common causes in the past is nothing but a manifestation of low entropy in the past. In a state of thermal equilibrium, or in a purely mechanical system, there isn’t a direction to time identified by causality.