“Thus is our treaty written; thus is agreement made. Thought is the arrow of time; memory never fades. What was asked is given; the price is paid.” -Robert Jordan
Why does time flow forwards and not backwards, in 100% of cases, if the laws of physics are completely time-symmetric? From Newton’s laws to Einstein’s relativity, from Maxwell’s equations to the Schrödinger equation, the laws of physics don’t have a preferred direction. Except, that is, for one: the second law of thermodynamics. Any closed system that we look at sees its entropy only increase, never decrease.
A system set up in the initial conditions on the left and let to evolve will become the system on the right spontaneously, gaining entropy in the process. Image credit: Wikimedia Commons users Htkym and Dhollm, under a c.c.-by-2.5 license.
Could this thermodynamic arrow of time be responsible for what we perceive as the forward motion of time? Interestingly enough, there’s an experiment we can perform: isolate a system and perform enough external work on it to force the entropy inside to *decrease*, an “unnatural” progression of entropy. What happens to time, then? Does it still run forward?
No matter how we change the entropy of the Universe around us, time continues to pass for all observers at the rate of one second per second. Public domain image.