Category: time’s arrow

No, Thermodynamics Does Not Explain Our Perceived Arrow Of Time

“As far as we can tell, the second law of thermodynamics is true: entropy never decreases for any closed system in the Universe, including for the entirety of the observable Universe itself. It’s also true that time always runs in one direction only, forward, for all observers. What many don’t appreciate is that these two types of arrows — the thermodynamic arrow of entropy and the perceptive arrow of time — are not interchangeable.

During inflation, where the entropy remains low and constant, time still runs forward. When the last star has burned out and the last black hole has decayed and the Universe is dominated by dark energy, time will still run forward. And everywhere in between, regardless of what’s happening in the Universe or with its entropy, time still runs forward at exactly that same, universal rate for all observers.

If you want to know why yesterday is in the immutable past, tomorrow will arrive in a day, and the present is what you’re experiencing right now, you’re in good company. But thermodynamics, interesting though it may be, won’t give you the answer. As of 2019, it’s still an unsolved mystery.”

No matter who you are, where you are, or what you’re doing, you’ll always perceive time running forward, from your frame of reference, at exactly the same rate: one second-per-second. The fact that this is true has led many to speculate as to what the cause of time’s arrow might be, and many, having noticed that entropy never decreases in our Universe, place the blame squarely on thermodynamics as the root of our arrow of time.

But that’s almost certainly not the case, and we can demonstrate that fact in a number of ways, including by decreasing entropy in a region and noting that time still moves forwards. The perceived arrow of time is still a mystery.