Category: arrow of time

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.

We Still Don’t Understand Why Time Only Flows Forward

“It’s true that entropy does explain the arrow of time for a number of phenomena, including why coffee and milk mix but don’t unmix, why ice melts into a warm drink but never spontaneously arises along with a warm beverage from a cool drink, and why a cooked scrambled egg never resolves back into an uncooked, separated albumen and yolk. In all of these cases, an initially lower-entropy state (with more available, capable-of-doing-work energy) has moved into a higher-entropy (and lower available energy) state as time has moved forwards. There are plenty of examples of this in nature, including of a room filled with molecules: one side full of cold, slow-moving molecules and the other full of hot, fast-moving ones. Simply give it time, and the room will be fully mixed with intermediate-energy particles, representing a large increase in entropy and an irreversible reaction.”

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.

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?

Find out the answer, and learn whether thermodynamics has anything to do with the passage of time or not!