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We Still Don’t Understand Why Time Only Flows Forward

The past is gone, the future not yet here, only the present is now. But why does it always flow the way it does for us?

Every moment that passes finds us traveling from the past to the present and into the future, with time always flowing in the same direction. At no point does it ever appear to either stand still or reverse; the “arrow of time” always points forwards for us. But if we look at the laws of physics — from Newton to Einstein, from Maxwell to Bohr, from Dirac to Feynman — they appear to be time-symmetric. universe science space earth science and theory science

In other words, the equations that govern reality don’t have a preference for which way time flows. The solutions that describe the behavior of any system obeying the laws of physics, as we understand them, are just as valid for time flowing into the past as they are for time flowing into the future. Yet we know from experience that time only flows one way: forwards. So where does the arrow of time come from? universe science space earth science and theory scienceuniverse science space earth science and theory science

A ball in mid-bounce has its past and future trajectories determined by the laws of physics, but time will only flow into the future for us.(Wikimedia commons users MichaelMaggs and (edited by) Richard Bartz) universe science space earth science and theory science

Many people believe there might be a connection between the arrow of time and a quantity called entropy. While most people normally equate “disorder” with entropy, that’s a pretty lazy description that also isn’t particularly accurate. Instead, think about entropy as a measure of how much thermal (heat) energy could possibly be turned into useful, mechanical work. If you have a lot of this energy capable of potentially doing work, you have a low-entropy system, whereas if you have very little, you have a high-entropy system. universe science space earth science and theory science

The second law of thermodynamics is a very important relation in physics, and it states that the entropy of a closed (self-contained) system can only increase or stay the same over time; it can never go down. In other words, over time, the entropy of the entire Universe must increase. It’s the only law of physics that appears to have a preferred direction for time. universe science space earth science and theory science

Still from a lecture on entropy by Clarissa Sorensen-Unruh. (C. Sorensen-Unruh / YouTube) universe science space earth science and theory science

So, does that mean that we only experience time the way we do because of the second law of thermodynamics? That there’s a fundamentally deep connection between the arrow of time and entropy? Some physicists think so, and it’s certainly a possibility. In an interesting, 2016 collaboration between the MinutePhysics YouTube channel and physicist Sean Carroll, author of The Big Picture, From Eternity To Here, and an entropy/time’s arrow fan, they attempt to answer the question of why time doesn’t flow backwards. Unsurprisingly, they point the finger squarely at entropy. universe science space earth science and theory science

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. universe science space earth science and theory science

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