It's one of those things that we take for granted—time moves forward and never backward. But did you ever stop to wonder why it moves in one direction, as opposed to the other?
The question continues to stump physicists. After all, there are certain physical processes that are actually time-reversible—they look the same no matter which way you run them.
For example, gravity operates the same way regardless of Time's Arrow; a planet will orbit a star in exactly the same way, just with the direction of that orbit reversed. But there is one aspect of the universe that is dependent on the direction of Time's Arrow: the Second Law of Thermodynamics. This states that the disorder of a closed system (such as our universe) must increase, never decrease.
It's commonly called "entropy," and it's why broken eggs don't suddenly reassemble themselves, or why dead things don't suddenly come back to life. Disorganization and chaos are downhill, order and complexity are uphill; complex systems like stars and planets and human beings may emerge locally, but they require an inordinate amount of energy to create, which only increases the overall entropy of the system.
This is why the Second Law of Thermodynamics is universally reckoned as the mechanism that imparts directionality to time—although, understanding the how of a thing is not the same as understanding the why of it.
A Dark Connection
In the quest to understand the origins of Time's Arrow, two Armenian physicists, A. E. Allahverdyan and V. G. Gurzadyan, decided to search for a link between so-called "dark energy" and the Second Law of Thermodynamics. Dark energy is a mysterious quantity that is proposed as an explanation for why the universe is continuing to expand, rather than decelerating and collapsing, as our current understanding of gravity dictates it should.
The scientists ran a simulation to test how the orbit of a planet would change, depending upon whether dark energy was absent, or present as in our own universe. What they found was intriguing, to say the least: if dark energy is absent, or if gravity pulls space together, a planet simply orbits its star in the accepted fashion, regardless of whether time runs "forward" or "backward." But introduce dark energy to push space apart, and eventually, over immense time scales, the planet is flung away from the star altogether.
Which means there is a temporal directionality when dark energy is introduced—in one direction, we see a planet escaping a star's gravity; in the other, a planet is captured by a star and becomes a part of its retinue.
There's nothing absolutely conclusive about the study, which was published in Physical Review E; the authors aren't saying dark energy is responsible for Time's Arrow. But there's something decidedly suggestive about it all—that the Second Law of Thermodynamics and dark energy might be two facets of the same phenomenon, some mysterious quantity of our universe that imparts or requires an arrow of time.
So next time you're late—just blame it on dark energy.
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