Entropy Is Why You Can't Unbreak An Egg. Is It Also Why Time Can't Go Backward?
When you wake up in the morning and break an egg into a frying pan only to realize you want oatmeal instead, there isn't much you can do. You can't exactly put the egg back in the shell. The principle that makes it impossible for you to unbreak that egg is the same one that makes it impossible for the universe to go through a reverse Big Bang—and may explain why time only goes forward, never backward. That principle is called entropy.
Wake Up And Smell The Entropy
Here's a surprising fact: Newton's laws of motion don't have a problem with your egg unbreaking. Neither does quantum mechanics. Both have a lot to say about how the universe behaves, but those behaviors can happen forwards or backwards, as far as they're concerned. Really, there's only one principle that takes issue with your half-fried mess turning back into a fully intact egg: the second law of thermodynamics. In contrast to that all-too-famous first law of thermodynamics, which says that energy can't be created or destroyed, the second law says that there's a natural tendency for things degenerate into increasing disorder, also known as entropy. Things go from low entropy—more order—to high entropy—more disorder, never the opposite.
Take ice, for example. Water molecules in an ice cube are in an ordered arrangement; that's how it can keep its shape, after all. But plunk an ice cube in a glass of lemonade, and it will eventually melt. That's entropy: there are more ways for the water molecules, and the energy they carry, to arrange themselves in a liquid state than in a solid state, so it's much more likely that the ice will melt than stay frozen. Similarly, there are so many more ways for an egg to arrange itself broken than whole that it's vanishingly unlikely—nay, impossible—that the broken egg would ever rearrange its molecules into an unbroken state.
Time And The Origin Of The Universe
There's an interesting implication in the second law of thermodynamics. Because all thermodynamic processes—anything involving the transfer or conversion of heat energy—must result in an increase in entropy, they're irreversible. In theory, there are interactions that don't involve heat transfer, but in practice, everything has inefficiencies that result in friction or other types of heat loss, so this law pretty much applies to everything. So if nothing is reversible, time isn't either. Time always flows in the direction of increasing entropy.
This extends to the very nature of our universe: it began as a very dense, hot mass of stuff—a low-entropy state—that expanded and cooled into a high-entropy state. But wait just a second. If entropy must always increase, then how did stars and planets form? Why isn't the universe just a chaotic mass of particles? As the BBC explains, "The answer is that gravity affects entropy, in a way that physicists still don't fully understand. With truly massive objects, being clumpy is higher entropy than being dense and uniform. So a universe with galaxies, stars and planets actually has a higher entropy than a universe filled with hot, dense gas."
Of course, that brings up a new problem: the universe, we can all agree, is "truly massive," but it began dense and uniform, which by that explanation would be considered high-entropy. The Big Bang, then, would have made it low-entropy. This is the kind of conundrum scientists are grappling with as they try to determine the nature and origin of the universe and the physical laws that govern it. There are plenty of possible explanations being thrown around, but for now, all we can do is ponder.
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