Recently, when we were talking about why time only moves forward, we got into talking about the Second Law of Thermodynamics. It says that disorder, otherwise known as entropy, must always increase. For instance, if you add cream to your coffee, you'll always get an even mix of the two. This is the most disordered state. And because entropy can never decrease, your coffee can never spontaneously unmix itself.

If that's true, and the universe is constantly getting more disordered, then why do we see ordered things like galaxies and planets and people? Surely if the universe always gets more disordered, we shouldn't expect complex things like ourselves to form. Well, over at MinutePhysics, physicist Sean Carroll explains how this apparent contradiction is possible:

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For one thing, the Second Law of Thermodynamics says that the total entropy in the universe has to increase. It still allows for local decreases in entropy as long as those decreases are balanced out by an increase in entropy somewhere else. So your freezer can lower the entropy in water by turning it into ice, as long as it increases the total entropy by emitting heat.

But more fundamentally, there's a distinct difference between complexity and entropy. In physics, entropy refers to the number of ways you can swap molecules and have the whole system remain relatively the same. It's possible for something to grow in complexity and become more disordered at the same time. In fact, that's usually how it works. Typically, as entropy increases, disorder increases, reaches a peak, and then decreases again.

This is how our universe works too. At the start, the universe had no entropy and was very simple. In the middle, where we live, it's highly complex and has medium entropy. At the end of the universe it will be simple again, but will have extremely high entropy.

Source: MinutePhysics

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