Eli5: what is entropy?


Eli5: what is entropy?

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Imagine you have a bunch of toys in a big toy box. If you keep all the toys in order and neatly stacked, the toy box looks very organized and clean. But if you throw all the toys in the box without sorting them, the toy box looks messy and disorganized.

Entropy is like how messy or disorganized the toy box is. If the toys are all stacked neatly, the entropy is low. But if the toys are all mixed up and disorganized, the entropy is high.

In technical terms entropy is the tendency of all ordered systems to proceed towards disorder. Now what that actually means can be explained less technically.

Let’s imagine we’re looking at a clear box full of 8 bouncy balls that are bouncing off the walls without stopping. The center line of that box has a big sharpie line draw around it. If we take a picture of the box at any single point in time there is most likely going to be 4 balls bouncing on each side of that line. Now let’s say we can move all the balls to one side of that line and start them bouncing again. We have introduced order into our disordered system. However over time entropy will take over. The balls will bounce off each other more often than they bounce off the walls and the balls will eventually end up evenly distributed inside the box again.

Now what does that mean in practical terms. Well energy can only travel one way. We burn coal to make electricity. So we’re taking the stores energy in coal and turning it into heat. That heat goes to generate power and gets vented off. So we have taken the highly compact energy of coal and spread it out as heat into the atmosphere. We can use that energy to cool things but we will never be able to remove more energy from the thing we’re cooking than the heat generated from burning that amount of coal.

The other important part of entropy besides “disorder” is that it can only go up in a closed system. That doesn’t mean it can’t go down, but that going down requires added energy from outside the system.

You don’t become increasingly disordered and fall apart because you aren’t a closed system. You eat food and that energy is used to keep your cells intact and in place. The Earth doesn’t become increasingly disordered because sunlight is constant energy input.

The entire universe doesn’t have outside input, so entropy always increases. The universe is also very big (citation needed), so proceeding towards a state of maximum entropy will take lengths of time that we can math out but can’t really intuitively understand.

Entropy is randomness.

The reason entropy breaks things down is that randomness, over time, breaks things, but most things don’t *un-break* on their own. If a rock splits in half, it stays split forever, functionally. Same with many other things, if a thing can eventually fall apart, then eventually, infinite random chance will mean the perfect conditions for it to do so happen.

Even things that *can* fix themselves break down eventually. You see, the processes of healing introduce randomness into the healed bits. So the new skin over a healed cut is ever so slightly different from the old. Over time, this *eventually* means it will “forget” how to heal as well, as the new skin is missing little bits of the process of self healing. This is the effects of aging, and is why older folks don’t heal as well.

So eventually, that means *everything*, eventually, falls apart. This obviously is a pretty long “eventually” though.

To sum up, randomness breaks things, and nothing we know of can fix itself forever. So everything will, eventually, be worn down.