This is the third post in a series. The story so far:
Thermodynamics was, for me, one of the most obnoxious parts of the physics curriculum. I hated it. Partly because it’s mathematically very different from a lot of other physics (and no, I don’t want to get into why, I promised not to burden this with math; just trust me) … but partly because it’s depressing.
You’ve probably heard of the Laws of Thermodynamics, and in particular you’ve likely heard of the Second Law. The Second Law talks about a thing called entropy. In particular, the Second Law basically says: “The entropy of a closed system must always increase.” Take any “system” of matter and energy that’s isolated enough from the rest of the universe, and the entropy in it will always go up, never down. But what is entropy?
Without getting into math, there’s two big ways to understand entropy:
- Unusable energy.
- Disorder.
Let’s look at these in turn.
Unusable Energy
Imagine a car’s engine. A spark plug ignites a bit of gasoline, which explodes and pushes the piston down, which turns the axle. But not all the energy of the explosion does useful work: some of it just gets dissipated as friction between the pistons and the engine block. The more efficient the engine, the less energy is wasted, but there’s always going to be some wasted. The energy from the gasoline which is wasted, doing no useful work, is a manifestation of entropy.
Couldn’t we put it to some use? Like, could we maybe use the heat from the engine block to do something useful, like heating the interior of the car in winter? Sure, you could try building some clever machinery around the engine which does that, but the laws of thermodynamics say that you’re never going to capture it all. Some of it is going to be wasted just heating up the hood, the axle, whatever.
This loss of energy is, to put it simply, why a perpetual-motion machine cannot exist. Because in any process involving the transfer of energy, some of it is always lost.
Disorder
Imagine a tank of water with a waterproof divider in the middle. The water in one half of the tank is dyed, say royal blue, and the other half is clear. Now we pull out the divider and wait. You can easily see that, left to its own devices, the dye will gradually diffuse through the whole tank, leaving the water a uniform paler shade of blue, even if no one deliberately stirs it. The water molecules themselves just sort of vibrate and bounce around – if they didn’t, it would be ice. So they will “stir” themselves.
When we started, the tank was a lot more orderly: you could definitely say that all the molecules of dye were confined to a specific volume. But now the whole tank is a uniform mix. The dye molecules are everywhere.
The Second Law
So let’s come back to the Second Law. Entropy always increases. If I showed you a video of the dye dispersing through the tank, you’d accept it as how things happened. But if I played it backwards and you saw the dye all going back into one half of the tank, you’d say “Oh, this is backwards.” The chances of the water and dye molecules just randomly moving into that configuration are … well, not actually zero, but so very unlikely that it’s nonsensical.
Similarly, if I showed you a video of a hammer smashing a teacup, no problem, but if I ran it backwards, you’d know. No one has ever pulled a hammer away from shards of porcelain and had the resulting motion pull them back together as a teacup.
To understand why this is, imagine cleaning your room.
Your room can be messy in far, far more ways than it can be tidy. Assuming your idea of “tidy” is close to normal, your clothes need to be folded up and in specific drawers, or at least in a hamper, for the room to be tidy. But for your room to be messy, they can be anywhere in the room. On the bed, under the bed, on your desk, hanging from the ceiling fan, you name it.
Many ways to be messy. Few to be tidy. So just as a matter of probability, imagine someone sort of picked up your room and shook it as one would shake a snowglobe. It’s not likely that most things will go into a more orderly configuration than before; probably it becomes a big mess.
Welcome to entropy. Entropy in a closed system always increases.
But wait, you say. I cleaned my room. Didn’t I reverse entropy? Well, yes, but you brought energy into your room (in the form of the energy you possess, which in turn came from your food) – so your room is not a closed system. You had to expend energy to fix the entropy in your room – and guess what, the CO2 you exhaled, and your other waste products, increased the entropy in the environment overall.
Another example is ice. If you want to turn liquid water into ice, which is a more orderly state, you have to expend energy. I’ve never yet seen a refrigerator which ran without being plugged in.
Wait a minute. There’s ice in space, right? No one’s running a fridge on the surface of Mars! Right, but the surface of Mars isn’t a closed system: heat radiates off of Mars right back into space. You want your fridge to be as close to a closed system as possible – and if it isn’t for long, your parents will yell at you for it.
Okay. There you have it. Entropy can be thought of simply as a measurement of either how much wasted energy there is in a system, or of how disorderly/chaotic the system is.
Now for the depressing bit: The entire universe is by definition a closed system, because there’s nothing outside of it for it to exchange energy with, by definition. And if the total entropy in any closed system must always increase … you guessed it. The total entropy in the entire universe will always increase … until the entire cosmos is nothing but wasted heat, dissipated so far as to be useless; nothing but disorder.
This is how things end: not with a bang, not even with a whimper precisely, but with a broken teacup and a messy room.
Okay. That was a little Egypt-free digression from our story. We’ll bring the myths back in the next post.