People often repeat the mindless slogan that entropy means disorder, disarray, crumbling and decay. It can't possibly mean any of those things, because Entropy is not a property of matter at all. It is only a property of Energy.
It is also said that the existence of Entropy (to be precise, the existence of the Second Law of Thermodynamics which defines Entropy) means that organized structures cannot arise spontaneously and therefore the very existence of "complex" structures has various bizarre philosophical implications. This is also pure bottled nonsense.
Entropy is a property of Energy and has nothing to do with matter, atoms, crystals, structures, buildings, organisms or automobiles except for the extent to which energy in its various forms is lurking inside those things.
Energy is nothing more than motion or the possibility of motion. One big car in motion has as much kinetic energy as two smaller cars that together weigh the same as the big car and that are moving at the same speed. One car moving 100 mph has the same energy as four cars identical to the first traveling 50 mph. Doubling the speed of something requires quadrupling its kinetic energy.
But the most important thing about energy is that it cannot come from nothing and it cannot disappear into nothing. Energy always comes from something else and always goes somewhere else. That is the First Law of Thermodynamics ("how energy works") in which Nature demands that all energy is always conserved.
But if energy is always conserved in every physical process, why do things seem to run out of energy after a while? AHA!!! This is where Entropy comes in.
Entropy is the property of Energy which states that Energy always tries to spread itself out as evenly as possible. Energy never gets destroyed; it always exists in exactly the same amount. But energy always gets more and more uniformly distributed as time goes on.
If you wind up a clockwork toy and let it go, the energy stored in the spring doesn't stay there: it gets used to move the parts, and eventually friction (heating) slows the toy down until it stops. The energy was transformed into heat, work (which eventually became heat) and sound waves (which eventually became heat). The exact same amount of energy still exists, but that energy became widely dispersed. Mission accomplished.
When energy spreads out, a numerical quantity known as Entropy increases. Entropy is a measure of the amount of evenness over which energy is shared among every possible location that the energy can possibly occupy.
How would that work? Imagine a box full of bouncy balls. And in this imaginary experiment, let's pretend that the balls and even the walls of this box are so bouncy that the balls never ever stop bouncing. Maybe this is a box of hydrogen atoms, or children.
Let's start out with all of the balls in this box not moving, just sitting quietly at their desks. Now, lets throw a really really fast-moving ball (lots of energy!) into the box. What happens? At first, the new ball is the only one moving, but as it ricochets off of other balls that were just sitting there minding their own ball business, they too will begin to move. Eventually, all of the balls will be bouncing around, but none of them as fast as the new ball had been when we first threw it in. In fact, all of the energy in the box, if added up, would only equal the energy that the new ball had when we threw it in.
At first, when only one ball was moving, Entropy was almost zero. The energy was not shared among all the possible places (balls) that the energy could exist. This was actually the MOST uneven arrangement imaginable. As the new ball began to share its kinetic energy with other balls, Entropy began to increase. Eventually, when all the balls were moving, Entropy was as high as it could get.
How likely do you think it would be for all the other balls to somehow bounce off the new ball again at exactly the right angle and at exactly the right time so that they all stop moving and the new ball once again has all of the energy to itself? Pretty much impossible, isn't it. Entropy never goes down by itself; only ever up.
"But," you say, "what about the example of the wind-up toy? We can wind up the toy some more and once again the entropy of that toy will be minimum, allowing it to run some more." Quite so. But how much did Entropy increase by you doing that? Was there heat in your fingers that transferred to the winding key? That's energy being spread around. Did doing that make any sounds? That also dispersed energy and therefore increased entropy. Every time entropy is temporarily reduced in one area, it has to increase by an even larger amount everywhere else.
That's why "perpetual motion of the second kind" - something running indefinitely without a source of energy, even if it's not producing any energy - is just as bogus as fantasy machines that people imagine "produce" energy from nothing. That's simply not how energy works.
How Energy works is really simple and really easy:
1. Energy always comes from somewhere and always goes somewhere.
2. Energy always gets spread out as evenly as it possibly can by any means.
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What about matter?
Getting energy to spread itself out more evenly sometimes results in matter having to arrange itself in complex structures, sometimes in ordered structures, and sometimes in random structures. So, it turns out that Entropy can accommodate ALL arrangements of matter, and not just the messy ones!
A perfectly ordered crystal is actually entropy in action, since the crystal forms as a direct result of energy trying to get to a stable minimum in one bunch of matter so that any excess can get spread around elsewhere. Complex organic structures also increase entropy by absorbing concentrated chemical energy or even sunlight and turning it into heat, waste, and motion.
A dog, for example, is an excellent device for turning useless Money into valuable hair, noise and poop. So in no way does biology violate the law of Entropy.
Entropy is easy. It's a property of Energy, not matter. It says nothing directly about how or whether matter is arranged. Measuring Entropy tells us whether Energy is in a useful form (highly concentrated in one place) or if it's unavailable (highly dispersed).
Other than being indestructible, the most basic fact about Energy is that it always spreads out. That's Entropy.
