eli5 What is entropy and what are its biggest implications?
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Entropy can be looked at from two angles, but I’ll take the statistics approach.
Say you’ve got a hundred coins. They’re laid flat on a table.
Now, I tell you that all of them are heads. There is only one way that they can all be heads.
What if, instead, fifty are heads? There are all sorts of different orders they can be laid out in, while still having exactly fifty coins on heads.
The fifty-heads order is “higher entropy”. There are more ways for the pennies to be laid out and have 50 heads than 100, so 50 heads is higher-entropy.
If we took the table and hit it, the pennies would bounce and maybe land differently. They will always tend to move towards the highest-entropy state, because that’s how entropy works in probability.
However, because physics and probability are related, that’s not all. We can actually calculate entropy in matter, and also change in entropy. This has a lot of important implications in our universe on *many* levels.
For something down to earth, it poses a limit on the efficiency of your car engine. Even if the gas has a lot of energy, some of that energy is wasted because the entropy of the engine must always increase.
Geez, there’s LOTS of ways to approach this. But at a fundamental level, entropy is the tendency for everything to become “disordered” if no energy is put toward maintaining order. Take a living human body for example – it is a highly ordered and complex system, with lots of very specific detailed parts doing very specific detailed things constantly in order to keep the living human body intact and living. This takes a huge amount of energy, which is why we need to eat often. When that person dies, the human body becomes less and less complex and less and less ordered. Pretty quickly, all the specific parts that used to do specific things stop doing much of anything and start to degrade. Formerly highly ordered complex molecules break down and with no repair system in place, they just degrade and degrade and degrade until the body is more or less a goopy mush with not much happening. Over longer time scales, the formerly highly ordered complex molecules break down into smaller and smaller constituents, and there is no order to what is left or how it’s “used.” It’s just a pile of elements and less complex molecules.
Another example is a skyscraper with lots of people inside, functioning utilities and waste removal, a cleaning and repair crew, etc. Lots of energy expended keeping the building functioning. Once it’s emptied and abandoned, utilities shut off, no longer maintained or repaired, it inexorably falls into a state of disrepair and disorder. Given enough time, what was once a hundred stories of glass, stone and steel arranged just so, becomes a steel skeleton jutting out from a pile of rubble. Given even more time, the steel rusts away and eventually, there’s hardly any structure or order left.
Essentially, everything tends toward disorder, and the only way to stop any particular thing from breaking down into a disordered heap is to expend energy maintaining its order. The kicker is that the energy has to come from somewhere, and the process of obtaining and using that energy unavoidably creates more disorder than the order preserved in the thing you’re working to keep ordered.
The implications are that the universe itself will tend to disorder more and more and more, and unless there’s a way to obtain energy from outside the universe or more energy simply comes into existence in ways we don’t understand (and don’t think possible), the inevitable result will be a completely disordered universe with no structure or complexity of any kind. Just endless nothingness where nothing ever happens, forever and ever and ever.
Entropy is chaos; it’s a disorder of a system. A plate is low-entropy. A shattered plate is high-entropy. You’ve seen plates shatter but you’ll never see shards reassemble themselves into a plate. That’s because entropy/chaos can only increase; that’s one of the laws of thermodynamics.
Ultimately, this means our universe is destined for a heat death.
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Let me see if I can fill it in a little along side these excellent answers.
Entropy is very much tied into temperature. That’s why the physical formulae that define entropy are called the laws of *thermodynamics*.
Consider ice and water. Ice is a very regular, very ordered, crystalline structure. Like all solids, it is able to hold its shape without any outside assistance. Water, on the other hand, isn’t so well ordered. The molecules in it are just kind of floating around. They can hold their volume steady but not their shape.
So, ice would be considered a low entropy situation and water would be higher, right? And how did that entropy get into the system? With heat. As temperature goes up, so does entropy.
Even in the plate example, it took energy entering the system to break the plate and, theoretically, you could measure a very small increase in temperature when it broke. This is one of the things that causes certain kinds of mints to fluoresce when you bite them. This is why metal that’s going through a bending machine come out pretty warm. This is what drives the piezo electric effect. This is why the motion of hot liquids is so mind bogglingly complex.
Heat, for most purposes, is entropy and entropy is heat.
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