ELI5-What is entropy?
In: 1763
You know the “random bullshit go” meme. It’s basically that.
Entropy is the measure of randomness of the system. Steam has more entropy than a block of ice because steam is a gas, and ice is a solid.
Imagine you’re playing the lottery, where you (say) pick 10 out of 100 numbers, and correspondingly 10 numbers get pulled. Entropy is the number of outcomes that are compatible with a specific criteria (e.g. all numbers pulled so far are among those you previously picked). If you could sell your lottery ticket during pulls, the value of your ticket will be linked to the entropy.
Imagine you have a box of toys, and all the toys are mixed up and scattered around inside. When the toys are all jumbled up and you don’t know where each toy is, we can say that the toys are in a state of high entropy.
Now, let’s say you start organizing the toys one by one, putting each toy in its proper place. As you do this, the toys become more ordered and less mixed up. Eventually, when all the toys are neatly organized and you can easily find each one, we can say that the toys are in a state of low entropy.
Entropy is a way to measure how messy or disordered things are. The higher the entropy, the more mixed up and unpredictable things are. But when things are organized and predictable, the entropy is lower.
Entropy can apply to things other than toys too. It can describe how messy a room is, how jumbled up a puzzle is, or how confusing a group of numbers or letters can be. It’s a way to understand how much disorder or randomness there is in the world around us.
Roughly speaking, entropy is the amount of information required to describe a system. For example, take a system of 10 coins, numbered 1 to 10. If the coins are showing all heads, you can simply say `10H` to describe the system. Thats 3 characters. Change the 5th coin to show tails. Now your description of the system will be `4H 1T 5H`, requiring 6 characters. If the distribution of the coins is completely random, only way for you to describe it is to write it out in full, requiring 10 characters. The last case has the most entropy, the first case the least.
Let’s see if I can ELI5 it.
Entropy is a word that describes the process of going from order to chaos.
Example: you have two cups of water. One has blue dye in it, the other has red dye in it.
You pour them into a third cup and it makes purple.
Now it’s easy to take red and blue and make purple, but virtually impossible to reverse that action by separating the purple water back into separate red and blue cups of water.
Take a deck of cards shuffle it. It’s in a disordered state. Organize it in whatever state you want it to be. There are far more arrangements if Decks of cards that are disorganized than there are arrangements for organized cards.
In deck of cards, the cards are uniquely identifiable. When it comes down to molecules and atoms, theres less uniqueness but significantly more particles. Entropy generally says the morely common organization is more likely to occur, and that is the more disorganised arrangement. So a shuffled but neatly stacked deck of cards is less entropic than scattering cards to the wind. However to make the cards in the first place, you have to cut down trees, collect the sawdust in a slurry to make paper, apply a plastics and paint or ink and then have systems to paint the cards and organize them and check them for defects. All of thise steps produce waste byproducts, heat, noise that add to the general chaos of the the universe.
Theres a handful of ways your room can be organized, but there are a ton of ways it can be messy.
So naturally your room will, over time, become messy. That‘s entropy. Nature‘s tendency for things to become messy.
The reason is actually pretty simple: if theres 1 way to be orderly and 99 ways to be messy then of course it‘s more likely to be messy.
I‘ve seen a lot of talk in the comments about energetic states so I wanna expand on that too.
– imagine an empty room with a chunk of coal on it. This room is organized; most of its energy is concentrated in a small part
– as you burn the coal you release its energy into the room. Once everything is burnt out you have a room filled with CO2. This room is messier, its energy is spread out.
– the room as a whole was never in a higher or lower energetic state. Its energy never increased or decreased. The only thing that changed is its entropy; the way the energy is distributed.
Imagine a vase. Now imagine throwing it on the ground, smashing it into thousands of pieces. Now imagine finding every single piece and gluing it back together, *perfectly*. What was easier and took less time and energy to do: smashing the vase into thousands of pieces or the act of gluing it *perfectly* back together?
The vase all together as one is a **low entropy state**, everything is super organized, there is a **low amount of disorder**. The vase in thousands of pieces is a **high entropy state**, the jumbled pile of glass shards is in a **high amount of disorder**.
Things in the universe prefer/tend to approach a higher state of entropy: farts spread out into a room rather than squeeze into a tiny space. This also helps determine the forward flow of time (farts come out of the butt and spread into a room as time goes by).
Brian Cox did a good explanation on one of his shows.
“While left to the elements, mortar crumbles, glass shatters and buildings collapse. A good way to understand how is to think of objects not as single things, but as being made of many constituent parts like the individual grains of sand that make up a pile of sand.
Entropy is a measure of how many ways I can rearrange those grains and still keep the sand pile the same. There are trillions and trillions of ways of doing that. Pretty much anything I do to this sand pile, mess the sand around, move it around, then it doesn’t change the shape or the structure at all. So this sand pile has high entropy.
But creating order in the universe, (using the sand, in a bucket, making a sand castle), there approximately the same amount of sand grains in the castle as there are in the sand pile. But now virtually anything I do to it will mess it up, will remove the order from this structure. Because of that, the sand castle has a low entropy. It is a much more ordered state.”
To a 5yo, I’d say, “It’s what makes everything fall apart and become disordered as time goes on.”
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