ELI5-What is entropy?
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Imagine you’re in a room. In this room every molecule of air is evenly spaced apart like a grid. It doesn’t naturally stay this way, the particles will move and change direction and get out of order.
There’s that one way to be organized, but trillions upon trillions of ways it can be unorganized. So, naturally, the room will tend towards the unorganized.
This tendency is called entropy.
Entropy is a complex mathematical concept that ties into a lot of things.
On a mathematical level, it’s about amount of disorder or randomness in the system.
On a thermal physics level, it mentions how heat and energy flows.
On other level it says how time flows.
The reason for all that confusion is that we are made of unimaginably high number(10^23) of atoms and molecules. And it’s impossible to figure out how each individual atom and molecule behaves in a system. But, we know the probabilistic behaviour of all atoms and molecules.
Consider a coin toss, ideally 50% of them should be heads and 50% tails. But if you only do one coin toss, it’s 100% one way or the other. If you do 10 coin toss, you’ll be closer to 50%, if you do 10000000 coin tosses, you’ll be extremely close to 50%.
So, for 100000000000000000000000 atoms and molecules, we go for probabilistic analysis instead of tracking each one individually. And probabilistic analysis says that disorder(50% heads and 50% tails) is way more likely than order (100% heads or 100% tails).
Antropy measures disorder in the system, that says how disorder always increases, or how heat moves which is just vibrating and colliding atoms, or time ‘flows’ in the same way disorder increases.
Entropy is our measure of our level of uncertainty of the state of a system that arises from our inability to discern the microscopic state. Particularly, the position and momentum. If both of these were known for each particle, there would be no uncertainty given that the trajectory would be known and therefore deterministic. In a simulation of atoms (eg MD), there is no entropy, the entire state of the system is known. Humans are just bad at knowing the microscopic state and formulated ideas based upon that.
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