What is entropy? Also, can anyone explain how it relates to matter, gravity, time, other physics terms?
In: Physics
“Disorder” is the ELI5 version of entropy and is honestly a pretty good summary in a single word.
The true nature of entropy isn’t necessarily about disorder, but about the nature of potential in a system. High potential means that the system can essentially either do useful work or can be biased towards it. A simple example of this is a room where all the temperature is 5000 degrees and spread the same – this has almost no potential to do work (heating or cooling). Meanwhile a room where one end is 10000 degrees and one end is 0 degrees has a huge amount of potential. Although both rooms have the same amount of “total energy”, the potential in the 2nd room is much greater and in human terms we would label it as more useful.
> Also, can anyone explain how it relates to matter, gravity, time, other physics terms?
Entropy is so closely connected to the passage of time that some theories abstract them to be “kind of” the same thing. One of the most bulletproof laws of the universe is that with the passage of time, the entropy of a system must increase.
When we talk about matter and entropy, often we’re talking about the relative potential or nature of energy in an atom, molecule or greater group of them. This isn’t only about the “temperature” (and localised temperature – i.e. what subatomic molecules actually have the kinetic “temperature” energy and in what % its split) but also about relative placements of atoms (it’s easy to imagine that the Earth would be a very different place if all the oxygen was only in one part of the sky, or if all the air was only on one half of the planet for some reason – that’s a massive pressure difference).
On a grander scale; entropy is a comment on the ultimate past and ultimate future of the universe. We might not know everything, but what we can be certain of is that the future will have higher entropy. This means that temperatures will become more spread out. It also means that the universe itself biases towards low-energy heat radiation instead of little packages of high energy (i.e. atoms and matter itself). It’s easy to see that some objects have inherently low entropy (stars being an extremely obvious example of this; they are so much hotter and have so much more localised energy than the empty space surrounding them) – and it is inevitable that things with low entropy will eventually transform into things with higher entropy.
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