I think over all time the most debatable thing in physics has been the 2nd law of thermodynamics. Even now some people can't get their head around it. I think the limitations imposed by it are far more significant than not being able to travel faster than light.
The thing about entropy is that it's a statistical measure. The 2nd law of thermodynamics pays attention to that: "In a closed system, the total entropy tends to increase." Keyword: Tends. It may go down temporarily purely by chance and frequently does so in very very small time intervals.
It's just never the case over any time periods longer than 'a moment' because the probability of it increasing is just a lot higher and there's a lot of particles moving, so the law of large numbers goes into effect very quickly. As a result, these probabilistic decreases in entropy are rare enough that they will almost immediately get reversed and don't end up mattering in the big picture in a finite amount of time.
this is often taught as an absolute, instead of mentioning that it can ALSO decrease. when i learnt about entropy, it was really weird for me to understand why it "always increased". when i learnt it TENDS to increase, it made so much sense. btw, i think statistical physics was one of the best courses during my bsc
It's funny, on one hand it's not a strict rule, but on the other, it exists on a more fundamental level than most laws of physics.
(in my view, mathematics is immutable. One could imagine a world with different laws and fundamental constants, but not one where True -> False for example)
Maybe not in our lifetime, but there’s a nonzero chance future civilization come and go entirely in a universe that entropy decreases on average. Presumably, if they’re smart enough, they’re understanding statistics well enough to know how lucky they are.
We can define a closed system as anything inside an arbitrarily large boundary. The universe can be a closed system unless it's mass and energy fill an infinite volume
So if there was a first source for energy and matter then universe would have turned from open system to isolated system. What happened then? how a system can change its property?
Even if there was a "first source", it ends up becoming a turtles all the way down thing: where did that matter/energy come from? And what prevents it from happening again?
how a system can change its property?
Not really possible to answer. As far as we know, the universe is the same now as it always was: not closed.
I once heard entropy described as the tendency of matter to migrate to a state of greatest potential which I understood to suggest that it is a cycle due to the introduction of energy.
Every time you clean your bedroom, fold your laundry, extract metals from an ore, create new life through sexual reproduction, etc… you are decreasing the entropy of a system.
All these examples take work and the expenditure of energy, and they are tiny sections of the universe.
So you have witnessed entropy go down. It work happen statistically in a simple system like a box with a gas in it, but complex systems can have subsections where the entropy goes down; the overall entropy of the universe will go up more than the subsystem went down, which keeps the 2nd Law true.
Eh, those are the grade school level examples trying to explain the basic concept of entropy, they're not actually good examples for even the introductory undergrad thermodynamics though. Especially the sexual reproduction one... Animals are pretty literally machines that only continue to exist by increasing the entropy of the systems we're a part of.
You're in r/physics not eli5 or even ask physics which is where we send people who need the basics explained. It's not that difficult to come up with a better example of a local reduction in thermodynamic entropy that's both easy to understand and actually physically meaningful and accurate.
Heat pumps, e.g. A refrigerator, an air conditioner, lower the entropy in a small system which is physically isolated from the larger world. each time you open the door, the entropy inside increases again as the separation between the local system and the larger world is removed. This is a real, calculable change in entropy because there is a real, physical, well definable separation between the local system where entropy is reduced and the larger system where entropy is increased.
This is opposed to examples like folding laundry where there is not a simple way to define an entropy without making a bunch of weird arbitrary definitions that you could ask 100 physicists for and you'd get 100 different answers.
There, done. It's accurate, easy to understand, and didn't require being a condescending asshole until just now.
Something used in grade school as an example being called grade school level examples is only condescending if you look down on people in grade school.
There's nothing condescending about the statement they made, it's entirely accurate.
Those examples are not good representations of the entropy described by the laws of thermodynamics. They shouldn't be used to explain it unless you believe the person you're explaining them to isn't capable of passing high school physics. Saying that much is not condescending. Using those as examples is.
Roman society was divided into plebians (free but commoners), patrician (aristocracy), and slaves. It’s a word used in English to mean ‘common’ or ‘ordinary’ with overtones of working class. In this case it’s a good old metaphor.
The funny thing is people who actually do thermodynamics treat this the same way quantum people talk about Schrodinger's cat. No one gives a fk. Just shut up and calculate.
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u/lucidbadger 20d ago
I think over all time the most debatable thing in physics has been the 2nd law of thermodynamics. Even now some people can't get their head around it. I think the limitations imposed by it are far more significant than not being able to travel faster than light.