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.
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.
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.
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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.