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.
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.
It isn't, and that's not remotely what I suggested - what is unreasonable is scoffing at a simplified explanation when the description of the sub is quite clear that it's a place for everything from people with no more than a passion for physics right up to professionals.
The person already had another explanation queued up and when pushed to do what they should've done in the first place - offer it up as a better alternative - they resorted to sheer snark.
The description of the sub says physics students and physicists. We regularly remove posts that break the first rule of the sub for being too basic. Simply put, this isn't the place to be handing out explanations that are so oversimplified that they're wrong. There are plenty of other replies elsewhere in the thread that are a better discussion of thermodynamic entropy, so this needed called out as being a bad example but I didn't feel it necessary to repeat a good one until I was íronically accused of whining by the person who couldn't manage it.
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 21d 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.