Laws of thermodynamics
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The laws of thermodynamics are a grand-sounding term often bandied around in discussions of science, pseudoscience and general woo. Despite being scientific laws themselves, they are often cited by pseudoscientists (e.g., creationists) as a reason for why some other bit of science must be wrong. The classic example of this is "evolution must be wrong as it violates the laws of thermodynamics". As well as being ironic, such claims are usually also bullshit.
Thermodynamics as a subject originated in the Industrial Revolution, more or less as a way for engineers to understand how steam engines worked, and how to make them more efficient. Since then it has developed and generalised into a rigorous mathematical treatment of energy and entropy, and is a bread-and-butter part of science courses such as physics and chemistry.
There are three 'Laws' of thermodynamics ("laws" in the sense that they describe how physical systems "must" behave), and also a "zeroth" law, which isn't really a law so much as a definition of what is meant by "temperature".
- Zeroth law of thermodynamics: "When two systems are in thermal equilibrium with a reservoir, they are in thermal equilibrium with each other."
- First law of thermodynamics: "The total energy of the Universe is constant."
- Second law of thermodynamics: "The entropy of an isolated system does not decrease."
- Third law of thermodynamics: "As the temperature of a perfect crystal approaches zero, its entropy approaches a constant."
The 1st, 2nd and 3rd Laws may be humorously summarized in non-scientific form as:
- You can't get something for nothing.
- You can't even break even unless you cool the temperature to absolute zero.
- It's impossible to actually reach absolute zero.
Or, if you are a poker player:
- You can't win.
- You can't break even.
- You can't get out of the game.
- Spin degeneracy may lead to two possible states for the system, but then the entropy approaches <math>k_B \ln 2</math>, which is incredibly small compared to the entropy of a macroscopic system.