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# Eliezer_Yudkowsky comments on The Second Law of Thermodynamics, and Engines of Cognition - Less Wrong

45 27 February 2008 12:48AM

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Comment author: 29 February 2008 03:21:02PM 16 points [-]

Isn't entropy a property of the system, not the observer?

Nope. It's a property of the observer, but one that behaves in such a lawful and inescapable way that it seems to you like a property of the system.

Your ignorance of next week's winning lottery numbers is a property of you, not just a property of the lottery balls, but good luck on ignoring your ignorance.

Someone elsewhere said: Almost all the time, I stick with this idea: Temperature of a gas is the mean kinetic energy of its molecules.

Aren't there vibrational degrees of freedom that also contribute to kinetic energy, and isn't that why different materials have different specific heats? I.e., what matters is kinetic energy per degree of freedom, not kinetic energy per molecule? So you actually do have to think about a molecule (not just measure its kinetic energy per se) to determine what its temperature is (which direction heat will flow in, compared to another material), even if you know the total amount of heat - putting the same amount of heat into a kilo of water or a kilo of iron will yield different "temperatures".

But the more important point: Suppose you've got an iron flywheel that's spinning very rapidly. That's definitely kinetic energy, so the average kinetic energy per molecule is high. Is it heat? That particular kinetic energy, of a spinning flywheel, doesn't look to you like heat, because you know how to extract most of it as useful work, and leave behind something colder (that is, with less mean kinetic energy per degree of freedom).

If you know the positions and speeds of all the elements in a system, their motion stops looking like heat, and starts looking like a spinning flywheel - usable kinetic energy that can be extracted right out.