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Epictetus comments on Open Thread, Feb. 2 - Feb 8, 2015 - Less Wrong Discussion

4 Post author: Gondolinian 02 February 2015 12:28AM

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Comment author: passive_fist 02 February 2015 02:20:59AM *  4 points [-]

In a previous thread, I brought up the subject of entropy being subjective and got a lot of interesting responses. One point of contention was that if you know the positions and velocities of all the molecules in a hot cup of tea, then its temperature is actually at absolute zero (!). I realized that the explanation of this in usual terms is a bit clumsy and awkward. I'm thinking maybe if this could be explained in terms of reversible operations on strings of bits (abstracting away from molecules and any solid physical grounding), it might be easier to precisely see why this is the case. In other words, I'm looking for a dynamical systems interpretation of this idea. I googled a bit but couldn't find any accessible material on this. There's a book about dynamical systems approaches to thermodynamics but it's extremely heavy and does not seem to have been reviewed in any detail so I'm not even sure of the validity of the arguments. Anyone know of any accessible materials on ideas like this?

Comment author: Epictetus 10 February 2015 09:45:33PM 0 points [-]

The lesson is that statistical methods are superfluous if you know everything with certainty. It's worth noting that classical mechanics is completely symmetric with respect to time (does not have a distinguished "arrow of time"), whereas thermodynamics has a definite arrow of time. You run into problems if you assume that everything behaves classically and try to apply thermodynamic notions.

Landau and Lifshitz's Statistical Physics has some discussion of issues with entropy.

Comment author: passive_fist 10 February 2015 10:03:53PM 0 points [-]

I understand what you're saying and I agree. Though it's worth mentioning that the 'arrow of time' in thermodynamics actually doesn't exist for closed, reversible systems.