3 ms·

I would argue that's exactly the wrong way to think about thermodynamic entropy. Consider this: Energy isn't just in your head (you can use it to do work, and y

by cygx 7y ago

I would argue that's exactly the wrong way to think about thermodynamic entropy. Consider this: Energy isn't just in your head (you can use it to do work, and you have to know where it is precisely if you want to do General Relativity). Temperature isn't just in your head (you can measure it with a thermometer, and it tends to manifest as an average energy). The laws of thermodynamics then connect energy and temperature to entropy, which implies that the latter also can't just be in your head unless you add some mubo-jumbo. Thermodynamics doesn't care about our ignorance of potential system configurations, but that these configurations are available for microscopic system evolution!

kgwgk 7y ago
> Thermodynamics doesn't care about our ignorance of potential system configurations, but that these configurations are available for microscopic system evolution! Thermodynamics equations are based on the macroscopic constraints on the system that we choose to measure and manipulate, implicitely admitting our ignorance about the potential microscopic configurations. Thermodynamic entropy, unlike energy, is not an intrinsic property of the physical system; it is a property of our macroscopic description of the system and the experimental processes that we decide to perform on it. See section VI. THE "ANTHROPOMORPHIC" NATURE OF ENTROPY in https://bayes.wustl.edu/etj/articles/gibbs.vs.boltzmann.pdf https://bayes.wustl.edu/etj/articles/gibbs.vs.boltzmann.pdf "Recognition that the "entropy of a physical system" is not meaningful without further qualifications is important in clarifying many questions concerning irreversibility and the second law. For example, I have been asked several times whether, in my opinion, a biological system, say a cat, which converts inanimate food into a highly organized structure and behavior, represents a violation of the second law. The answer I always give is that, until we specify the set of parameters which define the thermodynamic state of the cat, no definite question has been asked!"
jbay808 7y ago
Energy and entropy are quite different though; energy certainly is a property of the system itself. Temperature, interestingly, is also in our heads. It isn't actually a measure of average energy, but actually is defined as a function of energy and entropy. (Entropy comes first and temperature is defined in terms of it, not the other way around). T = dQ/dS You can create systems in real life with infinite temperature, or even with negative temperature, but of course they certainly don't have infinite energy. One way to do so is to pump a system into having a population inversion, such that its entropy decreases as its energy increases, because the number of possible high energy microstates is small. In general, only one microstate is ever "available" for a system to evolve into, which is the one that its current microstate will evolve into in accordance with the laws of physics. The trouble is that we don't know its current microstate, so we don't know its future microstate, so it seems to us that there are many available microstates, but that's all due to our own ignorance having only conducted summary statistical measurements and isn't an intrinsic property of the system.

by cygx 7y ago

kgwgk 7y ago

> Thermodynamics doesn't care about our ignorance of potential system configurations, but that these configurations are available for microscopic system evolution! Thermodynamics equations are based on the macroscopic constraints on the system that we choose to measure and manipulate, implicitely admitting our ignorance about the potential microscopic configurations. Thermodynamic entropy, unlike energy, is not an intrinsic property of the physical system; it is a property of our macroscopic description of the system and the experimental processes that we decide to perform on it. See section VI. THE "ANTHROPOMORPHIC" NATURE OF ENTROPY in https://bayes.wustl.edu/etj/articles/gibbs.vs.boltzmann.pdf https://bayes.wustl.edu/etj/articles/gibbs.vs.boltzmann.pdf "Recognition that the "entropy of a physical system" is not meaningful without further qualifications is important in clarifying many questions concerning irreversibility and the second law. For example, I have been asked several times whether, in my opinion, a biological system, say a cat, which converts inanimate food into a highly organized structure and behavior, represents a violation of the second law. The answer I always give is that, until we specify the set of parameters which define the thermodynamic state of the cat, no definite question has been asked!"

jbay808 7y ago

Energy and entropy are quite different though; energy certainly is a property of the system itself. Temperature, interestingly, is also in our heads. It isn't actually a measure of average energy, but actually is defined as a function of energy and entropy. (Entropy comes first and temperature is defined in terms of it, not the other way around). T = dQ/dS You can create systems in real life with infinite temperature, or even with negative temperature, but of course they certainly don't have infinite energy. One way to do so is to pump a system into having a population inversion, such that its entropy decreases as its energy increases, because the number of possible high energy microstates is small. In general, only one microstate is ever "available" for a system to evolve into, which is the one that its current microstate will evolve into in accordance with the laws of physics. The trouble is that we don't know its current microstate, so we don't know its future microstate, so it seems to us that there are many available microstates, but that's all due to our own ignorance having only conducted summary statistical measurements and isn't an intrinsic property of the system.