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Thermodynamical entropy is not a physical property of a system. It’s a property of our description of the system as a macroscopic state. Quantum (von Neumann)
by kgwgk 8y ago
Thermodynamical entropy is not a physical property of a system. It’s a property of our description of the system as a macroscopic state.
Quantum (von Neumann) entropy is a related but different concept. It’s worth noting that it is constant for a closed system.
Cosmological entropy can be defined in different ways. In summary, entropy means many things and not all “entropies” behave in the same way.
- eru 8y agoInteresting enough, even though entropy is in some sense a property of a description of a system, there are things like 'entropic forces' and they play a big role in physics. https://en.wikipedia.org/wiki/Entropic_force https://en.wikipedia.org/wiki/Entropic_force
- kgwgk 8y agoEntropic forces are not more real than centrifugal forces or the Coriolis force. They appear when we use a thermodynamic description of a system. That wikipedia page doesn’t make much sense, but note that the “mathematical formulation” is about macrostates and canonical ensembles. See also https://johncarlosbaez.wordpress.com/2012/02/01/entropic-forces/ https://johncarlosbaez.wordpress.com/2012/02/01/entropic-for...
- eru 8y agoYes, no more and no less.
- cygx 8y agoThermodynamical entropy is not a physical property of a system. From a thermodynamic point of view, it is: As ΔS = ΔU/T and assuming one holds temperature and inner energy to be physical properties, then so is entropy.
- kgwgk 8y agoThe thermodynamic point of view is a description of the system as a macroscopic state.
- cygx 8y agoAnd properties of macroscopic states are 'unphysical'?
- kgwgk 8y agoMacroscopic states are not a complete description of physical systems and the same microstate could be part of different macroscopic states. The thermodynamic variables are not properties of the microstate (the "physical reality"). They are properties of the macrostate, which is a model of the unknown "physical reality". If you don't like the original phrasing, let's say that "Thermodynamical entropy is not a fundamental property of a system. It’s a property of our description of the system as a macroscopic state."
- dr_dshiv 8y agoHow could the same microstate be part of different macrostate? Like, if I described the microstate of a drop of water, wouldn't it have, as a whole, one temperature? I thought that a macrostate could have many different microstates, but not vice versa
- kgwgk 8y agoConsider for example, a system in thermal equilibrium with a heat bath, with a fixed number of particles and a fixed volume. The macrostate is defined by the variables T, V and N. The energy of the microstates corresponding to the macrostate is not fixed. The probability of each microstate is proportional to the exp(-E/kT). https://en.wikipedia.org/wiki/Boltzmann_distribution https://en.wikipedia.org/wiki/Boltzmann_distribution The microstates don't have a temperature: they are more or less likely to happen depending on the temperature. And that's assuming a particular thermodynamical model. The same microstate could be looked at in the context of a different thermodynamical model as a different (or differents) macrostate(s) characterized by another set of thermodynamical variables.
- dr_dshiv 8y agoWhat are example physical properties of a system that can be separated from the properties of "our description of the system as a macroscopic state"?