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I have often heard people say "entropy depends on the observer."
Why should entropy depend on the observer?
"The entropy of a substance, its entropy change from 0 K to any T, is a measure of the energy that can be dispersed within the substance at T: integration from 0 K to T of ∫Cp/T dT (+ q/T for any phase change)." (Frank L. Lambert, "Entropy Is Simple, Qualitatively", J. Chem. Educ., 2002, 79 (10), p 1241)
Entropy is linked to energy through its original definition by Clausius, dS = dQ/T, where "d" connotes a very small change.
The question is: Does entropy depend on the observer?
When transfering a system from state 0 to state 1 (both characterized by a set of selected macroscopic observables), you can in principle think of any reversible process to define the entropy in state 1:
S1 = S0 + ∫δQrev/T (integration from 0 to 1)
The "subjective" part is merely the definition of the macroscopic observables you want to keep track of for the given system (temperature, pressure, volume, number of particles etc.).