Dependence of entropy and temperature on Planck's constant

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SUMMARY

The discussion centers on the relationship between entropy, temperature, and the reduced Planck's constant, denoted as \(\hbar\). It establishes that both entropy \(S\) and temperature \(T\) are directly proportional to \(\hbar\) in quantum systems, as derived from the definitions of statistical entropy in classical and quantum mechanics. The classical Gibbs entropy and quantum von Neumann entropy formulations are presented, highlighting the role of Boltzmann's constant \(k_B\) and the density matrix \(\rho\). The query raised concerns the direct proportionality of entropy and temperature to \(\hbar\) and the implications of \(\hbar\) approaching zero on the number of microstates.

PREREQUISITES
  • Understanding of statistical mechanics concepts, particularly Gibbs and von Neumann entropy.
  • Familiarity with quantum mechanics and the significance of the reduced Planck's constant (\(\hbar\)).
  • Knowledge of thermodynamic principles, especially the relationship between entropy and temperature.
  • Basic grasp of probability theory as it applies to microstates in physical systems.
NEXT STEPS
  • Explore the implications of the reduced Planck's constant on quantum statistical mechanics.
  • Study the derivation and applications of the von Neumann entropy in quantum systems.
  • Investigate the relationship between temperature and entropy in the context of quantum thermodynamics.
  • Examine the effects of varying \(\hbar\) on the number of distinguishable microstates in quantum systems.
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This discussion is beneficial for physicists, particularly those specializing in quantum mechanics and thermodynamics, as well as researchers exploring the foundations of statistical mechanics and quantum gravity.

spaghetti3451
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Consider the dependence of entropy and of temperature on the reduced Planck's constant (taken from page 23 of Thomas Hartman's lecture notes(http://www.hartmanhep.net/topics2015/) on Quantum Gravity):

$$S \propto \hbar, \qquad \qquad T \propto \hbar.$$

I do not quite see how entropy can depend on the reduced Planck's constant. To give credence to my claim, consider the definition of (statistical) entropy in classical and in quantum systems.

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In a classical system, the (statistical Gibbs) entropy for a macroscopic system (with a discrete set of microstates) is defined as

$$S = -k_\text{B}\,\sum_i p_i \ln \,p_i,$$

where ##k_{\text{B}}## is Boltzmann's constant and ##p_i## is the probability that the system is in microstate ##i## during the system's fluctuations.

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In a quantum system, the (statistical von Neumann) entropy for a macroscopic system (with a discrete set of microstates) is defined as

$$S = -k_\text{B}\,\text{Tr}\ (\rho \ln \rho),$$

where ##k_{\text{B}}## is Boltzmann's constant and ##\rho## is the density matrix of the system.

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How then does entropy depend on the reduced Planck's constant in quantum systems?

Why should temperature also depend on the reduced Planck's constant in quantum systems?
 
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I can see how the number of distinguishable miscrstates might be related to the inverse of h, but not directly proportional to it. (As h tends to zero the number of micro states would to infinity.)
 

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