In her article “The demons haunting thermodynamics” (Physics Today, November 2021, page 44), Katie Robertson concludes the introductory historical summary by saying that modern developments in quantum foundations have banished the demons “once and for all.” Unfortunately, no explanation or reference is given for that optimistic but controversial conclusion.

Robertson presents Erwin Hahn’s 1950 spin-echo experiments1 as the realization of Josef Loschmidt’s vision of reversing momentum. But Hahn clearly described his spin-echo experiments as the effect of traditional spin dynamics for noninteracting spins in a spatially inhomogeneous magnetic field. Although the detailed explanation involves many particular subtleties of NMR dynamics in liquids, Hahn’s interpretation does not imply any violation of the “second law”; it uses only the mild assumption that the spin observables are at thermal equilibrium before each start signal. Robertson’s misunderstanding clearly appears when she writes that “atomic spins that have dephased and become disordered are taken back to their earlier state by an RF pulse” and, a few lines later, “it turns out that the spin-echo experiment is a special case; most systems approach equilibrium instead of retracing their steps back to nonequilibrium states.” The spins have not become disordered: The phase of each spin remains directly related to the magnetic field at the spin’s location, and that relationship explains the echo.

Two illuminating articles by Won-Kyu Rhim, Alexander Pines, and John Waugh describe spin-echo experiments in which the irreversible time evolution of a coupled nuclear spin system in solids is apparently “reversed” for a limited duration.2 As the authors explain, the results arise from uniform spin manipulation and are still consistent with the laws of thermodynamics.

Another aspect of Robertson’s article that disturbed me is the lack of discussion of the relations between the actual experiments performed on large (macroscopic) systems and the microscopic-scale models used in attempts to make predictions about those results.

Consider a mixture of oxygen and hydrogen at atmospheric pressure and room temperature. Standard thermodynamics and statistical mechanics will lead to excellent predictions of the equation of state, specific heat, and the like. But the standard models ignore the possibility of the chemical reaction producing water. Improved models are needed if one is to allow for, say, a slow, isothermal catalytic reaction or—much more complicated—an explosion in an isolated system at constant volume. Spin systems offer a rich variety of experimental possibilities for external manipulation and observation, but the corresponding models are related to the real experimental situations by complicated transformations and approximations that must be chosen according to the situation under study.

Robertson invokes two models in the context of Maxwell’s demon. One is a biological machine using a ratchet-style mechanism. But in the cited article, the abstract carefully indicates that the evolution does not violate the second law because the microscopic mechanism is coupled to the exterior of the system.3 In the demon-style experiment of Robertson’s figure 3, a complete realistic discussion of an actual implementation of the experiment leads to the similar conclusion that the second law is not violated.

My conclusion is that demons and the related controversies are features of models and that the interpretation of actual experiments should be subjected to critical examination, preferably by those who performed the experiment and have a complete knowledge of all details.

2.
W.-K.
Rhim
,
A.
Pines
,
J. S.
Waugh
,
Phys. Rev. Lett.
25
,
218
(
1970
);
W.-K.
Rhim
,
A.
Pines
,
J. S.
Waugh
,
Phys. Rev. B
3
,
684
(
1971
).
3.
V.
Serreli
 et al,
Nature
445
,
523
(
2007
).
4.
K.
Robertson
,
Physics Today
74
(
11
),
44
(
2021
).