In classical statistical mechanics, the -theorem, introduced by Ludwig Boltzmann in 1872, describes the tendency of the quantity (defined below) to decrease in a nearly-ideal gas of molecules.L. Boltzmann, "Weitere Studien über das Wärmegleichgewicht unter Gasmolekülen ." Sitzungsberichte Akademie der Wissenschaften 66 (1872): 275-370. English translation: As this quantity was meant to represent the entropy of thermodynamics, the H-theorem was an early demonstration of the power of statistical mechanics as it claimed to derive the second law of thermodynamics—a statement about fundamentally irreversible processes—from reversible microscopic mechanics. It is thought to prove the second law of thermodynamics, albeit under the assumption of low-entropy initial conditions.Zeh, H. D., & Page, D. N. (1990). The physical basis of the direction of time. Springer-Verlag, New York
The H-theorem is a natural consequence of the kinetic equation derived by Boltzmann that has come to be known as Boltzmann's equation. The H-theorem has led to considerable discussion about its actual implications,Ehrenfest, Paul, & Ehrenfest, Tatiana (1959). The Conceptual Foundations of the Statistical Approach in Mechanics. New York: Dover. with major themes being: What is entropy? In what sense does Boltzmann's quantity correspond to the thermodynamic entropy? Are the assumptions (especially the assumption of molecular chaos) behind Boltzmann's equation too strong? When are these assumptions violated?
Name and pronunciation Boltzmann in his original publication writes the symbol E (as in entropy) for its statistical function. Years later, Samuel Hawksley Burbury, one of the critics of the theorem, wrote the function with the symbol H, a notation that was subsequently adopted by Boltzmann when referring to his "H-theorem". The notation has led to some confusion regarding the name of the theorem. Even though the statement is usually referred to as the "Aitch theorem", sometimes it is instead called the "Eta theorem", as the capital Greek letter Eta (Η) is indistinguishable from the capital version of Latin letter h (H). Discussions have been raised on how the symbol should be understood, but it remains unclear due to the lack of written sources from the time of the theorem. Studies of the typography and the work of J.W. Gibbs seem to favour the interpretation of as Eta.
Definition and meaning of Boltzmann's H
The value is determined from the function , which is the energy distribution function of molecules at time . The value is the number of molecules that have kinetic energy between and . itself is defined as
For an isolated ideal gas (with fixed total energy and fixed total number of particles), the function is at a minimum when the particles have a Maxwell–Boltzmann distribution; if the molecules of the ideal gas are distributed in some other way (say, all having the same kinetic energy), then the value of will be higher. Boltzmann's -theorem, described in the next section, shows that when collisions between molecules are allowed, such distributions are unstable and tend to irreversibly seek towards the minimum value of (towards the Maxwell–Boltzmann distribution).
(Note on notation: Boltzmann originally used the letter for quantity ; most of the literature after Boltzmann uses the letter as here. Boltzmann also used the symbol to refer to the kinetic energy of a particle.)
Boltzmann's H theorem
Boltzmann considered what happens during the collision between two particles. It is a basic fact of mechanics that in the elastic collision between two particles (such as hard spheres), the energy transferred between the particles varies depending on initial conditions (angle of collision, etc.).
Boltzmann made a key assumption known as the Stosszahlansatz (molecular chaos assumption), that during any collision event in the gas, the two particles participating in the collision have 1) independently chosen kinetic energies from the distribution, 2) independent velocity directions, 3) independent starting points. Under these assumptions, and given the mechanics of energy transfer, the energies of the particles after the collision will obey a certain new random distribution that can be computed.
Considering repeated uncorrelated collisions, between any and all of the molecules in the gas, Boltzmann constructed his kinetic equation (Boltzmann's equation). From this kinetic equation, a natural outcome is that the continual process of collision causes the quantity to decrease until it has reached a minimum.
Impact
Although Boltzmann's -theorem turned out not to be the absolute proof of the second law of thermodynamics as originally claimed (see Criticisms below), the -theorem led Boltzmann in the last years of the 19th century to more and more probabilistic arguments about the nature of thermodynamics. The probabilistic view of thermodynamics culminated in 1902 with Josiah Willard Gibbs's statistical mechanics for fully general systems (not just gases), and the introduction of generalized statistical ensembles.
The kinetic equation and in particular Boltzmann's molecular chaos assumption inspired a whole family of Boltzmann equations that are still used today to model the motions of particles, such as the electrons in a semiconductor. In many cases the molecular chaos assumption is highly accurate, and the ability to discard complex correlations between particles makes calculations much simpler.
The process of thermalisation can be described using the H-theorem or the relaxation theorem.
Criticism and exceptions
There are several notable reasons described below why the H-theorem, at least in its original 1871 form, is not completely rigorous. As Boltzmann would eventually go on to admit, the arrow of time in the H-theorem is not in fact purely mechanical, but really a consequence of assumptions about initial conditions.J. Uffink, "Compendium of the foundations of classical statistical physics." (2006)
Loschmidt's paradox
Soon after Boltzmann published his theorem, Johann Josef Loschmidt objected that it should not be possible to deduce an irreversible process from time-symmetric dynamics and a time-symmetric formalism. If the decreases over time in one state, then there must be a matching reversed state where increases over time (Loschmidt's paradox). The explanation is that Boltzmann's equation is based on the assumption of "molecular chaos", i.e., that it follows from, or at least is consistent with, the underlying kinetic model that the particles be considered independent and uncorrelated. It turns out that this assumption breaks time reversal symmetry in a subtle sense, and therefore begs the question. Once the particles are allowed to collide, their velocity directions and positions in fact do become correlated (however, these correlations are encoded in an extremely complex manner). This shows that an (ongoing) assumption of independence is not consistent with the underlying particle model.
Boltzmann's reply to Loschmidt was to concede the possibility of these states, but noting that these sorts of states were so rare and unusual as to be impossible in practice. Boltzmann would go on to sharpen this notion of the "rarity" of states, resulting in his entropy formula of 1877.
Spin echo
As a demonstration of Loschmidt's paradox, a modern counterexample (not to Boltzmann's original gas-related H-theorem, but to a closely related analogue) is the phenomenon of spin echo. In the spin echo effect, it is physically possible to induce time reversal in an interacting system of spins.
An analogue to Boltzmann's for the spin system can be defined in terms of the distribution of spin states in the system. In the experiment, the spin system is initially perturbed into a non-equilibrium state (high ), and, as predicted by the theorem the quantity soon decreases to the equilibrium value. At some point, a carefully constructed electromagnetic pulse is applied that reverses the motions of all the spins. The spins then undo the time evolution from before the pulse, and after some time the actually increases away from equilibrium (once the evolution has completely unwound, the decreases once again to the minimum value). In some sense, the time reversed states noted by Loschmidt turned out to be not completely impractical.
Poincaré recurrence
In 1896, Ernst Zermelo noted a further problem with the theorem, which was that if the system's is at any time not a minimum, then by Poincaré recurrence, the non-minimal must recur (though after some extremely long time). Boltzmann admitted that these recurring rises in technically would occur, but pointed out that, over long times, the system spends only a tiny fraction of its time in one of these recurring states.
The second law of thermodynamics states that the entropy of an isolated system always increases to a maximum equilibrium value. This is strictly true only in the thermodynamic limit of an infinite number of particles. For a finite number of particles, there will always be entropy fluctuations. For example, in the fixed volume of the isolated system, the maximum entropy is obtained when half the particles are in one half of the volume, half in the other, but sometimes there will be temporarily a few more particles on one side than the other, and this will constitute a very small reduction in entropy. These entropy fluctuations are such that the longer one waits, the larger an entropy fluctuation one will probably see during that time, and the time one must wait for a given entropy fluctuation is always finite, even for a fluctuation to its minimum possible value. For example, one might have an extremely low entropy condition of all particles being in one half of the container. The gas will quickly attain its equilibrium value of entropy, but given enough time, this same situation will happen again. For practical systems, e.g. a gas in a 1-liter container at room temperature and atmospheric pressure, this time is truly enormous, many multiples of the age of the universe, and, practically speaking, one can ignore the possibility.
Fluctuations of H in small systems
Since is a mechanically defined variable that is not conserved, then like any other such variable (pressure, etc.) it will show thermal fluctuations. This means that regularly shows spontaneous increases from the minimum value. Technically this is not an exception to the theorem, since the theorem was only intended to apply for a gas with a very large number of particles. These fluctuations are only perceptible when the system is small and the time interval over which it is observed is not enormously large.
If is interpreted as entropy as Boltzmann intended, then this can be seen as a manifestation of the fluctuation theorem.
Connection to information theory
H is a forerunner of Shannon's information entropy. Claude Shannon denoted his measure of information entropy after the H-theorem.Gleick 2011 The article on Shannon's information entropy contains an explanation of the discrete counterpart of the quantity H, known as the information entropy or information uncertainty (with a minus sign). By extending the discrete information entropy to the continuous information entropy, also called differential entropy, one obtains the expression in the equation from the section above, Definition and Meaning of Boltzmann's H, and thus a better feel for the meaning of H.
The H-theorem's connection between information and entropy plays a central role in a recent controversy called the Black hole information paradox.
Tolman's H-theorem
Richard C. Tolman's 1938 book The Principles of Statistical Mechanics dedicates a whole chapter to the study of Boltzmann's theorem, and its extension in the generalized classical statistical mechanics of Gibbs. A further chapter is devoted to the quantum mechanical version of the H-theorem.
Classical mechanical