History of entropy

Clausius' definitions

1854 definition

In his 1854 memoir, Clausius first develops the concepts of interior work, i.e. that "which the atoms of the body exert upon each other", and exterior work, i.e. that "which arise from foreign influences [to] which the body may be exposed", which may act on a working body of fluid or gas, typically functioning to work a piston. He then discusses the three categories into which heat Q may be divided:

1. Heat employed in increasing the heat actually existing in the body.
2. Heat employed in producing the interior work.
3. Heat employed in producing the exterior work.

Building on this logic, and following a mathematical presentation of the first fundamental theorem, Clausius then presented the first-ever mathematical formulation of entropy, although at this point in the development of his theories he called it "equivalence-value", perhaps referring to the concept of the mechanical equivalent of heat which was developing at the time rather than entropy, a term which was to come into use later. He stated:

the second fundamental theorem in the mechanical theory of heat may thus be enunciated:

If two transformations which, without necessitating any other permanent change, can mutually replace one another, be called equivalent, then the generations of the quantity of heat Q from work at the temperature T, has the equivalence-value:

${\displaystyle \frac{Q}{T}}$

and the passage of the quantity of heat Q from the temperature T₁ to the temperature T₂, has the equivalence-value:

${\displaystyle Q\left(\frac{1}{T_2}-\frac{1}{T_1}\right)}$

wherein T is a function of the temperature, independent of the nature of the process by which the transformation is effected.

In modern terminology, that is, the terminology introduced by Clausius himself in 1865, we think of this equivalence-value as "entropy", symbolized by S. Thus, using the above description, we can calculate the entropy change ΔS for the passage of the quantity of heat Q from the temperature T₁, through the "working body" of fluid, which was typically a body of steam, to the temperature T₂ as shown below:

If we make the assignment:

${\displaystyle S=\frac{Q}{T}}$

Then, the entropy change or "equivalence-value" for this transformation is:

${\displaystyle \mathrm{\Delta }S=S_{\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}}-S_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}}$

which equals:

${\displaystyle \mathrm{\Delta }S=\left(\frac{Q}{T_2}-\frac{Q}{T_1}\right)}$

and by factoring out Q, we have the following form, as was derived by Clausius:

${\displaystyle \mathrm{\Delta }S=Q\left(\frac{1}{T_2}-\frac{1}{T_1}\right)}$

1856 definition

In 1856, Clausius stated what he called the "second fundamental theorem in the mechanical theory of heat" in the following form:

${\displaystyle \int \frac{\delta Q}{T}=-N}$

where N is the "equivalence-value" of all uncompensated transformations involved in a cyclical process. This equivalence-value was a precursory formulation of entropy.

1862 definition

In 1862, Clausius stated what he calls the "theorem respecting the equivalence-values of the transformations" or what is now known as the second law of thermodynamics, as such:

The algebraic sum of all the transformations occurring in a cyclical process can only be positive, or, as an extreme case, equal to nothing.

Quantitatively, Clausius states the mathematical expression for this theorem is follows.

Let δQ be an element of the heat given up by the body to any reservoir of heat during its own changes, heat which it may absorb from a reservoir being here reckoned as negative, and T the absolute temperature of the body at the moment of giving up this heat, then the equation:

${\displaystyle \int \frac{\delta Q}{T}=0}$

must be true for every reversible cyclical process, and the relation:

${\displaystyle \int \frac{\delta Q}{T}\le 0}$

must hold good for every cyclical process which is in any way possible.

This was an early formulation of the second law and one of the original forms of the concept of entropy.

1865 definition

In 1865, Clausius gave irreversible heat loss, or what he had previously been calling "equivalence-value", a name:

I propose that S be taken from the Greek words, 'en-tropie' [intrinsic direction]. I have deliberately chosen the word entropy to be as similar as possible to the word energy: the two quantities to be named by these words are so closely related in physical significance that a certain similarity in their names appears to be appropriate.

Clausius did not specify why he chose the symbol S to represent entropy, and it is almost certainly untrue that Clausius chose S in honor of Sadi Carnot; the given names of scientists are rarely if ever used this way.

Later developments

In 1876, physicist J. Willard Gibbs, building on the work of Clausius, Hermann von Helmholtz and others, proposed that the measurement of "available energy" ΔG in a thermodynamic system could be mathematically accounted for by subtracting the "energy loss" TΔS from total energy change of the system ΔH. These concepts were further developed by James Clerk Maxwell [1871] and Max Planck [1903].

Statistical mechanics

In 1877, Ludwig Boltzmann developed a statistical mechanics evaluation of the entropy S, of a body in its own given macrostate of internal thermodynamic equilibrium. It may be written as:

${\displaystyle S=k_{\mathrm{B}}\ln \mathrm{\Omega }}$

where

k_B denotes the Boltzmann constant and

Ω denotes the number of microstates consistent with the given equilibrium macrostate.

Boltzmann himself did not actually write this formula expressed with the named constant k_B, which is due to Planck's reading of Boltzmann.

Boltzmann saw entropy as a measure of statistical "mixedupness" or disorder. This concept was soon refined by Gibbs, and is now regarded as one of the cornerstones of the theory of statistical mechanics.

Relation to living systems

Erwin Schrödinger made use of Boltzmann's work in his 1944 book What is Life? to explain why living systems have far fewer replication errors than would be predicted from statistical mechanics. Schrödinger used the Boltzmann equation in a different form to show increase of entropy

${\displaystyle S=k_{\mathrm{B}}\ln D}$

where D is the number of possible energy states in the system that can be randomly filled with energy. He postulated a local decrease of entropy for living systems when (1/D) represents the number of states that are prevented from randomly distributing, such as occurs in replication of the genetic code.

${\displaystyle -S=k_{\mathrm{B}}\ln (1/D)}$

Without this correction Schrödinger claimed that statistical mechanics would predict one thousand mutations per million replications, and ten mutations per hundred replications following the rule for square root of n, far more mutations than actually occur.

Schrödinger's separation of random and non-random energy states is one of the few explanations for why entropy could be low in the past, but continually increasing now. It has been proposed as an explanation of localized decrease of entropy in radiant energy focusing in parabolic reflectors and during dark current in diodes, which would otherwise be in violation of statistical mechanics.

Information theory

An analog to thermodynamic entropy is information entropy. In 1948, while working at Bell Telephone Laboratories, electrical engineer Claude Shannon set out to mathematically quantify the statistical nature of "lost information" in phone-line signals. To do this, Shannon developed the very general concept of information entropy, a fundamental cornerstone of information theory. Although the story varies, initially it seems that Shannon was not particularly aware of the close similarity between his new quantity and earlier work in thermodynamics. In 1939, however, when Shannon had been working on his equations for some time, he happened to visit the mathematician John von Neumann. During their discussions, regarding what Shannon should call the "measure of uncertainty" or attenuation in phone-line signals with reference to his new information theory, according to one source:

My greatest concern was what to call it. I thought of calling it ‘information’, but the word was overly used, so I decided to call it ‘uncertainty’. When I discussed it with John von Neumann, he had a better idea. Von Neumann told me, ‘You should call it entropy, for two reasons: In the first place your uncertainty function has been used in statistical mechanics under that name, so it already has a name. In the second place, and more important, nobody knows what entropy really is, so in a debate you will always have the advantage.

According to another source, when von Neumann asked him how he was getting on with his information theory, Shannon replied:

The theory was in excellent shape, except that he needed a good name for "missing information". "Why don’t you call it entropy", von Neumann suggested. "In the first place, a mathematical development very much like yours already exists in Boltzmann's statistical mechanics, and in the second place, no one understands entropy very well, so in any discussion you will be in a position of advantage.

In 1948 Shannon published his seminal paper A Mathematical Theory of Communication, in which he devoted a section to what he calls Choice, Uncertainty, and Entropy. In this section, Shannon introduces an H function of the following form:

${\displaystyle H=-K\sum _{i=1}^{k}p(i)\log p(i),}$

where K is a positive constant. Shannon then states that "any quantity of this form, where K merely amounts to a choice of a unit of measurement, plays a central role in information theory as measures of information, choice, and uncertainty." Then, as an example of how this expression applies in a number of different fields, he references Richard C. Tolman's 1938 Principles of Statistical Mechanics, stating that

the form of H will be recognized as that of entropy as defined in certain formulations of statistical mechanics where p_i is the probability of a system being in cell i of its phase space ... H is then, for example, the H in Boltzmann's famous H theorem.

As such, over the last fifty years, ever since this statement was made, people have been overlapping the two concepts or even stating that they are exactly the same.

Shannon's information entropy is a much more general concept than statistical thermodynamic entropy. Information entropy is present whenever there are unknown quantities that can be described only by a probability distribution. In a series of papers by E. T. Jaynes starting in 1957, the statistical thermodynamic entropy can be seen as just a particular application of Shannon's information entropy to the probabilities of particular microstates of a system occurring in order to produce a particular macrostate.

Popular use

The term entropy is often used in popular language to denote a variety of unrelated phenomena. One example is the concept of corporate entropy as put forward somewhat humorously by authors Tom DeMarco and Timothy Lister in their 1987 classic publication Peopleware, a book on growing and managing productive teams and successful software projects. Here, they view energy waste as red tape and business team inefficiency as a form of entropy, i.e. energy lost to waste. This concept has caught on and is now common jargon in business schools.

In another example, entropy is the central theme in Isaac Asimov's short story "The Last Question" (first copyrighted in 1956). The story plays with the idea that the most important question is how to stop the increase of entropy.

Terminology overlap

When necessary, to disambiguate between the statistical thermodynamic concept of entropy, and entropy-like formulae put forward by different researchers, the statistical thermodynamic entropy is most properly referred to as the Gibbs entropy. The terms Boltzmann–Gibbs entropy or BG entropy, and Boltzmann–Gibbs–Shannon entropy or BGS entropy are also seen in the literature.