The Information - James Gleick [129]
When the pressure and temperature of the system have become uniform the entropy is exhausted.
Within a few years, though, Maxwell turned about-face and decided to follow Clausius.♦ He rewrote his book and added an abashed footnote:
In former editions of this book the meaning of the term Entropy, as introduced by Clausius, was erroneously stated to be that part of the energy which cannot be converted into work. The book then proceeded to use the term as equivalent to the available energy; thus introducing great confusion into the language of thermodynamics. In this edition I have endeavoured to use the word Entropy according to its original definition by Clausius.
The problem was not just in choosing between positive and negative. It was subtler than that. Maxwell had first considered entropy as a subtype of energy: the energy available for work. On reconsideration, he recognized that thermodynamics needed an entirely different measure. Entropy was not a kind of energy or an amount of energy; it was, as Clausius had said, the unavailability of energy. Abstract though this was, it turned out to be a quantity as measurable as temperature, volume, or pressure.
It became a totemic concept. With entropy, the “laws” of thermodynamics could be neatly expressed:
First law: The energy of the universe is constant.
Second law: The entropy of the universe always increases.
There are many other formulations of these laws, from the mathematical to the whimsical, e.g., “1. You can’t win; 2. You can’t break even either.”♦ But this is the cosmic, fateful one. The universe is running down. It is a degenerative one-way street. The final state of maximum entropy is our destiny.
William Thomson, Lord Kelvin, imprinted the second law on the popular imagination by reveling in its bleakness: “Although mechanical energy is indestructible,” he declared in 1862, “there is a universal tendency to its dissipation, which produces gradual augmentation and diffusion of heat, cessation of motion, and exhaustion of potential energy through the material universe. The result of this would be a state of universal rest and death.”♦ Thus entropy dictated the universe’s fate in H. G. Wells’s novel The Time Machine: the life ebbing away, the dying sun, the “abominable desolation that hung over the world.” Heat death is not cold; it is lukewarm and dull. Freud thought he saw something useful there in 1918, though he muddled it: “In considering the conversion of psychical energy no less than of physical, we must make use of the concept of an entropy, which opposes the undoing of what has already occurred.”♦
Thomson liked the word dissipation for this. Energy is not lost, but it dissipates. Dissipated energy is present but useless. It was Maxwell, though, who began to focus on the confusion itself—the disorder—as entropy’s essential quality. Disorder seemed strangely unphysical. It implied that a piece of the equation must be something like knowledge, or intelligence, or judgment. “The idea of dissipation of energy depends on the extent of our knowledge,” Maxwell said. “Available energy is energy which we can direct into any desired channel. Dissipated energy is energy which we cannot lay hold of and direct at pleasure, such as the energy of the confused agitation of molecules which we call heat.” What we can do, or know, became part of the definition. It seemed impossible to talk about order and disorder without involving an agent or an observer—without talking about the mind:
Confusion, like the correlative term order, is not a property of material things in themselves, but only in relation to the mind which perceives them. A memorandum-book does not, provided it is neatly written, appear confused to an illiterate person, or to the owner who understands it thoroughly, but to any other person able to read it appears to be inextricably confused. Similarly the notion of dissipated energy could not occur