The Information (43 page)

Yet probability is enough: enough for the second law to stand as a pillar of science. As Maxwell put it:

The improbability of heat passing from a colder to a warmer body (without help from elsewhere) is identical to the improbability of order arranging itself from disorder (without help from elsewhere). Both, fundamentally, are due only to statistics. Counting all the possible ways a system can be arranged, the disorderly ones far outnumber the orderly ones. There are many arrangements, or “states,” in which molecules are all jumbled, and few in which they are neatly sorted. The orderly states have low probability and low entropy. For impressive degrees of orderliness, the probabilities may be
very
low. Alan Turing once whimsically proposed a number
N
, defined as “the odds against a piece of chalk leaping across the room and writing a line of Shakespeare on the board.”
^♦

Eventually physicists began speaking of microstates and macrostates. A macrostate might be: all the gas in the top half of the box. The corresponding microstates would be all the possible arrangements of all particles—positions and velocities. Entropy thus became a physical
equivalent of probability: the entropy of a given macrostate is the logarithm of the number of its possible microstates. The second law, then, is the tendency of the universe to flow from less likely (orderly) to more likely (disorderly) macrostates.

It was still puzzling, though, to hang so much of physics on a matter of mere probability. Can it be right to say that nothing in physics is stopping a gas from dividing itself into hot and cold—that it is only a matter of chance and statistics? Maxwell illustrated this conundrum with a thought experiment. Imagine, he suggested, “a finite being” who stands watch over a tiny hole in the diaphragm dividing the box of gas. This creature can see molecules coming, can tell whether they are fast or slow, and can choose whether or not to let them pass. Thus he could tilt the odds. By sorting fast from slow, he could make side A hotter and side B colder—“and yet no work has been done, only the intelligence of a very observant and neat-fingered being has been employed.”
^♦
The being defies ordinary probabilities. The chances are, things get mixed together. To sort them out requires information.

Thomson loved this idea. He dubbed the notional creature a demon:

“Maxwell’s intelligent demon,” “Maxwell’s sorting demon,” and soon just “Maxwell’s demon.” Thomson waxed eloquent about the little fellow: “He differs from real living animals only [
only!
] in extreme smallness and agility.”
^♦
Lecturing to an evening crowd at the Royal Institution of Great Britain, with the help of tubes of liquid dyed two different colors, Thomson demonstrated the apparently irreversible process of diffusion and declared that only the demon can counteract it:

The reporter for
The Popular Science Monthly
thought this was ridiculous. “All nature is supposed to be filled with infinite swarms of absurd little microscopic imps,” he sniffed. “When men like Maxwell, of Cambridge, and Thomson, of Glasgow, lend their sanction to such a crude hypothetical fancy as that of little devils knocking and kicking the atoms this way and that …, we may well ask, What next?”
^♦
He missed the point. Maxwell had not meant his demon to exist, except as a teaching device.

The demon sees what we cannot—because we are so gross and slow—namely, that the second law is statistical, not mechanical. At the level of molecules, it is violated all the time, here and there, purely by chance. The demon replaces chance with purpose. It uses information to reduce entropy. Maxwell never imagined how popular his demon would become, nor how long-lived. Henry Adams, who wanted to work some version of entropy into his theory of history, wrote to his brother Brooks in 1903, “Clerk Maxwell’s demon who runs the second law of Thermo-dynamics ought to be made President.”
^♦
The demon presided
over a gateway—at first, a magical gateway—from the world of physics to the world of information.

Scientists envied the demon’s powers. It became a familiar character in cartoons enlivening physics journals. To be sure, the creature was a fantasy, but the atom itself had seemed fantastic, and the demon had helped tame it. Implacable as the laws of nature now seemed, the demon defied these laws. It was a burglar, picking the lock one molecule at a time. It had “infinitely subtile senses,” wrote Henri Poincaré, and “could turn back the course of the universe.”
^♦
Was this not just what humans dreamed of doing?

Through their ever better microscopes, scientists of the early twentieth century examined the active, sorting processes of biological membranes. They discovered that living cells act as pumps, filters, and factories. Purposeful processes seemed to operate at tiny scales. Who or what was in
control? Life itself seemed an organizing force. “Now we must not introduce demonology into science,” wrote the British biologist James Johnstone in 1914. In physics, he said, individual molecules must remain beyond our control. “These motions and paths are un-co-ordinated—‘helter-skelter’—if we like so to term them. Physics considers only the statistical
mean
velocities.” That is why the phenomena of physics are irreversible, “so that for the latter science Maxwell’s demons do not exist.” But what of life? What of physiology? The processes of terrestrial life
are
reversible, he argued. “We must therefore seek for evidence that the organism
can
control the, otherwise, un-co-ordinated motions of the individual molecules.”
^♦

When life remained so mysterious, maybe Maxwell’s demon was not just a cartoon.

Then the demon began to haunt Leó Szilárd, a very young Hungarian physicist with a productive imagination who would later conceive the electron microscope and, not incidentally, the nuclear chain reaction. One of his more famous teachers, Albert Einstein, advised him out of avuncular protectiveness to take a paying job with the patent office, but Szilárd ignored the advice. He was thinking in the 1920s about how thermodynamics should deal with incessant molecular fluctuations. By definition, fluctuations ran counter to averages, like fish swimming momentarily upstream, and people naturally wondered: what if you could harness them? This irresistible idea led to a version of the perpetual motion machine,
perpetuum mobile
, holy grail of cranks and hucksters. It was another way of saying, “All that heat—why can’t we use it?”

It was also another of the paradoxes engendered by Maxwell’s demon.
In a closed system, a demon who could catch the fast molecules and let the slow molecules pass would have a source of useful energy, continually refreshed. Or, if not the chimerical imp, what about some other “intelligent being”? An experimental physicist, perhaps? A perpetual motion machine should be possible, declared Szilárd, “if we view the experimenting man as a sort of deus ex machina, one who is continuously informed of the existing state of nature.”
^♦
For his version of the thought experiment, Szilárd made clear that he did not wish to invoke a living demon, with, say, a brain—biology brought troubles of its own. “The very existence of a nervous system,” he noted, “is dependent on continual dissipation of energy.” (His friend Carl Eckart pithily rephrased this: “Thinking generates entropy.”
^♦
) Instead he proposed a “nonliving device,” intervening in a model thermodynamic system, operating a piston in a cylinder of fluid. He pointed out that this device would need, in effect, “a sort of memory faculty.” (Alan Turing was now, in 1929, a teenager. In Turing’s terms, Szilárd was treating the mind of the demon as a computer with a two-state memory.)