The Mechanical Demon

By HANS CHRISTIAN VON BAEYER

The conception of the "sorting demon" is purely mechanical ... It was not invented to help us deal with questions regarding the influence of life and of mind on the motions of matter.
-- William Thomson, Lord Kelvin

In December 1867, James Clerk Maxwell wrote to his friend Peter Guthrie Tait at the University of Edinburgh about the behavior of gases. Basing his reasoning on the billiard ball model, he tried to predict what would happen if he could control the molecules one by one. Since he obviously couldn’t, he invented a servant to do it for him. Thus Maxwell’s Demon, a "very observant and neat-fingered being" who could see and manipulate individual molecules, was born.

Maxwell imagined him as the gatekeeper at a little door in the wall between two boxes full of gas that are initially at one common temperature. According to the microscopic interpretation of temperature, both boxes contain balls with a wide spread of speeds, even though the average speeds on opposite sides of the wall are equal. The Demon allows only exceptionally fast balls to pass from the right into the left chamber, stopping all others. (To keep the numbers of particles in the two boxes equal, he might also be instructed to allow exceptionally slow molecules to pass in the opposite direction.) The result would evidently be that the left box gets warmer, and the right one cooler. Heat would begin to flow without a difference in temperature, like water flowing along a level trough, and then continue on uphill--a clear violation of the second law of thermodynamics.

Of course, Maxwell did not believe in the existence of microscopic sorting sprites. His fanciful creation of the Demon was not the conjuring of a spirit but the supposition of a mechanism that could test his theories.

As a research tool the Demon quickly proved his worth: As soon as he was born he began to raise difficult and significant questions. To begin with, one wonders whether such a mechanism can actually be constructed. If the answer is no, why not? On the other hand, if the answer is yes, what would that imply about the second law? Would the Demon invalidate the entropy principle as a universal law of nature? Maxwell did not know.

... when Richard Feynman, who liked to do things in his own way, set out to write his monumental textbook The Feynman Lectures on Physics, he invented a different version of the automated Demon. Feynman’s machine is a minuscule ratchet-and- pawl mechanism that allows a small weight hung from a string on a pulley to rise but not to fall. The ratchet wheel is attached to a tiny windmill in a vessel full of hot gas. Left to itself, the windmill would turn this way and that under the constant bombardment of gas molecules. However, the pawl--the little hinged tongue that keeps the ratchet wheel from slipping--should ensure that the weight can only rise. But alas, it won’t. Feynman showed that the pawl too warms up and bounces randomly, thereby allowing the weight to fall at random intervals. Careful calculation proved that this happens just often enough to save the laws of thermodynamics. Feynman wrote the blunt obituary of the dumb version of the Demon: "[It] must heat up. ... Soon it is shaking from Brownian motion so much it cannot tell whether it is coming or going, much less whether the molecules are coming or going, so it does not work."

... This conclusion in no way implies that the Demon’s life was a failure. On the contrary, the thoughts and debates on the subject of Brownian motion that he stimulated have enhanced our appreciation of that wonderful window through which Nature makes the motion of atoms visible to our poor human eyes. And, of course, our inability to see atoms was exactly the problem that frustrated Maxwell when he tried to understand the behavior of gases. In that sense, then, both the creator and his creature have succeeded splendidly.

Nor is the Demon really dead. Whenever he was tossed out a window, he has always managed to climb back in through another one in a new guise. At the end of a second paper on this problem in 1914, [Marian von] Smoluchowski announced the demise of the mechanical Demon, but added: "Such a device might, perhaps, function regularly if it were appropriately operated by intelligent beings." With that surmise he renewed the Demon’s lease on life.