nature.net
September 2005
Heat Demons
By Robert Anderson
My interest in thermodynamics was recently rekindled while reading a copy of Basil Mahon’s The Man Who Changed Everything: The Life of James Clerk Maxwell. Outside the physics community, he has never been celebrated the way Newton or Einstein are. Yet in the mid-1800s he brought his genius to bear on an extraordinary range of problems, from the composition of Saturn’s rings, to color vision, to thermodynamics—a field initially driven by the quest for more efficient steam engines. (The Web site www.clerkmaxwellfoundation.org offers a concise biography.)
I was particularly intrigued by Maxwell’s “demon,” an imaginary creature the Scottish physicist dreamed up as a thought experiment to illustrate how, in principle, one might get heat to flow from cold to hot without adding energy—in violation of the second law of thermodynamics. A demon who could see molecules, Maxwell thought, might act as a gatekeeper, allowing only fast-moving molecules into a hot region on one side of the gate and only slow-moving molecules into a cold region on the other side. On the Internet I found two sites where you can play the demon’s little game. At absolutist.com/online/demon/, you have to register to play (for free), but it’s more fun than playing at the site of Sitabhra Sinha, a professor at India’s Institute of Mathematical Sciences, where no registration is necessary.
At a site run by a group called Physics and Astronomy Online, I found a wonderful formula for remembering the three laws of thermodynamics, attributed on that Web site to the English scientist and writer C.P. Snow: One, you cannot win; two, you cannot break even; three, you cannot get out of the game. It sounds remarkably like a trip to Las Vegas.
The second law, despite Snow’s succinct summary, is particularly hard for many people to grasp. Yet its implications are enormous. To be slightly less succinct, the second law states that disorder increases with time, which in turn implies that whenever energy changes from one form into another, some usable energy is lost. A Web page titled “Energy Flow in Nature” gives a brief illustration of how, because of the second law, energy is lost in a food web every time it passes from one consumer to another.
An online slide show created by Mark C. Benfield of Louisiana State University in Baton Rouge illustrates how removing energy from an ecosystem can have dire consequences. In the 1970s, conservation efforts had led to the recovery of sea-otter populations off the west coast of North America. Then climate change and overfishing reduced food supplies—energy—for seals and sea lions that live in the deep ocean, causing those populations to crash. That forced orcas, or killer whales, which normally feed on seals and sea lions, to approach the coast and eat the sea otters. By the late 1990s the sea otter population had fallen by almost 90 percent of its 1970 level.
Some people maintain that evolution violates the second law, because it leads to increasingly complex, or more highly ordered, organisms. The Internet is full of sites that take this apparent inconsistency as proof that the theory of evolution is wrong. Other sites, however, unravel the misconception. Brig Klyce, for example, provides a history of the use of thermodynamics in biology. If your appetite is whetted for even more about that fascinating topic, check out a site developed by Frank Lambert, a professor emeritus of biology at Occidental College in Los Angeles. Or you can go back to playing Maxwell’s demon.
Robert Anderson is a freelance science writer living in Los Angeles.
Copyright © Natural History Magazine, Inc., 2005