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Are these things alive? That depends on how the term is defined. Surprisingly, there is no clear definition of "life." Most of the criteria put forward in the past are anthropocentric. Life on earth is carbon-based and built around the nucleic acids RNA and DNA, but that may be a historical accident. Most living things metabolize and multiply, but not all. Viruses have no metabolisms of their own; mules cannot reproduce. Many living things grow, but so do clouds and garbage dumps.
Still, most people have an intuitive sense of what it means to be alive. They know life when they see it. That is what is so disturbing about a good computer simulation. Take Craig Reynolds' flocking birds. By specifying a couple of simple rules -- keep a few wings' distance from your neighbors, try to fly as fast as they do -- Reynolds, a computer scientist at Symbolics, Inc., got bird-shaped objects on a screen to exhibit a flocking behavior that is absolutely convincing. The birds are artificial, but the flocking is real.
It is the same with life in general. Contends Langton: "Artificial life will be genuine life. It will simply be made of different stuff." This is the leap of faith made by a growing number of scientists, many of whom are associated with the Sante Fe Institute, a research facility that is the center of the artificial-life movement. "They feel like they are taking the first step into taboo territory," says Steven Levy, a New York City-based author who is writing a book on artificial life. "It's almost a religion."
Like religion, artificial life has evolved certain tenets. One is that lifelike behavior cannot be imposed from the top down. Rather, it emerges from the bottom up, like flocking among birds, when large numbers of parts obey a few simple rules. Another principle, derived from recent advances in the theory of chaos, is that when a system is sufficiently complex -- like the mix of chemicals in the primordial sea -- a lifelike order will spontaneously emerge.
Scientists have begun to think of possible uses for adaptive, self- replicating machines -- cleaning up toxic wastes, perhaps, or exploring outer space. There is a danger, though, that such machines could multiply uncontrollably, like the viruses that have disrupted computer networks. Doyne Farmer, a physicist at the Los Alamos lab, points to a cautionary science- fiction tale by Stanislaw Lem. In Lem's Fiasco, space explorers discover a Saturn-like planet with a ring around it. On closer inspection, the ring turns out to be a swarm of attack satellites and killer robots, part of a "star wars" defense shield that had reproduced itself over and over again. Artificial life, says Farmer, could turn out to be man's most beautiful creation. Or, like Lem's swarming robots, it could be a nightmare.