As fans of the Star Trek TV programs and movies know, crew members of the starship Enterprise like to relax by visiting the ship's Holodeck. There they can enter any landscape that their imaginations-or anyone else's-might dream up. They can relive a battle from an ancient war, stand on the swaying deck of a sailing ship tossed by a storm, or have conversations with fictional characters such as Detective Sherlock Holmes.

On the Holodeck, whatever place the crew members choose seems to be all around them. They see it, hear it, feel it, perhaps even taste and smell it. Most exciting of all, they themselves are part of the action. When they walk through a scene, their view of it changes, just as it would if they were walking in a real place. When they speak, characters in the scene...