ANTA FE, N.M. — With advances toward ultrastrong fibers, the concept of building an elevator 60,000 miles high to carry cargo into space is moving from the realm of science fiction to the fringes of reality. This month, the Los Alamos National Laboratory was a sponsor of a conference to ponder the concept. Yet, the keynote address was by a titan of science fiction, Arthur C. Clarke, speaking via satellite from his home in Sri Lanka. "I'm happy that people are taking it more and more seriously," said Mr. Clarke, whose novel "The Fountains of Paradise" (1978) revolved around such...