An elaborate fantasy which revolves about the theory of the relatively of time. Set in the fourth millennium, Robert Silverberg projects a dual world--the earth world a highly mechanized nightmare governed by a tangled bureaucracy, and the world of spacemen, whose trips to other planets isolate them from their mundane brothers. A curious time differential emphasizes the incompatibility of the two factions, with the earth people aging at a much more rapid rate than the spacemen. Pivoting on the conflict between twins, one, a seventeen-year-old and the other, an earth man of twenty-six, the time discrepancy is ultimately resolved, but not before the reader is given a complete tour of the future, in and out of space. Except for a rather clever device, there is nothing here of compelling interest, either in terms of character, plot or conception. A quite ordinary adventure made somewhat more stimulating by its scientific veneer, told professionally by the author of Revolt on Alpha C.