To say as Davies does that this world, this universe, might be just one of an infinite number of possible projections of superspace, coexisting with sibling universes, sounds like yet another fancy bit of science-cum-science-fiction speculation. Here, it is not. Davies is a prominent British astronomer/mathematician whose writing, moreover, is on a par with the exquisite science exposition for the layman associated with the best of Gamow or Hoyle. For the first half of the book he treads the familiar ground of paradoxes cast by quantum theory and special and general relativity: wave-particle dualities, mass-energy equivalences, space-time distortions--all the realities wrought by 20th-century physics that contradict the Newtonian world. In explaining them, Davies moves from the familiar and commonsensical to the bizarre and unreal through a series of logical steps. The Heisenberg uncertainty principle, for instance, never seemed more obvious. Antimatter? Davies' presentation of Dirac's thinking and Anderson's discovery of the positron unfolds with rare beauty and clarity. These explanations then serve as groundwork for discussion of tunneling effects and other quantum phenomena that seem to contradict the second law of thermodynamics. The second half of the book comes to grips with more philosophical issues; whole chapters dwell on the nature of reality, on time, and supertime. These chapters permit Davies to repeat some of the earlier exposition, now fleshed out with considerations of how the human thinker fits in. Will our descendants abandon traditional notions of free will and the unidirectional arrow of time--or ""could it be that our minds are more reliable than our laboratory instruments. . . ?"" Either prospect is awesome, Davies truly says. Following up his earlier, also estimable The Runaway Universe (1978), he has made the journey leading to such questions informative and exhilarating.