A Durable Discourse on Time

The Nature of Time: Raymond Flood and Michael Lockwood, eds. Basil Blackwell, New York, 1987. 187 pp. $19.95. In 1985 the Oxford University Department for External Studies sponsored a series of popular lectures on the nature of time by five physicists and three philosophers. The eight essays that make up this exceptionally well-edited book are based on these lectures. Although they span a wide range of topics and points of view, none presupposes a strong background in either physics or philosophy. Dennis Sciama's essay is a lucid account of time in Einstein's two great theories of space and time, special and general relativity. Michael Shallis writes about tachyons, hypothetical particles that travel faster than light and slow down as they acquire energy. Peter Atkins, explaining entropy and the law of entropy growth, argues that "the deep structure of change is decay." Paul Davies discusses issues raised by Niels Bohr and John Wheeler concerning the role of the observer in the physicist's description of "the external world." Short essays by W.H. Newton-Smith and J.R. Lucas seek to relate aspects of physical theories of time to philosophical questions. The longest and most interesting contributions are physicist Roger Penrose's "Big Bangs, Black Holes and 'Time's Arrow' "and philosopher Michael Dummett's "Causal Loops." The phrase time's arrow refers to the lack of symmetry in ordinary experience and in laws that govern such phenomena as the flow of heat between the direction of the past and the direction of the future. An asymmetry between past and future poses a problem for physicists because no corresponding asymmetry is apparent in the microscopic phenomena that underlie ordinary experience or in the laws that govern microscopic phenomena. Penrose adroitly exposes the logical incoherence of the notion, held by some cosmologists, that time's arrow is caused by the cosmic expansion. He points out that a uniform distribution of matter filling all space would have far less entropy per unit mass than a cosmic distribution of black holes. The gravitational collapse of matter generates entropy. Thus the dumpiness of the present-day universe—the fact that matter is concentrated in self-gravitating systems, not spread evenly throughout space—is at least consistent with the hypothesis that the universe began to expand from a uniform initial state, and with the law of entropy growth. The law of entropy growth doesn't, however, explain why an initially uniform distribution of matter must break up into self-gravitating clumps. May is not the same as must. The speculative and controversial part of Penrose's argument concerns his suggestion that the growth of entropy that accompanies gravitational collapse may be related to a decline of entropy during a hypothetical process that quantum physicists refer to as "the collapse of the wave function." His contribution is characteristically original and stimulating. In the most polished essay of the collection, Michael Dummett explores the relationship between temporal succession and causality. Among other topics, he discusses whether it is possible to have a coherent view of causality in which events taking place now can affect past events. Dummett argues persuasively that such an account is possible. If his argument is correct, the connection between the ideas of temporal succession and causation is weaker than most of us usually assume. Dummett doesn't, however, address the important question of whether current physical theories are consistent with reverse causation. Layzer is the Donald H. Menzel Professor of Astrophysics at Harvard University, Cambridge, MA 02138.

A Durable Discourse on Time

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