Three Cheers for Diversity INFINITE IN ALL DIRECTIONS by Freeman J. Dyson Harper & Row; 321 pages; $19.95

To most laymen, the explosions of scientific knowledge in the 20th century have been chiefly felt as ominous aftershocks. The splitting of the atom, after all, led to nuclear bombs. The breaking of the genetic code of the DNA molecule raises nightmares about malevolent new designer viruses escaping from laboratories and running wild. And the Big Bang theory of the universe's origin suggests two possible conclusions, both of them unpleasant: infinite expansion, with a concurrent dispersal of heat and an annihilating deep freeze; or eventual contraction and a horrendous Big Crunch.

Between this rock and a hard place, British-born Physicist Freeman Dyson makes a spirited stand for optimism. Will our species end in fire or ice? Fire, the author concedes, would pose a difficult problem, but man might be able to overcome ice: "It is easier to keep warm on Pluto than to keep cool on Venus." Will we blow ourselves up? Probably not: "We shall abolish nuclear weapons, not by a sudden outburst of peace and goodwill but by a slow process of erosion. The weapons will be abolished as the missions for which they were designed come to seem unnecessary or absurd." And what of tinkering around with life in test tubes? Dyson issues a warning: "Genetic engineering must stop short of monkeying around irresponsibly with the species Homo sapiens." Beyond that restriction, beneficent marvels proliferate: "There are no laws of physics and chemistry which say that potatoes cannot grow on trees or that diamonds cannot grow in a desert."

Dyson's good cheer seems rigorously earned. For 35 years he has been a professor at the Institute for Advanced Study in Princeton, N.J., where his colleagues have included the likes of Albert Einstein, J. Robert Oppenheimer, Kurt Godel and John von Neumann. Dyson has had an intimate look at upheavals of contemporary science ranging from advances in particle physics and molecular biology to space travel and artificial intelligence. His long career in the ivory tower has not made him a reflexive defender of his elite brotherhood. "I detest and abhor," he writes, "the academic snobbery which places pure scientists on a higher cultural level than inventors." Nor has he been content to converse solely with fellow specialists. Disturbing the Universe (1979), his autobiography, and Weapons and Hope (1984), a meditation on the threat of atomic warfare, both reached for and found a wide general audience.

So should Infinite in All Directions, even though it is a revised version of a series of academic talks delivered at the University of Aberdeen in 1985 and hence an unlikely candidate for popular appeal. But Dyson is not the first person to turn a Gifford lectureship in Scotland into a book; other products of this prestigious assignment include William James' The Varieties of Religious Experience and Alfred North Whitehead's Process and Reality. Anyone would be daunted by such illustrious predecessors, including Dyson: "Confronted with the fact that I was not William James or Alfred Whitehead, I decided to make a virtue of necessity. I talked about things which interested me."