Billions and Billions: Thoughts on Life and Death at the Brink of the Millenium.

For nearly two decades before his death in December 1996, Carl Sagan was arguably the most famous scientist in America. Author of numerous books and articles aimed at a general audience, host of the public-television series Cosmos, and frequent guest on The Tonight Show, Sagan addressed subjects ranging from human prehistory to the search for extraterrestrial intelligence to the consequences of nuclear war. He became virtually an all-purpose explainer of and spokesman for the scientific enterprise. His own research in planetary astronomy, respectable though it was, was distinctly secondary to his skills as a popularizer in ensuring his fame.

In many respects, Sagan was well-suited for the role of scientist-as-celebrity. His writing style was lucid, elegant, and often dramatic. At its best, it conveyed a sense of the epic quality of cosmic and biological evolution. He was an adroit public speaker who garnered attention through minor eccentricities such as the plosive "b" with which he pronounced "billions." (Yet, claims Sagan in Billions and Billions, he never mouthed the title's redundant phrase; rather, it was Johnny Carson who did so, in skits caricaturing his favorite astronomer.) Self-promotion was an obvious, and oddly appealing, feature of Sagan's public persona, as when the camera in Cosmos lingered endlessly on the host's awestruck heavenward gaze.

Sagan was an advocate as much as an expositor. He was a persuasive proponent of space exploration; unlike many astronomers, he envisioned a grand human future in space, not just a series of ever-more sophisticated data-gathering probes. He was a passionate debunker of astrology, alien abductions, channeling, and other forms of pseudoscience and irrationalism. Sagan pointed out repeatedly that widespread scientific illiteracy is a dangerous thing in a society heavily dependent on science and technology. He did more than his share to combat that danger

To be sure, at times he succumbed to the occupational hazards of the science popularizer: the oversimplification of esoteric ideas, the blurring of distinctions between speculation and established fact, and the dressing-up of personal or political views in the mantle of scientific authority. Sagan overstated the certainty of climate models showing a possible "nuclear winter," and erroneously predicted a spate of cold weather and darkened skies resulting from the oil fires of the Persian Gulf War. He also gave undue credence to the highly speculative notion, little-accepted among neuroscientists, of a "triune brain," in which the human cerebrum coexists uneasily with distinct vestiges of reptilian and early mammalian anatomy. The triune brain theory enabled Sagan to denounce behaviors of which he disapproved - such as Cold War-era defense spending - as emanations of primitive brain parts.

Billions and Billions, a posthumously published collection of essays, reflects Sagan's diversity of interests - as well as his tendency to combine brilliant scientific exposition with less-than-convincing political argument. The book is divided into three sections. The first, titled "The Power and Beauty of Quantification," contains largely apolitical essays on science and mathematics. The second, "What are Conservatives Conserving?," is a set of warnings of environmental dangers, with particular emphasis on the thinning ozone layer and global warming. The final part, "Where Hearts and Minds Collide," covers a range of topics at the intersection of science and politics, and ends with Sagan's reflections on the illness that eventually would take his life.

The book opens with its title essay, in which Sagan discusses the public's growing but imperfect familiarity with the large numbers used in astronomy, economics, and other fields. His own association with "billions" came at a time when "millions" had become a bit passe, he notes, and soon "trillions" will be commonly evoked in reports of national debts, distances to nearby stars, and more. This leads to an explanation of the workings and benefits of exponential notation in describing very large numbers. A subsequent essay elucidates the concept of exponential growth, drawing upon examples that involve chessboards, bacteria colonies, world population, and radioactive decay.

A discussion of wave phenomena, ranging from splashes in a bathtub to gamma rays in space, displays a similar inventiveness in the use of examples and analogies. Less successful is "Monday-Night Hunters," which draws links between modern sports and prehistoric survival tactics; this essay includes an odd, first-person vignette of life in the Pleistocene era. The book's first section closes with a tour of astronomy's fast-moving frontiers. Sagan sketches out current scientific evidence and speculation regarding planets in other solar systems; possible past, or present, life on Mars and Saturn's moon Titan; and the origin and fate of the universe. Sticking closely to his formal discipline, Sagan is at his most precise and authoritative.

In the section "What are Conservatives Conserving?," Sagan delivers a jeremiad about the environment, while disdaining to grapple with opposing arguments made by conservatives, libertarians, or anybody else. A highly pessimistic view of global warming is accompanied by a warning that powerful interests are beholden to fossil fuels and thus seek to deny the problem. The idea that environmentalists might overstate the problem for reasons of their own is not even contemplated, much less explored. On the issue of ozone depletion, an adept explanation of the chemistry involved is marred by Sagan's grotesque musing that there is some "remote cosmic justice" in lighter-skinned people getting skin cancer, since whites were mainly responsible for inventing ozone-thinning chlorofluorocarbons.

Sagan's environmental proposals emphasize the development of solar energy, wind power, and other "alternative" energy sources. He correctly notes the growing competitiveness of solar energy in certain market niches, such as rural electrification in the Third World. Yet his prescriptions are heavily weighted toward government mandates, research subsidies, fuel taxes, and the like. The environmental benefits of markets - or of market-oriented regulations such as tradable pollution allowances - receive little notice. His mindset is that of a Progressive-era planner, confident of his ability to guide new technologies into socially useful roles.

The book's final third is dominated by the matters of "life and death" evoked in Billions and Billions's subtitle. In an essay originally published in 1988 in Parade and the Soviet magazine Ogonyok, Sagan presents a grim recounting of the histories of the United States and the Soviet Union. Sagan strains to present the superpowers as more-or-less equally culpable in the Cold War - even placing minor or debatable American acts, such as the CIA's mining of Nicaraguan harbors, alongside vast Soviet atrocities, such as Stalin's forced collectivization of agriculture. Nonetheless, as Sagan laments, the glasnost-era Soviets saw fit to censor their published version, despite assurances to the contrary.

Also reprinted is a speech given by Sagan that same year at a rededication of the Eternal Light Peace Memorial in Gettysburg. His theme is the danger posed by modern weapons of mass destruction, which are orders of magnitude more powerful than the arms of the Civil War. Yet Sagan is far too much of a technophile to seek solutions in a wholesale dismantlement of industrial society. In "The Twentieth Century," he notes that modern technology also has wrought much good: longer lifespans, greater literacy, expanded leisure time. Furthermore, this century has brought vast advances in every branch of science - a fact that Sagan celebrates, while acknowledging science's tendency to strip away cherished assumptions and beliefs.

In an essay on abortion, Sagan makes his own effort at such stripping away. He argues that the fetus's capacity for rudimentary thought - measured by electrical activity in the brain - is the proper criterion of its personhood. Since fetal brain activity does not occur until the third trimester, this criterion would allow abortions during the first six months - and thus the position of Roe v. Wade is arrived at on completely different grounds. While this argument is highly unlikely to convince pro-life activists or, for that matter, supporters of late-term abortions, and is weakened by the tendentiousness of Sagan's tone, it is at least an attempt to bring fresh thinking into a deadlocked debate.

Among the high points of Billions and Billions is 'The Rules of the Game," a clever discussion of ethical principles. Sagan begins with an amusing summary of the well-known golden rule and less-familiar dicta such as the iron rule ("Do unto others as you like, before they do it unto you") and the tin rule ("Suck up to those above you, and abuse those below"). Then, drawing upon the work of social scientist Robert Axelrod and others, he recounts how a tit-for-tat strategy - a mixture of cooperation and retaliation - proved successful in computerized game-theory tournaments. Sagan concludes, among other things, that moral questions are not beyond experimental investigation.

Sagan's essay "In the Valley of the Shadow" recounts the ups and downs of his struggle against myelodysplasia, a bone-marrow disease so rare and little-understood that Sagan, for all his trans-disciplinary knowledge, had never previously heard of it. Sagan writes movingly about the emotional support he received from his family, friends, doctors, and people who had never met him. Despite his long-standing skepticism of religion, he was buoyed to learn that thousands had prayed for him in a Manhattan cathedral and at a Hindu gathering on the banks of the Ganges. His final entry, dated October 1996, strikes a note of cautious optimism about his recovery; some two months later, he was dead.

That loss is felt far beyond his immediate circle. Cati Sagan was interesting even when he was wrong, and even those who disagreed with him were compelled to recognize his formidable intelligence and intellectual curiosity. He provided a much-needed voice for science, and a living rebuke to the many scientists who regard public understanding of their work as a secondary or impossible mission. Future popularizers of science will be hard-pressed to match his eloquence and range.

Kenneth Silber ([e-mail]) writes about science, technology, and economics.