Dating the Earth

The title of "A Natural History of Time" (University of Chicago Press, 353 pages, $29) sounds like a paradox. If history is what is measured by time, how can time itself have a history? As Pascal Richet, a French professor of geophysics, points out in this fascinating and challenging book, that was the very problem that led Aristotle to deny that time was finite. "A beginning of time," according to Mr. Richet's paraphrase, "would imply an absence of time before time began; but one cannot say before unless one has already supposed the existence of time." What could have a history is the way that human beings measure time, or experience it, or think about it; but these subjects seem to belong more to philosophy and anthropology than to "natural history."

In fact, Mr. Richet's real subject is not the natural history of time, but the time of natural history. How much time did it take for the natural world to achieve the form we see today? Or, to put the question in its plainest form: How old is the Earth? Mr. Richet sets out to explore humanity's attempts to answer this most perplexing of questions, which acted as a spur and a baffle to human ingenuity for 2,500 years. Before it could be solved, we needed to invent chemistry and geology, astronomy and physics — to isolate the elements, read the sedimentary record, understand the evolution of species, and chart the movement of the stars. But unlike most historical mystery stories, this one actually has a happy ending. It took the combined effort and genius of many generations, but finally — and within living memory — human beings learned the true age of the Earth.

Because determining the age of the Earth drew on so many areas of scientific thought, "A Natural History of Time" sometimes reads like a condensed history of science. In such a short book, this means that many complex issues are dispatched briefly. Readers, like me, whose knowledge of math and physics dates back to high school, will find themselves wishing that Mr. Richet had taken more time to explain complex phenomena such as the precession of the equinoxes, or the radioactive decay of uranium. Add to this compression the fact that the book is translated from the French — capably, considering how much scientific terminology it contains, but not gracefully — and "A Natural History of Time" can be rough going.

Still, "A Natural History of Time" more that repays the effort it requires. Not only does it shed light on key advances in the history of science, from the ancient Greeks to the X-ray, it reminds us of the real heroism and nobility of the scientific enterprise. Today, science and technology have advanced to such a point that we tend to think mainly about their dangers — nuclear weapons, global warming, cloning. Yet our lives are supported by an immense edifice of scientific ingenuity, which we seldom understand or even think about. Mr. Richet reminds us that each acre of the continent of modern science was won back from an ocean of ignorance, by the hard work and intellectual courage of individuals.

That is why Mr. Richet's story is told as a series of individual stories, about the men and (more recently) women whose discoveries made it possible to date the Earth. One of the first of these was Hipparchus, "one of the greatest masters of astronomy, about whom we know practically nothing," except that he lived in Nicaea in the second century B.C.E. By comparing the position of a certain star in his own day with the position recorded by an Alexandrian astronomer a century earlier, Hipparchus was able to show that its longitude had changed over time. From this he deduced the so-called "precession of the equinoxes" — the slow revolution from west to east of the sphere of fixed stars, whose cycle takes 26,000 years to complete. This phenomenon tended to reinforce the ancient Greek view that time was endless and cyclical. Rather than a history, the Earth and the heavens had unchanging patterns, which made it impossible to try to assign them ages.

Hipparchus's data were correct, but his interpretation, of course, was not. This is a running theme in "A Natural History of Time": Again and again, Mr. Richet shows, it is our unspoken assumptions, the paradigms we use to process our observations, that lead human beings to error. Only once it was established that the Earth rotates around the sun, not vice versa, could the precession of the equinoxes be correctly understood. It is an artifact of the Earth's tilted orbit, which, like that of a wobbling top, makes the polar axis describe a circle over time. And not until Newton's discovery of gravity would it become clear that the cause of that wobble is that the Earth is not perfectly spherical. It took Newton to explain what Hipparchus saw — a meeting of the minds across 1,800 years.

Such intellectual encounters are found throughout Mr. Richet's book. The millennia-long attempt to date the Earth gave ample opportunity for them, because it involved just about every field of scientific endeavor. But for much of that time, the effort was hobbled by scientists' need to reconcile their observations with the biblical account of creation — whether because they genuinely believed in the Mosaic story, or because they feared persecution if they contradicted it. It was easy to see, for instance, that some of the fossils found on the tops of mountains resembled the skeletons of sea-creatures. But were these legacies of the flood, which had carried the bottom-dwellers up to the heights? Or were they the index of an inconceivably long period of sedimentation, the slow, regular work of the tides? Not until geologists had banished the biblical myth, at least from their own minds, could they learn to read the layers of the fossil record as a concrete history of the planet.

A similar hesitation plagued Count Buffon, the great 18th-century French naturalist. He took a different approach to figuring out the age of the Earth, one based on the rate of cooling of the planet's surface. By heating steel balls of various diameters, measuring how long it took them to cool, and applying the resulting formulas to the Earth's crust, Buffon concluded that the planet had to be at least 600,000 years old — or so he said in public. In fact, he confided to his notebook, the figure could be as high as 10 million years. He had deliberately underestimated the age of the Earth to avoid scandalizing his readers: "Instead of going back too far in the limits of the duration," he wrote, "I brought them in as close as possible without obviously contradicting the facts delivered in the archives of nature." Even so, his clerical opponents described his calculations as "the rantings of old age."

Those churchmen would have been surprised to learn that, in the 19th century, Buffon's timescale already looked laughably short. A long section of Mr. Richet's book — possibly too long — is devoted to the controversy that raged in Victorian England, between physicists on the one hand and biologists and geologists on the other. The latter, looking at the immense changes time had wrought in the surface of the Earth and the evolution of species, believed the planet had to be many hundreds of millions of years old. The physicists, led by the famous Lord Kelvin, reasoned that the heat source in the core of the Earth could only hold out for a fraction of that time — perhaps as few as 20 million years.

The debate involved a great deal of rancor and mutual recrimination, and Mr. Richet stages a mock "trial" by quoting from the various participants' writings. Thomas Huxley, the popularizer of evolution known as "Darwin's bulldog," took the geologists' side, writing: "we have exercised a wise discrimination in declining to meddle with our foundations at the bidding of the first passer-by who fancies our house is not so well built as it might be."

Once again, however, the governing assumptions of both sides proved to be wrong — and once again, even the greatest estimates of the Earth's age proved to be too short. When a number of scientists, in the last decade of the nineteenth century, converged on the discovery of radioactivity, it became clear that the earth had heat sources — in the form of uranium, radium, and the other radioactive metals — which Kelvin had not suspected. More important still, it was discovered that the radioactive elements had precise half-lives, so that the age of a mineral could be exactly dated by measuring the proportion of isotopes it contained. In effect, radioactivity offered the earth-clock that scientists had long been searching for.

That clock enabled Clair Patterson, an American geochemist who died as recently as 1995, to solve the mystery that had baffled Aristotle and Newton and Kelvin. By analyzing samples of terrestrial lead, and comparing them to lead from the great meteorite that created the Meteor Crater in Arizona, Patterson concluded that the Earth was formed 4.55 billion years ago. The immensity of the figure is staggering, not least because it renders absurd all anthropocentric accounts of the Creation and the universe. But if "A Natural History of Time" teaches one lesson, it is that any attempt to short-circuit the truth, in the name of a consoling myth, is destined to fail. Worse, it is an insult to human dignity. If there is anything miraculous about mankind, it is not our Scriptures, but our ability to read the text of the world. In a time when obscurantism speaks louder and louder, Mr. Richet reminds us that the truth, if anything, is what will set us free.

akirsch@nysun.com