by Livio, Mario
Strictly speaking, de Maillet’s calculations and the theory on which they were based were flawed in a number of ways. First, water never entirely covered the Earth—de Maillet did not realize that rather than the water receding, the land might rise. Second, his understanding of rock formation was seriously lacking. He further weakened his case by occasional wanderings into fantasy. For instance, to support his contention that all life-forms emerged from the sea (an idea that is actually consistent with present thinking), de Maillet relied on accounts of mermaids and men with tails. Nevertheless, de Maillet’s estimate of the age of the Earth marked a major shift in the thinking about this problem. For the first time, it was not the human lifetime by which the age of the Earth was determined but rather the rate of natural processes.
De Maillet humbly dedicated his book to the romantic French dramatist Cyrano de Bergerac, who died less than a year before de Maillet’s birth. He started his dedication this way: “I hope you will not take it ill, that I address my present work to you, since, I could not possibly have made choice of a more worthy Protector of the Romantic Flights of Fancy which it contains.” Today we can appreciate that de Maillet’s work was more than “romantic flights of fancy”—it contained the seeds of geochronology. Determining the age of the Earth by scientific methods was about to become a worthy intellectual challenge.
The Earth and Life Gain a History
In his masterwork Principia, first published in 1687, Isaac Newton noted that “a globe of red hot iron equal to our earth, that is, about 40,000,000 feet in diameter, would scarcely cool in an equal number of days, or in above 50,000 years.” Realizing he could not easily square this result with his religious beliefs, Newton was quick to add, “But I suspect that the duration of heat may, on account of some latent causes, increase in a yet less proportion than that of the diameter; and I should be glad that the true proportion was investigated by experiments.”
Newton was not the only seventeenth-century scientist to think about this problem. The famous philosophers Descartes and Gottfried Wilhelm Leibniz also discussed the cooling of the Earth from an initially molten state. However, the first person who appears to have taken seriously Newton’s advice about an experimental investigation—and who in addition was imaginative enough to attempt to use the cooling problem to estimate the age of the Earth—was the eighteenth-century mathematician and naturalist Georges-Louis Leclerc, Comte de Buffon.
Buffon was a truly prolific character who was not only an accomplished scientist but also a successful businessman. He is perhaps best known for the clarity and forcefulness with which he presented a new method for approaching nature. His monumental lifework, Histoire Naturelle, Générale et Particulière (Natural History, General and Particular)—thirty-six-volumes of which were completed during his lifetime (with eight more published posthumously)—was read by most of the educated people of the day in Europe and North America. Buffon’s aim was to deal in succession with topics ranging from the solar system, the Earth, and the human race to the different kingdoms of living creatures.
In his mental excursion into the Earth’s physical past, Buffon assumed that the Earth started as a molten sphere after having been ejected from the Sun due to a collision with a comet. Then, in the true spirit of an experimentalist, he was not satisfied with a purely theoretical scenario—Buffon proceeded immediately to manufacture spheres of different diameters and to measure accurately the time it took them to cool down. From these experiments he estimated that the terrestrial globe solidified in 2,905 years and cooled down to its present temperature in 74,832 years, even though he suspected that the cooling time could be much longer.
Eventually, however, it was not pure Newtonian physics that brought the problem of the Earth’s age into the limelight. The surge in the study of fossils in the eighteenth century convinced naturalists such as Georges Cuvier, Jean-Baptiste Lamarck, and James Hutton that both the paleontological and the geological records required the operation of geological forces over exceedingly long periods of time. So long, in fact, that, as Hutton has put it, he found “no vestige of a beginning, no prospect of an end.”
In view of the increasing difficulty of trying to cram the entire history of the Earth into the biblical mere few thousand years, some of the more religiously inclined naturalists (but not only them) opted to rely on catastrophes such as floods as agents of rapid changes. If great expanses of time were to be denied, catastrophes appeared to be the only vehicle that could significantly shape the Earth’s surface almost instantaneously. To be sure, the distribution of marine fossils provided clear evidence for the action of flooding and glaciation in the Earth’s geological past, but many of the ardent catastrophists were at least partially motivated by their unwavering loyalty to the biblical text rather than by the scientific attestation. Richard Kirwan—one of the well-known chemists of the day—articulated this position clearly. Kirwan pitted Hutton directly against Moses in describing how dismayed he was to observe “how fatal the suspicion of the high antiquity of the globe has been to the credit of Mosaic history, and consequently to religion and morality.”
The situation started to change dramatically with the publication of Charles Lyell’s three-volume Principles of Geology in the years 1830–33. Lyell, who was also Charles Darwin’s close friend, made it clear that the catastrophist doctrine was far too frail to last as a compromise between science and theology. He decided to put aside the question of the origin of the Earth and to concentrate on its evolution. Lyell argued that the forces that sculpted the Earth—volcanism, sedimentation, erosion, and similar processes—remained essentially unchanged throughout the Earth’s history, both in their strength and in their nature. This was the idea of uniformitarianism that inspired Darwin’s concept of gradualism in the evolution of species. The basic premise was simple: If there was one thing that these slow-acting geological forces needed in order to have an appreciable effect, it was time. Lots of it. Lyell’s followers have almost abandoned the notion of a definite age altogether in favor of the rather vague “inconceivably vast” time. In other words, Lyell’s Earth was one that was almost in a steady state, with snail’s-pace changes operating over a nearly infinite time. This principle starkly contrasted with the theological estimates of some six thousand years.
To a certain extent, the world view of an immeasurably extended geological age permeated Darwin’s The Origin, even though Darwin’s own attempt to estimate the age of the Weald—the eroded valley stretching across the southeastern part of England—turned out to be disastrously flawed, and he eventually retracted it. Darwin envisaged for evolution a long sequence of phases, lasting perhaps ten million years each. There was, however, one important difference between Darwin’s stance and those of the geologists. While he indeed required long periods of time for evolution to run its course, he insisted on a directional “arrow of time”; he could not be satisfied with a steady state or a cyclical progression, since the concept of evolution gave time a clear trend. But a controversy was starting to brew. It was not between Darwin and Lyell personally, nor even between geology and biology in general, but between a champion of physics on one side and some geologists and biologists on the other. Enter one of the most eminent physicists of his time: William Thomson, later known as Lord Kelvin.
Global Cooling
In 1897 the Vanity Fair Album, a compendium of highlights from the weekly British society magazine, published a eulogy of Lord Kelvin, part of which read as follows:
His father was Professor of Mathematics at Glasgow. Himself was born in Belfast seventy-two years ago, and educated at Glasgow University and at St Peter’s, Cambridge; of which College, after making himself Second Wrangler and Smith’s Prizeman, he was made a Fellow. Unlike a Scotchman, he presently returned to Glasgow—a Professor of Natural Philosophy; and since then he has invented so much and, despite his mathematical knowledge, has done so much good, that his name—which is William Thomson—is known not only throughout the civilized world but
also on every sea. For when he was a mere knight he invented Sir William Thomson’s mariner’s compass as well as a navigational sounding machine, that is, unhappily less well known. He has also done much electrical service at sea: as engineer for various Atlantic cables, as inventor of the mirror-galvanometer and siphon recorder, and much else that is not only scientific but useful. He is so good a man, indeed, that four years ago he was enobled as Baron Kelvin of Largs; yet he is still full of wisdom, for his Peerage has not spoiled him . . . He knows all there is to know about heat, all that is yet known about Magnetism, and all that he can find out about Electricity. He is a very great, honest, and humble Scientist who has written much and done more.
Figure 9
This was a fairly accurate, if humorous, description of the numerous accomplishments of the man dubbed by one of his biographers the “Dynamic Victorian.” On his ennoblement, in 1892, Thomson adopted the title Baron Kelvin of Largs, after the River Kelvin, which flowed close to his laboratory at the University of Glasgow. “Second Wrangler” referred to Kelvin having placed (to his disappointment) second in the final honors school of mathematics at Cambridge. Story has it that on the morning the examination results were to be posted, he sent his servant to find out “who is Second Wrangler?” and was devastated when he was told “You, sir!” There is no doubt that Kelvin was the foremost figure of the age that witnessed the end of classical physics and the birth of the modern era. Figure 9 shows a portrait of Lord Kelvin, possibly after a photograph taken in 1876. Appropriately, upon his death in 1907, he was laid to rest in a tomb alongside Isaac Newton in Westminster Abbey. What the eulogy did not capture, however, was the eventual collapse of Kelvin’s stature in scientific circles. As an old man, Kelvin developed a reputation as an obstructionist to modern physics. Often portrayed as someone who clung stubbornly to his old views, he resisted new findings about atoms and about radioactivity. More surprisingly, even though James Clerk Maxwell relied on some of Kelvin’s applications of energy principles when he developed his impressive theory of electromagnetism, Kelvin still objected to the theory, stating, “I may say that the one thing about it that seems intelligible to me, I do not think is admissible.” For the technically savvy person that he was, Kelvin made similarly astonishing declarations on technology, such as “I have not the smallest molecule of faith in aereal navigation other than ballooning.” It was this enigmatic man—brilliant as a young scientist, seemingly out of touch as an old one—who attempted to discredit the geologists’ views on the age of the Earth.
On April 28, 1862, Kelvin (then still Thomson) read to the Royal Society of Edinburgh a paper entitled “On the Secular Cooling of the Earth.” This paper followed closely on the heels of another article published just the month before, with the title “On the Age of the Sun’s Heat.” Thomson made clear from the opening sentence that this was not going to be just another forgettable technical essay. Here was a hard-line attack on the geologists’ assumption about the unchanging nature of the forces that had shaped the Earth:
For eighteen years it has pressed on my mind, that essential principles of Thermodynamics have been overlooked by those geologists who uncompromisingly oppose all paroxysmal hypotheses, and maintain not only that we have examples now before us, on the earth, of all the different actions by which its crust has been modified in geological history, but that these actions have never, or have not on the whole, been more violent in past time than they are at present.
While the phrase “pressed on my mind” was somewhat of an overdramatized exaggeration, it was certainly true that Kelvin’s first papers on the topics of heat conduction and the distribution of heat through the body of the Earth were written as early as 1844 (when he was a twenty-year-old student) and 1846, respectively. Even before his seventeenth birthday, Thomson succeeded in spotting a mistake in a paper on heat by an Edinburgh professor.
Kelvin’s point was simple: Measurements from mines and wells indicated that heat was flowing from the Earth’s interior to its surface, implying that the Earth was an initially hotter planet that was cooling. Consequently, Kelvin argued, unless some internal or external energy sources could be shown to compensate for the heat losses, clearly no steady state, or repeating, identical geological cycles, were possible. Charles Lyell was actually aware of this problem, and in his Principles of Geology he proposed a self-sustaining mechanism by which he believed that chemical, electric, and heat energy could be exchanged cyclically in the Earth’s interior. Basically, Lyell envisaged a scenario in which chemical reactions generated heat, which drove electrical currents, which in turn dissociated the chemical compounds into their original constituents, thus starting the process anew. Kelvin could barely hide his contempt. He demonstrated unambiguously that such a process amounted to some sort of perpetual motion machine, violating the principle of dissipation (and conservation) of energy—when mechanical energy is transformed irreversibly into heat, as in the case of friction. Lyell’s mechanism therefore violated the basic laws of thermodynamics. To Kelvin, this was the ultimate proof that the geologists were completely ignorant of physical principles, and he remarked caustically:
To suppose, as Lyell, adopting the chemical hypothesis, has done, that the substances, combining together, may be again separated electrolytically by thermoelectric currents, due to the heat generated by their combination, and thus the chemical action and its heat continued in an endless cycle, violates the principles of natural philosophy in exactly the same manner, and to the same degree, as to believe that a clock constructed with a self-winding movement may fulfill the expectations of its ingenious inventor by going for ever.
At its core, Kelvin’s calculation of the age of the Earth was straightforward. Since the Earth was cooling, he explained, one could use the science of thermodynamics to calculate the Earth’s finite geological age: the time it took the Earth to get to its current state, since the formation of the solid crust. The idea itself was not entirely new; the French physicist Joseph Fourier had developed the mathematical theory of thermal conductivity and of the Earth’s cooling process at the beginning of the nineteenth century. Realizing the theory’s potential, Kelvin engaged in 1849 in a series of measurements of underground temperatures (together with the physicist James David Forbes), and in 1855 urged that a complete geothermal survey be conducted, precisely to enable the calculation of the Earth’s age.
Kelvin assumed that the mechanism that transported heat from the interior to the surface was the same type of conduction that transfers heat from an iron skillet on an open fire to its handle. Still, in order to apply Fourier’s theory to the cooling Earth, he needed to know three physical quantities: (1) the initial internal temperature of the Earth, (2) the rate of change in the temperature according to depth, and (3) the value of the thermal conductivity of the Earth’s rocky crust (which determines how fast heat can be transported).
Kelvin thought that he had a fairly good handle on two of these quantities. Measurements by a number of geologists have shown that while results varied from location to location, in the mean, the temperature toward the Earth’s center increased roughly by one degree Fahrenheit for every fifty feet of descent (this quantity is known as the temperature gradient). Concerning the thermal conductivity, Kelvin relied on his own measurements for two types of rocks and for sand to give him what he regarded as an acceptable average. The third physical quantity—the Earth’s deep internal temperature—was extremely problematic, since it couldn’t be measured directly. But Kelvin was not a man easily deterred by such difficulties. Putting his analytic mind to work, he was eventually able to deduce an estimate for the unknown internal temperature. The entangled intellectual maneuvering that he had to perform to achieve this result presented Kelvin at his best—and his worst. On one hand, his virtuosic command of physics and his ability to examine potential alternatives with a razor-sharp logic were second to none. On the other, as we shall see in the next chapter, due to his overconfidence, he could sometimes be completely b
lindsided by unforeseen possibilities.
Kelvin started his assault on the problem of the Earth’s internal temperature by analyzing a variety of possible models for the cooling Earth. The general assumption was that the Earth’s initial state was molten, as a result of the heat generated by some collision—either with a number of smaller bodies, such as meteors, or with one body of nearly equal mass. The subsequent evolution of this molten sphere depended on a property of rocks that was not known with certainty: whether upon solidifying, molten rock expanded (as in the case of freezing water) or contracted (as metals do). In the former case, one could expect the solid crust to float over a liquid interior, just like ice on the surface of lakes in winter. In the latter, the denser solid rocks forming near the Earth’s cooler surface would have sunk down, eventually forming perhaps a solid scaffolding that could support the surface crust. While the empirical evidence was scarce, experiments with melted granite, slate, and trachyte all seemed to point in the direction of molten rock contracting both upon cooling and solidifying. Kelvin used this information to chart a new scenario. He proposed that before complete solidification took place, the cooler surface liquid had sunk toward the center, thus maintaining convection currents similar to those generated in the oil in a frying pan. In this model the convection was assumed to sustain a nearly uniform temperature throughout. Consequently, Kelvin assumed that at the point of solidification, the temperature everywhere was roughly the temperature at which rock melts, and he took that to be the Earth’s internal temperature (assuming that the core had not cooled by much since). This model implied that the Earth was nearly homogeneous in its physical properties. Unfortunately, even this ingenious scheme did not fully solve the problem, since the value of the fusion temperature of rock was not known in Kelvin’s time. He was, therefore, forced to adopt an educated guess of seven thousand to ten thousand degrees Fahrenheit for an acceptable range. (Seismic measurements performed in 2007 gave a temperature of about 6,700 degrees Fahrenheit for a region that is about 1,860 miles below the Earth’s surface.)