Authors: Mario Livio
How often these vicissitudes of decay and renovation have been repeated it is not for us to determine; they constitute a series of which, as the author of this theory [Hutton] has remarked, we neither see the beginning nor the end, a circumstance that accords well with what is known concerning other parts of the economy of the world . . . in the planetary motions where geometry has carried the eye so far both into the future and the past, we discover no mark either of the commencement or the termination of the present order.
It is unreasonable, indeed, to suppose
that such marks should exist anywhere
[emphasis added]. The Author of nature has not given laws to the universe which, like the institutions of men, carry in themselves the elements of their own destruction. He has not permitted in His works any symptoms of infancy or of old age, or any sign by which we may estimate either their future or their past duration. He may put an end, as He, no doubt, gave a beginning to the present system, at some determinate time; but we may safely conclude that the great
catastrophe
will not be brought about by any of the laws now existing, and that it is not indicated by anything which we perceive.
Kelvin’s reaction to this excerpt was merciless. “Nothing,” he said, “could possibly be further from the truth.” Explaining again his argument in layman’s terms, he noted:
The earth, if we bore into it anywhere, is warm, and if we could apply the test deep enough, we should, no doubt, find it very warm. Suppose you should have here before you a globe of sandstone, and boring into it found it warm, boring into another place found it warm, and so on, would it be reasonable to say that that globe of sandstone has been just as it is for a thousand days? You should say, ‘No; that sandstone has been in the fire, and heated not many hours ago.’ It would be just as reasonable to take a hot water jar, such as is used in carriages, and say that that bottle has been as it is for ever—as it was for Playfair to assert that the earth could have been for ever as it is now, and that it shows no traces of a beginning, no progress towards an end.
To strengthen his argument further, Kelvin decided not to rely just on his old reasoning about the Earth and the Sun. He came up with yet a third line of evidence, based on the Earth’s rotation around its axis. The concept itself was ingenious and easy to understand. An initially molten Earth would have assumed, due to its spin, a
slightly oblate shape: more flattened at the poles and more bulging at the equator. The faster the initial spin, the less spherical the resulting shape. This form, Kelvin inferred, would have been preserved upon the Earth’s solidification. Precise measurements of the deviation from sphericity could therefore be used to determine the original rate of rotation.
Since tides caused by the Moon’s gravity were expected to act like friction and slow down the rotation, one could estimate how much time was required to reduce the initial spin rate to the present one rotation every twenty-four hours.
While the idea was fascinating, turning it into an actual number for the age of the Earth was extremely tricky. Kelvin himself admitted,
“It is impossible, with the imperfect data we possess as to the tides, to calculate how much their effect in diminishing the earth’s rotation really is.” Nevertheless, Kelvin felt that even the mere fact that one
could
place a limit on the age of the Earth, no matter how uncertain, was sufficient to refute the uniformitarian notion of an inconceivably vast time. Referring to his own numerical estimate of a 22-second retardation per century in the Earth’s rotation period, he concluded, “[Whether] the earth’s lost time is 22 seconds, or considerably more or less than 22 seconds, in a century, the principle is the same. There cannot be uniformity. The earth is filled with evidences that it has not been going on for ever in the present state, and that there is a progress of events toward a state infinitely different from the present.”
Disappointingly for Kelvin, the estimate based on the Earth’s spin rate did not last for very long, at least not in any quantitative way. As fate would have it, none other than George Howard Darwin, Charles Darwin’s fifth child, showed the argument to be useless for an age estimate.
George was a physicist with considerable mathematical dexterity. He attacked the problem of the spinning Earth with infinite patience and attention to detail. In a series of papers published mainly between 1877 and 1879, the younger Darwin was able to demonstrate that, contrary to Kelvin’s expectation, the Earth could continue to gradually change its shape even as its rotation rate was slowing down. This was a consequence of the fact that
even a solidified Earth was not completely rigid. The bottom line
was unequivocal. Darwin showed that given the many uncertainties about the Earth’s interior, there was no reliable way to calculate the planet’s age from its spin.
Needless to say,
Charles Darwin was delighted to discover that his own son managed to “stagger” the great Kelvin, and he exclaimed, “Hurrah for the bowels of the earth and their viscosity and for the moon and for the Heavenly bodies and for my son George.”
But George Darwin’s work did not affect Kelvin’s main claims—it only established that Kelvin’s third argument (concerning the Earth’s rotation) could not be used to support the
value
of the estimate of the age of the Earth. There was another sense, however, in which Darwin’s work was revealing. It showed that even the august Lord Kelvin was not infallible. As we shall see in the next chapter, this may have helped to open the door for further criticism.
Deep Impact
To describe the age-of-the-Earth controversy as a battle to the death between physics and geology would be a mistake. While certainly there was tension along disciplinary lines, Kelvin saw himself so much in the mainstream of British geology that in his address at the Glasgow Geological Society meeting in 1878 he did not hesitate to declare,
“We, the geologists
[emphasis added], are at fault for not having demanded of the physicists experiments on the properties of matter.” This “flexible” self-identification reflected the less compartmentalized scientific world of the nineteenth century. Victorian scientists freely attended meetings of societies that formally represented other branches of science. Rather than a dispute between disciplines, therefore, the age-of-the-Earth debate was largely a clash between Kelvin and the doctrine of
some
geologists.
One may wonder what it was that motivated Kelvin to examine this problem in the first place. The answer is actually quite simple. Even a cursory examination leaves little doubt that the publication of Darwin’s
The Origin
in 1859 provided the main impetus for Kelvin’s direct attack on the estimates of the ages of both the Sun and
the Earth. To be clear, Kelvin did not object to the theory of evolution per se. In his 1871 presidential address to the British Association for the Advancement of Science, for instance, he expressed moderate support for some of Darwin’s conclusions in
The Origin
. However, he completely rejected natural selection because he had
“always felt that this hypothesis does not contain the true theory of evolution, if evolution there has been, in biology.” Why not? Because, he explained, he was
“profoundly convinced that the argument of design has been greatly too much lost sight of in recent zoological speculations.” In other words, even this committed mathematical physicist, who passionately declared that the “essence of science . . . consists in inferring antecedent conditions, and anticipating future evolutions, from phenomena which have actually come under observation,” still believed that “overpoweringly strong proofs of intelligent and benevolent design lie all around us.” In fact, Kelvin held that the laws of thermodynamics themselves were a part of that universal design. Nevertheless, we should remember that even if Kelvin felt a certain emotional attachment to the concept of “design,” there is no doubt that he anchored his scathing criticism of the geologists’ practices in genuine physics, not in his religious beliefs.
What was Kelvin’s impact on geology? Until the 1860s, geologists were much more preoccupied with discussions on whether the Earth’s interior was solid or fluid than with the Earth’s chronology. By the mid-1860s, however, quite a few of the influential geologists
started to pay serious attention to Kelvin’s claims. Foremost among these were John Phillips, Archibald Geikie, and James Croll. Based on studies of sediments, Phillips himself had suggested in 1860 an age of about ninety-six million years for the Earth. By 1865, he was publicly supporting Kelvin. Geikie, the new director of the Geological Survey for Scotland, more or less assumed the role of a conduit and mediator between physics and geology. On one hand, he criticized Kelvin’s assertion that the Earth’s geological past had been more active, citing evidence that seemed to show that if anything “the intensity . . . has, on the whole, been augmenting.” On the other, in a paper published in 1871, he essentially abandoned
uniformitarianism and stated that based on research in physics “about 100 millions of years is the time assigned within which all geological history must be comprised.” Croll, an impressive self-taught physicist and geologist, was entirely convinced by Kelvin’s calculation of the cooling Earth, and even though he was extremely skeptical about Kelvin’s estimate of the age of the Sun, accepted one hundred million years for the Earth’s age.
Often you can judge whether a certain scientific theory has had an impact by the vehemence with which the heavyweights with something at stake announce their objections to it. In Kelvin’s case, the sure sign that the opposition had taken notice came when biologist Thomas Huxley attacked Kelvin’s calculation in February 1869.
Huxley had earned the title “Darwin’s Bulldog” because of his aggressive support of the theory of evolution and his eagerness to debate in its defense. Huxley loved controversy as much as Darwin hated it.
He is perhaps best known for his legendary, brief verbal encounter with Samuel Wilberforce, bishop of Oxford, on June 30, 1860. The event took place at Oxford University’s New Museum library as part of the annual conference of the British Association for the Advancement of Science.
The story was told in colorful, although probably partly imaginary, detail in the October 1898 issue of
Macmillan’s Magazine
. The writer reminisced:
I was happy enough to be present on the memorable occasion at Oxford when Mr. Huxley bearded Bishop Wilberforce . . . Then the Bishop rose, and in a light scoffing tone, florid and fluent, he assured us there was nothing in the idea of evolution; rock-pigeons were what rock-pigeons had always been. Then, turning to his antagonist with a smiling insolence, he begged to know, was it through his grandfather or his grandmother that he claimed his descent from a monkey? On this Mr. Huxley slowly and deliberately arose. A slight tall figure stern and pale, very quiet and very grave, he stood before us, and spoke those tremendous words—words which no one seems sure of now,
nor I think, could remember just after they were spoken, for their meaning took away our breath, though it left us in no doubt as to what it was. He was not ashamed to have a monkey for his ancestor; but he would be ashamed to be connected with a man who used great gifts to obscure the truth. No one doubted his meaning and the effect was tremendous. One lady fainted and had to be carried out.
Even though there are many versions of the precise wording of this impromptu exchange, Huxley’s oratorical skills and the rising sentiments against the interference of men of the church with science have helped this legend grow.
Historian of science James Moore even went so far as to state, “No battle of the nineteenth century, since Waterloo, is better known.”
Huxley decided to come to the geologists’ defense in his 1869 presidential address to the Geological Society of London. First, he took advantage of the fact that Kelvin’s condemnation happened to be directed at the rather old text by Playfair to make the questionable claim
“I do not suppose that, at the present day any geologists would be found to maintain absolute Uniformitarianism.” He continued by asking rhetorically whether any geologist had ever required more than one hundred million years for the action of geology. This was really a sleight of hand, since Huxley’s own “master,” Darwin himself, wrongly estimated an age of three hundred million years for the Weald. Finally, after a few more dubious, if eloquent, assertions, Huxley pronounced his own summary that “the case against [geology and biology] has entirely broken down.”
Huxley’s address drew a furious response from one of Kelvin’s most staunch supporters: Peter Guthrie Tait. The mathematician, who never missed an opportunity for a good fight, wrote a review of Kelvin’s and Huxley’s addresses in which he wrapped insults directed at Huxley in a few polite sentences. Then, to deliver an even more punishing blow, Tait decided to cite a number for the age of the Earth that not only had no physical justification whatsoever but also was even shorter than Kelvin’s most extreme estimates:
We find that we may, with considerable probability, say that Natural Philosophy already points to a period of some ten or fifteen millions of years as all that can be allowed for the purpose of the geologist and paleontologist; and that it is not unlikely that, with better experimental data, this period may be still further reduced.
The net result of Tait’s provocative statements was an increasing sense of discontent among the geologists, who felt that in spite of their efforts to come to terms with Kelvin’s limitations, the physicists were not reciprocating by any concessions to geological evidence. These details notwithstanding, however, there was no question that, conceptually at least, Kelvin had won the battle, and the notion of a limited rather than immeasurable time for the age of the Earth had triumphed. By the end of the nineteenth century, the idea of a steady state Earth had given way to the realization that calculating the age of the Earth using physical principles was part of what geology was all about.