TIMELINE
Darwin was so troubled that in the third edition of On The Origin of Species he removed his Weald calculation altogether, but even this did not quiet the criticism from Kelvin and his allies. In April 1869, Darwin was moved to warn Lyell theatrically: ‘Having burned my fingers so consumedly with the Wealden, I am fearful for you ... for heaven’s sake take care of your fingers: to burn them severely, as I have done, is very unpleasant.’
‘The grandest mill’
Darwin may have been running scared, but his self-appointed bulldog, T.H. (Thomas Henry) Huxley (see pages 38–47), was ever ready to take up the gauntlet on his behalf. In his 1869 presidential address to the Geological Society of London, Huxley defended the views of the ‘old Earthers’ and pointed out the basic flaw in Kelvin’s approach: ‘Mathematics may be compared to a mill of exquisite workmanship, which grinds your stuff to any degree of fineness; but, nevertheless, what you get out depends on what you put in; and as the grandest mill in the world will not extract wheat flour from peas cods, so pages of formulae will not get a definite result out of loose data.’ In other words, Kelvin’s calculations might be unimpeachable, but if he had got his starting assumptions wrong then his conclusions would also be wrong.
The scientific world now began to gather behind the respective banners of Kelvin and Huxley. And although the physicist P.G. Tait used a new method to calculate that the Sun was around 20 million years old and the Earth only 10 million, in the tenth edition of his Principles of Geology, Lyell accepted that the age of the Earth was finite but dated the Cambrian era to around 240 million years ago. Although many scientists sought some middle ground, attempting to prove that evolutionary and geological processes might act relatively quickly, operating within Kelvin’s timescale, it was becoming increasingly obvious that both sides could not be correct; someone must be making basic errors.
The geophysicist Osmond Fisher suggested that the error was Kelvin’s, proposing a new (and prescient) model of the structure of the Earth – a thin crust over a plastic substratum – that would destroy the basic assumptions upon which Kelvin had based his calculations. Fisher further pointed out that it was a form of scientific arrogance to disregard the clear evidence of geology and biology: ‘I think we cannot but lament, that mathematical physicists seem to ignore the phenomena upon which our science founds its conclusions, and, instead of seeking for admissible hypotheses the outcome of which, when submitted to calculation, might agree with the facts of geology, they assume one which is suited to the exigencies of some powerful methods of analysis, and having obtained their result, on the strength of it bid bewildered geologists to disbelieve the evidence of their senses.’
In seeking to approach the question of the age of the Earth from the point of view of ‘proper scientists’, the mathematical physicists were actually betraying one of the cardinal rules of science: if the facts do not fit the theory, the theory must be modified or discarded, not the other way round.
The fire within
With opposition to his views mounting, Kelvin was stung to respond. In 1897, he addressed the Victoria Institute with a talk entitled ‘The age of the Earth as an abode fitted for life’. Many assumed he would modify his views or relent. In fact, he was more dogmatic and intransigent than ever, revising his uppermost estimate of the age of the Earth to 24 million years, and talking grandly of ‘certain truths’.
The Badlands of Dakota. This was once the bed of an inland sea. Layers of rock have been revealed by aeons of gradual erosion – or perhaps by a Biblical deluge?
Unfortunately for Kelvin, new discoveries were at hand that would invalidate his fundamental assumption that the Earth had started with limited heat energy and lost heat ever since. Radioactivity had been discovered in 1896, and in 1903 the French chemist Pierre Curie realized the potential geological significance of heat generation by radioactive elements in rocks. The following year, the greatest of the new generation of physicists, Ernest Rutherford, lectured at the Royal Institution of Great Britain on the topic of radium and of radioactive elements as a source of heat energy. Noticing Lord Kelvin in the audience, he realized he was ‘in for trouble at the last part of my speech dealing with the age of the Earth, where my views conflicted with his. To my relief, Kelvin fell fast asleep, but as I came to the important point, I saw the old bird sit up, open an eye and cock a baleful glance at me! Then a sudden inspiration came, and I said Lord Kelvin had limited the age of the Earth provided no new source of heat was discovered. That prophetic utterance refers to what we are now considering tonight, radium! Behold, the old boy beamed upon me.’
In practice, Kelvin continued to deny that radioactivity had rewritten the rules of the debate. However, by the time he died in 1907, radioisotope dating was already being used to make direct measurements of the age of rocks, with samples dated at up to 2.2 billion years old. By 1931, the geologist Arthur Holmes was able to assure a US National Research Council meeting that ‘the age of the Earth exceeds 1,460 million years [and] is probably not less than 1,600 million years’. Modern dating techniques reliably prove that the Earth is around 4.55 billion years old (see box below).
‘As Lord Kelvin is the highest authority in science now living, I think we must yield to him and accept his views.’
MARK TWAIN, LETTERS FROM THE EARTH, 1909
EVIDENCE FOR THE AGE OF THE EARTH
How do we know how old the Earth is?
There are three primary methods, all of which measure the age of rock by comparing the ratios of isotopes they contain (radiometric dating). The oldest rocks so far discovered on Earth are around 3.9 billion years old, and some include minerals that are even older (around 4.2 billion years old). This puts a lower limit on the age of the Earth, but not an upper one, since none of the original surface of the Earth still exists, thanks to its molten nature and subsequent processes of erosion and crustal recycling.
More direct means of calculating the age of the Earth are based on the assumption that all the rocky material in the solar system formed at the same time, and from the same pool of material (as a giant disc of dust and gas coalesced into solid matter). Different isotopes of uranium decay into different isotopes of lead and, by measuring the ratios of these isotopes in Earth rock and meteorites, it is possible to plot a graph of the values and calculate from it the amount of time that has elapsed since the original pool of matter became separated into discrete objects. This method, known as lead isochron dating, gives a figure of around 4.55 billion years, as does the other direct method, which is radiometric dating of meteorites (asteroids that have fallen to Earth). Unlike the Earth, asteroids do not undergo geological processes and therefore may date back to the formation of the Solar System. Around 100 meteorites have been dated, and the ages obtained are almost all around 4.5 billion years.
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WEGENER