Yes, But How Old Is the Earth and How Will It Die? Darwin vs. Kelvin


“Are we entitled to say that Earth’s age is 4.55 billion years, and its trajectory an ellipse centred on the Sun, with an average radius of 150 million kilometres?” writes Hubert Krivine in the introduction to his important new work of scientific history, The Earth: From Myths to Knowledge. It’s a better question than you might think. Creationism, whether we like it or not, makes recourse to “proof” and its own historical lineage — it has fronted itself with the awning of “logic.” And from the other side, scientific inquiry is often reduced to what it produces, or how it applies. “As for the general public,” Krivine writes, “they know science only through its applications, the worst as well as the best, which is why the euphoria that it generated in the nineteenth century has given way today to scepticism, at least in the rich countries.”

So our conception of the Earth, if we stop to think about it, is under attack from two sides. That’s why Krivine’s new book should be read, distributed, and defended: it outlines — allowing only for the uncertainties of science inquiry — how we know what we know about the earth. In the process, too, it uncovers strange debates in the history of science, like the one between Charles Darwin and Lord William Thompson Kelvin over the age of the earth, captured below. And once that debate is settled, if just for the moment, another question quickly arises: if we know the age of the earth, can we also predict its death? In this exclusive excerpt, Krivine examines both Darwin and Kelvin’s lines of thought.

Darwin versus Kelvin

Charles Robert Darwin (1809–82), founder of the modern theory of the evolution of species, was also highly influenced by Thomas Malthus (1766–1834). We have seen how Darwin, working as a geologist, had recognized an age for Earth of at least 300 million years in the wake of his study of the erosion of the hills of the Weald. The same Darwin, as palaeontologist, despite accepting this ill-founded estimate, would none the less require more time, but without giving figures. The expression ‘a vast amount of time’ often recurs in his letters, as well as in his major work The Evolution of Species. The geological strata several kilometres thick had needed immense time to sediment. It was necessary then to explain:

why we do not find [in the geological record] closely graduated varieties between the allied species which lived at its commencement and at its close … Although each formation may mark a very long lapse of years, each perhaps is short compared with the period requisite to change one species into another.

This is the famous problem of ‘missing links’, which led Darwin to reject Kelvin’s figures for Earth’s age, despite being unable to propose any definitive alternative. His argument can be summed up as follows: since the geological strata do not generally contain intermediate links, and since such links must nevertheless exist, the times of sedimentation of these geological strata, already colossal – certainly tens of millions of years, if not hundreds – are small in comparison with the time needed to account for the actual evolution of flora and fauna. In other words, 50 million years is neither here nor there in relation to geological time. If we add that exceptional conditions are needed to ensure a lasting fossilization, it is understandable why the traces left in geological history the beginning of the modern age are not significant: they represent only a negligible fraction of the past. The chance discovery in 1861 of a link between dinosaur and bird, the archaeopteryx, possessing both feathers and teeth, was a great support for Darwin. Similarly, in the lineage of our distant ancestors, only one Lucy has been found (in 1974) and one Toumaï man (in 2001). For the moment, we have only two individuals, a derisory representative of populations that must have counted several hundred thousand individuals (no one knows exactly how many) over millions of years.

His fingers burned by his study of the Weald, Darwin refrained from giving figures, but he viewed Kelvin’s range as too short to be credible. As a theorist of biological evolution he suspected a mistake in the great physicist’s procedure, but where exactly? He suggested that the Sun might be able to transfer magnetic energy which heated Earth. This was not a good hypothesis, but the idea of a different source of heat was formulated; Kelvin should not be left a monopoly on physics, a man who suspected geologists and other biologists of not accepting the universality of its laws.

Kelvin’s authority at this time was so great, however, that his estimate of Earth’s age was accepted. The majority of geologists, whose culture tended to be uniformitarian, bent themselves into all kinds of contortions so as to compress their estimates, with the justification that a hotter Earth would have made possible a more rapid evolution of chemical reactions and material actions.

This antagonism between Darwin and Kelvin was abundantly used by Darwin’s adversaries. It was not so much Darwin’s datings that they were concerned to ruin (those of Kelvin, though shorter, were no more compatible with a literal reading of the Bible), but rather his theory of evolution, which was viewed as an abomination, supposedly having man descend from the ape. It reduced the story of Adam and Eve, as well as original sin, to the status of myth. Russell gleefully cites various quotations from Anglican bishops attacking Darwin. But the situation within the institution was rather more subtle. A crisis was already brewing in the Church of England before Darwin came along, with less literalist tendencies (such as the ‘broad church’) already developed. Though Galileo had no right to a tomb, or Bruno to a statue, Darwin had the exceptional honour of a burial alongside Newton in Westminster Abbey.

One might have hoped that this polemic from over a century ago would today be closed, as even Pope John-Paul II declared to the Pontifical Academy of Sciences in 1996 that Darwin’s theory was ‘more than an hypothesis’. Yet this is not the case. The weight of creationists in the United States is such that George Bush, in the course of his electoral campaign, declared, according to the New York Times of 22 October 2000: ‘I think that, for example, on the issue of evolution the verdict is still out on how God created the Earth.’ No further comment is needed.

One might say that Kelvin was the predecessor of today’s ‘intelligent design’, the short timescale he defended being insufficient for the development of life without a preconceived (divine?) plan. But that would be an anachronism: the world of the mid nineteenth century was not that of the twenty-first!

The Death of Earth

Once the concept of Earth’s origin is posited, it is quite coherent to predict its death. If we leave aside the Apocalypse and the Last Judgement, there are several scientific hypotheses.

Thermic death. This idea derives from the findings of nineteenthcentury physics. Earth is inexorably cooling as a function of the laws of thermodynamics. The planet will not die, but its inhabitants will perish from cold.

The Sun as red giant. In the Sun’s cycle of thermonuclear combustion, a moment will come (in 4 billion years) when it will undergo a tremendous explosion, expanding to beyond the size of Earth’s orbit. Earth’s inhabitants will be burned to death.

Change in Earth’s orbit (chaos). On everyday timescales of hundreds of thousands of years, Earth’s movement around the Sun is the very symbol of regularity; it was even used for a long while to define the legal unit of time. But on timescales of a billion years or more, the theory of chaos teaches us that the existence of other planets and the Moon makes prediction hazardous.

Collision with a heavenly body. Though highly improbable on a short timescale, this becomes likely in the long term. We should not forget that the Moon was born in the same way more than 4 billion years ago, from a collision between Earth and an object the size of Mars, which has even been given a name: Theïa.