Lord Kelvin's Blunder
From SkepticWiki
Contents |
[edit] Introduction
In the nineteenth century, Lord Kelvin calculated a maximum age of the Earth based on its temperature gradient, which was erroneous because he omitted to take account the heat produced by radioactive elements. His result, being wrong, is still of interest to Creationists today.
[edit] Kelvin's calculation
Given that Kelvin didn't know about radioactivity, his reasoning was reasonable enough. He knew that the temperature of the Earth was higher than could be accounted for by the heat supplied the Sun alone; moreover, we know from observation of deep mines that the temperature of the Earth increases the deeper one goes into the Earth's crust, at a rate of about 3°C every 100 meters.
But Kelvin knew of no source for this heat. Consequently, the only explanation that he could think of was that the Earth had started off hot and had not yet cooled down to thermal equilibrium (i.e. the point at which the temperature of the Earth could be accounted for simply by the heat supplied by the Sun.)
His next step was to make a mathematical model that took into account the thermal properties of the rocks that make up the Earth, the input of heat from the Sun, and the insulating effects of the atmosphere, to produce a model of the cooling Earth. Given such a model, he was able to calculate not only how the Earth should cool in the future, but also how hot it must have been in the past. By extrapolating in this way, he could calculate how long ago the Earth must have been so hot that it was actually molten --- which would give a maximum age for life on Earth and the formation of rocks.
His results varied between 500,000,000 years and 20,000,000 years as he tinkered with the details of his model. These results, especially the low estimates that he came to prefer, were sharply opposed to the figures preferred by geologists, who knew that such a short space of time (short by geological standards) was not nearly enough time for the processes of deposition and erosion evident in the geological record to have taken place.
[edit] Kelvin's blunder
Where Kelvin went wrong, of course, was in assuming that there was no source of heat in the Earth, and that its present temperature must be the result only of heat from the Sun and of cooling from its initial hot state. But there is a source of heat in the Earth --- the radioactive elements contained in its rocks. These elements are not terribly abundant, but then they don't need to be. When, in 1903, Pierre Currie showed that radium can heat its own weight in water from freezing to boiling in an hour, and could go on doing so day in, day out, this discovery led Ernest Rutherford to recalculate that if radium alone formed just 1 part in 2,000,000,000,000 of the composition of the Earth, this, together with the heat from the Sun, would account for the temperature of the Earth without needing to appeal to Kelvin's hypothesis of residual heat of a recently molten Earth. And radium is, of course, by no means the only radioactive element present in the Earth.
Rutherford was in some trepidation at announcing results contradicting so senior and respected a physicist as Lord Kelvin. As he recalled it later:
- I came into the room, which was half dark, and presently spotted Lord Kelvin in the audience and realized that I 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. (Ernest Rutherford, quoted in A. S. Eve, Rutherford, Cambridge University Press, 1939; emphasis in the original.)
Lord Kelvin published no further papers on the age of the Earth, and his calculations became, almost overnight, of no interest to anyone except historians of science.
We have described Kelvin's calculations as a "blunder", but, to be fair to Kelvin, when he made his calculations he didn't know about radioactivity, and so can hardly be blamed for his mistake. His model exemplifies the principle known as GIGO (Garbage In, Garbage Out): however accurate your calculations are, if you put in the wrong data you will get the wrong result.
[edit] Creationism: making the zombie walk
If creationists didn't cling to discredited arguments, they'd have none left; and the fact that this mistake is, for once, associated with a real scientist with some genuine scientific achievements gives it a particular appeal for them.
Their problem is, of course, that we know perfectly well that Lord Kelvin was wrong, and why he was wrong. The creationist solution? Fudge the figures. So in 1978 the creationists Slusher and Gamwell attempted to redo the calculations in such a way that the radioactive materials in the Earth would make no difference to the result. (See Harold S. Slusher and Thomas P. Gamwell, Age of the Earth, Institute for Creation Research Technical Monograph No. 7).
The way they achieved this was to assume, without a shred of evidence, that most of the radioactive material in the Earth is contained within the top 10 kilometers of the Earth! To put that another way, they assume that most of the Earth's radioactivity is concentrated in only 0.00025% of its volume! There is no reason to make this bizarre assumption except that it is necessary to produce the answer desired by creationists.
Their work has been severely criticized on the basis of their unsubstantiated assumptions:
- What, then, of the conclusion of Slusher and Gamwell that consideration of the Earth’s heat budget indicates that the Earth is very young? They have reached this conclusion by ignoring most of what is known about the chemistry, physics, and history of the Earth. --- G. B. Dalrymple, 1984. "How Old Is the Earth? A Reply to "Scientific Creationism"", in Proceedings of the 63rd Annual Meeting of the Pacific Division, AAAS 1, Part 3, California, AAAS. pp. 66-131 (reproduced here on the talkorigins archive)
This has not prevented Slusher and Gamwell's fellow Young-Earthers from swallowing their model hook, line, and sinker. Noted creationist propagandist Walt Brown writes:
- If the earth had initially been molten, it would have cooled to its present condition in much less than 4.6 billion years. This conclusion holds even after one makes liberal assumptions on the amount of heat generated by radioactive decay within the earth. The known temperature pattern inside the earth is only consistent with a young earth. — Walt Brown, In the Beginning (1989), p 17, emphasis added.
To assume that the 99.99975% of the Earth we can't see is completely different in abundance of radioactive elements from the small proportion we can see is not, of course, a "liberal assumption". Just as Kelvin did, Slusher and Gamwell have put garbage into their model and gotten garbage out: the difference is that they wished to produce Young-Earth garbage, whereas Lord Kelvin's mistake was honest and excusable.
