Today [17th December 2007] marks the 100th anniversary of the death of the brilliant physicist and engineer William Thomson (a.k.a. Lord Kelvin).
As one of the great minds behind the Laws of Thermodynamics—the second of which is almost certainly the most important law in the whole of science—Thomson knew a thing or two about temperature, devising the absolute temperature scale which still bears his chosen aristocratic name.
It was Thomson's expertise in thermodynamics which led to his crossing swords with supporters of Charles Darwin. In 1863, he published a paper entitled On the Secular Cooling of the Earth in which he calculated the age of the earth at 98 million years (later revised downward). He did this by working backwards from the earth's current temperature, assuming that it had cooled according to the Laws of Thermodynamics. The amount of time that the earth would have been hospitable to life would have been considerably less than this.
Darwin knew that this was a serious, potentially fatal, objection to his theory: life on earth as we know it needed much more time to have evolved through Natural Selection. Darwin and his supporters expended much effort trying to overcome Thomson's result, but Darwin went to his grave having never done so satisfactorily. This is hardly surprising as Thomson's calculations were pretty sound, according to the known laws of science at the time.
But Thomson's calculation made a number of assumptions which later turned out to be incorrect. Perhaps the most significant of these was his stated assumption that there were no unknown sources of energy providing additional heat to the earth. Thomson reckoned without radioactivity.
It wasn't until 1904 that, in one of the great dramatic moments in the history of science, Ernest Rutherford announced to the Royal Institution that a new source of energy had been found. The announcement was dramatic because the elderly Thomson (now Lord Kelvin) was sitting in the audience. As Rutherford later recounted:
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 balefule 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 energy) was discovered. That prophetic utterance refers to what we are now considering tonight, radium! Behold! the old boy beamed upon me.
Although Kelvin never publicly acknowledged that radioactivity was a missing factor in his calculation of the age of the earth, he is said to have acknowledged in privately to his colleague and (sur)namesake J.J. Thomson. If so, Kelvin must have gone to his own grave a few feet from Darwin's in Westminster Abbey knowing that the other grand old boy of science had been vindicated after all.