Atomic clocks

At the TLS, Richard Fortey has a particularly lovely piece on Pascal Richet's A Natural History of Time:

Charles Darwin and T. H. Huxley and their many supporters needed plentiful time to achieve the gradual transformation of life – and their reckoning ran to hundreds of millions of years. However, Lord Kelvin, a physicist with the authority of a minor god, had other ideas. Using a more sophisticated version of Buffon’s terrestrial cooling model, by the late nineteenth century Kelvin had settled on no more than a few tens of millions of years for the age of the earth, and he didn’t budge from his conclusion. Huxley, “Darwin’s bulldog”, demanded a dozen times the span. Even estimates based on the time taken to accumulate the thickness of sedimentary rocks then known – the whole pile added together – yielded figures of an order of magnitude higher than Kelvin’s. An impasse had been reached.

The deadlock was broken by the discovery of radioactivity. The story of the age of the earth now became part of the tale of understanding the constitution of matter itself. Michael Faraday had remarked in 1866 that “to discover a new element is a very fine thing, but if you could decompose an element and tell us what it is made of – that would be a discovery indeed worth making”. The time for dissection of matter had arrived. N-rays and radiations allegedly emitted by psychics were perhaps the last of the distractions from that denouement. The production of energy from radioactive decay rewrote all the equations that Lord Kelvin had used for his estimates. The earth was a boiler, not a cooling potato. The instincts of the geologists and the palaeontologists had been correct after all; and the apparent certainties of physics had been revealed as inadequate. The earth could, after all, be very old. When it was realized that many chemical elements could exist as different isotopes, it became clear that radioactive decay converted one form of uranium into another of lead at a predictable rate. Here, at last, was the objective “clock” that had been sought since the time of Buffon. Decay of elements ticked off geological time in millions of years. Different decay routes provided a double check on any results. Margins of error tumbled as one piece of “kit” was replaced by yet another, still more sophisticated, and eventually capable of counting the very atoms themselves. Through shared technical advances, the story of the age of the earth then became entangled with the tragedy of Hiroshima.