07. The Age of the Earth — The Man Who Also Removed the Lead

A wrong answer from a correct calculation

In the nineteenth century Lord Kelvin computed the age of the Earth by treating it as a hot body cooling from a molten start. The physics was sound, the arithmetic was careful, and he arrived at figures in the tens of millions of years, tightening over his career to around twenty million. Geologists and Darwin's biology both needed far longer, and Kelvin's authority made the disagreement a genuine crisis rather than a footnote. He was wrong because radioactivity had not been discovered, and decaying isotopes supply heat that keeps the interior warm far longer than a simple cooling model allows. Arthur Holmes applied uranium-lead decay to rocks from 1911 and published "The Age of the Earth" in 1913, pushing the figure into the billions against resistance. A rigorous calculation from an incomplete inventory of the physics produces a confident wrong answer, which is more dangerous than an admitted unknown.

Meteorites, and what the measurement cost

Clair Patterson settled it in 1956 by dating the Canyon Diablo meteorite using lead isotope ratios, on the reasoning that meteorites and the Earth formed together. He reported 4.55 ± 0.07 billion years, and the modern accepted value of about 4.54 billion years has barely moved since. Getting there required laboratories clean enough to defeat lead contamination, because ordinary air, dust, glassware and reagents carried far more lead than his samples did. Building that clean room forced Patterson to discover how much lead was in the ordinary environment, and the answer was that industrial lead — overwhelmingly from petrol additives — had raised background levels far above natural ones. He spent the rest of his career campaigning against it, opposed by the industry that funded much of the field. After leaded petrol was phased out in the United States, the CDC recorded a fall in mean blood lead levels in the population across successive national surveys.

When contamination is the finding

Patterson's contamination problem was not an obstacle to the measurement but the second discovery inside it. An experiment sensitive enough to date the planet turned out to be sensitive enough to detect the industrial poisoning of the atmosphere, and only because he refused to accept an anomalous background as noise. Treating what is ruining your measurement as data rather than nuisance is a habit with an unreasonable record of success.

Example: A microphone picking up a hum can be replaced, or the hum can be traced — and sometimes the hum is the story.

Verdict. The Earth is about 4.54 billion years old, established by Patterson in 1956, whose pursuit of the contamination threatening his measurement also produced the case against leaded petrol.

Overturned if: lead-isotope dating of meteorites is shown to rest on a false assumption of simultaneous formation with the Earth.

Sources. 1956 · Clair C. Patterson, "Age of Meteorites and the Earth," Geochimica et Cosmochimica Acta 10:230 — ADS · 1994 · CDC, "Blood Lead Levels — United States, 1988–1991," MMWR — CDC