Hello there. If you haven't read Episodes 1 to 3, head on over there and read them, for the rest of this series is a continuation of that grand story. Episode 1 is here.
Isaac Newton followed on from Rene Descartes with his Principia which revolutionised physics. Edmond Halley thought he could calculate the age of the Earth from the amount of salt in the sea, but realised he would need to sample every 100 years. Georges Buffon heated and cooled steel balls to get an idea of Earth's rate of cooling. Lord Kelvin huffed and puffed and said that Earth could not be older than 24 million years. Arthur Holmes pushed that date back to 3 billion years, and it was Clair Patterson who nailed down to 4.55 billion years. Read on for the full story.
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Newton then swept away Descartes’ imprecise theories with his Principia – the laws governing the motions of the heavenly bodies were now defined in minute detail. However he seems to have wasted his intellect and the latter years of his life trying to tie the history of ancient kingdoms and biblical chronology into the record of astronomical events – eclipses and the arrival of comets – but with extremely limited success.
His friend Edmond Halley then came up with the theory that the age of the Earth could be estimated by the amount of salt in the sea. By measuring the rate of increase of the sea’s salinity, and assuming that salt had been washed into the oceans at a constant rate, it would be possible to work backward to a time when the seas contained no salt at all, which would then give the age of the Earth.
Clever, hard working, and somewhat eccentric
In 1715, this was a fantastic idea, but no one, least of all Edmond Halley, knew how to measure increases in salinity year on year, and he concluded that the observations would need to be made a century apart to be meaningful. However Methuselah, in spite of his famed longevity, was no longer alive, so this was impossibility.
Georges Louis Leclerc Buffon was the next to estimate the age of the Earth. He was well connected with the French court and had the ear of several of the crowned heads of Europe. Clever, hard working, and somewhat eccentric, he proposed the idea that the Earth’s age could be estimated by the rate at which it cooled.
He recanted his heresies
He heated up metal spheres and then estimated the rate of heat loss as they cooled, and then extrapolated this to the larger Earth, settling on the age of 75 000 to 168 000 years. This was a quantum leap from the 6000 year old theory and he nearly got excommunicated for his troubles. Being a politically astute animal, he recanted his heresies but then proceeded to repeat his assertions for the rest of his days.
This date proved contentious
Charles Darwin, apparently unhappy with creating controversy, but brave enough to do so anyway, suggested that the Earth was 300 million years old. This date proved so contentious that he actually withdrew it from his Origin of the Species. The great Lord Kelvin, who towered over Victorian science like a colossus, took Darwin to task for some of his assertions, and declared that the Earth could be no older than 24 million years, which were downward revisions of an original figure of 400 million.
Thanks to the pioneering work by Marie and Pierre Curie
However, in spite of Kelvin’s phenomenal intellect, he was not aware of the invisible heat source deep with the Earth – radioactivity – which drives the engine of our planet. Radioactivity is a concept to which we have become accustomed to thanks to the popular press, nuclear power and its use in the medical field. But in Kelvin’s day it was still unknown, and it was thanks to the pioneering work by Marie and Pierre Curie that radioactive material was first recognised and isolated.
The properties of the atom and the elements became increasingly well understood
Earnest Rutherford then provided the world with an explanation of how it all worked, and of the energies released in the process – which then provided a mechanism for keeping the interior of the Earth at elevated temperatures. Other great names of nuclear physics come from this time – Bohr, Heisenberg, Thompson, Chadwick and of course Einstein. The properties of the atom and the elements became increasingly well understood as well as the chemistry and isotopes of radioactive elements.
Atoms decay at a rate predictable enough to allow them to be used as clocks
This understanding of radioactive half lives became fundamental to the dating of rocks, Professor Arthur Holmes of Durham University took up the cudgels and pioneered the methods which are used to this day. The technique was based on Rutherford’s 1904 work, when he discovered that atoms decay from one element into another at a rate predictable enough to allow them to be used as clocks.
Holmes measured the rate of decay of uranium to lead, and used this to calculate the age of the Earth. In spite of a lack of funds and sophisticated equipment, he announced in 1946 that the Earth was at least 3 billion years old. His methods met with praise, but his date was not. But he was far closer to the mark than Kelvin had been
It would be easy to find the age of the Earth
Building on Holmes' pioneering work, Clair Patterson of the University of Chicago took up the challenge. The problem with dating the Earth is that one needs rocks almost that old, and these are rare thanks to the ongoing recycling of crust due to plate tectonics. In addition one needs uranium-bearing crystals within these rocks. Patterson made the assumption that if all the lead on Earth came from radioactive decay, then it would be easy to find the age of the Earth. The more lead a rock contained, the older it had to be.
And you’ll be famous, because you have will have measured the age of the Earth
In reality all the lead on Earth did not come from uranium and it was impossible to separate out the ‘primordial’ lead – that which had been around from the creation of the Earth - from that derived from radioactive decay. To sidestep this thorny issue, another brilliant assumption was made, that meteorites were left-over building material dating back to the early days of the solar system, and thus their lead content would be the same as that of the early Earth. Perhaps more importantly they contained no uranium to upset the clock. None of the lead in meteorites then would have come from radioactive decay. Harrison Brown, Patterson’s doctoral supervisor said this to him:
“Pat, you just go in and get an iron meteorite – I’ll get it for you. We’ll get the lead out of the iron meteorite. You measure its isotopic composition and you stick it into the equation. And you’ll be famous, because you have will have measured the age of the Earth.”
Dated the Earth at 4550 million years
It took Patterson 7 years to build a completely lead free laboratory and to obtain a result, which dated the Earth at 4550 million years. After three hundred years we had at last a date for the beginning of the World. And that figure still stands to this day.
The End
