When Physics Met Geology

C14 dating, as mentioned in my last blog, is based on carbon. Which means, as we learnt at school, it can only be used for dating organic materials. How then are does one find the age of inorganic (non-Carbon) materials? Like the age of the earth, for example?

That makes for an interesting tale. One always associates radioactivity with terrible things like cancer, nuclear bombs and Chernobyl. And yet, it was the principle of radioactivity that helped figure out the age of the earth!

Radioactivity is the decay of one element into another, along with the emission of the catastrophic radiation. The rate of decay for different radioactive materials is fixed. That property means radioactive materials can be used like clocks. The idea was simple: uranium decays into lead. So if you found the ratio of uranium to lead in really, really old rocks, you could derive the age of the earth.

The devil lay in the execution of the idea. How do you find uncontaminated really, really old rocks? Even if you found such rocks, how could you be sure that there weren’t even older rocks that you had missed? And that’s when Clair Patterson, who was working on his dissertation project in 1948, found that it was almost impossible to find truly ancient rocks anywhere on earth. This made no sense: where could all the ancient rocks have possibly gone? Man had even managed to step into space before that question could be answered: the answer was plate tectonics. But that’s another story.

So if you couldn’t find ancient rocks on earth, it would appear that you’ve hit a dead end. It required some inspiration to move forward and this is where Patterson made a bold, and as it turned out accurate, assumption. He felt that many meteorites are left over building blocks from the time the solar system was formed. And so their age must be approximately the same age as the earth. So he decided to find meteor samples on earth (from the meteors that had hit fairly recently and thus not gotten “lost” like the ancient rocks that were part of the earth itself). And then find the uranium to lead ratio in those meteor samples. It took Patterson 7 years to find such samples, and to perform the necessary measurements.

And the number he arrived at (4.5 billion years) is the accepted number for the age of the earth even 50 years after Patterson’s calculations.

It’s a tribute to physicists that they accepted Patterson’s number because the age of the earth exceeded the (then) accepted age of the universe (2 billion years)! Talk about synergy: physics, via radioactivity, helped geologists date the earth. And geology in turn pushed physics to re-evaluate the age of the universe.

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