Earth, Our Environment - Class Notes
The Age of Earth Controversy

Often, the battle over the age of Earth is portrayed as a competition between religion and science. In fact, the interesting and important battle was waged between geologists and physicists. As Lyell was laying the foundation for geological inquiry, the successful and established physics community enjoyed a place at the uppermost echelon of science social structure.

At the time, physicists were developing and refining the theory of thermodynamics. Using their mathematical methods and several assumptions, they could construct a model of the Earth's internal heat. In 1863, Lord Kelvin (William Thomson, 1824-1907) published a calculation "demonstrating" that Earth was probably about 100 million years old, and certainly not more than 200 million years in age.

Kelvin was perhaps the most famous scientist of his day and he had great influence. However, his young age of the Earth conflicted with the observations of geologists. Soon, based on the work of Darwin (1809-1882), biologists joined the debate in favor of an Earth older than estimated by Kelvin. [Darwin was a student of geology and Lyell and during his historical voyage, he carried a copy of Lyell's book. He later dedicated one his books, "The Voyage of the Beagle", to Lyell.]

However, Kelvin had a good point - the Earth was not an infinite source of energy, and heat was observed flowing out of Earth which meant it was cooling. Geologists began to consider a finite-age Earth and soon were able to reconcile their observations with an Earth age on the order of one hundred million years.

But as geologists move closer to Kelvin's initial age estimate, Kelvin kept revising the age downward!


Kelvin's Age (Millions of years)











The arguments were fierce and "hard feelings" between the two communities formed. For some time, traditional geologists remained suspicious of mathematical models and modelers! (The bitterness is gone, but the history exists in benign forms such as jokes and general "ribbing" between different "specialties" in the geosciences).

Where Kelvin went Wrong

If we look at Kelvin's assumptions, with hindsight, we can spot the problem that eluded one of the greatest scientists of all time. Kelvin assumed:

The first assumption was correct, the third is clearly a simplification, but not a source of too much discrepancy. The second assumption is the real culprit in underestimating Earth's age.

Around the turn of the last century, Marie Curie (originally Manja Skoodowska, 1867-1934, and the winner of two Nobel Prizes, one in chemistry and one in physics) discovered radioactivity, and soon after, scientists recognized that the decay of radioactive elements would heat up the Earth. These new observations led to the modern estimates of Earth age: 4.55 billion years (Kelvin never recanted his age estimates and refused to believe that radioactivity was sufficient to heat the Earth).

What the geologists contributed to our view of the expanding applicability of the Principle of Universality is that the same laws of nature that we observe and study today, operated throughout Earth history. In the last century, astronomers and physicists have applied the principle of universality all the way back to the beginning of the Universe.


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Prepared by: Charles J. Ammon
January 1997