Eventually, radiometric dating became one of the most reliable techniques to determine the ages of minerals, rocks, and other geological features, including the Earth itself. Generally, a radioactive element decays into another radioactive element at a rate determined by its half-life: the period of time it takes for the initial amount of radioactive material to decrease by half. The decay series continues until it reaches a stable element. By measuring and comparing the relative abundances of naturally occurring radioactive isotopes and all of their decay products, and coupling those data with the known half-lives, geologists have been able to determine the Earth’s age to high precision. Rutherford was one of the pioneers of this technique, as the following story documents: Rutherford was walking on campus with a small black rock in his hand, when he met his Canadian geologist colleague Frank Dawson Adams. “Adams,” he asked, “how old is the earth supposed to be?” Adams answered that several methods had given an estimate of one hundred million years. Rutherford then commented quietly, “I know that this piece of pitchblende [a mineral that is a source of uranium] is seven hundred million years old.”

Most if not all descriptions of the age-of-the-Earth controversy would have you believe that Kelvin’s dramatically wrong age estimate was a direct consequence of the fact that he ignored radioactivity. If this were the whole truth, Kelvin’s error would not have qualified as a blunder in my book, since Kelvin could not have considered a previously undiscovered source of energy. However, it is actually mistaken to attribute the erroneous age determination entirely to radioactivity. It is true that radioactive decays within the entire volume of the Earth’s mantle (down to a depth of about 1,800 miles) do indeed produce heat at a rate that is roughly equal to half the rate of heat flow through the planet. But not all of this heat can be tapped readily. A careful examination of the problem reveals that, given Kelvin’s assumptions, had he even included radioactive heating, he really should have considered only the heat generated inside the Earth’s outer 60-mile-deep skin. The reason is that Kelvin showed that only heat from such depths could be effectively mined by conduction in about one hundred million years. Geologists Philip England, Peter Molnar, and Frank Richter demonstrated in 2007 that when this fact is taken into account, the inclusion of radioactive heat deposition would not have altered Kelvin’s estimate for the age of the Earth in any significant way. Kelvin’s most serious blunder was not in being unaware of radioactivity (even though, once discovered, ignoring it was certainly not justified), but in initially ignoring and later objecting to the possibility raised by Perry of convection within the Earth’s mantle. This was the true source of the unacceptably low age estimate.

How could a man of such intellectual powers as Kelvin be so sure that he was right even when he was dead wrong? Like all humans, Kelvin still had to use the hardware between his ears—his brain—and the brain has limitations, even when it belongs to a genius.

Since we can neither interview Kelvin nor image areas of his functioning brain, we will never know for sure the precise reasons for his misguided stubbornness. We do know, of course, that people who have spent much of their working lives defending certain propositions do not like to admit that they were wrong. But shouldn’t have Kelvin, the great scientist that he was, been different? Isn’t changing one’s theories based on new experimental evidence part of what science is all about? Fortunately, modern psychology and neuroscience are beginning to shed some light on what has been termed the “feeling of knowing,” which almost certainly shaped some of Kelvin’s thinking.

I should first note that in his approach to science and crusade for knowledge, Kelvin was more akin to an engineer than to a philosopher. Being on one hand an effective mathematical physicist, and on the other, a gifted experimentalist, he always sought a premise with which he could calculate or measure something rather than an opportunity to contemplate different possibilities. At the very basic level, therefore, Kelvin’s blunder was a consequence of his belief that he could always determine what was probable, not realizing the ever-present danger of overlooking some possibilities.