John Perry, Lord Kelvin, and radioactive decay

I’ll admit I am behind on Cosmos. I’ve watched some of the originals on Netflix, and plan on binge watching the Neil deGrasse Tyson version soon. Tyson is one of the best ambassadors of science we’ve heard from in a long time, and the clips I’ve seen are promising. It is big enough and good enough to make the anti-sciece crowd grumbly, which is a good sign. What I want to talk about today though isn’t directly from the show, but was instead inspired by a discussion I heard on the radio about a recent episode. My local NPR station has a regular feature about Cosmos, basically wrapping up the most recent episode with a physicist and a local radio host. Tonight they were talking about the age of the earth, a topic near and dear to my heart. The conversation came around to early attempts to date the earth, especially the attempts to use the cooling of the earth to determine its age. The most famous scientist to have tried this was of course Lord Kelvin, and the story of his “too-young” dates, the discovery of radioactivity, and the disapproval and ultimate validation of the geologic community is something that is repeated in most introductory courses. This was a story I’d heard, one that I’d repeated, and one that I now know is fricking wrong.

The story we often learn is that Kelvin calculated his age based on a continuously cooling body, and that his big mistake was in assuming that the earth was only losing heat. At the time we knew next to nothing of radioactivity, so it made sense that he didn’t include the heat generated from radioactive decay in his equations. We are told then, that Kelvin was incorrect primarily because he didn’t include this source of heat. This is a nice story, geologists roundly thought his dates were way too young, and serves as both a nice victory story for the geologists, and as a cautionary tale about how important it is to know the right input variables into your calculations.

I learned this, I taught this, I used it as a point of pride for geology (take that physicists!) Then in 2007, I read this paper, and I learned that I was wrongedy wrong wrong.


England, P., Molnar, P., and Richter, F., 2007, John Perry“s neglected critique of Kelvin”s age for the Earth: A missed opportunity in geodynamics: GSA Today, v. 17, no. 1, p. 4–9.

In 2007, Phil England and his super famous co-authors published a short paper in GSA Today that kind of blew my mind. They present two main points: first, a simple illustration of why the heat generated from radioactive decay is simply not sufficient to explain why Kelvin was wrong, and second, a discussion of work done by John Perry, a contemporary of Kelvin’s, that should be the awesome story we tell when we teach. I’ll let you read the paper, but in short, John Perry (Kelvin’s one-time assistant) correctly pointed out in an 1895 paper that Kelvin’s assumption of a purely conductive earth could invalidate his conclusion. It turns out that if you ignore radioactive decay and model the cooling age of the earth assuming a layered planet with a convective layer, you actually can get very close to the true age of the earth. Kelvin’s big omission wasn’t radioactivity, but convection. As England et al (2007) summarize:

The story of Kelvin and the age of the Earth is often told as a David-and-Goliath struggle, with the geologists in the role of the underdog armed only with the slender sword of geological reasoning, while Lord Kelvin bludgeoned them with the full force and prestige of mathematical physics. Kelvin’s come- uppance is often taken as evidence that simple physics ought not to be applied to geological problems, but there have been numerous occasions when simple physical models have had great explanatory power in geology. Perry’s critique of Kelvin’s calculation reminds us that even well-posed physical models can sometimes be misleading, but recognition of their flaws may lead to major advances.

I love papers like this, simple illustrations of simple concepts. Since 2007, when I teach the age of the earth, I now always teach about John Perry.

This entry was posted in earth science, geochronology, Great Contributions, teaching, Things I Wish I'd Thought of, Uncategorized. Bookmark the permalink.

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