# My Scientific American article on Kelvin and Darwin

In lieu of a post, I will refer you to an article of mine that was just published in Scientific American magazine titled When Lord Kelvin Nearly Killed Darwin’s Theory. It deals with an an interesting historical period in the second half of the 19th century that pitted two scientific giants against each other in which the age of the Earth was the key factor in determining the final outcome.

Enjoy! And let me know in the comments what you think.

1. Rob Grigjanis says

I don’t have handy references, but I remember reading that Kelvin agreed that Perry’s calculations could vastly increase the age of the Earth. However, I think Kelvin was bound to lower numbers by his treatment of the sun’s age. It wasn’t until some years after Kelvin’s death that nuclear fusion was proposed as a possible source of stellar energy.

2. Was this the debate about how the Sun’s mass could only produce a certain amount of energy through endothermic chemical reactions, therefore if the sun was still “burning” then it must be equal to or less than a certain age, which was much less than the age that geology and other studies were beginning to lead us to suspect?

IIRC, this fight was later settled by the discovery of nuclear reactions and radioactivity, which provided a previously unknown source of energy which converted much more mass into energy and thus allowed the finite mass of the sun to provide the same level of energy output over a vastly longer period of time.

It’s interesting to frame this as a Kelvin/Darwin fight. I’m not at all familiar with the personalities involved in the argument, but I do remember that the debate was settled late enough that both would be dead. If I get the chance, I’ll be happy to read the article.

3. Reginald Selkirk says

Their work was first presented in a joint paper in 1858.

I disagree with the characterization of it as a “joint paper” since it was not a work co-authored by the two. Rather, two selections by Darwin were read, including a portion of an 1857 letter to Asa Gray, and the paper Wallace had sent to Darwin was read. This was at the meeting of the Linnean Society, with the works presented by Lyell and Hooker.

4. Reginald Selkirk says

It was the discovery of radioactivity that decisively changed the picture; it led to an entirely new way of measuring the age of the Earth, by allowing scientists to calculate the ages of rocks.

It certainly was radioactivity that tipped the scales, but I think your description of how that transpired is a bit lacking. Kelvin’s calculations were based on heat flow and the temperature of the Earth. Radioactivity within the Earth provided a new source of heat that burned his calculations to a crisp. That radioactive dating of rocks could provide independent dates was a double whammy.

5. Rob Grigjanis says

CD @2: Kelvin proposed gravitational, not chemical, energy as the source of the sun’s heat. But he wasn’t particularly dogmatic about it. My bolding;

It seems, therefore, on the whole most probable that the sun has not illuminated the earth for 100,000,000 years, and almost certain that he has not done so for 500,000,000 years. As for the future, we may say, with equal certainty, that inhabitants of the earth can not continue to enjoy the light and heat essential to their life for many million years longer unless sources now unknown to us are prepared in the great storehouse of creation.

https://zapatopi.net/kelvin/papers/on_the_age_of_the_suns_heat.html

6. Reginald Selkirk says

@2 It’s interesting to frame this as a Kelvin/Darwin fight. I’m not at all familiar with the personalities involved in the argument, but I do remember that the debate was settled late enough that both would be dead.

True enough. Kelvin died in 1907. The suggestion that radioactive decay provided a source of heat was suggested in 1903 by George Darwin and John Joly. The first calculations of the age of the Earth based on radioactive dating were by Boltwood and Rutherford in the 1905-1907 time frame.
Wikipedia

7. mnb0 says

8. Holms says

“Meh, I already knew this” -- valuable comment, mnbo. So did many of us, but without the weird brag.

9. consciousness razor says

Was this the debate about how the Sun’s mass could only produce a certain amount of energy through endothermic chemical reactions, therefore if the sun was still “burning” then it must be equal to or less than a certain age, which was much less than the age that geology and other studies were beginning to lead us to suspect?

It wasn’t just about the Sun. Kelvin was also trying to calculate how long it may have taken for Earth to cool after it initially formed in a hot molten state. That did also rely on an assumption that there wasn’t another source of energy (as it did for the Sun and fusion). It was not known that then that radioactive decay was another such source. But apparently, that only makes a relatively small difference in the end — not enough to get geologists (and Darwin) the amount of time they needed.

So more importantly, Perry showed that convection allowed for a much older Earth, and Kelvin wasn’t properly accounting for that. That more or less did the job. And I suppose if the Sun’s age had turned out to be less than Earth’s, you could imagine some other kind of scenario in which they didn’t form together, since it’s not really necessary that they’re the same age. (But it is nicer that way I guess.)

10. Owlmirror says

@Reginald Selkirk:

It certainly was radioactivity that tipped the scales, but I think your description of how that transpired is a bit lacking. Kelvin’s calculations were based on heat flow and the temperature of the Earth. Radioactivity within the Earth provided a new source of heat that burned his calculations to a crisp.

No. Mano read the paper I linked for him, which shows that radioactivity alone doesn’t contribute that much heat to the Earth. Differential cooling does allow for billions of years for primordial heat to cool. Once again:

We now know that the crust does not contain enough radioactive heat to explain the surface heat flux; nevertheless, it is still frequently stated that, because the discovery of radioactive heat undermined an assumption behind Kelvin’s calculation, it also undermined his conclusion. This statement is logically incorrect; Kelvin’s conclusion would be undermined by that discovery only if incorporation of the Earth’s radioactive heat into his calculation produced a substantially different age for the Earth.

[ . . . ]

even if Kelvin had included radioactive heat in his calculation—his estimate of the age of the Earth would have been unaffected (Richter, 1986). Thus, the discovery of radioactivity did not invalidate Kelvin’s calculation for the age of the Earth. In a rigid Earth, with or without radioactivity, heat is delivered to the surface by conduction through a shallow layer, which can maintain a rate of heat loss comparable to today’s for only a small fraction of what we now know to be the Earth’s age.

(These paragraphs actually come earlier in the paper than the above)

Instead of focusing on Kelvin’s calculations, Perry suggested, one should examine his assumptions. In Kelvin’s model, the present supply of heat to the Earth’s surface is derived from the cooling of a shallow outer layer of thickness, $\sqrt{\pi \kappa t}$ (Equation 3). If, however, the thermal conductivity inside the Earth were much higher than at the surface, then the deep interior would also cool, providing a large store of energy to maintain the surface heat flux. In that case, Kelvin’s estimate of the age of the Earth would be too low, potentially by a large multiple.

Perry had two reasons for postulating a higher conductivity in the interior. First, experimental evidence showed an increase, if modest, in conductivity of rocks with temperature; in addition, the Earth’s increase in density with depth implies a greater proportion of iron and other materials that conduct heat better than do silicates. More radically, he argued (Perry, 1895a) that convection in the fluid, or partly fluid, interior of the Earth would transfer heat much more effectively than would conduction: “… much internal fluidity would practically mean infinite conductivity for our purpose.”

The above quotations are all from:
England, P.; Molnar, P.; Righter, F. (January 2007). “John Perry’s neglected critique of Kelvin’s age for the Earth: A missed opportunity in geodynamics”. GSA Today. 17 (1): 4–9. doi:10.1130/GSAT01701A.1. (PDF)

This is a less technical explanation of the topic:

Philip C. England , Peter Molnar and Frank M. Richter. “Kelvin, Perry and the Age of the Earth”. American Scientist. Vol. 95, No. 4 (JULY-AUGUST 2007), pp. 342-349 (PDF)

11. Mano Singham says

Crip Dyke @#2 and Reginald @#4:

While radioactivity did provide a new and hitherto unknown source of energy, it had not been shown that there was enough radioactive material in the Earth’s crust to provide sufficient heat to significantly change Kelvin’s calculations. Inhomogeneity and convective flow were more significant in throwing his numbers into doubt.

Here is a passage from a paper on the subject:

“We now know that the crust does not contain enough radioactive heat to explain the surface heat flux; nevertheless, it is still frequently stated that, because the discovery of radioactive heat undermined an assumption behind Kelvin’s calculation, it also undermined his conclusion. This statement is logically incorrect; Kelvin’s conclusion would be undermined by that discovery only if incorporation of the Earth’s radioactive heat into his calculation produced a substantially different age for the Earth.”

(England, Philip, Peter Molnar, and Frank Richter. 2007. “John Perry’s Neglected Critique of Kelvin’s Age for the Earth: A Missed Opportunity in Geodynamics.” GSA Today 17, no. 1, 4–9. https://doi.org/10.1130/GSAT01701A.1.)

[p.s. My response overlapped with Owlmirror’s @#10. They were the one who sent me that link and for which I am grateful because it enabled me to include Perry’s work in my book and the article.]

12. John Morales says

Excellent article for a general audience, IMO.

I rather like the understated dig at religious recalcitrance, too.

13. Rob Grigjanis says

John @12:

I rather like the understated dig at religious recalcitrance, too

What ‘dig’ are you referring to? I must have missed it.

14. John Morales says

The radical change in the status of human beings implied by their model, from being specially created in the image of God to just another accidental byproduct of the evolutionary process just like all other species, was an immediate source of controversy because it challenged a key religious tenet that human beings were special. This was why natural selection aroused such opposition even while evolution was accepted. Many scientists of that time were religious and believed in theistic evolution that said that a supernatural agency was guiding the process to produce the desired ends.

15. Rob Grigjanis says

Ha, I did miss that paragraph.

16. rs says

Enjoyed reading the article. Hope to see more interesting articles like this on the history of science.