In the spring of 1896, the French physicist Henri Becquerele discovered that the decay of unstable atomic nuclei is accompanied by spontaneous emission of particles and radiation. The phenomenon became known as radioactivity. Seven years later, physicists Pierre Curie and Albert Laborde communicated that the decay of radium salts provided a previously unknown source of heat. It took the amateur astronomer William E. Wilson less than four months from the Curie and Laborde announcement to come up with the speculation that this property of radium “may possibly afford a clue to the source of energy in the sun and stars.” Wilson estimated that just “3.6 grams of radium per cubic meter of the sun’s volume would supply the entire output.” While Wilson’s extremely short note to Nature received relatively little attention from the scientific community, the potential implications of an unanticipated source of energy did not escape George Darwin. This mathematical physicist, who ceaselessly looked for ways to free geology from the straitjacket imposed by Kelvin’s chronology, declared emphatically in September 1903: “The amount of energy available [in radioactive materials] is so great as to render it impossible to say how long the sun’s heat has already existed, or how long it will last in the future.” The Irish physicist and geologist John Joly embraced this pronouncement enthusiastically and immediately applied it to the problem of the age of the Earth. In a letter to Nature published on October 1, Joly pointed out that “a source of supply of heat [the radioactive minerals] in every element of material” would be equivalent to an increased transfer of heat from the Earth’s interior. This was precisely what Perry had shown was needed in order to increase the age estimates. Put differently, in Kelvin’s scenario, the Earth was merely losing heat from its original reservoir. The discovery of a new source of internal heat seemed to undermine the entire basis for this scheme.

One of the key figures in the ensuing frantic research on radioactivity was the young New Zealand–born physicist Ernest Rutherford, who later became known as the “father of nuclear physics.” At the time, Rutherford was working at McGill University in Montreal (he later moved to the United Kingdom), where he concluded on the basis of scores of experiments that the atoms of all of the radioactive elements contained enormous amounts of latent energy that could be released as heat. One journal welcomed the announcement by Rutherford that the Earth would survive much longer than Kelvin had estimated with the headline: “DOOMSDAY POSTPONED.”

On his part, Kelvin showed great interest in the discoveries concerning radium and radioactivity, but he remained unconvinced that these would alter his age estimates. Refusing to admit, at least initially, that the source of energy of the radioactive elements could come from within, he wrote, “I venture to suggest that somehow ethereal waves may supply the energy to the radium while it is giving out heat to the ponderable matter around it.” In other words, Kelvin proposed that the atoms simply collect energy from the ether (ether was supposed to permeate all space), only to release it back upon their decay. In 1904, however, with considerable intellectual courage, he abandoned this idea at the British Association meeting, although he never published a retraction in print. Unfortunately, for some unclear reason, he again lost touch with the rest of the physics community in 1906 when he rejected the notion that radioactive decay transmuted one element into another, even though Rutherford and others had accumulated solid experimental evidence for this phenomenon. Throughout this period, Rutherford’s one-time collaborator Frederick Soddy lost his patience. In an acerbic exchange with Kelvin in the pages of the London Times, he declared disrespectfully, “It would be a pity if the public were misled into supposing that those who have not worked with radioactive bodies [alluding to Kelvin] are as entitled to as weighty an opinion as those who have.” Even before that altercation, in a book he had published in 1904, Soddy did not hesitate to firmly assert that “the limitations with respect to the past and future history of the universe have been enormously extended.”

Rutherford was a little more generous. Many years later, he told and retold an anecdote related to a lecture on radioactivity that he had given in 1904 at the Royal Institution:

I came into the room, which was half dark, and presently spotted Lord Kelvin in the audience and realized that I was in for trouble at the last part of the speech dealing with the age of the Earth, where my views conflicted with his. To my relief he fell fast asleep but as I came to the important point, I saw the old bird sit up, open an eye and cock a baleful glance at me! Then sudden inspiration came, and I said Lord Kelvin had limited the age of the Earth, provided no new source of heat was discovered. That prophetic utterance refers to what we are now considering tonight, radium! Behold! The old boy beamed at me.