Brilliant Blunders: From Darwin to Einstein - Colossal Mistakes by Great Scientists That Changed Our Understanding of Life and the Universe (14 page)

Perry’s challenge caused Kelvin to spend the following couple of months conducting experiments with heated basalt, marble, rock salt, and quartz. These experiments seemed to show, in agreement with new results by the Swiss geologist Robert Weber, that the conductivity either did not change much or even decreased slightly with increasing temperature. Unfortunately for Perry, Weber’s new results contradicted those of his own previous experiments—the very experiments Perry had used to support his case.
The overjoyed Kelvin published the results in
Nature
on March 7, 1895, breaking the news that “Prof. Perry and I had not to wait long . . . to learn that there was no ground for the assumption of greater conductivity of rock at higher temperatures.” Kelvin further cited a conclusion of the American geologist Clarence King, who (without considering the possibility of convection by a fluid) stated: “We have no warrant for extending the earth’s age beyond 24 millions of years.” Kelvin pronounced gleefully that he was “not led to differ much from his [King’s] estimate of 24 million years.”

Perry, however, was not convinced. Concentrating on
possible
internal conditions, rather than trying, like Kelvin, to guess what the most
probable
conditions might be, he noted that King’s conclusion was still constrained by the assumption of a solid, homogeneous Earth. In a paper that appeared in
Nature
on April 18, 1895, Perry summarized his views on the impasse: “Now it is evident that if we take any probable law of temperature of convective equilibrium at the beginning and assume that there may be greater conductivity inside than on the surface rocks, Mr. King’s ingenious test for liquidity will not bar us from almost any great age.” Perry’s logic was clear: His goal was to demonstrate that the Earth could be older than Kelvin’s estimate, even if he was unable to identify the precise flaw in Kelvin’s argument, due to uncertainties concerning the Earth’s internal structure. The measurements of the conductivity of heated rocks might have disproved one of the ways in which heat could be transported more readily at great depths, but other possibilities were still open. In particular, convection by fluidlike mass was an attractive alternative.

Perry’s intuition turned out to be visionary. He continued to maintain that the failure of Kelvin’s model to produce greater ages was a direct consequence of Kelvin’s assumption of a homogeneously conductive Earth, and that this limitation could be overcome if one allowed the Earth’s mantle to convect. It took the geologists of the twentieth century a few decades to prove Perry right. The realization that convection was possible, even within what appeared to be a rather solid mantle, played an important role in the eventual acceptance of the idea (first introduced in 1912 by the German scientist Alfred Wegener) of plate tectonics and continental drift. Not only can heat be transported by fluidlike motion but also entire continents can move horizontally over long periods of time. The precise conditions at the interface between the Earth’s inner core and the outer part continues to be a hot topic (no pun intended) of research even today.

Perry concluded his last article on the subject of the age of the Earth with an unambiguous statement:

From the three physical arguments [tidal retardation of the Earth’s spin; the cooling of the Earth; and the age of the Sun], Lord Kelvin’s higher limits are 1,000, 400, and 500 million years. I have shown that we have reasons for believing that the age, from all three, may be very considerably under estimated. It is to be observed that if we exclude everything but the arguments from mere physics, the
probable
age of life on the earth is much less than any of the above estimates; but if the palaeontologists have good reasons for demanding much greater times, I see nothing from the physicist’s point of view which denies them four times the greatest of these estimates.

Perry saw nothing wrong with 4 billion years for the Earth’s age, fairly close to today’s determination of about 4.5 billion years.

Perry’s work created the first crack in Kelvin’s seemingly unshakable calculations, by challenging the postulates that Kelvin made
concerning the Earth’s solidity and homogeneity. There was, however, another crucial hypothesis in Kelvin’s estimate of the age of the Earth: that there were no unknown internal or external energy sources that could compensate for the heat losses. Events toward the end of the nineteenth century demolished this premise too.

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.

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.