Neuroscientists of the past two decades have developed sophisticated imaging techniques that allow them to see in detail which parts of the human brain light up in response to pleasing tastes, music, sex, or winning at gambling. The most commonly used techniques are positron-emission tomography (PET) scans, in which radioactive tracers are injected and then followed in the brain, and functional MRI (fMRI), which monitors the flow of blood to active neurons. Studies showed that an important part of the reward circuitry is a collection of nerve cells that originate near the base of the brain (in an area known as the ventral tegmental area, or VTA) and communicate to the nucleus accumbens—an area beneath the frontal cortex. The VTA neurons communicate with the nucleus accumbens neurons by dispatching a particular chemical neurotransmitter called dopamine. Other brain areas provide the emotional contents and relate the experience to memories, and to the triggering of responses. The hippocampus, for instance, effectively “takes notes,” while the amygdala “grades” the pleasure involved.

So how does all of this relate to intellectual endeavors? To embark on, and persist in, some relatively long-term thought process, the brain needs at least some promise of pleasure along the way. Whether it is the Nobel Prize, the envy of neighbors, a salary raise, or the mere satisfaction of completing a Sudoku puzzle labeled “evil,” the nucleus accumbens of our brain needs some dose of reward to keep going. However, if the brain derives frequent rewards over an extended period of time, then just as in the case of those self-starving rats, or with people who are addicted to drugs, the neural pathways connecting the mental activity to the feeling of accomplishment become gradually adapted. In the case of drug addicts, they need more drugs to get the same effect. For intellectual activities, this may result in an enhanced need for being right all the time and, concomitantly, in an increasing difficulty to admit errors. Neuroscientist and author Robert Burton suggested specifically that the insistence upon being right might have physiological similarities to other addictions. If true, then Kelvin would no doubt match the profile of an addict to the sensation of being certain. Almost a half century of what he surely regarded as victorious battles with the geologists would have strengthened his convictions to the point where those neural links could not be dissolved. Irrespective, however, of whether the sensation of being certain is addictive or not, fMRI studies have shown that what is known as motivated reasoning—when the brain converges on judgments that maximize positive affect states associated with attaining motives—is not associated with neural activity linked to cold reasoning tasks. In other words, motivated reasoning is regulated by emotions, not by dispassionate analysis, and its goal is to minimize threats to the self. It is not inconceivable that late in life, Kelvin’s “emotional mind” occasionally swamped his “rational mind.”

You may recall that earlier I referred to Kelvin’s calculation of the age of the Sun. I do not consider his estimate to be a blunder. How is that possible? After all, his estimate of less than one hundred million years was wrong by as much as his value for the age of the Earth.

In an article on the age of the Earth written in 1893, three years before the discovery of radioactivity, the American geologist Clarence King wrote, “The concordance of results between the ages of the sun and earth certainly strengthens the physical case and throws the burden of proof upon those who hold to the vaguely vast age derived from sedimentary geology.” King’s point was well taken. As long as the age of the Sun was estimated to be only a few tens of millions of years, any age estimates based on sedimentation would have been constrained, since for sedimentation to occur, the Earth had to be warmed by the Sun.

Recall that Kelvin’s calculation of the age of the Sun relied entirely on the release of gravitational energy in the form of heat as the Sun contracts. This idea—that gravitational energy could be the source of the Sun’s power—originated with the Scottish physicist John James Waterston as early as 1845. Ignored initially, the hypothesis was revived by Hermann von Helmholtz in 1854, and then enthusiastically endorsed and popularized by Kelvin. With the discovery of radioactivity, many assumed that the radioactive release of heat would turn out to be the real source of the Sun’s power. This, however, proved to be incorrect. Even under the wild assumption that the Sun is composed largely of uranium and its radioactive decay products, the power generated would not have matched the observed solar luminosity (as long as chain reactions not known at Kelvin’s time were not included). Kelvin’s estimate of the age of the Sun had served to strengthen his objection to revising his calculation of the age of the Earth—as long as the problem of the age of the Sun existed, the discrepancy with the geological guesstimates would not have been resolved fully. The answer to the question of the Sun’s age came only a few decades later. In August 1920, astrophysicist Arthur Eddington suggested that the fusion of hydrogen nuclei to form helium might provide the energy source of the Sun. Building on this concept, physicists Hans Bethe and Carl Friedrich von Weizsäcker analyzed a variety of nuclear reactions to explore the viability of this hypothesis. Finally, in the 1940s, astrophysicist Fred Hoyle (whose groundbreaking work we shall investigate in chapter 8) proposed that fusion reactions in stellar cores could synthesize the nuclei between carbon and iron. As I noted in the previous chapter, Kelvin was therefore right when he declared in 1862: “As for the future, we may say, with equal certainty, that inhabitants of the earth can not continue to enjoy the light and heat [of the Sun] essential to their life for many million years longer unless sources now unknown to us are prepared in the great storehouse of creation [emphasis added].” The solution to the problem of the age of the Sun required no less than the combined genius of Einstein, who showed that mass could be converted into energy, and the leading astrophysicists of the twentieth century, who identified the nuclear fusion reactions that could lead to such a conversion.