As esoteric as Kelvin’s work might seem, it made him rich when he applied it to designing the transatlantic cables that joined Europe to North America by telegraph. And sometimes, when Kelvin was bobbing on a cable ship in the middle of the Atlantic, he would think about how old Earth was. Kelvin was a devout man, but he didn’t accept that Earth was a few thousand years old simply because someone decided that the Bible said so. He thought that it should be possible instead to put an upper bound on the age of the Earth scientifically, by studying its heat.
Miners, Kelvin knew, found that the deeper they dug, the hotter the rocks became. To account for this heat, Kelvin speculated that Earth formed from the collision of miniature failed planets, and the energy of their impact created a molten blob (a speculation that later proved to be right). Kelvin assumed that once the impacts ended, there would be no way for the planet to receive any new heat, so it would gradually cool like a dying ember. Its surface cooled off the fastest, while its interior remained warm to this day. Only at some point in the distant future would Earth’s core become as cold as its surface.
Kelvin and other physicists had worked out equations to accurately predict how objects cool, and he applied them to the entire planet. By measuring how fast heat escaped from rocks, and how hot the deepest mine shafts became, he came up with an estimate for the age of the planet. He concluded in 1862 that Earth had been cooling no more than 100 million years.
Kelvin’s original motive had been to show how sloppy geology was compared to physics. But after reading Origin of Species, he was happy to use his results to attack Darwin. Steeped as Darwin was in Lyell’s ancient geology, he assumed the gradual changes of natural selection could take as long as they needed to alter life. But Kelvin’s results wouldn’t allow him enough time. Kelvin himself wasn’t a rabid anti-evolutionist—for all he knew, all of life might have started as a germ—but he saw life today as evidence of design, of the handiwork of God. He used his estimate for Earth’s age to cut Darwin down with a single scimitar slice.
Huxley tried to defend Darwin by making a compromise—something Huxley rarely did with critics. He said that biologists had to accept an age for Earth that geologists and physicists decided on and figure out how evolution could work in that span of time. If Earth was only 100 million years old, then evolution must be able to work at high speed. Wallace went further, suggesting that at times evolution could work far faster than it does today. As Earth wobbles on its axis, the planet might experience harsh climates that would make evolution run at high speed.
Darwin wasn’t satisfied. “I am greatly troubled at the short duration of the world according to Sir W. Thomson,” he wrote in a letter. Kelvin, meanwhile, was getting new reports on the planet’s temperature, and kept revising his estimate—each time shortening Earth’s life span. By the time he was done, he had winnowed it down to only 20 million years. All the while, Darwin could do nothing but grit his teeth. As he struggled to flesh out his theory of evolution, “then comes Sir W. Thomson like an odious specter.”
Lord Kelvin had based his calculation of Earth’s age on a fundamental (and, it would turn out, false) assumption: the planet had no source of heat of its own. But there was a hidden heat inside the planet that Kelvin hadn’t counted on. In 1896, 14 years after Darwin died, a French physicist named Henri Becquerel wrapped a piece of uranium salt in a photographic plate. When he developed the plates, he found sharp, bright dots on them. Uranium, he realized, released rays of energy. Seven years later Pierre and Marie Curie showed that a lump of radium released a constant supply of heat.
Becquerel and the Curies had found a source of energy in the basic structure of atoms. Atoms are made out of three building blocks: protons, neutrons, and electrons. Electrons, which carry a negative charge, flit around the edge of atoms, while positively charged protons sit at the center. Each element has a unique number of protons. Hydrogen has one proton, helium has two, and carbon has six. Alongside protons, atoms have neutrally charged particles called neutrons. Two atoms of the same element may have a different number of neutrons. The most common form of carbon on Earth has six protons and six neutrons (known as carbon 12), but there are trace amounts of carbon 13 and carbon 14 as well. These different versions of atoms, known as isotopes, make it possible to tell geological time.
The protons and neutrons in an atom are a bit like piles of oranges at a grocery store: in some arrangements they’re perfectly stable, but in others they will sooner or later fall apart. Orange piles are held together by gravity, but protons and neutrons are held together by other forces. When an unstable isotope breaks down, it releases a burst of energy, along with one or more particles (otherwise known as radiation). In the process it may become a different element. Uranium 238, for example, breaks down by releasing a pair of neutrons and a pair of protons, and turns into thorium 234. But thorium 234 is unstable as well and decays into protactinium 234, which also decays. Through a chain of 13 intermediates, uranium 238 finally settles into a stable form, lead 206.
You can’t predict exactly when a particular atom will decay, but a large collection of them will obey certain statistical laws. In any given period of time, an atom has a certain probability of decaying. Let’s say a pebble has 1 million radioactive isotopes inside it, and this particular kind of isotope has a 50 percent chance of decaying in a year. After the first year, 500,000 of the isotopes will be left. Of those 500,000 isotopes, 50 percent will decay in the second year, leaving 250,000. Year after year, half of the remaining isotopes disappear, until about 20 years later the last isotope disappears. Physicists capture this trailing off in a measurement known as the half-life: how long it takes for half of any given amount of a radioactive element to decay. Uranium 238, for example, has a half-life of 4.47 billion years; other elements have half-lives lasting tens of billions of years, while others have half-lives of only minutes or seconds.
The laws that govern atoms don’t submit to any simple intuitive sense, but they work. If they didn’t, computers wouldn’t crunch numbers and nuclear bombs wouldn’t explode. And long before computers and nuclear bombs were invented—in fact, within a few years of the work of Becquerel and the Curies—physicists realized that these laws exposed a fatal flaw in Kelvin’s claims of a young Earth. Uranium and other radioactive elements such as thorium and potassium can be found in the Earth, where they decay and give off heat. Kelvin thought that Earth was young because it hadn’t cooled down very much from its origin. Radioactivity allows the planet to stay warm far longer.
A physicist named Ernest Rutherford was the bearer of these bad tidings. Rutherford worked out many of the fundamentals of radioactivity, showing that it was a natural alchemy that could transform one element into another. In 1904 he traveled from Montreal, where he taught at McGill University, to England to give a talk about the new discoveries.
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, Kelvin 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 a 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 upon me.