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Physics LibreTexts

11.4 Kelvin and the Sun's Age

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Charles Darwin. Click here for original source URL.

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William Thomson, 1st baron Kelvin. Click here for original source URL.

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Thomas Huxley. Click here for original source URL.

Lord Kelvin, for whom the Kelvin temperature scale is named, was one of the most famous scientists of his day. His scientific achievements ranged through the study of energy and heat. He was a towering intellect, and he held strong views on the age of the Sun and the Earth based on his understanding of thermodynamics. Lord Kelvin was not amused by the ideas introduced by Charles Darwin. His displeasure was aroused at a meeting of the Geological Society of London in 1871, as he debated the age of the Earth with Thomas Huxley, a stand in for Charles Darwin. Darwin a shy young man who had proposed a radical new theory of evolution. Species changed in response to the pressures of the environment, Darwin said. He believed that the process of natural selection caused species to pass on survival traits to their offspring and so continually to adapt to the world. Darwin suspected that the gradual evolution of life on Earth from single-celled organisms to the present array of elephants and eagles and whales must have taken a very long time. Huxley presented Darwin's hypothesis that life had been evolving on Earth for billions of years.

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Hermann von Helmholtz. Click here for original source URL.

Kelvin strongly opposed this radical, old-Earth idea of Darwin on a basis of temperature. The great physicist had a different way to estimate the age of the Earth and the Sun. He focused on the ideas of thermodynamics, the science of the measurement and transfer of heat. The Sun is the incubator that keeps all life on Earth alive. However old the Earth is, the Sun must be at least that old. Kelvin knew that the answer depended on the energy source of the Sun. Earlier physicists had speculated that the Sun was burning fossil fuels, like a gigantic furnace. However, it would have lasted only a few thousand years with a chemical energy source. Following a suggestion of Hermann von Helmholtz, Kelvin studied whether the Sun may in fact be fueled by an alternative energy source: gravitational contraction. If the material of the Sun is slowly settling towards its center, he reasoned, gravitational energy would be converted into heat and then radiated into space. Using these ideas, Kelvin calculated a maximum age for the Sun of 500 million years.

The Kelvin-Huxley debate ended inconclusively. Kelvin's age estimate was still less than the span of time believed to be needed for the diversity of species to be produced by natural selection. Darwin was bitterly disappointed. He died not knowing if the age of the Earth would accommodate his theory. This halting progress — the clash of ideas and the difficulty of choosing between hypotheses — is typical of how science works in new fields where data is insufficient.

Kelvin was a brilliant man. He had entered the University of Glasgow at age ten, published his first research paper at sixteen, and was a full professor by the age of twenty-two. For all the things he had done right in his career, in this one instance he was very wrong. It turned out that Kelvin was missing something vital: an understanding of the processes of nuclear decay.

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Ernest Rutherford. Click here for original source URL.

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Henri Becquerel. Click here for original source URL.

During nuclear decay, energy is released when atoms either fragment, or undergo beta decay. This wasn't known in Kelvin's time. It would be another 25 years before the radiation of nuclear decays was discovered by Henri Becquerel, and 40 years before Ernest Rutherford would discover the atomic nucleus. Later scientists would harness this energy for splitting atomic nuclei. The energy produced through the fusion of atomic nucleus is vastly more efficient at producing energy than the chemical fuels that heat our homes and powers our cars. Our star the Sun is powered by the fusion of hydrogen nuclei. The Sun keeps shining as long as its fuel reserves are not depleted. That’s about 10 billion years, and long enough to encompass Darwin's estimate of the timescale of biological evolution.