A Scottish physicist, born in Ireland, who proposed the thermodynamic temperature scale (1848), now measured in kelvin, and deduced the so-called heat death of the universe based on an extrapolation of the second law of thermodynamics (heat cannot flow spontaneously from a cooler object to a hotter one). Kelvin estimated the age of Earth by calculating how long it would take for an Earth-sized ball of rock to cool from its initial molten state. His value – 20 to 400 million years – was much too low because he knew nothing about the heat still being generated inside our planet by radioactive decay. He also estimated the Sun's age, based on the most efficient energy source he could imagine, which was the slow release of gravitational energy by contraction (see Kelvin-Helmholtz contraction). Again, he had no way of knowing that, in nuclear fusion, there is a vastly more potent way of generating heat and light.
Quote by Kelvin
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
