Lord Kelvin | On the Age of the Sun’s Heat

Lord Kelvin | On the Age of the Sun’s Heat.

William Thomson was one of the giants of 19th Century physics, playing an important role in all the major innovations of that crowded Century – thermodynamics, electricity, telegraph and geology. Not everything he proclaimed the verdict of physics on was found to be the way he had assumed, but he pushed physical insight to its limits. Famously he is apocryphally said to have claimed physics was over in about 1900 – though no source can ever be found where he says something so short-sighted. He’s also said to have disbelieved x-rays, though what isn’t mentioned is he became a firm supporter of Roentgen’s work after getting his own hand x-rayed. Experiment trumped theory for William Thomson, Lord Kelvin.

Infamously he challenged Charles Darwin over the age of the Earth. But Thomson was correct, within the limits of knowledge of the day, that the Earth couldn’t be as internally warm nor the Sun as bright for as long as Darwin and the geologists wanted… unless both were internally different to what was inferred. Sure enough, heat transport within the Earth proved more complicated than simple heat conduction. Convection of the mantle changes the thermal profile, so that below a certain depth the rate of change in temperature declines at a much slower rate with depth. Similarly the Sun is chiefly powered by a source other than the gravitational contraction that Thomson had invoked – nuclear fusion. But none of Thomson’s work on the Sun – or the Earth – was invalidated, merely extended. A newly forming star is powered by gravitational contraction energy until its core becomes sufficiently dense and hot to initiate proton-proton fusion. And the Earth’s thermal profile is much as Thomson described until it reaches a depth where the mantle softens enough to convect.

So, rightly, Thomson is remembered for his foundational work in thermodynamics and for his principled work in applying new physical insights as they became established. We owe the first successful Trans-Atlantic telegraph cables to him too. Quite an amazing life really.