A scientist calculated the coldest possible temperature using nothing but gas laws

In 1848, William Thomson (later Lord Kelvin) proposed a temperature scale with a true zero point, independent of any substance, building on Sadi Carnot's theory of heat engines. Studying how gases contract as they cool, he calculated this 'absolute zero' at about -273°C — within a fraction of a degree of the modern value, worked out decades before anyone got remotely close to it in a lab. The Kelvin scale, and its unit, are now the SI standard for scientific temperature.

— William Thomson (Lord Kelvin), On an Absolute Thermometric Scale — 1848