The coldest place on Earth was a Dutch street

There was a time — a specific date, 10 July 1908 — when the coldest point of the known universe was not the South Pole, not a cloud in Siberia, but a laboratory on Steenschuur, Leiden.

The physicist was Heike Kamerlingh Onnes, and for years before 1908 he had been doing with his Leiden laboratory what almost nobody else thought worth the effort: cooling gases, stepwise, one ladder rung at a time. On that day in July he reached the last rung. Liquid helium: about four degrees above absolute zero. The coldest temperature ever achieved was a matter of one street, one lab, one stubborn campaign. The Pieterskerk is a two-minute walk away. The coldest place on Earth, near the parish church.

The carpenter's son behind the cold

And one correction that matters: the cold came from more than one man. Kamerlingh Onnes steered his experiments by the equations of Johannes Diderik van der Waals — born in Leiden in 1837, the carpenter's son whose name is now in every chemistry textbook. Van der Waals had written down how real gases really behave: molecular size, molecular attraction. The two Leiden Nobelists' work met at four degrees above absolute zero. Three years on, almost as a byproduct, the same lab found superconductivity. The cold was Onnes' work. The equations behind it were van der Waals'.

Kamerlingh Onnes won the Nobel Prize in Physics in 1913. The walk stops at the wall on Steenschuur where the laboratory stood, and the science museum in Leiden that keeps the instruments of that campaign has all the details: rijksmuseumboerhaave.nl — check it for current hours, because like every museum in this town it charges entry only if you go inside.

The relationship between the two Leiden Nobelists, one street's worth of physics and what to notice on the door you walk past, is in the walking book. Related walks: the carpenter's son who won the Nobel Prize and the hidden gems from the street.