In 1908, Leiden was officially the coldest spot on Earth. This was the year that Heike Kamerlingh Onnes succeeded in liquefying helium. He was able to lower the temperature of helium to -272°C, close to absolute zero ( -273.15°C). This led to the discovery of superconductivity; Onnes found that the electrical resistance of certain metals disappears at very low temperatures. Superconductivity is still used today, for instance in MRI scanners.

The Netherlands is still known for ‘its’  tulips (Carolus Clusius’ tulips). At the end of the 16^th century, botanist Clusius brought the first tulips to Leiden. The bulbs originally came from Turkey and he cultivated all sorts of new varieties in the Hortus Botanicus. His research formed the foundation for the present Dutch bulb-growing industry. Hundreds of years later, his influence is still present in the flower bulb region between Leiden and Haarlem.

The Big Bang is another of Leiden’s discoveries. In 1917, Willem de Sitter published a number of articles about the origins and expansion of the Universe. This Leiden professor based his descriptions of the cosmos on Albert Einstein’s general theory of relativity, which was published earlier that year. De Sitter immediately appreciated what this meant for our understanding of the Universe.

Not just the Big Bang but also the size and shape of the galaxy were discovered by Leiden researchers. Jan Hendrik Oort calculated that the distance to the centre of the Milky Way galaxy is 30,000 light years. He also proved that the Milky Way is an orbiting galactic disc with two spiral-shaped arms. The famous Oort cloud, the origin of comets, was named after him. Oort was director of the Leiden Observatory and the first astronomer to describe the existence of dark matter.

Although Galileo Galilei did not come up with his theories in Leiden, the city did play an important role in the dissemination of his ideas. The Leiden publisher Lodewijk Elsevier took a leap when others in Europe wouldn’t: in 1638 he published Galilei’s famous Discorsi. In this book Galilei described two new sciences: materials sciences and kinematics. The Elsevier publishing house was situated in a building adjacent to the Academy Building and published over 2000 books including very influential academic works.