As every physicist knows, the speed of light in a vacuum is about 300, 000 kilometres per second. Although light travels fractionally slower through matter, it still propagates at speeds measured in thousands of metres per second. However, by exploiting a quantum effect called 'electromagnetically induced transparency', Lene Vestergaard Hau of the Rowland Institute for Science in the US and co-workers have reduced the speed of light in a gas of ultracold sodium atoms to 17 metres per second - 20 millions times slower than the speed of light in vacuum (Nature 397 594).