The Discovery of Superfluidity

T of the greatest physics discoveries in the first half of the 20th century were superconductivity and superfluidity. Superconductivity involves the frictionless conduction of electricity in wires, and superfluidity the frictionless flow of superfluid liquid helium through channels. Both discoveries led to the arcane phenomenon of macroscopic quantization: the manifestation of the laws of quantum mechanics not just on atomic dimensions but also on the scale of common laboratory apparatus. Both discoveries required the liquefaction of helium, which was first done in 1908. Superconductivity was discovered almost immediately after liquid helium became available—by 1911—and its importance in the construction of magnets was quickly recognized. Although there were hints as early as 1911 that something was peculiar about liquid helium near 2 K, it was not until 1938 that superfluidity was discovered and named. (Figure 1 is a remarkable photograph of a superfluid-helium fountain, one of the hallmarks of superfluidity.) Analogies between the two phenomena played an important part in the discovery. Why was it that these closely related discoveries occurred more than a quarter of a century apart? No technical barrier stood in the way of the discovery of superfluidity, and as we shall see, suggestions of a new phenomenon were definitely present.