The study of matter when it is subjected to very low temperatures has provided surprising results. For example, when we try to liquefy helium. It was achieved by the Dutch physicist Heike Kamerlingh Onnes in 1908. A great achievement considering that helium boils at -269º C and then the cryogenization methods were too rudimentary. We must take into account that helium has the lowest boiling temperature and at normal atmospheric pressure it never freezes . Just as cars use water as a coolant, Onnes used liquid helium to freeze other substances, such as mercury , which solidifies at -38.89º C. Measuring its electrical conductivity, he found, as expected, that the lower the the temperature of the mercury, the better it conducted the current. But when it reached -269º C, he discovered that its electrical resistance disappeared completely : Kamerlingh Onnes had just discovered superconductivity . This means that they do not heat up and, therefore, there are no losses of electrical energy when carrying the current from one side to the other, something that is priceless for any electrical company. In fact, it is estimated that if superconducting materials were discovered at room temperature, 20% of the losses in energy transported from power plants to homes would be saved.
The drawback of these materials lies in the critical temperature that must be reached to make a material superconducting : for metallic compounds it is around -250º C. In 1986 it was discovered that not only metals and metal alloys had this property : at the IBM Laboratories in Zurich they found that when copper and lanthanum oxide, which is an insulator, is doped with barium, it becomes superconductive at -237º C. Years later ceramic materials were found that also exhibited superconductivity: they are known as high-temperature superconductors , -150º C. Are there superconducting substances at room temperature? They are eagerly sought because there is no theoretical impediment that prevents it. On the other hand, superconductors also exhibit another fascinating property, called the Meissner-Ochsenfeld effect : they completely expel the magnetic field inside them, causing a magnet to float on top of them. Will we one day have high-speed trains levitating on superconducting rails?
The surprises do not end here. Against all odds, Kamerlingh Onnes missed one of the most amazing properties of liquid helium. In 1938 the Russian Pyotr Kapitsa and the Canadians John Allen and Austin Misener found that below -271º C liquid helium became an excellent conductor of heat, 200 times better than copper . And not only that, but it had a viscosity less than one ten-thousandth of that of gaseous hydrogen: it is the phenomenon of superfluidity.
All liquids present opposition when flowing: it is the viscosity, product of the frictional forces that appear between the liquid molecules themselves and between these and those of the surface on which they slide. Some, like shampoo or honey, are very viscous. Others, like water, are not so. Nineteenth-century scientists found the equations that describe the behavior of a fluid, but soon realized that introducing viscosity was going to be extremely complicated. So after several attempts they did what everyone who makes physical models does: eliminate unnecessary complications, although as in this case it is a defining characteristic of a liquid. The brilliant physicist John Von Neumann was repulsed by a fluid dynamics that turned a deaf ear to viscosity and branded those who developed it as dry land theorists . However, those equations were useless for all liquids in nature except one: liquid helium at -271 degrees Celsius. Below that temperature its viscosity practically disappears, becoming superfluid. This allows us to see how the helium literally climbs up the walls of the glass that contains it and spills out. This fact has important technological applications, such as locating microholes in ducts and pipes: superfluid helium can easily ‘sneak’ through holes smaller than 2 ten-thousandths of a millimeter .
In this peculiar race to discover surprising phenomena at low temperatures, a group from the Joint Institute for Laboratory Astrophysics (JILA), in Boulder, Colorado, culminated in 1995 a two-decade effort by scientists from around the world to experimentally verify a prediction made almost 80 years ago by Albert Einstein and Hindu Satyendra Nath Bose. At normal temperatures, the atoms of a gas are distributed throughout the volume of the container that contains them. But at extremely low temperatures, of the order of a few billionths of a degree above absolute zero, the atoms lose their individual identity (they become impossible to distinguish between them) and behave as if they were a single “superatom”: it is the Bose condensate. Einstein , the state of matter that is below the solid. The JILA group managed to cool 2,000 rubidium atoms below 100 billionths of an absolute degree for 10 seconds, creating the first Bose-Einstein condensate in history.
We might wonder why physicists always talk about absolute degrees of temperature or degrees Kelvin. What is special about this value of -273.16º C? Can’t there be anything colder? The answer is no. Temperature is simply a measure of the agitation of atoms and molecules. The more they move, the more temperature we will measure. At absolute zero all movement stops and therefore you cannot go any lower.
Onnes, H. K. (1911) The Superconductivity of Mercury., Comm. Phys. Lab. Univ. Leiden, 122-124
Rubinin, P. E. (1997) The discovery of superfluidity. Letters and documents, PHYS-USP, 40 (12), 1249-1260