Superconductivity is the set of physical properties observed in certain materials when cooled below a characteristic temperature. It was discovered by Dutch physicist Heike Kamerlingh Onnes on April 8, 1911 in Leiden. He found that the electrical resistance of mercury suddenly vanished when it was cooled to 4.2 Kelvin (−268.95 °C). Under certain conditions, electrons can pair up and flow without resistance in a superconductor.
There are several theories that attempt to explain how and why superconductivity occurs. The most widely accepted theory is the BCS (Bardeen-Cooper-Schrieffer) theory which explains superconductivity in terms of electron pairs forming a condensate which flows without resistance below a critical temperature. Another important theory is the Ginzburg-Landau theory which provides a more macroscopic description of superconductors and their behaviour near the critical temperature.
The ability of some materials to become superconducting has many potential applications including high efficiency electrical power transmission, levitating trains and magnetic resonance imaging (MRI). Superconductivity also has implications for our understanding of quantum mechanics as it appears to violate one of its key principles – that no information can travel faster than the speed of light.