Einstein predicted a new state of matter in 1924

Person: Albert Einstein
Lived: 1879–1955
Field: Physics
From: German-American

The story

In 1924 Albert Einstein received a paper from a young Indian physicist, Satyendra Nath Bose, describing a new way to count particles of light. Bose had derived Planck's law of radiation using only the statistics of light quanta, without any reference to classical physics. Einstein was impressed. He translated the paper from English into German himself and submitted it on Bose's behalf to the Zeitschrift für Physik, which published it in 1924. His handwritten translation, long thought lost, was found in a library at Leiden University in 2005.

Einstein then extended Bose's method from light to matter in two further papers in 1924 and 1925. The result was the concept of a Bose gas, governed by what became known as Bose–Einstein statistics. These describe particles that are allowed to share the same quantum state, now called bosons in Bose's honour; they include photons and certain atoms such as helium-4 and rubidium-87.

Einstein drew a startling conclusion. If such atoms were cooled to a temperature very close to absolute zero, a large share of them should drop into the lowest possible energy state together, forming a new state of matter in which quantum effects that are usually hidden at the scale of single atoms would become visible on a much larger scale. In 1938 Fritz London suggested that this kind of condensation might explain the strange frictionless flow of superfluid helium.

For seventy years the condensate itself remained a prediction, because the temperatures needed were far beyond reach. Advances in laser cooling finally made it possible. On 5 June 1995 Eric Cornell and Carl Wieman at the NIST–JILA laboratory of the University of Colorado in Boulder produced the first gaseous condensate from rubidium atoms cooled to 170 billionths of a degree above absolute zero. Shortly afterwards Wolfgang Ketterle at MIT made one from sodium atoms. The three shared the 2001 Nobel Prize in Physics.

Their work founded the field of ultracold atoms. Hundreds of research groups around the world now routinely make Bose–Einstein condensates, and the technique has been extended to molecules and even to light, giving physicists a laboratory in which quantum behaviour can be studied directly.

Why it matters

Bose–Einstein condensates let scientists see quantum effects on a much larger scale than single atoms, opening a field of research used to test fundamental physics and explore new quantum technologies.