Ketterle, Wolfgang, << KEHT er luh, VOHLF gahng >> (1957-…), a German physicist, won a share of the 2001 Nobel Prize in physics for his experiments on a state of matter known as a Bose-Einstein condensate (BEC). A BEC is a cluster of atoms that behave somewhat as if they were a single atom. Ketterle shared the prize with the American physicists Eric A. Cornell and Carl E. Wieman. BEC’s are named for the physicists Satyendra Nath Bose of India and Albert Einstein of Germany, who proposed in the 1920’s that BEC’s could be created.
The creation and behavior of a BEC can be explained in terms of the wave-particle duality of matter. We commonly consider an atom to be made up of solid particles. But an atom also has properties of a wave. One of those properties is a wavelength, the distance between successive wave crests. The wavelength of an atom depends on the atom’s momentum—that is, its total amount of motion. The smaller the momentum, the larger the wavelength. An atom’s momentum equals the mass of the atom times the atom’s velocity. The atom’s mass is its amount of matter; its velocity is its speed in a particular direction.
In June 1995, Cornell and Wieman became the first to create BEC’s. Working independently of Cornell and Wieman, Ketterle created BEC’s two months later.
The scientists produced BEC’s by decreasing the velocity of atoms in a gas. As the velocity decreased, the wavelengths of the atoms became larger. Eventually, the wavelengths became approximately as large as the distances between the atoms. When that occurred, all the atoms suddenly began to move at one velocity. The entire cluster of atoms took on the properties of a single wave, thereby becoming a BEC.
The researchers created their BEC’s by cooling a gas to a temperature near absolute zero (–459.67 °F or –273.15 °C). Cooling a gas is equivalent to decreasing the velocity of its atoms; the temperature of a gas is a measure of the average energy of motion of its atoms.
The scientists decreased the velocity of the atoms by bombarding them with light. According to wave-particle duality, light can be thought of as consisting of waves or of particles called photons. A photon has zero mass, but it possesses momentum. The researchers forced photons to collide head-on with the atoms, thereby slowing the atoms.
Wolfgang Ketterle was born in Heidelberg, Germany, on Oct. 21, 1957. He received a Ph.D. degree in physics from the University of Munich in 1986. He did postdoctoral work at the Max-Planck Institute for Quantum Optics in Garching, Germany; at the University of Heidelberg in Germany; and at Massachusetts Institute of Technology (MIT) in Cambridge. In 1993, he joined the MIT faculty as an assistant professor. He became a professor in 1997.
