A New Form of Matter

Says Ketterle: "Pictures of BEC's can be regarded as photographs of wave functions" - that is, solutions to Schrodinger's equation.

Working independently in 1995, Eric Cornell (National Institute of Standards & Technology) and Carl Wieman (University of colourado) also created BEC's; theirs were made of super-cold rubidium atoms. Cornell and Wieman shared the 2001 Nobel Prize with Ketterle "for the achievement of Bose-Einstein condensation in dilute gases of alkali atoms, and for early fundamental studies of the properties of the condensates."

Bose-Einstein condensates were predicted by Indian physicist Satyendra Nath Bose and Albert Einstein in the 1920's when quantum mechanics was still new. Einstein wondered if BEC's were too strange to be real even though he himself had thought of them.

Image courtesy MIT BEC's form when the atoms in a gas undergo a transition from behaving like the "flying billiard balls" of classical physics to behaving as one giant matter-wave.

Now we know Bose-Einstein condensates are real. And Einstein was right: they are strange.For example, notes Ketterle, if you create two BEC's and put them together, they don't mix like an ordinary gas or bounce apart like two solids might. Where the two BEC's overlap, they "interfere" like waves: thin, parallel layers of matter are separated by thin layers of empty space. The pattern forms because the two waves add wherever their crests coincide and cancel where a crest meets a trough - so-called "constructive" and "destructive" interference, respectively. The effect is reminiscent of overlapping waves from two stones thrown into a pond.

"That means... we have the remarkable effect that an atom (in one BEC) plus an atom (in another BEC) gives no atom. It's destructive interference," says Ketterle. "Of course we didn't destroy matter, it just appeared somewhere else in the pattern, so the total number of atoms is conserved."

Not all atoms can form Bose-Einstein condensates - "only those that contain even numbers of neutrons plus protons plus electrons," says Ketterle. Ketterle made his BEC's from sodium atoms. If you add the number of neutrons, protons and electrons in an ordinary sodium atom, the answer is 34 - an even number suitable for Bose-Einstein condensation. Atoms or isotopes of atoms with odd sums can't form BEC's Strange, but true.