We are a new form of matter: Bose-Einstein condensates

[Guest guest]

New Form of Matter

Scientists have created a new kind of matter: It comes in waves and

bridges the gap between the everyday world of humans and the micro-

domain of quantum physics.

Bose-Einstein condensates ( " BECs " for short) aren't like the solids,

liquids and gases that we learned about in school. They are not

vaporous, not hard, not fluid. Indeed, there are no ordinary words to

describe them because they come from another world -- the world of

quantum mechanics.

 

Quantum mechanics describes the bizarre rules of light and matter on

atomic scales. In that realm, matter can be in two places at once;

objects behave as both particles and waves (a strange duality

described by Schrodinger's wave equation); and nothing is certain:

the quantum world runs on probability.

 

Although quantum rules are counter-intuitive, they underlie the

macroscopic reality we experience day-to-day. Bose-Einstein

condensates are curious objects that bridge the gap between those two

realms. They obey the laws of the small even as they intrude on the

big.

 

BECs 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.

 

A BEC is a group of a few million atoms that merge to make a single

matter-wave about a millimeter or so across. In 1995, Ketterle

created BECs in his lab by cooling a gas made of sodium atoms to a

few hundred billionths of a degree above absolute zero -- more than a

million times cooler than interstellar space! At such low

temperatures the atoms became more like waves than particles. Held

together by laser beams and magnetic traps, the atoms overlapped and

formed a single giant (by atomic standards) matter wave.

 

Says Ketterle: " Pictures of BECs 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 Colorado) also

created BECs; 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 BECs were too strange

to be real even though he himself had thought of them.

 

Now we know Bose-Einstein condensates are real. And Einstein was

right: they are strange.

 

For example, notes Ketterle, if you create two BECs and put them

together, they don't mix like an ordinary gas or bounce apart like

two solids might. Where the two BECs 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 BECs 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

BECs. Strange, but true.

 

One of the most extraordinary aspects of Bose-Einstein condensates is

that they are quantum creatures big enough to see. And there lies

much of their promise. Many of today's cutting-edge technologies --

smaller, faster computer chips, micro-electro-mechanical systems

(MEMS) and quantum computers -- lie in the twilight zone between the

quantum world and the macroscopic world. Scientists hope that

studying BECs will advance those technologies and create others.

 

Ketterle is already experimenting with one: a pulsed atom-laser.

 

" In an ordinary gas, atoms move around randomly, they flit around in

all directions. But in a BEC, all the atoms march lock-step, "

Ketterle explains. " They are just one single matter-wave propagating

in one direction. "

 

Atom-lasers are akin to light-lasers, which are beams of photons that

likewise " march lock-step. " But there are differences: For instance,

atom-laser beams have mass so they will bend downward in Earth's

gravitational field. Light-laser beams are massless; they bend, too,

but the effect is very small. Furthermore, light-lasers pass through

air with ease. Atom-laser beams will be substantially scattered by

air molecules.

 

" Atom lasers need a vacuum to retain their properties, " notes

Ketterle. As a result they won't be used in the same way as light-

lasers. They won't improve CD players or supermarket scanners, for

instance. But atom-lasers will doubtless find uses of their own --

" like better atomic clocks [which will improve spacecraft

navigation -- a boon to NASA], atomic optics or very fine

lithography, " says Ketterle.

 

Who knows where BECs will lead? After all, humans evolved on this

planet with solids, liquids and gases all around, and we're still

figuring out innovative uses for them. With Bose-Einstein

condensates ... we're just getting started.

 

" And when the two become one " .....bob