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.
by Patrick Barry and Dr Tony
It's not often that you get to be around
for the birth of a new kind of matter, but when you do, the excitement
"To see something which nobody else
has seen before is thrilling and deeply satisfying. Those are the
moments when you want to be a scientist," says Wolfgang Ketterle,
a physicist at MIT and one of the first scientists to create a new
kind of matter called Bose-Einstein condensates.
Bose-Einstein condensates ("BEC's"
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.
Image © 2002 The Nobel Foundation.
prizing-winning scientists used lasers and magnetic fields
to create a new form of matter.
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.
A BEC is a group of a few million atoms
that merge to make a single matter-wave about a millimetre or so
across. In 1995, Ketterle created BEC's 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)
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
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.
Now we know Bose-Einstein condensates
are real. And Einstein was right: they are strange.
Image courtesy MIT
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.
"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.
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 BEC's will advance those technologies
and create others.
pulses produced in Ketterle's lab. The curved shape of the
pulses was caused by gravity and forces between the atoms.
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 BEC's 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.