In a discovery that experts are calling breathtaking and beautiful--and of "Nobel Prize caliber"--physicists at the University of Colorado at Boulder have created an entirely new state of matter. It exists only in the coldest spot in the universe, which is currently a carrot-size tube in the laboratory of physicists Carl Weiman and Eric Cornell.
Albert Einstein predicted more than 70 years ago that atoms chilled to sufficiently frigid temperatures should "freeze" into this new state, just as liquid water freezes into solid ice. More compact even than a solid, the new state of matter contains several thousand atoms all merged into one.
"What's unbelievable is that it happened just the way we hoped it would," said Weiman. "It's downright magical."
Dan Kleppner at the Massachusetts Institute of Technology, who has also been working for years to achieve the super-cold state, said the Colorado experiment was "clear and convincing, good enough for a textbook, although obviously we would have loved to [do] it first."
Experiments on the new matter will give physicists a deeper understanding of how atoms behave when all the residual quivering motion of heat is taken away and atoms sing out pure tones, undistorted by the static noise that normally pervades the universe.
Although no practical applications are foreseen for the immediate future, a clearer understanding of atomic behavior has always led to startling advances--including lasers, computers and medical diagnostics. Most directly, the discovery will allow researchers to explore the frequently bizarre world of super cold, where liquid helium flows uphill and electricity runs in currents forever without resistance. "They don't call them super for nothing," said Cornell.
The experiment is described in today's issue of the journal Science.
"The most exciting thing is that the phenomenon is so different from anything ever seen before," Kleppner said. "It gives us an opportunity to investigate an area which is ripe for discoveries."
Einstein and Indian physicist Satyendra Nath Bose speculated that atoms of matter could condense into a single "superatom" at sufficiently cold temperatures. Called the Bose-Einstein condensate after the theoreticians, this new state of matter can exist only at a whisper above absolute zero--the ideal but unreachable limit to cold, where no motion exists apart from the innate restlessness of subatomic particles.
The cluster of atoms in Weiman's tube registered just 170 billionths of a degree above absolute zero--the coldest temperature in the universe. Since the background radiation left over from the Big Bang still warms even empty space to 3 degrees above absolute zero (roughly equivalent to minus 454 degrees Fahrenheit), no known place in the universe is as cold as that tube.
Weiman and Cornell produced their record cold by combining several approaches used by different groups, including Kleppner's. First, they slowed rubidium atoms by bombarding them from all sides with laser beams. (Cold, in the atomic world, means slow.) Then they turned on a magnetic trap and allowed the faster (hotter) atoms to evaporate, just as the fastest (hottest) molecules escape first from a cup of coffee. What was left was the most sluggish atomic brew in creation.
These super-chilled atoms then went through a kind of alchemy, transforming from hard matter-like particles into compressible particles like light. Most atoms have a natural tendency to hold each other at arm's length, a result of their built-in architecture in which clouds of electrons push off other atoms like springs. However, particles of light or gravity are naturally gregarious, and crowd together without limit.
A major challenge was figuring out how to see the super-cold atoms, because any light shined on them would destroy the condensate. (The term condensate describes the same phenomenon as water condensing out of steam on a bathroom mirror.)
The researchers solved this problem by shining a flash of laser light so short that it acts like a strobe, snapping the picture before the atoms have time to disperse. "It blows the condensate away," said Cornell, "but the flash is so short we can still take the picture."
The new state does not form at once, but freezes rather gradually at first, like water freezing in an ice tray. Once it gets going, says Cornell, "it's a runaway process."
The glob of condensed atoms stays in the trap for about 20 seconds before flying apart as room-temperature atoms--which are a trillion times hotter--leak in. The new matter appears as a bright glow in the center of a diffuse cloud, just like freezing water.
Rubidium was the atom of choice, said Weiman, "because we could use these real cheap lasers," the same diode lasers used in CD players. Since he set out to make the new form of matter six years ago, Weiman has insisted on doing it at a bargain-basement price. "From the beginning," he said, "I've thought what would make this exciting would be if lots of people could easily study it."
The carrot-size tube containing about a thousand atoms sits inside a complex of lasers--each about the size of a toaster oven--and magnets. The whole experiment rests on a picnic-size table. Disregarding the salaries of Weiman, Cornell and about a dozen graduate students over six years, the experiment cost only $50,000--peanuts for fundamental physics.
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A New State of Matter
By getting a thousand atoms to march in step as a single unit, physicists have imposed a new level of order on the universe. Only very cold atoms can accomplish this feat, because heat is equivalent to motion, and disorder.
* Room temperature: Atoms whiz around at high speeds in random directions.
* Very cold: Exposure to cold slows the atoms' motion.
* Extreme cold: Particles become orderly, freezing into a single atom. This orderliness makes the normally hidden wave nature of atoms visible, as shown in this computer model.
Sources: University of Colorado, Boulder.