It wasn’t so long ago that the Coldest Spot in the Universe was drifting out in the vast emptiness of space between galaxies. Warmed only by the meager crackling of energy left over from the Big Bang, atoms out there manage no more than a brisk 454 degrees below zero Fahrenheit.
Today the Coldest Spot in the Universe resides in Boulder, Colorado. Hardly anyone is complaining, though. Business remains brisk in downtown Boulder’s hip eateries and boutiques, and hikers haven’t recorded any negative impact on the town’s treasured scenic resources. No wonder: the Coldest Spot is just a crumb of space a quarter-inch wide sitting in a lipstick-size glass tube. The tube is surrounded by a miniature forest of lenses, vacuum pumps, and laser beams, and the whole deal is neatly tucked away in Carl Wieman’s modest lab in a small, towerlike physics building at the University of Colorado.
The crumb of space enfolds a wisp of vapor consisting of a mere 200 million atoms--less than a billionth as many as in a normal chunk of air that size. But the cesium atoms in the Coldest Spot do something that no other atoms anywhere else have ever done: nothing. Or at least very close to nothing. While other atoms bounce and dance and slam and careen, Wieman’s atoms lazily float. Which is to say his atoms are cold. To be exact, they are within a millionth of a degree of absolute zero, the unreachable point on the temperature scale--close to minus 460 degrees Fahrenheit--where all matter would, if it could, come to a perfect standstill.
Other physicists, too, including Daniel Kleppner of MIT and Steven Chu of Stanford, are racing to quiet atoms to the point where they’re as cold as inhumanly possible. This desire to reach the stillest possible state of matter is driven by more than mere competitiveness; theory predicts there’s a strange new state of matter at the limit of coldness--a state unlike anything that has ever existed on Earth. Perhaps unlike anything in the universe. Atom stuff, Wieman calls it, for lack of a better word. Its properties, he says, are a mystery.
Matter, after all, has an odd way of completely changing its character as it heats up or cools down. Take ordinary water. Warm it up a bit and it changes to vapor, a gas. Heat it even more and the water molecules dissociate into oxygen and hydrogen atoms; what was wet and largely sedentary is now two fairly volatile gases. Heat it more and the atoms split apart into electrons and nuclei. Atoms, in fact, no longer exist. The disordered mixture, called a plasma, is electrically charged; under the right conditions it would glow. Of course, you can continue: in a particle accelerator you can split the nuclei into protons and neutrons; you could then, perhaps, disassemble the protons into quarks. Some physicists think that at the very top of the temperature scale there’s something called quark matter, a sea of disconnected quarks that’s so weird no one knows what its properties might be.
Conversely, the journey down the temperature scale is a tale of increasing order. As matter grows cold, it congeals, condenses--growing ever more settled and sedate. Molecules of fluid water crystallize into solid ice. More ordered even than a solid is an exotic state of matter called superconductivity, and its partner, superfluidity. In these states atomic particles practically march in step, taking on seemingly magical properties: electric currents flow without resistance; liquids flow up and out of bottles or down through the bottoms of ceramic containers. At root, this behavior is the result of unnatural orderliness: groups of particles move as one, magnifying submicroscopic effects that are normally masked by the random motion that is heat.
And some physicists believe these bizarre behaviors are merely the tip of an ultracold iceberg. The territory is largely unexplored, however, because the fancy refrigerators traditionally employed to cool matter off simply aren’t capable of getting to temperatures where really interesting things are expected to happen. But new approaches involving lasers, high-powered magnetic fields, and sophisticated evaporation techniques are now bringing physicists tantalizingly close to the end of the line, temperature-wise. There matter will lose all its motion except for a minimum residual buzz, the visible manifestation of the laws of quantum mechanics at work. At this point, the physicists expect the atoms to condense into a single entity; somehow, all the atoms will go schlump, to use Wieman’s words, all occupying the same place at the same time. It’s like one big fuzzy atom, says Kleppner. An identity crisis for matter.
This theorized transition is known as Bose-Einstein condensation after the two physicists, Satyendra Nath Bose and Albert Einstein, whose calculations led to the prediction of this exotic state. Their calculations made it clear that another species of particle existed in nature, with properties quite distinct from those of protons, electrons, and other ordinary particles being studied by their colleague Enrico Fermi--and thus all called fermions. Bosons, as the new species of particle came to be called, differ from fermions in subtle--but significant--ways. One of these ways involves a property called spin. It won’t get you anywhere to worry about what particle spin is--even its discoverer, Wolfgang Pauli, said it was classically not describable. Particle spin is not at all like the spin of a top; it’s one of those friendly terms physicists use, like charmed quark, when they run out of everyday language to describe indescribable things. But like charm, particle spin can be precisely measured, and it has different values for bosons and fermions. Bosons possess spin in units of whole integers: for example, zero, one, two, and so on. Fermions have half-integer spin: 1/2, 3/2, and so forth. For reasons that fall out of the mathematics, this means that bosons can clump together in unlimited numbers, whereas fermions always stay out of each other’s orbits. It’s the standoffish nature of fermions that gives atoms their structure, and the social nature of bosons that allows enormous quantities of light particles, for example, to congregate in the same place. It’s like hermits versus people who love crowds, says Wieman.
A group of particles, say an atom, also has spin, a spin that is simply the sum of the spins of the constituent particles. Atoms, which are composed of particles like electrons and protons--which have half-integer spins--can be either fermions or bosons, depending on whether the total spins add up to a whole or a half integer. (A hydrogen atom, for example, is a boson: the spins of its single-proton nucleus and single orbiting electron add up to one.) At normal temperatures, the difference between boson atoms and fermion atoms isn’t visible. The energy of heat keeps even bosons from collapsing together. But take away the heat and suddenly the strange way in which particle spin influences the properties of matter becomes visible. That’s why I think it’s so neat, says Wieman.
A critical requirement is that the atoms overlap. If they don’t overlap, they can’t condense with other atoms, because they don’t know the other atoms are there. Just how cold temperature creates overlapping atoms is accounted for by yet another counterintuitive concept of quantum mechanics. Subatomic particles are notoriously hard to pin down. In fact, the better you measure one quantity about them (say, velocity), the less you know about another (position). A particle at very low temperature has a very precise velocity, of course--close to zero. That means its position is highly uncertain. Physicists describe this probability of a particle being here or there as its wave function, and the wave function of a very cold particle gets so blurred and spread out that it’s highly likely to bump into one of its fellows under the right circumstances.
If you can imagine a boson as a buffalo, suggests Wieman, then all this becomes a lot easier to understand. Let’s say there are one hundred buffalo happily roaming the state of Colorado, oblivious of one another’s existence. They could be social animals or not; no one would know. But let’s say something happens to make their territories begin to overlap--a shortage of food, for example. Now the animals would start wandering around looking for something to eat, probably picking up one another’s scent.
As the territories overlap, a distinct change will occur. The buffalo will learn there are other buffalo around; they will want to come together in the same place and move around together. The herd is a sort of buffalo Bose condensate, says Wieman. Another species--say, mountain lions--could be roaming the same territory. These might turn out to be fermions; in that case, they would stay in their individual territories no matter what.
Low temperature, in this instructive metaphor, is analogous to a scarcity of food: it smears out the probability of finding an atom, or a buffalo, in any one place. Its wave function now overlaps those of its fellows. If the atoms are bosons, and there’s no heat energy to keep them wiggling away from their neighbors, they will merge into one. And, as Wieman is quick to point out, the atoms don’t even have to be atoms. In principle, you could do this with locomotives: If you took two locomotives at normal temperatures and put them on the same place on the same track, you’d get a giant crash. But if you got precisely identical locomotives cold enough, and the combined spins of all the particles that made them up added up to an integer, you could put a whole pile of them together.
Since Bose condensation of a vapor has never happened in a laboratory--indeed, it may never have happened anywhere in the universe-- physicists don’t know exactly how this strange new stuff will behave. It might even spontaneously change its properties from one instant to the next. Whatever it does, Bose condensate should be very different from ordinary matter. If nothing else, it presents a rare opportunity to come face-to-face with one of nature’s walls. There is some good, exciting physics in the interaction of atoms at low temperatures, says Wieman. But the really big payoff will be if we can get to Bose condensation. That’s the sort of experiment that wins a Nobel Prize.
The chance to probe the edge of physics has kept Wieman’s and Kleppner’s teams, as well as Chu’s, straining to creep closer and closer to absolute zero. Chu, a laser cooling pioneer, is focused on achieving the coldest possible temperatures primarily to explore potential applications-- from superaccurate atomic clocks to superprecise means of measuring gravity. Wieman and Kleppner are more interested in getting clouds of atoms cold and dense enough to make the transition to Bose condensation. Despite Wieman and Kleppner’s shared goal, however, the physicists’ approaches are very different: Wieman and his students in Boulder slow cesium atoms by bombarding them with laser light, while Kleppner and his team at MIT are chilling hydrogen atoms in a 15-foot-tall thermos bottle with an evaporative process akin to sweating. Either group could achieve Bose condensation before year’s end.
And either way, it’s going to be a struggle. The closer you get to absolute zero, the harder it is to get any colder, as the moribund atoms become ever more prone to boosting their temperature by soaking up energy from anything in their immediate environment. Absolute zero itself is out- of-bounds: it is a limit that one can get infinitely close to but never reach. The uncertainty built into quantum mechanical behavior means that a minimum energy always remains. But that’s okay with Wieman and Kleppner; they just want to get close enough to trigger Bose condensation. It’s the lowest possible energy state for matter, says Wieman. For all practical purposes, it is absolute zero; there is no physics beyond it.
Wieman is already closer than anyone else to absolute zero. He chills cesium atoms in part because they are well suited to absorbing laser light. Each kind of atom absorbs light of only certain energies, like a picky eater who will swallow only certain bite-size pieces of food. A laser, in turn, produces a well-ordered stream of these tiny energy packets, or photons. The size of the packets can be finely adjusted. Wieman tunes his laser so that the photons have just the right amount of energy to react with the cesium atoms. (Photons of more or less energy go by unnoticed; the atoms simply don’t see them.) The photons bombard the atoms and gradually slow them down, like a stream of Ping-Pong balls bouncing off a bowling ball.
The slower the atoms, the cooler. At room temperature, atoms rocket around at more than 1,000 feet per second; but when Wieman’s laser gets through with them they are drifting along at about a half-inch per second. In addition, to trap these chilled, lazy atoms--that is, to keep them closely herded together and prevent them from floating into a wall and warming up--Wieman’s laser beams hem the atoms in like opposing fire hoses trained on a soccer ball. The laser-trapping is assisted by magnetic fields that get stronger near the edge of the cloud, pushing stray atoms back toward the center.
The entire cooling process can be watched live on television in Wieman’s lab via two cameras trained on the glass tube. Normally, atomic physics isn’t very mediagenic; in most experiments with atoms, there are too few of them and they are too spread out to bounce back enough photons to register much of an image. Indeed, Wieman’s tube-encased vapor is invisible too, until he turns on the laser beams. Within a second or two the atoms are chilled to within a few millionths of a degree of absolute zero and packed tightly enough to reflect the laser light toward the camera. On the television screen, a vague greenish haze appears and builds into a distinct spherical cloud, an eerie image of the coldest atoms ever to exist.
Wieman is a slight, amiable man who, though 41, looks awfully boyish for someone trying to create a new form of matter. In addition, he’s achieved his record low temperatures on a shoestring, all the while disdaining the outsize budgets that characterize so much of contemporary experimental physics. His setup consists of a few thousand dollars’ worth of equipment that sits on a lab table, while Kleppner and others have spent hundreds of thousands of dollars and filled rooms. I’ve always been sort of cheap, shrugs Wieman.
For recreation, Wieman takes off on foot with his wife into the parklands surrounding their home in the hills outside Boulder. This gives him a much-needed opportunity to kick back and talk about, well, laser cooling. His wife, Sarah Gilbert, is a physicist at the nearby National Institute of Standards and Technology who also traps and cools atoms. We rewrite each other’s papers, he says. It’s a situation I’d highly recommend to any dedicated physicist.
Clearly, Wieman is focused. He became so taken with lasers and atomic physics as an undergraduate at MIT, recalls Kleppner (a former Wieman mentor), that he managed to get himself excused from half his classes to do lab research, even sleeping in the lab. There he built lasers. Lasers create intense beams of identical photons through a cascade effect that can be compared to a raft stacked with bowling balls floating in a pool. The raft will float calmly until a single ball falls off into the pool; then the waves from that event will rock the raft, causing more balls to fall, which will set up stronger waves, which will cause even more balls to fall, so that soon balls are falling off the raft in a cascade. In a similar way, the atoms in a laser don’t do very much until a few photons come streaming through, shaking the atoms and causing them to emit photons, which cause more atoms to emit photons, and so on. The trick is to get the photons emitted in synchrony, which produces a highly concentrated beam. Wieman focused on a new type of laser that employed fluorescent dye to emit large quantities of photons. Kleppner remembers that Wieman’s contact with the dye often left him walking around with Day-Glo orange hands.
After MIT, Wieman received his Ph.D. at Stanford and then spent seven years at the University of Michigan before coming to the University of Colorado in 1984. At Michigan, Wieman was already doing work with lasers and atoms that earned him recognition as one of the country’s top young scientists. His research had to do with confirming theories of parity nonconservation, which relates to the properties of particles and their mirror images. At the time he moved to Boulder, he was looking into the possibility of adding more lasers to his lab in an effort to improve the accuracy of his experiments. But the dye lasers used by physicists cost about $80,000 each and are extremely sensitive. If someone sneezed during one of my experiments, everything had to be recalibrated, Wieman explains.
As a long-shot alternative, Wieman decided to check out the tiny, $200 diode lasers used in CD players, which produce cascades of photons in a chunk of semiconductor material such as gallium arsenide instead of in a gas (a diode is an electrical component etched into the semiconductor to help direct the flow of electric current). These solid-state devices were far less susceptible to sneezes than the delicate arrays of optical equipment used in other lasers. Physicists had not embraced these little mass-produced lasers, because their beams were weak--that is, they emitted relatively few photons per second. They were also difficult to tune. But Wieman realized that the strength of the beam wasn’t important for the applications he had in mind; and as for the tuning problems, he worked out a number of simple tricks that made the wavelength of the laser light much easier to control.
It wasn’t long before he could tune the cheap diode lasers nearly as accurately as standard lasers. As a bonus, the diode lasers could be detuned--that is, the wavelength of the light could be shifted--much faster than a standard laser. That property came in handy when dealing with atoms that absorbed different wavelengths of light depending on which excited state they had jumped into.
By this time Wieman had become aware of the laser-cooling experiments going on at several labs around the world. It occurred to both me and one of my students that we might be able to do these experiments just as well--and a lot more cheaply--with diode lasers, he says. In 1986 Wieman decided to throw himself into laser cooling. By that time, a group led by AT&T; Bell Labs physicist Steven Chu (now at Stanford) had already employed dye lasers to obtain temperatures as low as 240 microkelvin, or millionths of a degree on the Kelvin scale (the Kelvin scale is zero at absolute zero, with a degree equal to a centigrade degree). Within a year, Wieman’s group had matched that temperature with diode lasers.
In fact, it was in the context of friendly competition with Chu that Wieman got seriously hooked on laser cooling. Chu’s group noticed that atoms were being lost from their traps. Studies designed to uncover the cause suggested that whatever was going on, the intensity of the laser beam was unrelated. This result didn’t sound right to Wieman. His own group, after probing the problem, came up with contradictory results. In the end, Wieman turned out to be right.
That was more than a little ironic. The highly respected Chu hadn’t until recently even acknowledged Wieman as serious competition; after all, laser cooling and trapping is Chu’s longtime specialty, whereas Wieman, with his bargain-basement equipment, came out of left field. Carl’s major contributions are in parity nonconservation, Chu insists. Still, the two remain good friends; at a recent atomic physics conference in Italy, they took the opportunity to head off into the mountains together to hike and compare notes. Wieman, however, remarks: We were undoubtedly competing to see who could get to the top first; we’re rather famous for being so competitive.
Between 1988 and 1991 Wieman’s group, Chu’s group, a group at the National Institute of Standards and Technology led by William Phillips, and a group at the Ecole Normale Supérieure in Paris led by Claude Cohen- Tannoudji took turns pushing the state of the art in laser cooling, each getting its cloud just a little colder. As one group would find a better technique for tuning the laser to the right energy at just the right time-- or a way to set up the magnetic field so it would do a better job of pushing the atoms in toward the center of the cloud--the lowest temperature record, already down near 100 microkelvin, would drop by another dozen or so microkelvin--with the record being seized only months later by one of the other groups. In 1990 the French group seemed to have decisively trounced the rest of the field with a temperature of 2 microkelvin, a feat that stunned physicists. But three weeks later, Wieman’s group announced it had achieved a temperature of just over one microkelvin. That record still stands.
Besides lowering the temperature record, Wieman had also dramatically lowered the budget required to perform laser cooling experiments. His lasers, for example, cost about one four-hundredth as much as those that people thought were required for the job.
This meant that laser trapping and cooling was suddenly accessible to assistant professors around the world. Wieman has even been designing a laser cooling setup intended for undergraduates. Chu is quick to credit Wieman on this score: He’s turned laser cooling from a gee-whiz experiment into a people’s experiment.
Wieman had been aware of Bose condensation, but he hadn’t thought of it as an interesting goal. Then, in 1990, after giving a talk on his laser cooling work in Santa Fe, Wieman was approached by Berkeley physicist Raymond Chiao. He was all excited about the idea that I might be able to get to Bose condensation, recalls Wieman. He was convinced a lot of important physics might fall out of it. That just hadn’t occurred to me. Chiao’s enthusiasm proved infectious. Within a few months Wieman and his group had decided to go for the Bose. It has turned out to be a much more difficult achievement than Wieman first thought; as close as he is to absolute zero, he is not quite close enough, and so far his trapped cloud of cesium atoms has resisted all efforts to coax the last traces of heat from them. I thought it would take me a year, he says. I was naive.
He might have known better; his mentor Kleppner has been trying to reach Bose condensation for 14 years, using a rather daunting cooling apparatus--essentially a giant thermos bottle. Kleppner, a mild-mannered man with thin white hair immune to combs, built his first cooling chamber in 1980, injecting a cloud of hydrogen atoms into the chamber and cooling them with supercold liquid helium. He got the hydrogen down to three-tenths of a degree Kelvin, but still the cloud was only one-thousandth as dense as is necessary for Bose condensation. After all, Bose condensation occurs when atoms overlap, so that in addition to being cold and fuzzy the atoms have to be close together.
To put the squeeze on the atoms, in 1982 a team led by Kleppner and MIT’s Thomas Greytak built a piston inside the chamber to compress the hydrogen cloud, taking care to do it slowly enough to avoid heating the cloud more than a tenth of a degree. That made the cloud 50 times as dense, but they were still short by a factor of 20. When they tried to increase the density further, the atoms started clumping into molecules, effectively leaving the cloud. Dropping to still lower temperatures would have allowed them to get closer to Bose condensation without increased density. But at ultralow temperatures, the atoms would stick to the walls of the trap. For several years the team struggled without success to get around this obstacle. We really felt stuck, recalls Kleppner.
Luckily, a postdoc named Harald Hess had an idea. If hydrogen was sticking to the walls, he suggested, then why not get rid of the walls and use a magnetic trap to confine the cloud within the chamber? Magnetic trapping wasn’t a radical notion in itself, but it had never been applied to a relatively large cloud of atoms inside a sealed chamber. Nevertheless, Kleppner and his group gave it a shot, surrounding their device with a set of superconducting magnets.
As a bonus, Hess realized the magnetic trap would allow them to use a powerful new way to cool the cloud, a trick known as evaporative cooling. In a drop of liquid, the fastest atoms tend to jump right out into the air, leaving behind the slower, cooler molecules; then the fastest of these fly off, and so on, until the drop becomes relatively cool. That’s how sweating works, and how a cup of coffee cools off. Cooling naturally stops when none of the molecules have enough energy to leave the liquid. But Hess pointed out that by altering the strength of the trap as the atoms cool, the cooling could be forced to continue at lower temperatures. The scenario goes something like this: Hydrogen is injected into the trap. The fastest atoms immediately leap out, leaving behind only the slower, cooler ones. Then Kleppner’s team slowly lowers the strength of the trap; now the fastest remaining atoms can wiggle out, leaving behind an even cooler cloud. As the trap’s strength keeps falling, successively slower and slower fastest atoms jump out of the trap, and the temperature keeps sliding down. Now Kleppner and his team--which also includes graduate student Jon Sandberg--are down to a temperature of 100 microkelvin.
That’s 100 times warmer than Wieman’s coldest temperature, but Kleppner doesn’t have to go nearly as low as Wieman does. As it turns out, extremely light atoms such as hydrogen are even more susceptible than heavier atoms to the uncertainty inherent in quantum mechanics. With only one-hundredth the mass of cesium, a hydrogen atom becomes spread out with less prodding. Therefore, the atoms will begin to overlap their neighbors at a higher temperature than will cesium atoms. Kleppner believes his group will reach Bose condensation at 30 microkelvin.
First, however, the researchers need a better way to observe what the atoms are actually doing. At present, the only way they can study the atoms in the trap is to dump them out and measure the heat they give off as they group into H2 molecules. Once they can actually see the cloud, they’ll be able to fine-tune such details as the size of the cloud and the rate at which the trap’s strength is lowered during evaporative cooling. To see the cloud, they’re employing Wieman’s favorite tool: lasers. The chilled hydrogen atoms absorb the photons from laser light beamed into the trap; the atoms then emit the photons as ultraviolet radiation. An ultraviolet detector picks up and records a signal every time a photon hits it. By moving the beam around inside the chamber, the researchers can tell where the atoms are and thus determine the shape of the cloud. When Bose condensation occurs, the shape should change abruptly as the atoms condense into the center.
The laser will also give Kleppner’s team a direct measure of the velocity of the hydrogen atoms: essentially they’ll see a Doppler shift. That is, the wavelength of the light absorbed by an atom moving away will be stretched, while the wavelength absorbed by an atom moving closer will be squeezed together--just as the sound waves from a train whistle get stretched (lower frequency) as they die off into the distance and get squeezed (higher frequency) as they approach. By varying the frequency of the laser beam and then monitoring the ultraviolet detector, the team will be able to tell just which frequencies of light the atoms are picking up. When the atoms condense into the Bose state, there will be no detectable shift. You’ll find that a large fraction of the atoms are simply standing still, Kleppner says. That’s very different from what you’d normally see.
Wieman plans on doing a little borrowing himself: he hopes to proceed by enlisting evaporative cooling. Right now Wieman’s vapor isn’t dense enough. Under normal conditions a cloud of vapor would become denser as it gets colder, but laser cooling tends to thin the cloud. That’s because the laser photons used to cool the atoms can also knock them farther apart. They rarely encounter each other on their leisurely travels. But collisions are crucial for evaporative cooling, which works by allowing just the hotter-than-average atoms to escape. If there are no collisions, the cloud will quickly run out of hotter-than-average atoms; the cloud will be a little cooler, but the process will grind to a halt. If, on the other hand, the atoms collide after the hottest ones escape, then their temperatures will be rescrambled by the collisions and a new group of hotter-than-average atoms will be formed.
At the moment the low density of Wieman’s cloud precludes sufficient collisions. To boost the density, Wieman and his group have built a new cooling setup consisting of two separate but connected chambers, one equipped with laser beams and the other with a magnetic trap. The idea is to get the atoms cold and dense in the laser chamber, then transfer them to the magnetic chamber for temporary storage; meanwhile, a new batch of atoms would be cooled in the laser chamber and then added to the first batch in the magnetic chamber. We thought we could just keep packing the magnetic trap with more and more atoms until they were dense enough for evaporative cooling, explains Wieman. So far, the approach has fallen short of its potential for two reasons: a vacuum pump failed, and the atoms spread out too much during the transfer between chambers. The first problem is easily fixable. But the second requires the designing of magnets whose size, shape, and placement will create a magnetic field that focuses the spreading stream of atoms in the same way a magnifying lens focuses the rays of the sun. Such a magnetic lens could take months, or even years, to build.
Recently a newly designed magnetic trap allowed the group to achieve a density of more than 100 trillion atoms per cubic centimeter, or three times the previous density--impressive, but no evaporative cooling. For a few days we thought we had something ten times as big as that, but we were wrong, he says. We were pretty disappointed.
The group is placing a lot of hope in the magnetic trap, though: a new theory suggests that if they can hit precisely the right magnetic field strength, the atoms will experience a sort of resonance, akin to the way a perfectly pitched voice can shatter crystal. According to theoretical calculations, this resonance will suddenly increase the number of collisions by a factor of 100 or more. If that theory is right, we should be able to get to evaporative cooling much sooner, says Wieman, adding that this could mean a matter of a few months. But if the trick doesn’t pan out, it could take a few more years yet.
The good news is that once evaporative cooling begins, the atoms should keep getting denser and denser on their own, as already happens in Kleppner’s trap. The situation is similar to a bunch of bumper cars running around a bowl-shaped floor: if the cars start to lose speed (energy), they’ll naturally tend to settle closer and closer to the bottom of the bowl. Wieman’s and Kleppner’s traps are essentially magnetic versions of a bowl, pushing the atoms toward the center with magnetism instead of pulling them with gravity. Once the atoms begin to lose their heat energy, they also lose their ability to overcome the magnetic forces of the trap.
If Wieman’s or Kleppner’s cloud does get cold and dense enough for Bose condensation, it would first occur right at the center of the trap, where the atoms are most closely packed together. What’s more, the density of the Bose condensate will be higher than that of ordinary vapor, with more atoms to scatter light, so the researchers won’t have any trouble telling when they’ve reached their goal. When we hit Bose condensation, Wieman explains, we’ll suddenly see this incredibly bright, shiny ball right in the middle of the cloud.
If Wieman doesn’t see the shiny ball, his group will just try another approach. That’s the advantage we have over Kleppner’s group, says Wieman. If they want to change something, it takes them six months to take their experiment apart, make the change, and put it back together. With ours, we can have it changed by tomorrow.
Meanwhile, neither Wieman nor Kleppner can be completely confident that Bose condensation will occur under any conditions. The theory seems pretty solid, but you can’t be sure until you see it, says Wieman, noting that physicists don’t agree on how to calculate the properties of atoms that are on the verge of Bose condensation. In fact, they’re only measuring the bright reflectivity of the cloud because, explains Wieman, it’s so mundane we’re certain that it will happen. There’s been a suggestion that the cloud will shine even more brightly than can be explained simply by increased density. The prediction came from equations, and even Wieman doesn’t know whether to take it seriously. It came from one of the few papers I have seen that wasn’t obviously wrong; it comes out of the mathematics. We don’t understand why. Physicists are just starting to think about this stuff.
Some theorists say that even if the Bose condensate exists, the atoms would take forever to make the transition. In effect, that puts it as off-limits as absolute zero itself. Just in case, two team members prepared a consolation prize for themselves should Bose condensation prove elusive: a toy version of their magnetic trap that holds a piece of magnetized rock eerily suspended in midair. We’ve already applied for a patent, says former Wieman postdoc Eric Cornell. If Bose condensation doesn’t make us famous, this will make us rich.
Wieman isn’t ready to settle for consolation prizes. I could be wrong about what’s happening here, he says, but my own gut feeling is that we’ll condense. Kleppner professes optimism, too. We’d love to see Bose condensation happen here, he says. But in any case, it’s nice to be on such good terms with one’s competition.
