Attractive atoms pick up repulsive habits Talk about doing as you’re told. Physi-

cists now have made rubidium atoms act contrary to their nature. The atoms intrin- sically attract each other, but new experi- ments near absolute zero have induced the atoms to repel one another instead.

In those experiments, Carl E. Wieman, Eric A. Cornell, and their colleagues ob- served surprising rubidium explosions that have yet to be explained. The small bursts are faintly akin to supernovas, say the researchers, who are at JILA, an insti- tute in Boulder, Colo., jointly run by the University of Colorado and the National In- stitute of Standards and Technology (NET).

In 1995, a team headed by Cornell and Wieman made scientific history by cre- ating the first Bose-Einstein conden- sate-a dilute cloud of ultracold atoms that all share the same quantum state (SN: 7/15/95, p. 36). No property of the condensates surprises Wieman as much as the recently observed explosions. “We’re into territory that’s totally un- charted,” he says.

The new findings are “really exciting,” comments William D. Phillips of NIST headquarters in Gaithersburg, Md. Switching atoms from attractive to repul- sive is “something that nature didn’t allow us to do before.”

In threedimensions (top) and projected onto a plane (bottom), a plot shows the density of the condensate after it exploded. Ridges indicate two outgoing sprays of atoms detectable from a spherical burst.

When Cornell, Wieman, and their coworkers made the first Bose-Einstein condensate, they used rubidium-87, an isotope whose atoms naturally repel each other. To create condensates, physi- cists generally choose repulsive atoms such as sodium or hydrogen (SN: 7/25/98, p. 54) so the gaseous condensate won’t easily collapse.

However, by using very few atoms, sci- entists at Rice University in Houston have succeeded in producing conden- sates of lithium-7 atoms, which attract their fellow atoms.

In the new experiments, which will be

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described in an upcoming issue of PHYSI- CAL REVIEW LETTERS, the JlLA team tinkered with the naturally attracting atoms of an- other rubidium isotope, Rb-85.

Using lasers and evaporative methods to cool the atoms to record low tempera- tures of only a few nanokelvins, the team trapped the atoms in a magnetic field whose strength can be tuned with excep tional precision, Wieman says. By in- creasing that strength, the JlLA re- searchers lowered the characteristic, discrete energies at which the rubidium atoms bond into molecules.

Earlier studies had indicated that re- ducing a characteristic energy level far enough would increase the attraction be- tween nearby rubidium atoms. And that ought to boost their chances of forming molecules. Continuing to lower the level, however, should suddenly render the atoms mutually repulsive and then even- tually attractive again.

The JlLA researchers not only made their Rb-85 atoms repulsive but also were

able to fine-tune the magnetic field to keep the atoms in that state, making a stable condensate possible.

The reversible flip from attraction to repulsion stems from quantum interfer- ence among condensate atoms, which behave as waves, explains JILA’s Jacob L. Roberts. “We can pretty much dial up what we want”- attractive or repulsive, weak or strong interactions, he adds.

The fireworks began when the scien- tists raised the magnetic field strength still further. The condensate suddenly re- verted to attraction, shrank beyond de- tection, and exploded, blowing off about two-thirds of its 10,000 or so atoms. So far, “nobody has a convincing explana- tion” for the tiny burst, Wieman says.

Dying stars, in titanic blasts called su- pernovas, similarly implode before eject- ing material and leaving a dense core be- hind. Pushing that analogy is probably not good science, Wieman contends. Af- ter all, the conditions for the two events are vastly different.

Nonetheless, he’s found an unassail- able way to use the analogy: He’s dubbed the eruptions “bosenovas.” -I? Weiss

Depression may play a role in stroke risk Feelings of hopelessness and other

signs of major depression markedly raise a person’s likelihood of suffering a stroke, according to a new analysis of data from a long-term study.

Over an average follow-up period of 16 years, the initial presence of symp- toms of major depression raised the rel- ative risk of incurring a stroke about as much as did a 40-point increase in sys- tolic blood pressure, say epidemiolo- gists Bruce S. Jonas and Michael E. Mus- solino, both of the National Center for Health Statistics.

Severe depressive symptoms posed a greater statistical risk for suffering a stroke than did 13 other factors, including having a high blood concentration of c h e lesterol or high systolic blood pressure, smoking cigarettes, drinking alcohol, shunning exercise, or being overweight, black, diabetic, or elderly. Being male was the only risk factor for stroke that exhibit- ed about the same power as depression.

The relative risk of stroke rose less steeply, but still substantially, among people who cited moderate depressive symptoms, Jonas and Mussolino report in the July/August PSYCHOSOMATIC MEDICINE.

The researchers obtained data from a nationally representative sample of 6,095 individuals who took part in a study of health and nutrition. Participants, ages 25 to 74 years, first completed a medical ex- amination and questionnaires on various topics, including mood. Four items as- sessed the extent to which they felt sad, downhearted, low in spirits, and hopeless to the point of wanting to give up.

Researchers then performed three

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follow-up surveys and reviews of medical and hospital records over a span of as many as 22 years.

Depression’s association with strokes held for men and women, whites and blacks, and younger and older people, Jonas and Mussolino say.

It’s unlikely that vascular disease first led to depression and then to stroke, they add. The findings remained nearly as strong after excluding either volun- teers who suffered a stroke in the first 10 years of the study or those who began the study with recognized heart disease or diabetes.

Earlier prospective studies of depres- sion and strokes have yielded inconsis- tent results. Other data link depression to an elevated risk of hypertension and heart disease (SN: 7/31/93, p. 79).

Depression alters brain, endocrine, and immune activity. However, it’s un- clear what specific physiological changes resulting from depression might promote strokes, Jonas and Mussolino say. In some cases, depression may cause hy- pertension, which subsequently leads to strokes, they suggest.

The new evidence that preexisting de- pression markedly raises one’s chances of suffering a stroke “is likely to be im- portant for public health,” comments psychiatrist Lewis L. Judd of the Univer- sity of California, San Diego, in the same journal.

“Effective treatment of depressive symptoms may reduce the risk of stroke, but this intriguing suggestion awaits prospective and controlled investiga- tion.” he asserts. -B. Bower

AUGUST 12,2000