This Week in Science

Science  20 Apr 2001:
Vol. 292, Issue 5516, pp. 393
  1. Stable Vortex Lattices in Bose-Einstein Condensates


    In macroscopic quantum systems, the application of an external parameter—a magnetic field in the case of superconductors, or an induced rotation in the case of superfluids—results in the formation of vortices. These vortices penetrate the system and dissipate energy, and they also possess quantized units of angular momentum. Abo-Shaeer et al., p. 476 (see the 23 March news story by Voss) now show that large, ordered arrays of vortices can be formed and observed in a rotating Bose-Einstein condensate. The vortices are much more stable than had been predicted and may prove a useful testing ground of vortices in superfluids.

  2. Eros: A Beaten Asteroid

    The NEAR-Shoemaker spacecraft has mapped the asteroid 433 Eros for about 1 year. In October 2000, the spacecraft completed a close-approach to the asteroid surface and obtained images with a resolution of 1 to 5 meters with the multispectral imager (Veverka et al., p. 484) and collected complementary topographic details with the laser rangefinder (Cheng et al., p. 488). The fine-scale observations include meter-sized blocks, degraded craters, smooth, flat-floored craters, a lack of small craters, and evidence for linear fractures. These features suggest that Eros has a complex regolith produced by multiple impact events in which the size and strength of the impactors varied to produce its heterogeneous surface character.

  3. An Excited Bose-Einstein Condensate

    The formation of a Bose-Einstein condensation (BEC), whether as a liquid in the case of helium (He) or as weakly interacting, dilute atomic gases (seen for H, Li, Na, and Rb), dissipates energy during cooling. This process would seem to require that the atoms come together in their lowest energy or ground state, but Robert et al. (p. 461; see the Perspective by Inguscio and the 23 March news story by Voss) now show that a cloud of metastable helium atoms, excited some 20 electron volts in energy into the 2 3S1 triplet spin state, can be coaxed into forming a BEC. In their experiment, a magnetic field spin-polarizes the atoms in cloud and forces out any atoms with the opposite spin. With the spins all pointing the same direction, inelastic collisions that would otherwise destroy the delicate condensate are suppressed. They have also implemented a single-atom detection scheme with a multichannel plate that represents a large step on the road to quantum atom optics. This work may open a route to forming a dilute-gas BEC of ground-state He.

  4. Quantum Solutions to Difficult Problems

    There are certain problems, for example, factoring or searching for the shortest route connecting several points (the traveling salesman problem), that classically appear to grow exponentially in computational time with the number of digits (in factoring) or points that can be tried (in a search). So far, classical algorithms for solving the wide range of related “NP-complete” problems that could yield answers in polynomial time have remained elusive. From simulations, Farhi et al. (p. 472; see the news story by Anderson) show that quantum computers may be more effective at solving such problems. At least for the number of qubits they could simulate on their classical computer, they show that the adiabatic evolution of their quantum computer would yield a result to some examples of NP-complete problems in polynomial time.

  5. Adsorption on Very Small Clusters

    Small metal clusters have been characterized through a number of spectroscopic methods, but there is still great interest in the details of how these clusters react with small molecules. Nauta et al. (p. 481) have made complexes between magnesium atoms and small clusters (two and three atoms) and HCN in liquid helium droplets. In this environment, high-resolution infrared laser spectroscopy of the C-H vibrational band reveals rotational state information that allows much structural information to be deduced. A qualitative change in the adsorbate-metal cluster bonding occurs between the Mg2-NCH and Mg3-NCH complexes that is reflected in a change in Mg-N bond length.

  6. How Cells Sense Oxygen

    Mammalian cells are exquisitely sensitive to changes in oxygen concentration. When oxygen becomes limiting (hypoxia), the cells increase the transcription of genes that enhance oxygen delivery or that facilitate metabolic adjustment to reduced oxygen availability. This adaptive response is mediated by hypoxia-inducible factor (HIF), which is stable under hypoxic conditions but is degraded in the presence of oxygen by a ubiquitin ligase containing the von Hippel-Lindau (VHL) tumor suppressor protein. Ivan et al. (p. 464) and Jaakkola et al. (p. 468) have found that VHL binds to a specific domain of the HIF-1 subunit only when a conserved proline in that domain is hydroxylated (see the Perspective by Zhu and Bunn). The enzymes that catalyze prolyl hydroxylation require oxygen as a substrate, which suggests that this protein modification plays a key role in cellular oxygen sensing. This discovery could open up new therapeutic possibilities for the many diseases in which hypoxia plays a crucial role, including cancer, ischemic heart disease, hypertension, and stroke.

  7. Big Mama Knows Best

    African elephants live in matrilineal family groups that range over large areas and that frequently interact with other similar groups. McComb et al. (p. 491; see the cover and news story by Pennisi) show that the oldest female, or matriarch, acts as a repository for the group's social knowledge. Multiple playback experiments were performed during a period of 9 years to test vocal discriminatory abilities. Families with older matriarchs were better than those with young matriarchs at distinguishing the contact calls of other groups well or poorly known to them. These superior abilities appear to correlate with greater reproductive success on families with older matriarchs and suggest that age and experience may influence reproductive success through its effects on the acquisition of social knowledge. However, older elephants have the larger tusks prized by hunters and poachers; thus, whole populations may be affected by the removal of a few key individuals.

  8. What Parents Will Do for Children

    Understanding why species differ in their parental care tactics has been a central goal for evolutionary ecologists studying life history evolution. Ghalambor and Martin (p. 494; see the news story by Pennisi) test the prediction that life-history differences between bird species result in differences in how parents resolve the trade-off between feeding their young and reducing the predation risk to their offspring and to themselves. In a survey of nearly 200 bird species, they show that clutch size is negatively correlated with adult survival. They also manipulated predation risk to phylogenetically paired species in North and South America and show that parents with smaller clutch sizes and greater chances of survival are less willing to place themselves at risk than those with large clutch sizes.

  9. More Efficient in Groups


    Competition between organisms that share a common glucose resource leads to an evolutionary dilemma analogous to the “tragedy of the commons”—overexploitation of a common resource. Pfeiffer et al. (p. 504; see the Perspective by Cox and Bonner) present simulation models and a range of examples to show that organisms using pathways that produce adenosine triphosphate (ATP) with high rate but low yield (anaerobic) can outcompete those that produce ATP with low rate but high yield (aerobic). This situation leads to an inefficient use of a vital resource. They also suggest that the evolution of respiratory sugar metabolism required cooperation between cells, which suggests that an energetic bonus was reaped when the transition was made from single cells to multicellularity. This study is an example of how concepts derived from whole-organism ecology and evolution can apply to a very different sphere—the evolution of biochemical pathways.

  10. The Protein Code Turns 21

    All organisms construct proteins from the same set of 20 amino acids (the only known variants being formyl-methionine and selenocysteine, although many amino acids can be modified after their incorporation into polypeptide chains). Is it possible to extend the set of 20 amino acids in vivo to include new chemical functionalities in proteins. Two papers report success toward this goal in Escherichia coli (see the Perspective by Böck). Using genetics, Döring et al. (p. 501) identified mutations in the editing function of valyl-tRNA synthetase that result in high levels of incorporation of the nonnatural amino acid aminobutyrate. Wang et al. (p. 498) used a combination of genetics and molecular biology to add an orthogonal tyrosyl tRNA-tyrosyl tRNA synthetase pair with altered substrate specificity so that they could incorporate O-methyl-L-tyrosine at high fidelity.

  11. Sharing the Roundabout

    The midline of the fruit fly and the zebrafish share more than just topology. Fricke et al. (p. 507) show that zebrafish use a receptor encoded by the gene astray for guiding axons from the developing eyes to the brain, crossing the midline on their way to form the optic chiasma. The astray receptor is similar to the receptor encoded by the gene roundabout that is responsible for guidance of growing axons across the midline in Drosophila. Chimeras made by exchanging eyes of mutant and wild-type zebrafish demonstrated that the relevant site of expression of astray is in the eye.

  12. Small-Diameter Inorganic Nanotubes

    The bulk of work related to the synthesis of nanotube structures has been on carbon. Because of the wealth of unusual properties associated with carbon nanotubes, efforts are being made to create nanotube structures from compounds that resemble graphite. Using C60 as a catalyst and transport agent during the growth process, Remskar et al. (p. 479) report on a synthesis route for preparation of bundles of molybdenum disulfide tubes with subnanometer diameter. The low-wear and low-friction properties of MoS2 may find use in tribological applications.

  13. Seeing and Being Aware of It

    The processing of visual signals from the retina of the eye all the way to the highest brain centers has been studied in great detail for many years. What is still unknown, however, are where the cortical signals are that underlie awareness, and when they travel. Pascual-Leone and Walsh (p. 510) used dual transcranial magnetic stimulation, in both area V1 and area MT, in human subjects to investigate the role of feedback projections in visual awareness. Stimulation strength in V1 was adjusted to disrupt the feedback from MT to V1. With MT stimulation alone, the subjects perceived moving flashes of light. However, when V1 was stimulated shortly (5 to 45 milliseconds) after MT, the perception of moving dots was weak or diminished. This finding indicates that recurrent fast feedback connections into area V1 are necessary for visual awareness.