May 2009 Archives

Traveling smoothly through a turbulent medium is no mean feat, as anyone who regularly flies in an airplane can attest. Scientists have investigated how fish navigate through turbulent currents, but until recently they had not addressed the analogous issue of animal flight through turbulent air. Now biologist Stacey Combes has filmed male orchid bees (genus Euglossa) flying in turbulent airstreams and, with colleague Robert Dudley, has described the effects of the turbulent air on the bee’s flight stability and maximum speed. Combes induced the bees to fly in a turbulent airstream by luring them with an attractive scent. As the airspeed increased, the bees found it increasingly difficult to avoid the rolling illustrated in the left image. When the airspeed was high enough and maintaining stable flight difficult enough, the bees extended their hind legs, as depicted in the right photograph.

That move increased the moment of inertia about the roll axis by roughly 50% and improved stability, but it also increased body drag and energy expenditure by about 30%. In a second experiment, Combes altered the turbulence of the stream by inserting different geometric grids. Bees flying in the lower-turbulence environment were able to reach higher speeds before instabilities caused them to be ejected from the air stream. (S. A. Combes, R. Dudley, Proc. Natl. Acad. Sci. USA, doi:10.1073/pnas.0902186106.) —Steven K. Blau

Related link: Dragonfly Flight, Z. Jane Wang, Physics Today October 2008, page 74.

In an effort to reduce the pervasive smog in Beijing (see photo), Chinese authorities imposed measures to restrict traffic and close factories around the city during the 2008 Olympics. Were those efforts successful in reducing total atmospheric aerosol? Climate scientists Jan Cermak and Reto Knutti at ETH Zürich in Switzerland attempted to find out. They began by comparing absolute values of aerosol optical thickness—transmittance measurements from the Moderate-Resolution Imaging Spectroradiometer aboard NASA's Terra satellite—for the years 2002–08. They found that within a 150-km radius of Beijing, the average 2008 AOT value was more than 14% lower than the previous years. But what would it have been without the mandated emissions reductions? To answer that question, the researchers used a neural network approach: With data from the preceding six summers, they trained a model to predict AOT as a function of relative humidity, wind velocity, and precipitation. The model then predicted that within a 500-km radius of the city, AOTs in 2008 would have been 10%–14% higher than the actual observed values; the model was less accurate when larger regions were analyzed. Although the magnitude of the reductions was lower than expected, the emissions restrictions did have a statistically significant local impact. (J. Cermak, R. Knutti, Geophys. Res. Lett. 36, L10806, 2009, doi:10.1029/2009GL038572. Photo by Michael Silverman, 6 August 2006.)—Jermey N. A. Matthews

Late Stone Age metal smiths added a little tin to copper to usher in the eponymous Bronze Age; over the ensuing five millennia, many new combinations and applications of the two metals have appeared. Today, for example, a thin tin coating on a copper substrate often serves to interconnect electronic components of various kinds, such as are found in medical devices and satellite equipment. Unfortunately, micron-sized tin whiskers (see figure) sometimes arise spontaneously and can short out the equipment, with great technological and economic repercussions. After decades of widespread effort, the actual mechanism underlying such whisker growth has only now been elucidated. Led by Eric Mittemeijer, a group from the Max Planck Institute for Metals Research in Stuttgart, Germany, working with the Robert Bosch company, examined growing whiskers and their crystallographic environment. Using Laue diffraction measurements made at the Advanced Photon Source at Argonne National Laboratory in Illinois, the researchers noted that at the Cu–Sn interface, Cu₆Sn₅ develops along the tin grain boundaries and is most pronounced directly beneath a whisker's root. That observation, coupled with residual strain measurements, led the team to propose the following mechanism: Deep penetration of Cu₆Sn₅ into the 3-μm-thick tin layer induces in-plane compressive strains near the Cu–Sn interface and in-plane tensile strains nearer the surface. Out-of-plane and in-plane strain gradients—not the strains themselves—then provide the driving force that leads to whisker growth by transporting Sn atoms to the whisker nucleation site as a strain-relief mechanism. (M. Sobiech et al., Appl. Phys. Lett., in press.) —Stephen G. Benka

Last fall, the ATIC balloon collaboration reported a tantalizing peak near 500 GeV in its measured spectrum of high-energy cosmic-ray electrons (Physics Today, January 2009, page 16). The peak suggested that 500-GeV dark-matter particles of the kind predicted by extra-dimensional extensions of standard particle theory might be annihilating each other in nearby accumulations of dark matter to produce energetic electron-positron pairs. Now NASA's recently launched Fermi Gamma-ray Space Telescope has measured the electron spectrum out to 1 TeV with much higher statistics (see the figure). Fermi is designed primarily to record high-energy gammas, but it can also detect electrons.  The Fermi data show no narrow spectral feature near 500 GeV, nor anywhere else. But above 100 GeV, they do exhibit a growing excess over the predictions of a conventional diffusive model of electrons from very distant astrophysical sources. The positron spectrum measured by the orbiting PAMELA magnetic spectrometer showed a similar excess above 10 GeV. (Neither ATIC nor Fermi can distinguish electrons from the much rarer positrons.) Taken together, the Fermi and PAMELA results suggest that our local galactic neighborhood harbors either an undiscovered astrophysical electron-positron source (most likely a pulsar) or a dense concentration of unidentified, heavy dark-matter particles. Much should be revealed when the Fermi collaboration reports electron spectral data beyond 1 TeV and the spectrum of the diffuse gamma-ray background out to 1 TeV. (A. A. Abdo et al., Fermi collaboration, Phys. Rev. Lett. 102, 181101, 2009; O. Adriani, PAMELA collaboration, Nature 458, 607, 2009.) —Bertram Schwarzschild

Earth's atmosphere at altitudes between 80 km and 110 km is a no man’s land, accessible to neither the highest research balloons nor the lowest orbiting satellites. But it is substantial enough to vaporize billions of meteors—most smaller than a grain of sand—that intersect Earth’s orbit every day. Tons of metal atoms ablated from those meteors circulate in pervasive winds that can reach hurricane speeds up to 150 m/s and create enormous sheers. Measurements of those speeds have been made over the past half century using rockets to disperse luminescent tracers that can be tracked as they’re swept up in the winds. Meers Oppenheim and colleagues from Boston University, Los Alamos National Laboratory, and Jicamarca Radio Observatory (JRO) east of Lima, Peru, now offer an alternative approach that avoids expensive rocket launches and can be performed nearly continuously: They use radar to track the meteors’ plasma trails that are also swept up in the winds. Four years ago and again in 2007, the team gained access to JRO's phased array of nearly 19 000 dipole antennas (shown here). Located at the geomagnetic equator, the radar facility provides sufficient power to capture nonspecular reflections from ionization trails as they evolve over many seconds. And interferometry allowed the team to build a vector profile of wind speeds at different altitudes using data from many meteors. The spatial resolution of the profile is a few hundred meters, comparable to that from the tracer experiments. (M. M. Oppenheim et al., Geophys. Res. Lett., in press.) — R. Mark Wilson

A tokamak traps plasma inside a helical magnetic field that winds around, and delimits, a bagel-shaped confinement vessel. For a range of reasons—some fundamental, some practical—the magnetic field can't be made strong enough to prohibit escape. Confinement is therefore imperfect: If the plasma becomes concentrated in eddies or other energetic instabilities, it can burst out of the confining field like a hernia. Now, a team from MIT's Alcator tokamak (shown here) has demonstrated a method to stabilize tokamak plasma. Rotation is the key. When the whole plasma is made to rotate in its vessel, instabilities at the edge of the plasma are suppressed. And if the rotation has shear, turbulent eddies within the plasma are broken up. Modest rotation has been observed before at various tokamaks as an unintended consequence of the methods used to heat plasma. The MIT team, led by Yijun Lin and John Rice, has found a way to boost rotation twofold using VHF radio waves. The waves resonate with the cyclotron gyration of helium-3 ions, which are spread throughout the tokamak's deuterium fuel as a minority component. It's not clear how the momentum is transferred from the waves to the ³He²⁺ ions and then to the D⁺ ions. Still, by adjusting the proportion of ³He, the MIT team found they could maximize the rotation gain. In principle, the MIT method can be applied at ITER, whose design is being finalized at its site in Cadarache, France. (Y. Lin et al., Phys. Plasmas 16, 056102, 2009.) — Charles Day

To build a quantum computer, you need to pull off a delicate balancing act. Whatever objects embody the qubits must preserve their quantum coherence—which requires isolation. But the qubits must also be initialized, entangled, manipulated, and read out—which requires violating their isolation. Michael Biercuk, Hermann Uys, and John Bollinger of NIST in Boulder, Colorado, and their collaborators have just demonstrated a method for preserving qubit coherence using trapped ions, one of the leading qubit contenders. The NIST method is based on spin-echo, a venerable technique borrowed from nuclear magnetic resonance. Environmental noise causes the information encoded in spins, electronic or nuclear, to decay and would induce fatal errors in a spin-based quantum computer. If the noise fluctuates slowly, administering a single short electromagnetic pulse of the right frequency and duration—a spin echo pulse—will send the spins back to their original state; in principle, a sequence of such pulses should boost coherence times. In 2007, Götz Uhrig proposed that modifications to a standard sequence of spin-echo pulses could prolong coherence by orders of magnitude, even if the noise fluctuates rapidly. To test Uhrig’s scheme, the NIST team loaded about 1000 beryllium ions into a Penning trap (see image; the spacing is 10 μm). Optical and microwave pulses initialize the ions in a superposition of spin-up and spin-down states. Without intervention, information embodied by the relative phase between spin-up and spin-down ions would be lost. The NIST experiment not only validated Uhrig's prediction, it also extended his work. Using measurement feedback, the NIST team found sequences that could forestall dephasing for arbitrary and unknown noise spectra. (M. J. Biercuk et al., Nature 458, 996, 2009.) — Charles Day