Physics Update

Earliest astrophysical object yet seen

2009-07-02T13:49:31Z

A gamma-ray burst detected in April by NASA’s Swift orbiter has a higher redshift (z = 8.26 ± 0.08) than any other celestial entity for which a redshift has been measured—except for the cosmic microwave background (CMB) at z ≈ 1100. That means the massive star whose collapse to a black hole the GRB is presumed to manifest was significantly more distant than any star or galaxy yet observed. Its demise provides a glimpse of the cosmos just 625 million years after the Big Bang. Beyond revealing that such stars already existed back then and providing a first approximation to their formation rate, the discovery adds a potentially powerful new probe to the search for the first generation of stars and the investigation of how UV radiation from early stars reionized the intergalactic medium. After the first moment of cosmic transparency, signaled by the CMB, and before there were stars, almost all the primordial hydrogen and helium was unionized. To reconstruct the history of cosmic reionization, one seeks to measure the absorption by neutral atomic hydrogen of light arriving from sources at various very high redshifts. Such observations with quasars have revealed that cosmic reionization was essentially complete by z = 6 (950 Myr after the Big Bang). But high-redshift GRBs seem to be essential for tracing its earlier stages. GRBs are briefly luminous enough to be seen at much greater distances than quasars. (N. R. Tanvir et al., http://arxiv.org/abs/0906.1577; R. Salvaterra et al., http://arxiv.org/abs/0906.1578.)—Bertram Schwarzschild

Tides in Jupiter and Io

2009-06-29T14:06:09Z

The Moon's gravity raises tides in Earth's oceans. Because Earth's rotation pushes the tidal bulge slightly ahead of the Moon–Earth line, the gravitational attraction between the Moon and the bulge pulls the Moon forward in its orbit and slows Earth's rotation: Our days are getting longer, and the Moon is gaining orbital energy and thus receding. The same phenomenon happens with Jupiter and its four large moons—particularly Io, the innermost of those moons. Conversely, Jupiter also raises a tidal bulge in Io, which causes Io to lose orbital energy. (The friction generated by that ever-moving bulge is thought to be responsible for Io’s dramatic volcanic activity.) Any change in Io’s orbital motion strongly affects the orbits of the next nearest large moons Europa and Ganymede, due to the 1:2:4 ratio, or Laplace resonance, among their orbital periods. Now the Paris Observatory’s Valéry Lainey and colleagues have teased out the previously unknown magnitudes of the Jovian system’s tidal interactions by studying the moons’ orbits. Using a model that explicitly accounts for tidal effects, they fitted numerically integrated orbits to observations made between 1891 and 2007 and solved for the tidal susceptibilities of Jupiter and Io. They found that over the 116-year period, Io’s orbital energy decreased while Europa’s and Ganymede’s increased. In the short term, therefore, the moons are evolving out of their Laplace resonance, but the longer-term trend is unclear. (V. Lainey et al., Nature 459, 957, 2009.) —Johanna Miller

Measuring a quark-antiquark mass difference

2009-06-24T14:00:26Z

In any Lorentz-invariant local field theory, particle and antiparticle masses must be identical. That equality has been verified to high precision for leptons and hadrons, but not for quarks. With one exception, it's impossible to measure quark masses directly because a newly created quark "dresses itself" in other quarks and gluons to form a hadron within 10⁻²² seconds. And hadron masses yield, at best, only rough estimates of the quark masses. The exception is the top quark. Almost 200 times heavier than the proton, the top is by far the most massive quark. Its lifetime of 10⁻²⁴ seconds is much too brief to form a hadron. Thus by measuring its decay products, experiments at Fermilab's Tevatron collider have determined the top mass (173 GeV) with a precision of better than 1%. Those experiments were based on the production of top–antitop pairs, and the analyses assumed that the masses were equal. Now the DZero collaboration at the Tevatron has reanalyzed its data to look for a possible mass difference between the two. One can distinguish the top from the antitop by the charge of an energetic lone decay lepton (a muon, electron, or positron) in the event. The reanalysis yields a mass difference of 3.8 ± 3.7 GeV, consistent with zero. But that wasn't a foregone conclusion. The ultimate unified theory of particle interactions might well be something other than a local field theory—perhaps a string theory. (V. M. Abazov et al., http://arxiv.org/abs/0906.1172.)—Bertram Schwarzschild

A natural quasicrystal

2009-06-22T14:28:12Z

The hallmark of a conventional crystal such as table salt is translational symmetry. Quasicrystals do not have that symmetry and so can exhibit a wider structural variety than their more constrained brethren. But quasicrystals, like crystals, do have long-range correlations and display sharp, structure-revealing diffraction patterns. To date, more than 100 quasicrystals have been synthesized in the lab. Now Luca Bindi of the Natural History Museum of Florence has teamed up with Paul Steinhardt and colleagues from Princeton and Harvard universities to present evidence for a natural version of one of those quasicrystals: icosahedral Al₆₃Cu₂₄Fe₁₃. The material, a 100-μm grain, is from a mineral assemblage (left figure) excavated from the Koryak Mountains in Russia and now housed in the Florence museum; the very complexity of the sample argues for its natural formation. In consultation with his US-based colleagues, Bindi identified the sample as possibly hosting a quasicrystal. The US team then probed a small piece of it with transmission electron microscopy. Diffraction patterns such as shown in the right figure identified quasicrystal regions; the 10-fold symmetry cannot be generated by crystals. Subsequent analysis of x rays scattered off pure quasicrystal grains determined the material’s chemical formula. Geologists and physicists have much to learn about the conditions under which quasicrystals form. The study of natural materials can help address that question and may turn up new, never-before contemplated structures. (L. Bindi et al., Science 324, 1306, 2009.) —Steven K. Blau

Superconducting qubits show promise

2009-06-18T15:28:58Z

Quantum computing is a goal that both excites and challenges researchers, who are working on a wide variety of physical realizations of the basic building block: the quantum bit, or qubit.

One type is the superconducting qubit made from one or more Josephson junctions. The biggest advantage of superconducting qubits is their strong coupling to microwave signals, which can control the qubits and mediate their interactions. The greatest limitation is their short coherence lifetime.

Despite that limitation, recent experiments have demonstrated the kind of precise control that will be needed to make progress toward a viable quantum computer.

In one experiment, Max Hofheinz, John Martinis, Andrew Cleland and colleagues from the University of California, Santa Barbara, showed that they could impose on a microwave resonator any desired superposition of photon-number states. (M. Hofheinz et al., Nature 459, 546, 2009.)—Barbara Goss Levi

Evolution of gene regulation

2009-06-15T14:34:05Z

Lactose isn't present in our guts all the time. To ingest it and other occasional sources of nutrition, Escherichia coli (see figure) must detect the molecules and then make the proteins that help harvest them. That process of on-demand protein production is called gene regulation. It's the subject of a new quantitative analysis by physicists Ulrich Gerland of the University of Munich, Germany and Terence Hwa of the University of California, San Diego. E. coli uses two modes of gene regulation. In (+ +) control, proteins called transcription factors float freely in the cell. When a TF molecule meets its molecular target—lactose, say—it locks onto the appropriate region of the bacterium's DNA and triggers the production of the appropriate protein. In (− −) control, the TF is usually bound to the DNA and blocks protein production until TF's molecular target arrives to detach the TF and lift the block. Both modes are equally effective. When does evolution favor one over the other? The answer, according to Gerland and Hwa, depends on a tug of war between two competing selection principles. The use-it-or-lose-it principle favors (+ +) control during feasts and (− −) control during famines, whereas the wear-and-tear principle favors the opposite. Both selection principles mitigate the adverse effects of genetic mutation but, as Gerland and Hwa found, whether one prevails over the other depends on the size and age of the colony and on how rapidly the food supply fluctuates. Besides quantifying gene regulation, Gerland and Hwa's analysis might help pharmacologists understand and combat the resistance of bacteria to antibiotics. One strain of E. coli, called mar, is resistant to tetracycline, an otherwise potent antibiotic, due to the working of two transcription factors. (U. Gerland, T. Hwa, Proc. Natl. Acad. Sci. USA 106, 8841, 2009.) —Charles Day

Metal-insulator transition in vanadium dioxide

2009-06-11T14:38:40Z

Below 68 °C vanadium dioxide is an insulator. Above that temperature it’s a metal. The nature of the transition has long remained elusive, though, because bulk VO₂ has a domain structure that complicates its behavior. David Cobden and colleagues from the University of Washington have found an elegant way to avoid that difficulty and map the effective phase diagram. The team grew rectangular nanobeams that were thinner than the characteristic domain size of a few microns. They then suspended each beam between two electrical contacts. The metallic and insulating phases differ in lattice constant, but the constrained geometry creates a uniform stress field in a VO₂ beam such that the two phases coexist in a range of temperatures between 68 °C and 105 °C. Thanks to the dramatic change in optical properties that accompanies the transition, Cobden’s team could visually track the nucleation and growth of the metallic phase as a function of temperature. The figure here shows five snapshots of a 20-μm-long beam (red indicates the insulating phase). By measuring the electrical resistance in the coexistence regime, the researchers found that the resistivity of the insulating phase is independent of temperature. That remarkable result, they argue, implies that the phase transition occurs at a fixed carrier density in the material and is consistent with a picture in which electron–electron interactions drive the transition. (J. Wei, Z. Wang, W. Chen, D. H. Cobden, Nat. Nanotech., doi:10.1038/nnano.2009.141, in press.) —R. Mark Wilson

SQUID helps to probe glasslike dynamics in solid helium

2009-06-08T13:50:10Z

Decades ago, theorists predicted that under some circumstances, solids could flow like superfluids. In 2004 Moses Chan and Eun-Seong Kim found evidence of such so-called supersolidity: When a torsion oscillator filled with solid helium was cooled below 200 mK, its resonant frequency increased. Some of the He’s mass appeared to have decoupled from the rest. But subsequent experiments revealed a more complicated picture with many aspects unexplained. For example, the oscillator’s dissipation (related to the damping strength) depended on temperature in a way that the theory hadn’t predicted. Now, Cornell University’s Séamus Davis and colleagues have developed a torsion oscillator, shown in the figure, whose position sensor is a superconducting quantum interference device rather than the usual capacitor. The SQUID allows them to measure the dissipation more accurately and to explore a broader range of frequencies and amplitudes than was previously possible. Among their results is the discovery that when the temperature is abruptly lowered, the oscillator’s resonant frequency and dissipation share the same response time constant, which increases steeply with decreasing temperature—much like the characteristic flow time of cooling molten window glass. Some theorists have postulated that He’s behavior results from an ordinary glass transition, not from supersolidity. The relative magnitudes of the changes in frequency and dissipation rule out that possibility. But the ultraslow low-temperature behavior suggests that a glasslike phase may be involved. (B. Hunt et al., Science 324, 632, 2009.) —Johanna Miller

Entangled mechanical oscillators

2009-06-04T13:36:39Z

Entanglement is one of the hallmarks of quantum mechanics and is a key tool in the burgeoning field of quantum information processing. Generating entangled states has become routine in the quantum realms of photons and of electron and atomic spins. Now John Jost and colleagues at NIST in Boulder, Colorado, the Weizmann Institute of Science, and Lockheed Martin Corp have demonstrated entanglement in separated mechanical oscillators. Each oscillator consists of a pair of ions—one ⁹Be⁺ and one ²⁴Mg⁺—that behave like two unequal masses connected by a spring 4 µm long. The pairs are separated by 240 µm, so their individual vibrational motions are decoupled. To entangle those vibrational modes, the researchers cool the four ions in one zone of a multizone ion trap (shown here) while coaxing them with electrode voltages to line up in a specific order: a Be ion at each end. They next entangle the spins of the two Be ions and then separate the pairs into different trap zones. Lasers tuned to the Mg ions recool the separated pairs while maintaining the Be entanglement. The team finally uses laser pulses to coherently transfer the entanglement from the Be spin states onto the pairs' motional states. The end product is the entangled superposition of vibrational oscillations in the pairs' ground and first excited states. Along the way, the team also demonstrated the entanglement between one ion's spin state and the motion of the other ion pair. Mechanical entanglement and the tools developed to achieve it will be important ingredients for scaling up quantum information processing with trapped ions. (J. D. Jost et al., Nature 459, 683, 2009.)—Richard J. Fitzgerald

Arctic seawater flows south along an unexpected route

2009-06-01T13:51:46Z

Textbooks depict the Gulf Stream, the Kuroshio, and other great ocean currents as smooth, river-like streams. Reality is messier. Gravitationally bound to the spinning globe, the oceans constitute a complex, turbulent system. How turbulence influences one particular ocean current is revealed in a new field and computational study led by Amy Bower of Woods Hole Oceanographic Institution. In 2003–05 her team released 76 floats off the Newfoundland coast at a rate of about six every three months. Her aim was to trace the southward flow of cold arctic water in the Deep Western Boundary Current (DWBC), which hugs the continental slope of the Eastern Seaboard and eventually meets the Gulf Stream off Cape Hatteras. The floats recorded their courses by triangulating signals from a set of moored sound beacons along the route. Like nuclear submarines, the floats surfaced at the end of their voyage and beamed up their recordings to a satellite. To their surprise, Bower and her colleagues found that only seven floats followed the DWBC's coastal route. Most took a wide, irregular path farther east in the Atlantic interior. The same behavior showed up in the team's simulation of the two-year field study. Emboldened by that resemblance, the researchers simulated a further 13 years of flow and found a richer, more complex pattern than appears in textbooks. Understanding such patterns in the present-day ocean, says Bower, is an essential ingredient for predicting the effects of global warming on Earth's future climate. (A. S. Bower, M. S. Lozier, S. F. Gary, C. W. Böning, Nature 459, 243, 2009.)—Charles Day

Bees trade energy efficiency for stability when flying in turbulent winds

2009-05-28T13:53:10Z

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.

Smog reduced for Beijing Olympics

2009-05-21T19:21:36Z

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

How tin whiskers grow

2009-05-18T14:30:08Z

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

Cosmic-ray electron spectrum

2009-05-14T18:04:45Z

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

Tracking thermospheric wind

2009-05-11T14:48:51Z

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