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Albert Einstein’s description of Bose–Einstein condensation is based on a statistical argument. In a gas of identical bosons, the statistical weighting of each state is such that the total occupation of the excited states is capped at an upper bound N_c. If the number of identical particles exceeds N_c, all additional particles must occupy the ground state. That textbook picture doesn’t include interparticle interactions, but it does assume thermal equilibrium, which cannot exist without some form of interaction, so it’s no surprise that the picture doesn’t describe real systems exactly. Now, Zoran Hadzibabic (Cambridge University, UK) and colleagues have taken a closer look at how the role of interactions in a Bose–Einstein condensate relates to the textbook picture. The researchers found, as shown in the figure, that for a gas of potassium-39 atoms in an optical trap, the thermal component (excited-state atoms) consistently exceeded the textbook upper bound. Qualitatively, they attribute the difference to repulsion between the thermal and condensed components, which turns the harmonic trap into a Mexican hat potential and thereby increases the number of thermally accessible excited states. Quantitatively, they found that when they tuned the interparticle interaction strength, by applying a magnetic field near a Feshbach resonance, an extrapolation to the zero-interaction limit recovered the textbook picture. (N. Tammuz et al., Phys. Rev. Lett., in press.)—Johanna Miller

As massless and chargeless particles of integer spin, photons are the simplest of bosons. Moreover, Satyendra Nath Bose had photons in mind in 1924 when he proposed a new way of counting indistinguishable particles, work that led to Albert Einstein’s prediction of the state of matter known as a Bose–Einstein condensate (BEC): When a collection of bosons becomes cold enough, a macroscopic fraction of them congregate into a single quantum state. But until now, a BEC of photons has never been observed. Blackbody photons, when cooled in a cavity, simply disappear into the walls. And, unlike atoms, photons don’t usually interact with each other, which prevents them from reaching thermal equilibrium apart from a blackbody. Martin Weitz and colleagues from the University of Bonn have now overcome those obstacles to a photon BEC. By confining laser light within an optical cavity filled with dye, they create the conditions for light to equilibrate as a gas of conserved particles. The cavity mirrors’ reflectivity ensures that photons live long enough to scatter among dye molecules, which exchange energy with the photons by repeatedly absorbing and reemitting them. To make a BEC, Weitz and company increase the number of scattered photons in the cavity by dialing up the laser power. Above a critical density, the photon energy spectra exhibit an emerging Bose peak at the cavity’s lowest-energy mode, alongside a broad thermal distribution of uncondensed photons. Shown here, beside the spectra, are spatial profiles of the light emission, below the onset of condensation (top) and above it (bottom). (J. Klaers et al., Nature 468, 545, 2010.)—R. Mark Wilson

To a significant extent, the cells, tissues, organs, and so forth that make up a living organism act independently as they perform their related tasks. According to spin-glass models of evolution, modular structures analogous to those in biological systems generically arise for systems in changing environments. Moreover, such structures help ensure that a system is better able to cope with changes to come. Now Michael Deem of Rice University and his student Jiankui He have applied those conclusions to global trade. To do so, they came up with a parameter—the cophenetic correlation coefficient—that quantifies the modularity of the global trade network. As the figure shows, the system becomes more modular (the CCC increases) in response to environmental change—in this case, global recession. (The red bars indicate more severe recessions; the green bars, less severe. Data for the 2008 recession were not available.) But the overall trend during the past 40 years has been a mostly steady decline in the CCC as insular trade blocs—the modules—dissolved in favor of freer trade. According to evolution theory, and in contrast to much current economic thinking, the decreasing modularity implies that the global trade network is becoming less resistant to recessionary shocks such as a 1% dip in the US gross domestic product. And indeed, looking back at US recessions that have occurred during the past 30 years, Deem and He find that the most recent ones have had the greatest and longest lasting global impact. (J. He, M. W. Deem, Phys. Rev. Lett. 105, 198701, 2010.)—Steven K. Blau

Thermodynamics teaches that the efficiency of a heat engine operating between a hot reservoir at temperature T_h and a cold one at T_c can be no greater than the Carnot value η_C = 1 - T_c/T_h. To achieve its theoretical maximum, the engine must run infinitely slowly and generate zero power—surely an unsatisfactory state of affairs in the real world. Now Massimiliano Esposito (Free University of Brussels) and colleagues have derived efficiency bounds for engines operating at maximum power. They assume that the engine operates in a Carnot cycle and interacts with the hot reservoir for a finite time τ_h, presumed much greater than the duration of the adiabatic steps. They then express entropy as a sum of the standard term of heat over temperature and a term of the form a_h/τ_h (the cold reservoir is treated analogously); placing the interaction time in the denominator ensures that the reversible-process result is obtained in the infinite-interaction-time limit. After deriving the maximum power as a function of the interaction times, Esposito and company can readily calculate the efficiency, which depends in particular on a_c/a_h. The figure shows the allowed range of efficiencies at maximum power. The upper bound corresponds to a_c/a_h = 0; the lower bound to an infinite ratio. The points in the figure give observed efficiencies for several heat engines worldwide. Those engines may not satisfy the assumptions of the Esposito model or even run in Carnot cycles; still, their efficiencies lie within or near the idealized bounds. (M. Esposito et al., Phys. Rev. Lett., in press; available at http://arxiv.org/abs/1008.2464.)—Steven K. Blau

One hallmark of Albert Einstein’s genius is his 1905 theory that the kinetic energy of pollen grains, dust, and other similarly sized objects in thermal equilibrium depends solely on temperature—the classic definition of Brownian motion. Einstein then concluded that the instantaneous velocity of such particles would be impossible to physically measure, and for more than a century, it seemed that he was right. But now, Mark Raizen and his colleagues at the University of Texas at Austin have used optical tweezers in a vacuum chamber to trap a 3-μm-diameter silica bead, observe its ballistic (inertial) motion at short time scales, and determine its instantaneous velocity. The bead is held at the focal point of two noninterfering laser beams, similar to the setup in the image. When the bead makes a random move, it deflects the beams, which allows its position to be traced and the instantaneous velocity to be measured. From those measurements, the researchers calculated root mean square velocities; even when taken at varying air pressures, the results agreed with each other and with the theoretically predicted value, proving that in the ballistic regime, the bead’s mean velocity is solely dependent on temperature and not on pressure or the inertial effects of the surrounding air molecules. Raizen says they will next attempt to cool the particle’s motion to the quantum ground state and confirm that the kinetic energy will be nonzero even at 0 K. (T. Li et al., Science, in press, doi:10.1126/science.1189403.)—Jermey N. A. Matthews

In a combustion engine, work is produced from heat liberated by burning the fuel. In hydrocarbon fuel cells, the fuel is directly converted into electricity. Both types of engines, however, waste heat and emit gas byproducts that are considered useless—or even pernicious, as in the case of the greenhouse gas carbon dioxide. But Martin Gellender, an environmental officer for the state government of Queensland, Australia, makes the case for exhaust gases as an energy source: In a conceptual paper, he explores the overlooked entropy increase that occurs when concentrated gases isothermally mix with air. As illustrated in the schematic, if an exhaust gas mixture containing, for example, CO₂ at a high concentration is separated from air by a piston-membrane barrier that selectively blocks CO₂ passage, the concentration gradient performs work on the piston until the CO₂ concentrations on both sides are equal. According to Gellender’s calculations, a secondary entropy engine could theoretically recover up to 7% of the fuel’s energy and could provide a power boost to the primary engine: up to 1.5% for combustion engines and up to 3.5% for fuel cells. He says that new fuel-cell designs and material advances could lead to a practical entropy engine that reduces the fuel consumption of power plants. (M. Gellender, J. Renew. Sust. Energy, in press.)—Jermey N. A. Matthews

The partitioning of the continuous visible spectrum into a small number of basic colors is done differently in different languages. But the variation is less than would be expected by chance, as statistical analysis of the World Color Survey's data set has shown. Several computational approaches have been taken toward understanding how languages’ color categories develop. Among them is the work of Andrea Baronchelli (Polytechnic University of Catalonia, Barcelona, Spain) and his collaborators. They performed computer simulations in which individuals in a population, beginning with no words to describe colors at all, were tasked with describing different colors to one another. The individuals independently invented words and categories and, based on the success or failure of their communications, adjusted their categories and vocabularies to match those around them. Eventually, each population came to a near-consensus, as shown in two examples in the top panel of the figure. Now, the researchers have revised their model to include a real property of human vision, the “just noticeable difference” (JND; shown in the bottom panel), or wavelength resolution. In the new simulations, individuals were not required to distinguish between colors that a human couldn't tell apart. The categories produced by the JND-based simulations clustered together in color space to the same degree as the World Color Survey results did. The researchers hope that the quantitative agreement between their simple model and empirical data will pave the way for greater use of synthetic modeling in studying language development. (A. Baronchelli et al., Proc. Natl. Acad. Sci. USA, in press, doi/10.1073/pnas.0908533107.) —Johanna Miller

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

The Indus Valley civilization, in what is now eastern Pakistan and northwestern India, flourished circa 2500-1900 BCE. To this day its writing, as in the figure, has not been deciphered. Indeed, scholars are unsure if the Indus script represents a language. Other, superficially similar ancient texts are thought to be either rigidly prescribed strings, such as a hierarchical list of deities, or nonlinguistic strings in which order is unimportant. Now computer scientist Rajesh Rao (University of Washington) and colleagues from several Indian institutions have studied the correlations of neighboring tokens (symbols or words) with a statistical tool—the conditional entropy—that reliably distinguishes natural languages from token strings in which the ordering is rigid or unimportant. The Indus script, they conclude, has the structure of a language. Like the conventional entropy, the conditional entropy involves the logarithm of a probability—in this case the conditional probability that a specified token appears, given its immediate antecedent. Rao and colleagues identified the N most common tokens in the Indus script, various languages, and nonlinguistic systems and plotted the conditional entropy against N. The curves for the Indus system and the natural languages bunched in the middle and were clearly distinct from those corresponding to rigid or unimportant orderings. And the conditional entropy of the Indus system seemed especially closely related to Old Tamil, consistent with the conclusions of scholars who have analyzed the Indus script with more conventional means. (R. P. N. Rao et al., Science, 2009, doi:10.1126/science.1170391. Photo courtesy of J. M. Kenoyer / Harappa.com.) — Steven K. Blau

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Ancient Scripts: Indus Script

In his "Milkdrop Coronet," strobe-photography pioneer Harold Edgerton famously captured the splash produced by a milk droplet falling into a saucer. But our understanding of the underlying physics remains poor. It's known that before a liquid droplet splashes upward from a surface, a thin sheet of liquid spreads out from the impact point. Four years ago experiments by Sidney Nagel and colleagues at the University of Chicago showed, surprisingly, that splashing on a dry surface can be suppressed by reducing the ambient air pressure. The researchers concluded that compressible effects in the air are responsible for the splashing (L. Xu, W. W. Zhang, S. R. Nagel, Phys. Rev. Lett. 94, 184505, 2005). Now Michael Brenner and coworkers at Harvard University have further looked into the air's role in how droplets splash on a dry surface. Taking into account the compressibility and viscosity of the gas and the surface tension of the liquid, they modeled the behavior of the approaching droplet as it reaches the surface. They find that instead of spreading out over the surface, the liquid spreads over a very thin film of air. When the droplet nears the surface, pressure builds beneath it and the bottom of the droplet deforms by flattening and then becoming dimpled. The droplet's bottom perimeter develops a kink that, still over a layer of air, moves out and creates capillary waves. The calculations don't, however, show any indications of splashing; the researchers suggest that other parameters, such as the droplet viscosity and thermal transfer, must become important after the initial spreading phase. (S. Mandre, M. Mani, M. P. Brenner, Phys. Rev. Lett., in press.) — Richard J. Fitzgerald

In typical loudspeakers, a coil surrounds the apex of a flexible cone; when a varying current flows through the coil, the cone moves toward and away from a fixed permanent magnet and produces pressure waves we hear as sound. But researchers from Tsinghua University and Beijing Normal University have demonstrated a radically simpler loudspeaker design based on nanotubes: They showed that a thin film of nanotubes can reproduce sounds over a wide frequency range--including the full human audible range--with high sound pressure level, low total harmonic distortion, and no magnets. The team created the film by drawing nanotubes from a so-called superaligned array grown on a wafer, a technique the group introduced six years ago (see also PHYSICS TODAY, October 2005, page 23). The resulting film, only tens of nanometers thick but up to 10 cm wide, is transparent and has a nearly purely resistive impedance. When electrodes are placed along its ends and an alternating current is applied, the film produces clear tones that can be as loud as a conventional speaker. Moreover, since the film is flexible, the nanotube speaker can be configured into arbitrary shapes or mounted onto curved substrates; the figure shows an omnidirectional cylindrical loudspeaker 9 cm in diameter and 8.5 cm high. The film can even be stretched with essentially no degradation of the sound reproduction. The researchers attribute the sound generation not to vibration but to a thermoacoustic effect first proposed nearly a century ago: Thanks to the nanotube film's extremely low heat capacity per unit area, changes in the current flowing through the film are reflected in the film's temperature, and those temperature changes excite pressure waves in the surrounding air. The mechanism is independent of the sign of the current, which leads to a frequency doubling of the input signal, but that drawback can be overcome by applying a constant current bias. The movie shows a nanotube loudspeaker being periodically stretched with almost no noticeable effect on the sound intensity. (L. Xiao et al., Nano Lett., in press, doi:10.1021/nl802750z.) -- Richard J. Fitzgerald

Sodium is volatile. It easily burns and boils and diffuses. Meteorites are hardy, and the type known as chondrites are also primitive, dating back to the very early solar system. Chondrites contain a high density of so-called chondrules—roughly millimeter-sized spheres like the one shown here in polarized light—that were flash-melted at temperatures around 2000 K and subsequently cooled and incorporated into a meteorite's parent object, typically an asteroid. The heating mechanism is unknown but could involve shocks or lightning. Mostly made of silicate minerals such as olivine and pyroxene and of the metals iron and nickel, chondrules are expected to be deficient in volatile elements like sodium. But researchers at the Carnegie Institution of Washington, the US Geological Survey, and the American Museum of Natural History say it isn't so. Using electron microprobe spectroscopy, they studied 26 chondrules from the Semarkona meteorite that fell in India in 1940 and found significant sodium throughout. The only way that could happen, they say, is if the chondrules formed as closed systems at densities in the solar nebula (the disk of gas and dust from which the planets formed) that were far higher than previously thought. That way, the cooling droplets would be crowded together in an area of saturated sodium vapor. The required ambient densities range from 10 to hundreds of grams per cubic meter, far exceeding the standard assumption of 0.1 g/m³ or less. At the much higher densities, astronomically tiny regions just a few thousand kilometers across can collapse under their own gravity. Thus chondrule formation seems to be intimately linked to planetesimal formation, the first step in making planets like Earth. (C. M. O'D. Alexander et al., Science 320, 1617, 2008 [MEDLINE].) — Stephen G. Benka

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Department of Terrestrial Magnetism

At the May Conference on Lasers and Electro-Optics in San Jose, California, University of Colorado graduate student Mark Siemens reported on studying how tiny parcels of heat, called phonons, spread in a crystal. He and his colleagues used a near-IR laser to heat a grating of nickel lines—each 20 nm high and 1 µm
wide—grown on a sapphire substrate that acted as a heat sink. Then, by recording the transient diffraction of 10-fs pulses of coherent soft x rays from the sample, the researchers could monitor with picometer (10^-12 m) precision the displacement of the heated nickel nanostructure. The transport of heat is considered "ballistic" if the characteristic distance over which a phonon moves—about a micron in this case—is smaller than its mean free path before scattering off another phonon. At room temperature a typical phonon's mean free path in sapphire is a mere 150 nm but grows to more than a micron when the sample is cooled below 130 K. At that temperature the data show a clear transition from thermally diffusive to ballistic behavior. One reason for trying to understand how heat moves away from a nanoscale interface, says Siemens, is to manage the thermal environment of future advanced high-speed transistors. — Phillip F. Schewe

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Nanoscale Probes of Materials

In conventional superconductivity, electrons combine into Cooper pairs, and those pairs collectively enter into a single quantum state in which current can flow with zero electrical resistivity; there is no current dissipation and no Joule heating of the material. A multinational collaboration led by Valerii Vinokur of Argonne National Laboratory in the US and Tatyana Baturina of the Institute of Semiconductor Physics in Russia recently reported on an analogous but opposite situation in which electrical current is vanishingly small, effectively zero. The group studied a thin film of superconducting titanium nitride. Below critical values of temperature and applied voltage, the system went through an abrupt transition from an insulator with normal, linear resistivity to one with apparently infinite resistivity. What's more, the transition could be crossed by tuning a magnetic field for a given threshold voltage, as shown in the figure. As with a superconductor, the superinsulator has zero Joule loss—but now because there is no current rather than no resistance. The experimental system was successfully modeled and analyzed as an array of superconducting islands or droplets connected by Josephson weak links. The researchers conjecture that such a network is also essential to the superconductor-to-insulator transition in thin films. (V. M. Vinokur et al., Nature 452, 613, 2008 [MEDLINE].) — Phillip F. Schewe