APS Physics News Ticker

Wednesday, November 6, 2013

How To Get Women to Pursue Physics Careers

Of five common approaches to encouraging women to pursue physics careers, only discussing the reasons women don't enter the field appears to be effective.

Phys. Rev. ST Physics Ed. Research 9, 020115

There are many hypotheses regarding factors that may encourage female students to pursue careers in the physical sciences. Using multivariate matching methods on national data drawn from the Persistence Research in Science and Engineering (PRiSE) project (n=7505), we test the following five commonly held beliefs regarding what factors might impact females’ physical science career interest: (i) having a single-sex physics class, (ii) having a female physics teacher, (iii) having female scientist guest speakers in physics class, (iv) discussing the work of female scientists in physics class, and (v) discussing the underrepresentation of women in physics class. The effect of these experiences on physical science career interest is compared for female students who are matched on several factors, including prior science interests, prior mathematics interests, grades in science, grades in mathematics, and years of enrollment in high school physics. No significant effects are found for single-sex classes, female teachers, female scientist guest speakers, and discussing the work of female scientists. However, discussions about women’s underrepresentation have a significant positive effect.

Contact:
James Riordon
riordon@aps.org
301-209-3238

APS Physics Tip Sheet – Nov 5, 2013

In this issue: Measuring Slow Blood Flow, Cities Fragment Under Traffic Burden, An Electron Bucket Brigade, Chaos in a Transient World, Building a SQUID with Bose-Einstein Condensates, and More

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Measuring Slow Blood Flow

A new laser-based scheme allows the visualization of blood flowing at very small speeds.

The measurement of blood flow in human tissues is important for the diagnosis and assessment of many diseases. Current ultrasound techniques use the Doppler shift of the sound waves’ frequency to measure the motion of blood cells. But the effect is undetectable for blood moving slower than about ten millimeters per second. Now, researchers in the US have demonstrated an ultrasound-based technique that can measure speeds as low as a quarter of a millimeter per second. The scheme uses laser pulses to track the sound waves generated by locally heating the blood with ultrasound. The method may find use in biomedical applications ranging from functional brain imaging to the detection of cancer and atherosclerotic plagues

* Lidai Wang (contact author) et al, “Ultrasonically encoded photoacoustic flowgraphy in biological tissue”, Physical Review Letters (expected publication date: Nov 12)

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Cities Fragment Under Traffic Burden

Traffic congestion may be the root cause for why cities become more decentralized as they grow.

Most modern cities tend to develop several centers around which inhabitants organize their lives. A new model proposed by French researchers suggests that the transition from a monocentric to a polycentric structure is driven by traffic congestion. The authors find that the formation of multiple activity centers results from an instability due to the tension between the desire for better-paid jobs and the dread of long commutes. The model provides quantitative predictions that might help urban planners prepare for city growth.

* Rémi Louf (contact author), M Barthelemy, “Modeling the polycentric transition of cities”, Physical Review Letters (expected publication date: Nov 6)

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An Electron Bucket Brigade

Arrays of nanopillars could harvest energy from ambient vibrations with high efficiency.

Inside piezoelectric shoes or bounce backpack dynamos, certain electromechanical devices convert the energy harvested from ambient motion into electricity. An important limit to their efficiency is posed by the fact that such harvesters can only absorb a narrow spectrum of frequencies from the input motion. A team of researchers from South Korea, Germany, Spain and the US has demonstrated a new scheme based on nanometer-sized vertical rods that wave back and forth in response to motion, thereby generating an electric current. Since the mechanism works over a broad range of frequencies, these nanorod shuttles may improve the efficiency of energy harvesting by several orders of magnitude.

* C Kim, Marta Prada (contact author), G Platero, RH Blick, “Realizing broadbands of strong nonlinear coupling in nanoelectromechanical electron shuttles”, Physical Review Letters (expected publication date: Nov 5)

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Chaos in a Transient World

A new model suggests chaotic behavior could emerge even in systems in which all motion dies out due to the effects of dissipation

Chaotic systems exhibit a number of characteristic signatures, such as the butterfly effect (a pronounced sensitivity to initial conditions that makes long-term predictions impossible). Such behavior has always been studied in systems that perpetually evolve: they either don’t lose energy or they are constantly subject to external forces. But what would happen in a dissipative system without any energy input, in which all motion eventually dies out? A new study by researchers in the US and Hungary shows that the hallmarks of chaos could also be observed in these systems. The results imply that processes like the evolution of chemical reactions toward equilibrium or the coalescence of binary stars as they lose energy to gravitational waves could be chaotic – and thus far less predictable than expected.

* Adilson E Motter (contact author), M Gruiz, G Károlyi, T Tél, “Doubly Transient Chaos: The Generic Form of Chaos in Autonomous Dissipative Systems”, Physical Review Letters (expected publication date: Nov 7)

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Building a SQUID with Bose-Einstein Condensates

SQUIDs (superconducting quantum interference devices) are the basis of today’s most sensitive magnetometers. A research team at the Los Alamos National Lab has now built an analog of a SQUID made of a Bose-Einstein-condensed atomic gas. Since in this atomic SQUID rotation plays the same role as the magnetic field in a SQUID magnetometer, the device has potential as an ultrasensitive rotation sensor.

* C Ryu, PW Blackburn, AA Blinova, MG Boshier (contact author), “Experimental realization of Josephson junctions for an Atom SQUID”, Physical Review Letters (expected publication date: Nov 11)

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Wednesday, October 30, 2013

Most Invisibility Cloak Schemes Would Make Objects More Visible, Not Less

"Making an object invisible to red light, for instance, may actually make it bright blue, increasing its overall visibility."

Add caption

Phys. Rev. X 3, 041005 (2013): From ancient times, humanity has been fascinated by the concept of invisibility, and recently, scientists have moved a step closer to bringing this idea to reality by exploiting engineered artificial materials, or metamaterials. Several recent studies have indeed shown that a properly tailored metamaterial cover can, in principle, render an object invisible when illuminated by an electromagnetic wave oscillating at the specific frequency of interest. Yet, experimental realizations and theoretical investigations have consistently shown that reducing the visibility of an object with a passive cloak in a specific window of the electromagnetic spectrum is generally accompanied by a drastic increase of its visibility in other frequency ranges. Making an object invisible to red light, for instance, may actually make it bright blue, increasing its overall visibility.

In this paper, we quantitatively assess the potentials and limitations of passive cloaks in terms of overall visibility, integrated over the entire frequency spectrum. Quite surprisingly, our results show that any linear, causal, and passive invisibility cloak, without special superconducting features, is deemed to increase the scattering and visibility of the original uncloaked object, when integrated over all frequencies. This result confirms that the most popular cloaking devices actually scatter more, not less, when considered over a sufficiently broad frequency range, allowing easy detection using, e.g., pulsed excitation.

Our general theorem holds a relevant exception if specific covers with a strong static diamagnetism are considered, and, based on this principle, we propose a technique to reduce the global scattering, as well as the local response around a frequency of interest, using diamagnetic and superconducting thin cloaking layers. More generally, our results provide a quantitative measure to compare the overall performance of different cloaking devices and generally assess their detectability. These findings may open important research directions in the quest for invisibility, not only in the electromagnetic domain but also for acoustic, mechanical, and matter waves.

Contact:
James Riordon
riordon@aps.org
301-209-3238

Tuesday, October 29, 2013

APS Physics Tip Sheet – Oct 29, 2013

In this issue: Planet Search Finds No Dark-Matter Black Holes, Secure Quantum Commitment, A Single-Atom Switch, and More

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Planet Search Finds No Dark-Matter Black Holes

(Image credit: NASA/Kepler mission/Wendy Stenzel)

Using data from a planet-hunting mission, scientists place new limits on a supposed population of moon-sized black holes that could act as dark matter in our galaxy

According to recent theoretical studies, small, primordial black holes (PBHs) formed during the universe’s early expansion could be responsible for the gravitational effects attributed to dark matter. A team of researchers from the US and Taiwan has searched for PBHs using observations by Kepler - a NASA satellite designed to look for Earth-like planets orbiting other stars. Over 4 years, Kepler monitored ~150,000 stars in the Milky Way. If a primordial black hole passed in front of one of these stars, the star would become temporarily brighter due to the hole’s gravitational lensing. The analysis revealed no black-hole lensing events, ruling out the possibility that primordial black holes could account for a major fraction of the dark matter that is known to exist in our galaxy.

* Kim Griest (contact author), AM Cieplak, and MJ Lehner, “New limits on primordial black-hole dark matter from an analysis of Kepler source microlensing data”, Physical Review Letters (expected publication date: Oct 31)

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Secure Quantum Commitment

Researchers have demonstrated a secure quantum commitment scheme - a cryptographic protocol that allows one to commit to a chosen value, while keeping it secret until the value is revealed.

Secure commitment schemes could find use in bidding or voting systems, in which no one should learn anyone else's intent before an appointed time. Inspired by the success of quantum cryptography techniques like Quantum Key Distribution, researchers have turned to quantum mechanics to develop inherently secure commitment protocols. Some theoretical work seemed to demonstrate that a secure quantum commitment scheme was impossible, but according to a 2012 proposal, a solution could be found if relativistic effects were present. Now, a group of researchers from Switzerland, Singapore, the UK and Canada has experimentally realized this proposal. In an optical fiber link between Geneva and Singapore, the team showed that a committed bit could be kept secret for up to 15 milliseconds - a time that may be sufficient for high-speed applications such as stock trading.

* T Lunghi, Felix Bussières (contact author) et al, “Experimental bit commitment based on quantum communication and special relativity”, Physical Review Letters (expected publication date: Nov 1)

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A Single-Atom Switch

A research team at the Vienna Center for Quantum Science and Technology has demonstrated an optical switch made of a single rubidium atom placed in a microscopic cavity. Controlled by the atom position, the switch can be used to reroute optical signals from an optical fiber to different output fiber ports.

* D O’Shea, C Junge, J Volz, Arno Rauschenbeutel (contact author), “Fiber-optical switch controlled by a single atom”, Physical Review Letters (expected publication date: Nov 4)

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Physicists Detect the "Kick" from Light Hitting a Mirror

From the authors of manuscript LF14494:

When a reflective object is pushed by a pulse of light, various types of mechanical waves are launched from the illuminated surface. In terms of decreasing amplitude, a high-intensity laser pulse gives rise to the following: ablation-induced waves (AIWs) resulting from material recoil, thermoelastic waves (TEWs) caused by light absorption and the subsequent thermal expansion, and the ubiquitous, but weakest, light-pressure-induced elastic waves (LIWs) emanating solely from the linear momentum transfer during the photon recoil. Even though the macroscopic motion of the object is the superposition of all these waves, only AIWs and LIWs are capable of displacing the object’s center of mass. Until now, LIWs were indiscernible from the dominating AIWs and TEWs. However, we have succeeded, for the first time, in detecting LIW by striking a defocused laser pulse with a fluence of 1 J/cm² on the front surface of a 99.999% ultra-high-reflectivity mirror. Its reverberations, causing picometer-large displacements of the mirror’s rear facet, were observed with a calibrated piezoelectric sensor. Again, the light heating had to be suppressed, as was the case over a century ago, when the radiation pressure was experimentally validated with a delicate vane, similar to a light mill.

Friday, October 25, 2013

Twisting Ribbons

From one of the authors of manuscript LD14446:

Twisting a ribbon is at once familiar in decorative arts, geometry, and science. It is one of the simplest and fundamental deformations one can apply to an elastic material. Our investigations show that a wide variety of shapes and instabilities can be obtained by simply varying the applied twist and tension. The observed structures which include helicoids with and without longitudinal and transverse wrinkles, and spontaneous creases, are far richer than the looped and tubular shapes that were previously well known. We obtain a phase diagram of the observed structures, and characterize the evolution of the curvature with applied stress using micro-focus X-ray tomography. We show that the various shapes are organized in a phase diagram using only two control parameters: the initial tension and the twist angle. We develop scaling arguments based on a balance between stretching and bending energies stored in the ribbon which provides an understanding of the observed buckling and post-buckling behavior. Our study has both impact on fundamental theory of elastic materials and tests the limits of Föppl-von Kàrmàn equations, and development of smart, reliable, and efficient strategy to build complex structures starting with graphene sheets, and flat semiconductor nanoribbons and biomaterials.

Tuesday, October 22, 2013

APS Physics Tip Sheet – Oct 22, 2013

In this issue: Predicting Extreme Events, Atomic Butterflies, Surfaces that Control Gas Flow

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Predicting Extreme Events

Experiments using electronic circuits as models of complex systems show that a special class of extreme, catastrophic events (“dragon kings”) can be predicted and prevented.

In many complex systems, catastrophic events (such as financial crises, earthquakes or power blackouts) are assumed to be unpredictable. But a study by researchers from Brazil, Switzerland and the US now suggests a special class of extreme events may be forecasted. Following a recently proposed theory, the authors argue extreme events can be of two types: 1) Events generated through the same mechanisms as non-extreme events, but amplified to a larger scale. 2) “Dragon Kings”: large-scale events (“kings”) that belong to a completely different species (“dragons”), thus carrying special signatures that could help identify them while they develop. The authors show that an experimental model of complex systems based on coupled electrical oscillators exhibits extreme events (large deviations from the normal circuit behavior) that can be classified as Dragon Kings and can be reliably predicted and prevented. The approach may suggest similar strategies for more complex real-world applications.

* Hugo LD de S Cavalcante (contact author) et al, “Predictability and suppression of extreme events in a chaotic system”, Physical Review Letters (expected publication date: Oct 30)

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Atomic Butterflies

Researchers have engineered a lattice of cold atoms that could be used to see Hofstadter’s butterfly – a beautiful, butterfly-shaped structure in the atoms’ energy spectrum.

In 1976, Douglas Hofstadter (the author of “Gödel, Escher, Bach”) predicted that electrons experiencing both the periodic electric field in a crystal and a magnetic field would have an energy spectrum with a beautiful fractal shape reminiscent of a butterfly. The effect has only been seen in a small number of materials because large magnetic fields are needed. Two independent teams in Germany and the US have now engineered an analogous system, based on cold atoms trapped in the lattice created by the superposition of several lasers. With additional lasers, they forced the atoms to undergo circular motion, mimicking the motion of electrons in a magnetic field. These atomic systems, essentially free of defects, realize the physics described by Hofstadter and, at colder temperature, may provide the cleanest view of Hofstadter’s butterfly.

* M Aidelsburger, Julio T Barreiro (contact author) et al, “Realization of the Hofstadter Hamiltonian with ultracold atoms in optical lattices”, Physical Review Letters (expected publication date: Oct 28)

** H Miyake, Colin J Kennedy (contact author), Wolfgang Ketterle et al, “Realizing the Harper Hamiltonian with laser-assisted tunneling in optical lattices”, Physical Review Letters (expected publication date: Oct 28)

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Surfaces that Control Gas Flow

Researchers at Virginia Tech have demonstrated a new method for controlling how gas flows through a narrow channel. The scheme works thanks to a thin organic film whose roughness depends on temperature. The researchers deposited the film on two nearly-touching glass surfaces and measured the flow of gas in the channel between them. Heating the film made it smoother and thus eased the gas flow. The results indicate that raising the temperature from 18 C to 40 C could double the flow rate in a micron-wide tube. The authors suggest the principle could be used to control the flight of microrobotic aircraft or to throttle the flow of gas or fluids in micron-sized “labs-on-a-chip” for biological and chemical applications.

* Dongjin Seo, William A Ducker (contact author), “In-Situ Control of Gas Flow by Modification of Gas–Solid Interactions”, Physical Review Letters (expected publication date: Oct 25)

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Journal articles and preprints are available to journalists on request.

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

Matteo Rini, PhD

Deputy Editor, Physics

mrini@aps.org

http://physics.aps.org

Monday, October 21, 2013

APS Physics Tip Sheet – Oct 15, 2013

In this issue: What’s Inside a Black Hole’s Horizon?, Cosmic-Ray Positrons Limit Dark Matter Models, Heisenberg Vindicated

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What’s inside a black hole’s horizon?

New theoretical work rekindles the controversy over whether black holes have a “firewall” that would destroy any observer crossing the event horizon.

In 2012, a group of physicists at the University of Santa Barbara proposed that an observer falling inside a black hole would be destroyed by a firewall - a sea of high-energy quanta at the event horizon. Their idea was intended to solve certain inconsistencies in black-hole theory, but it sparked a heated debate among theoretical physicists: firewalls violate Einstein’s well-established equivalence principle, which says that an observer can’t distinguish between inertial motion and free fall and therefore shouldn’t be able to tell if he has passed the event horizon. Two of the original firewall proponents have now provided further arguments - based on a theoretical reconstruction of the interior of a black hole - to support the firewall hypothesis.

* Donald Marolf, Joseph Polchinski (contact author), “Gauge-gravity duality and the black hole interior”, Physical Review Letters (expected publication date: Oct 21)

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Cosmic-Ray Positrons Limit Dark Matter Models

An analysis of recent data from the Alpha Magnetic Spectrometer (AMS) provides new constraints on the nature of dark matter particles.

In spring 2013, the space-borne AMS - a detector on the International Space Station designed to look for unusual matter (antimatter and dark matter) - delivered its first data, confirming an unexplained excess of cosmic-ray high-energy positrons (the antiparticles of electrons). According to certain models, these positrons could be a signature of the decay or annihilation of dark-matter. A team of researchers from Sweden, Germany, the US and the Netherlands has now analyzed the AMS data to derive a set of constraints on the nature of possible dark matter particles, providing a 100-fold improvement of the current limits on certain dark matter parameters. The results significantly limit the range of models that may contain a viable candidate for dark matter

* L Bergström, Torsten Bringmann (contact author) et al, “New limits on dark matter annihilation from AMS cosmic ray positron data”, Physical Review Letters (expected publication date: Oct 21)

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Heisenberg Vindicated

Heisenberg originally formulated his uncertainty principle in terms of the “observer effect”: a relationship between the precision of a measurement and the disturbance it creates. But in a number of recent experiments, researchers have claimed experimental errors below the Heisenberg limit, arguing the precision-disturbance relationship can be violated. Now, a team of researchers from the UK, Finland and Germany has reported a new formulation of the uncertainty principle, which should have general validity provided that the measurement error is properly defined.

* Paul Busch (contact author), P Lahti, RF Werner, “Proof of Heisenberg’s error-disturbance relation”, Physical Review Letters (expected publication date: Oct 17)

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Journal articles and preprints are available to journalists on request.

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

Matteo Rini, PhD

Deputy Editor, Physics

mrini@aps.org

http://physics.aps.org