October 11, 2011
Physics News Highlights of the American Institute of Physics (AIP) contains summaries of interesting research from the AIP journals, notices of upcoming meetings, and other information from the AIP Member Societies. Copies of papers are available to journalists upon request.
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Using previously published data on the time-stamped locations of 100,000 anonymous cell-phone users, a researcher from Duke University has identified three distinct patterns of human mobility for short, medium, and long distance trips. In 2008, a separate research team that was not involved in the current study published a paper in which they had plotted data on cell-phone users’ movements, and then fitted the data with a single, downward-sloping curve. The curve captured an intuitive relationship: the longer a trip, the less likely it was to occur. Nicola Scafetta, however, thought deeper patterns might be hidden by the simple curve. In the AIP’s journal Chaos, Scafetta proposes a finer-resolution analysis of the cell-phone data. He divided the data set up into three separate sections, one each for short (from 1 to 10 km), medium (from 10 to 300 km), and long (above 300 km) distance trips. He then fit each chunk of data with a separate curve. Surprisingly, the exponents from the three separate curve fits were simple numbers – 1, 2 and 3 – that illustrated a different relationship between distance and trip frequency for each zone. For all three zones, the likelihood of a trip decreases with increased distance, but the rate of decrease is faster in the higher-numbered zones. Scafetta offers a physical and statistical explanation for this pattern. In zone one, people are running short-distance errands within an urban area, and may just consider one cost mechanism, like the time or the fuel cost of the trips, when deciding where and when to go. In the more distant zone two people are, for example, taking day-trips to nearby towns of specific interest. These trips might require travelers to consider both time and fuel costs in their decisions. And in zone three, people take multi-day trips and may consider time and fuel costs, as well as additional overnight lodging costs. The increase in the number of considered costs for each zone could help explain the increase in the curve-fit exponent for each zone. Scafetta also rescaled the model and found that it could be used to interpret data gathered on the movements of volunteers who walked to their destinations, either in zone one (within 200 m) or zone two (from 200 to 1000 m). The critical benefit of the alternative fitting method, Scafetta writes, is that it suggests clear physical and geographical mechanisms to explain the observations. Accurate models of human displacement have applications in traffic forecasting, urban planning, and in the study of social networks and the spread of diseases.
Article: “Understanding the complexity of the Lévy-walk nature of human mobility with a multi-scale cost/benefit model” is accepted for publication in Chaos: An Interdisciplinary Journal of Nonlinear Science.
Authors: Nicola Scafetta (1).
(1) ACRIM and Duke University, Durham, North Carolina
In the particle identification business, two pieces of information are vital: energy and spatial location. By measuring its energy you can work out the mass of your mystery particle. From its spatial location on the surface of a detector, you can work out where the particle came from – and therefore how big the event was that produced the particle in the first place. For the range of energies close to one million electron volts (1 MeV) – a popular energy range to probe, with uses in a variety of fields from cancer treatment research to astrochemistry – there are currently two leading methods of detecting particles. But both are limited in the types of molecules they can detect, and both sacrifice one type of information – spatial location or energy measurements – for the other. Now a group of nuclear physicists and molecular scientists from the Université Paris Sud and Hamamatsu Photonics have demonstrated a new type of detector that can do both of these jobs at the same time. Their device uses the CCD image sensor chip in a particular off-the-shelf X-ray camera. In the study, described in a paper accepted to the AIP’s Review of Scientific Instruments, the experimenters accelerated charged atoms (or ions) of carbon at various energies above 1 MeV, then “caught” those atoms with the camera. A single ion impact with the camera produced a bright spot on the image sensor. They also accelerated molecules containing carbon and hydrogen. Unfortunately, these bigger particles overwhelmed the CCD chip, wiping out the details. To avoid saturating the sensor, the researchers came up with the solution of putting a piece of thin carbon foil in front of it. The foil breaks up the projectile molecules that collide with it and sends them, like shrapnel, to the sensor to be counted. The foil also allowed them to separate different types of molecules from one another when the molecules’ signatures would otherwise have overlapped. The researchers say they hope their new detector will open the door to a new class of tools in the study of complex molecules using high-energy accelerators.
Article: “Detection of atomic and molecular MeV projectiles using an X-ray CCD camera” is accepted for publication in the Review of Scientific Instruments.
Authors: M. Chabot (1), G. Martinet (1), K. Béroff (2), T. Pino (2), S. Bouneau (1), B. Genolini (1), X. Grave (1), K. Nguyen (1), C. le Gailliard (1), P. Rosier (1), G. Féraud (2), H. Friha (2), and B. Villier (3).
(1) Institut de Physique Nucléaire d’Orsay, Université Paris Sud, France
Diamond, nature’s hardest known substance, is essential for our modern mechanical world – drills, cutters, and grinding wheels exploit the durability of diamonds to power a variety of industries. But diamonds have properties that may also make them excellent materials to enable the next generation of solid-state quantum computers and electrical and magnetic sensors. To further explore diamonds’ quantum computing potential, researchers from the University of Science and Technology of China tested the properties of a common defect found in diamond: the nitrogen-vacancy (NV) center. Consisting of a nitrogen atom impurity paired with a ‘hole’ where a carbon atom is absent from the matrix structure, the NV center has the potential to store information because of the predictable way in which electrons confined in the center interact with electromagnetic waves. The research team probed the energy level properties of the trapped electrons by cooling the diamonds to an extremely chilly 5.6 degrees Kelvin and then measuring the magnetic resonance and fluorescent emission spectra. The team also measured the same spectra at gradually warmer increments, up to 295 degrees Kelvin. The results, as reported in the AIP’s journal Applied Physics Letters, show that at temperatures below 100 Kelvin the electrons’ transition energies, or the energies required to get from one energy level to the next, were stable. Shifting transition energies could make quantum mechanical manipulations tricky, so cooler temperatures may aid the study and development of diamonds for quantum computation and ultra-sensitive detectors, the authors write.
Article: “Temperature dependent energy level shifts of nitrogen-vacancy centers in diamond” is accepted for publication in Applied Physics Letters.
Upcoming Conferences of Interest
Physics Today: October Articles
1. Binary black hole mergers: Solving the equations of general relativity presents unique challenges. Nowadays many of those have been met, and new numerical simulations are revealing surprising astrophysical phenomena.