News Release

Physics tip sheet #32 - January 23, 2003

Peer-Reviewed Publication

American Physical Society

1) New best limits on photon mass and photon charge (two papers)
J. Luo, L.-C. Tu, Z.-K. Hu/Y. K. Semertzidis, G. T. Danby, D. M. Lazarus
Physical Review Letters (to appear)/Physical Review D (Print issue: January 1, 2003)

Photons have no mass. It's one of the fundamental mantras of physics. But how do we know it is true? The only way to know is to weigh it and increasingly sophisticated measurements have been performed to determine if what we take as given is really true. A new experiment still finds the photon mass is zero but with the tightest limits yet achieved, 20 times better than the previous record. The experiment, conducted in an underground cave, involves a torsion balance, a type of pendulum that rotates around its axis rather than swinging back and forth. A specific type of magnetic field was turned on near the balance and any rotation was carefully measured using a laser. The whole apparatus was situated on a turntable to remove any effects due to its orientation with respect to the Earth. If photons have any mass, there should be some effect on the rotation but none was observed. Although it is impossible to show conclusively that the photon has no mass, this experiment means that the mass of a photon must be less than 1.2*10^-51 grams or less than one millionth of a billionth of a billionth of the mass of an electron. Meanwhile, the photon charge was also constrained to be less than 8.5*10^-17e, or 85 billionths of a billionth of the charge of the electron. This is the best constraint on photon charge yet from a laboratory experiment. (Cosmological data have provided tighter constraints on the charge.)

Photon mass journal article: Available on request
Photon charge journal article: http://link.aps.org/abstract/PRD/v67/e017701

2) First Results from KamLAND: Evidence for Reactor Antineutrino Disappearance
K. Eguchi, et al.
Physical Review Letters (Print issue: January 17, 2003)

After recent experiments concluded that neutrino flavors mix together and, hence, have mass, investigations have shifted to how the flavors mix. To do this, neutrinos are fired over a large distance to a neutrino detector to give them time to "oscillate" to new flavors. The first results from the Japanese KamLAND detector, with a 180 kilometer shooting distance, rule out all but one possible scenario for the way neutrinos mix. This leaves just the "large mixing angle" solution to the solar neutrino problem.

Journal article: http://link.aps.org/abstract/PRL/v90/e021802

3) Swifter Switching (two papers)
H.W. Schumacher, C. Chappert, P. Crozat, R. C. Sousa, P. P. Freitas, J. Miltat, J. Fassbender, and B. Hillebrands
Physical Review Letters (Print issue: January 10, 2003)

A collaboration of researchers has flipped micron-sized magnets as quickly as their fundamental speed limit will allow. In two papers, the researchers describe applying an ultrafast magnetic field pulse to reverse micromagnets almost as fast as the switching of a bit on a Pentium chip. But unlike silicon circuitry, a magnetic switch preserves data without the need for a constant power supply. The work strengthens hopes that fast and efficient magnetic memory could soon replace technology used in mobile phones and digital cameras.

Physical Review Focus: http://focus.aps.org/story/v11/st3
Journal article 1: http://link.aps.org/abstract/PRL/v90/e017201
Journal article 2: http://link.aps.org/abstract/PRL/v90/e017204

4) Scattered Chatter
A. Derode, A. Tourin, J. de Rosny, M. Tanter, S. Yon, M. Fink
Physical Review Letters (Print issue: January 10, 2003)

Tall buildings are often the bane of cell phone users when making calls in big cities, but such scattering structures may someday enhance communication. Researchers at the Laboratoire Ondes et Acoustique in Paris recently demonstrated the effect with ultrasonic antennas in a water-filled tank. When the space between a 23-element transmitter array and an array of 5 receivers was devoid of scattering structures, the error rate in transmission of a set of 5 messages sent simultaneously to the receivers was about 28%. By placing a forest of randomly arranged steel rods between the transmitter and receivers, the researchers found that they could transmit the same messages without any error at all.

Physics News Update: http://www.aip.org/enews/physnews/2003/split/621-1.html
Journal article: http://link.aps.org/abstract/PRL/v90/e014301

5) The spaser: a laser analog for nanotechnology
D. J. Bergman, M. I. Stockman
Physical Review Letters (Print issue: January 17, 2003)

Just as a laser can fire tightly focused light at a point, a new proposed device, the "spaser" will shoot surface plasmons (localized energy fluctuations on the surface of materials) on a nanoscale. The spaser beams could be used for probing nanostructures where a laser would not be suitable. Laser light is created in an optical cavity consisting of mirrors whereas the spaser emission comes from a quantum dot, acting as an effectively pointlike source of energy, driven by electrical current. The process by which the spaser works is analogous to the laser – they both use the "aser" parts of their names, Amplification by the Stimulated Emission of Radiation.

Journal article: http://link.aps.org/abstract/PRL/v90/e027402

6) The Indian Railway – a real-world small-world network
P. Sen, S. Dasgupta, A. Chatterjee, P. A. Sreeram, G. Mukherjee, S. S. Manna
Physical Review E (to appear)

What is the best way to organize train travel? On one hand, travelers want to change trains as few times as possible, but on the other, they want to get to the destination as fast as possible. The first criterion could be achieved by a train that visits all stations but would very slow. To satisfy the second, passengers could change at every station they pass through to guarantee the shortest distance traveled. In reality there is a balance in between. The Indian Railway network has over 8000 stations serviced by 10,000 trains and has evolved to find this balance. An analysis of the network shows that it is a small-world network, the type of network much discussed in recent years and common in diverse contexts from Internet structure to friendship networks.

Journal article: Available to journalists on request

7) Extreme edges of the net are Russia and the US Military
K. A. Eriksen, I. Simonsen, S. Maslov, K. Sneppen
Physical Review Letters (to appear)

The structure of the Internet displays a breakdown into separated modules focused in geographic regions, according to this study. The area of the Internet most isolated is a module in Russia. Furthest from that module, at the other extreme of the global network is the US military network. Measuring the modularity of the Internet is important for understanding its stability. Highly isolated areas can be cut off relatively easily but a few well-placed extra links can considerably strengthen the robustness of the Internet. Furthermore, knowing more about the structure of the net could assist in improving net tools such as search engines like Google that rely on analyzing network structure.

Journal article: Available to journalists on request

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