Physics News Update, Number 442

Number 442, August 9, 1999 by Phillip F. Schewe and Ben Stein

GRAVITY WAVE ANALYSIS FROM LIGO PROTOTYPE The Laser Interferometer Gravitational-wave Observatory (LIGO), when fully deployed, will consist of two facilities (Hanford, WA and Livingston, LA). At each site laser beams pass up and down two perpendicular 4-km-long vacuum pipes, reflecting repeatedly from mirrors hung from wires. The presence of a passing gravitational wave would announce itself by a flexing of space-time which would very slightly lengthen the path of light in one arm and shorten the path in the other arm, causing a subtle change in the interference pattern made by the converging light beams from the two arms. The full LIGO, by about November 2001, should be able to detect a strain, defined as the fractional change in the position of the mirrors divided by the length of the arm (4 km), of 10^-21. This is the expected disturbance one expects from the gravity waves emitted by the coalescence of two solar-sized stars at a distance from Earth of 30-50 million light years. But before LIGO scientists possess their full instrument, they do have a 40-m prototype at Caltech, built for doing engineering studies but also capable of sensing gravity waves, albeit with the lesser strain sensitivity of a few times 10^-19. Thus the LIGO team, while testing methods for searching (directly via gravity waves) for binary coalescences, have thereby rendered an upper limit for such events of less than one every two hours in our galaxy. This result is useful for the test of the procedures, but is not significant for astronomers, who have previously established more stringent upper bounds with electromagnetic waves (visible and radio). (Contact Barry Barish at Caltech, 626-395-3853 or 818-601-2643; Stan Whitcomb 626-395-2131; or Bruce Allen, University of Wisconsin-Milwaukee, 626-893-2003 or 414-229-6439; Allen et al., 23 August 1999 article in Physical Review Letters.)

AT THE INTERNATIONAL PHYSICS OLYMPIAD, held in July, the US team had its second-best showing since it started competing in 1986, with 3 gold medals and 2 silver medals brought home by the 5 high school students who participated. In informal rankings, the US placed 3rd out of the 62 countries that competed, after Russia and Iran. Taking place this year in Padua, Italy, where Galileo discovered the 4 Jupiter moons named after him, the Olympiad contains two days of grueling theoretical and experimental problems amounting to what is the world's most difficult high-school physics test. For example, the students had to compute the precise trajectory of a space probe that uses Jupiter's gravity as a slingshot - a technique used in real-life spacecraft such as Cassini. Gold medalists included Peter Onyisi (Arlington, VA), who had the tenth highest overall score out of the approximately 300 competitors at the Olympiad, Benjamin Mathews (Dallas, TX), and Andrew Lin (Wallingford, CT). Silver medalists include Jason Oh (Baltimore, MD) and Natalia Toro (Boulder, CO), who earlier this year also became the youngest person (at 14 years of age) ever to win the top prize of the Intel (formerly Westinghouse) Science Talent Search. (More information at www.aip.org/releases/1999/release05.html)

IN-PLANE-GATE (IPG) TRANSISTORS can be excavated using nanomachining techniques. IPG transistors, in which the source, drain, and gate all lie in a plane rather than in a sandwich, might be especially useful for high-frequency applications. Scientists at the University of Hannover (Hans Werner Schumacher, 011-49-511-762-2523, schumach@nano.uni-hannover.de) have carved out an IPG structure in a semiconductor surface using the probe from an atomic force microscope (see figure at Physics News Graphics). The probe makes an incision into the material extending down about halfway toward a buried interface where, lodged between GaAs and AlGaAs layers, a reservoir of electrons is confined to a plane. The incisions from above do not penetrate into this two-dimensional electron gas (2DEG) but they do shape (and can even pinch off) the conduction of the electrons. The Hannover researchers have also used their inscribing approach to make single-electron transistors (SETs), devices that register the coming and going of single electrons. (Schumacher et al., Applied Physics Letters, 23 August 1999.)