Number 355 (Story #2), January 20, 1998 by Phillip F. Schewe and Ben Stein|
THE DURATION OF SONOLUMINESCENCE (SL) PULSES has been determined by researchers at the University of Stuttgart and UCLA. Sonoluminescence is the conversion of sound into light, in which acoustic waves aimed at a water tank create bubbles which collapse to release light flashes lasting less than a billionth of a second. Previously, researchers could only establish an upper limit for the length of SL flashes, but not their actual duration. Recently, researchers at the University of Stuttgart (Bruno Gompf, email@example.com) adapted a powerful "single-photon-detection" technique to SL and announced the first measurements of SL pulse length, and presented evidence that the length of the pulse is identical in the red and the UV parts of the spectrum (B. Gompf et al., Physical Review Letters, 18 August 1997). Applying the Stuttgart technique, UCLA researchers (Seth Putterman, 310-825-2269) were able to determine that the entire spectrum of colors in an SL flash shines for the same amount of time--from 35-380 picoseconds for mixtures of various gases in the water. (Hiller et al., upcoming article in Physical Review Letters.) This rules out the "adiabatic heating" hypothesis, in which an imploding bubble would first emit red, then add higher-energy colors as it collapsed further and got hotter. It supports a picture in which the collapsing bubble launches a shock wave which heats up the bubble to form a dense, relatively cold plasma. In this scenario, the light would be produced by a process called "thermal Bremsstrahlung," in which the plasma electrons collide with each other, and as they speed up and slow down at different rates they would create light of all different colors.