Sonoluminescence

NEW THEORY OF SONOLUMINESCENCE . Sound energy, in the form of a beam of ultrasonic waves, can be partly converted into light energy by aiming the sound at an air bubble in a sample of water. The sound causes the bubble to collapse and to emit sharp (less than 12 picosecond) light pulses. The light's spectrum implies that the source of the radiation is similar to a black-body object at a temperature of tens of thousands of kelvins. Theorists have tried to explain sonoluminescence by saying, for example, that the radiation comes from a plasma formed by the collapse of the bubble. But mostly the mechanism behind the production of the pulses remains a mystery. Now Claudia Eberlein of Cambridge University (cce20@phy.cam.ac.uk, 44-1223-337-458) offers a more daring explanation. She believes the light is being emitted by the vacuum surrounding the bubble. Modern quantum theory holds that unseeable virtual photons abound in the vacuum. The behavior of these "zero-point fluctuations" is influenced by the properties of the surrounding medium. The rapidly moving air-water interface (where two media different indices of refraction come together) may facilitate the conversion of virtual photons into real photons. In fact, Eberlein says, sonoluminescence may represent the first observable manifestation of quantum vacuum radiation. This scenario can be compared to the "Unruh effect," a hypothetical phenomenon in which photons are emitted by a mirror accelerating through a vacuum. "Hawking radiation," the hypothetical emission of particles from black holes, is yet another example of energy seemingly coming out of nowhere; at the black hole's Schwarzschild radius (inside of which, light cannot escape), space is so warped that energy from the black hole can be converted into particle-antiparticle pairs; one particle falls back into the hole while its partner escapes. Eberlein asserts that researchers can put her theory to an experimental test and compare the results to other models of sonoluminescence. (Claudia Eberlein, Physical Review Letters, 13 May 1996.)That is the best I can do.