Sonoluminescence is a strange phenomenon in which light is emitted when bubbles in liquids collapse. It was thought that the light came from a tiny region of plasma, created and heated by the collapse, but a paper that appeared in Physical Review Letters in May proposes a more exotic mechanism, and has attracted an extraordinary amount of attention. Bubble collapse has been known since the beginning of this century to lead to propeller damage on ships, indicating the dramatically strong forces produced. The fact that the collapsing bubbles emit light was discovered in 1934 by Frenzel and Schultes in Cologne. In 1990, Gaitan and Crum showed how a single bubble in water could be trapped stably and made to expand and contract periodically (typically at about 25 kHz) and to emit light in time with the sound pulses driving the bubble (see figure). But the big surprise came with the discovery in 1991, by the group in Los Angeles headed by Seth Putterman, that each light pulse in this controlled sonoluminescence lasts less than 50 picoseconds!
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