SONOLUMINESCENCE BUBBLES COLLAPSE AT MORE THAN FOUR TIMES THE SPEED OF SOUND, new experiments have shown. Sonoluminescence is the still- mysterious process in which sound waves aimed at a water tank cause bubbles to collapse and generate ultrashort light flashes which represent a trillionfold concentration of the original sound energy. UCLA researchers (Seth Putterman, 310-825-2269) determine the speed of bubble collapse by measuring the amount of laser light scattered from a bubble during different points of its implosion. The amount of scattered light is proportional to the square of the bubble radius. Previous experiments could only establish that the bubble collapsed faster than the speed of sound; using ultrashort (100-fsec) laser pulses has now enabled the researchers to ascertain that the bubble collapses at a speed greater than Mach 4 (more than 1 km/s for this tiny bubble). This confirms a major prediction of the leading explanation for sonoluminescence known as the shock-wave model but does not rule out competing explanations because this and other experiments to date can only probe the outer surface of the bubble, not what is happening inside. In addition, the UCLA team determined that the bubble accelerates by at least 10^11 g when the bubble stops compressing and starts expanding; amazingly, the bubble remains intact during this massive acceleration. (K.R. Weninger et al., upcoming article in Physical Review Letters.)