A detailed discussion of the physics of can be found in the article "Sonoluminescence: Sound into Light," by Seth J. Putterman (Scientific American, February 1995). In it, the author outlines a different interpretation of the phenomenon from the one given below, though he agrees that the likelihood of getting fusion to occur in sonoluminescence bubbles is insignificant.
Andrea Prosperetti in the department of mechanical engineering at the Johns Hopkins University has studied this question in detail. He responds:
"It must first of all be stressed that the 'extremely high temperatures' referred to are, at least for now, speculation. While many researchers would concede temperatures of up to, say, 10,000 kelvins (which is way too low for nuclear fusion), a much smaller number would feel comfortable with temperatures in the millions of degrees range. The computations that indicate such extreme conditions inside a pulsating bubble are based on rather extreme idealizations.
"The most fundamental one is the fact that the bubble remains absolutely spherical during its radial oscillations. On theoretical grounds, there are many reasons to doubt this premise: a collapsing sphere is highly unstable (which is the reason why attempts at producing fusion by causing the implosion of gas-filled micro-balloons with powerful pulses of laser light have so far failed), and liquid jets may develop that span the bubble.
"Furthermore, experiment suggests that the light emitted by a bubble has a weak directional asymmetry, which would be incompatible with perfect sphericity. Hence, while it is not absolutely possible to rule out the occurrence of nuclear reactions inside a pulsating bubble on the basis of the present knowledge, the actual occurrence of such reactions is, to say the least, doubtful."
The Johns Hopkins University has also provided this official statement on Dr. Prosperetti's work:
Sonoluminescence, the puzzling glow emitted by a bubble in a field of high-pitched sound waves, may be caused by a tiny jet of liquid that shoots across the interior of the bubble at supersonic speed and slams into the opposite side, a Johns Hopkins researcher has proposed. At the point where this powerful jet strikes the bubble wall, it "fractures" the liquid, releasing energy in the form of light, says Andrea Prosperetti, an internationally respected expert on the mechanical properties of bubbles.
Prosperetti's theory appears in the April 1997 issue of the Journal of the Acoustical Society of America. His paper offers an alternative to the widely held view that the bubble glows because of shock waves that concentrate energy in its center as it shrinks.
His theory also deflates the hope among some researchers that sonoluminescence generates enough pressure and heat to produce nuclear fusion, a potential source of cheap, clean energy. Some scientists have speculated that bubble temperatures during sonoluminescence exceed 2 million degrees Fahrenheit, near the levels needed for fusion. This idea became a key plot point in the motion picture "Chain Reaction," starring Keanu Reeves. But if Prosperetti's theory holds true, the heat inside the bubbles would peak at about 10,000 degrees F, the level found at the sun's surface. "It's enough to explain the chemical activity, but it's far below the amount needed to produce nuclear fusion," says Prosperetti, who is the Charles A. Miller, Jr. Distinguished Professor of Mechanical Engineering at Hopkins.
Sonoluminescence was discovered in 1934 by two German physicists who immersed powerful ultrasound generators in a vessel of water, creating a cloud of tiny bubbles that gave off a glow. Scientists were intrigued but found it was too difficult to study in detail the unwieldy mass of short-lived bubbles. In 1989, however, Lawrence Crum, then a professor at the University of Mississippi, and his graduate student, Felipe Gaitan, were able to induce sonoluminescence in a single bubble trapped within a sound field inside a cylinder of water.