Inferno in a Bubble

Abstract

Seth J. Putterman compares the blue speck of light glowing in a flask of water to a star in the sky. Indeed, the glow seems cool and soothing, like starlight on a summer evening. Yet just as twinkling stars belie scorching suns, the speck, a tiny bubble of air blasted by sound waves, reaches hellish temperatures. The gas within may even hit millions of degrees Celsius as hot as the inside of our sun. Making light from sound, known as sonoluminescence, has generated a whirl of activity among some scientists in recent years. Creating the bubble entails little more than wrapping a flask of water in a couple of small loudspeakers, or transducers, then tuning the transducers to certain high frequencies. Yet somehow the sound energy gets condensed inside the bubble to one-trillionth of its original density. The bubble expands, collapses, and flashes 30,000 times per second, generating light that can be seen without darkening the room. No theory can fully explain how this happens or why the bubble flashes so briefly and as steadily as a clock. Though research suggests that temperatures and pressures in the bubbles soar astronomically, no one has yet figured out exactly how high. If the bubble is as hot and as dense as we think it is, then we have a dense plasma. That's an interesting state of matter, says Putterman, a physicist at the University of California, Los Angeles. Some scientists think the bubbles get so hot that they can use sonoluminescence to weld atoms of hydrogen isotopes. This process, known as fusion, holds the potential for providing a boundless source of energy. Meanwhile, just trying to understand what happens in the bubble is raising questions more quickly than researchers can answer them. You have to keep in mind that nobody knows, really, what is going on in this thing, says Michael Moran, an experimental physicist at Lawrence Livermore (Calif.) National Laboratory.