For 10 years scientists have struggled to explain the physics behind sonoluminescence, a phenomenon characterized by a tiny dot of light emanating from a solitary, sonically driven bubble. In a paper that appeared in the April 1 issue of Nature, researchers led by Sascha Hilgenfeldt of Harvard University in Cambridge, Mass., present an explanation. Simply put, a sound wave causes a bubble to grow from about 5 µm in diameter to about 70µm. When the sound field becomes compressive, the bubble collapses from the inertia of the surrounding water. As the collapse accelerates, the gas inside the bubble compresses, and its temperature rises to 20,000 to 30,000 K, creating a plasma of ions, electrons and neutral atoms. As the energy density of the bubble is reduced, photons are emitted. The process is repeated about every 20 µs.