Sonoluminescence: why do collapsing bubbles flash light

Still open in the literature as of 2026.

Sonoluminescence - the emission of a brief flash of light from a gas bubble in liquid violently driven to collapse by sound waves - was first reported by Frenzel and Schultes in 1934, but for decades was studied only in multi-bubble, hard-to-control conditions. Felipe Gaitan's 1990 discovery of stable, single-bubble sonoluminescence (a single bubble trapped in a standing acoustic wave, flashing once per cycle with remarkable regularity) transformed the field by enabling precise, repeatable experiments. Despite extensive study summarized in a comprehensive 2002 Reviews of Modern Physics review by Brenner, Hilgenfeldt and Lohse, favoring adiabatic compression heating and partial ionization as the dominant thermal mechanism, the exact process converting collapse energy into a picosecond flash of light - including true peak temperatures and precise emission mechanism - remains actively debated and is not considered fully resolved.

A speck of gas trapped in water, rattled by nothing more than a loudspeaker, collapses so violently each cycle that it flashes light with clockwork regularity, and physicists still argue about exactly how sound turns into light inside it.

Explanations that circulated

• Claims that single-bubble sonoluminescence definitively demonstrates bubble-fusion/thermonuclear reactions inside collapsing bubbles (notably experiments reported by Rusi Taleyarkhan in 2002 and 2005) have been met with sustained skepticism and were not independently reproduced by other groups, and are not an accepted explanation of the light-emission mechanism.
• A simple 'the bubble just gets hot enough to glow like a blackbody' explanation is incomplete - spectral measurements suggest thermal/bremsstrahlung-like emission at a few thousand kelvin, but this doesn't fully explain all observed spectral and timing details, and more exotic proposals (e.g., Schwinger's dynamical-Casimir-effect vacuum radiation) have been proposed and largely discounted on energy-budget grounds.

References

• Single-bubble sonoluminescence, M. P. Brenner, S. Hilgenfeldt, D. Lohse, Reviews of Modern Physics, 2002