Scientists can turn sound into light — and nobody fully knows how

If you blast a tiny bubble in water with intense sound waves, something bizarre happens: the bubble collapses so violently that it produces a flash of light. This phenomenon is called sonoluminescence, and it remains one of the most puzzling mysteries in modern physics.

Here's what we know: sound waves cause the bubble to expand and contract rapidly. During collapse, the bubble shrinks to a fraction of its original size in mere microseconds, and temperatures inside spike to an estimated 20,000°C — hotter than the surface of the Sun. That extreme heat causes the gas inside to ionize into plasma, which emits a brief burst of visible light.

What makes this so strange is the sheer concentration of energy. A relatively weak input — sound — gets focused into a tiny point that produces temperatures rivaling a star. Scientists have proposed explanations ranging from quantum vacuum effects to miniature shockwaves, but no single theory fully accounts for every observation. Some researchers have even explored whether sonoluminescence could be harnessed for tabletop nuclear fusion, though that idea remains highly speculative.

The phenomenon was first discovered accidentally in 1934 during sonar research, but it took until the 1990s for scientists to reliably produce single-bubble sonoluminescence — a lone bubble flashing with clockwork regularity, thousands of times per second.