Light from sound
At UCLA's Physics and Astronomy department, researchers in the Putterman Lab investigate a phenomenon straight out of science fiction: sonoluminescence. This process transforms the energy of sound waves into flashes of light. The typical experiment involves a spherical glass flask filled with water that has been partially degassed. Piezoelectric transducers attached to the flask generate a standing acoustic wave, creating a point of low pressure at the center. At this pressure antinode, a single, tiny bubble of gas, just micrometers in diameter, can be trapped.
The sound wave, often oscillating at frequencies between 20 and 40 kilohertz, forces the bubble to expand and then collapse violently once per cycle. During the collapse, the bubble's contents are compressed and heated to extraordinary conditions. The process is a dramatic example of energy focusing, where the diffuse energy of an acoustic field becomes intensely concentrated in a microscopic space. This violent implosion heats the gas and water vapor inside the bubble to a plasma state, causing a brief flash of light.
The flashes are incredibly short, lasting anywhere from 35 to a few hundred picoseconds—trillionths of a second. Despite their brevity, the flashes are remarkably stable and can repeat with the precision of a clock.
A sun in a bubble
The conditions inside the collapsing bubble are extreme. Spectroscopic measurements suggest that the temperature of the plasma reaches at least 20,000 Kelvin, nearly four times hotter than the surface of the sun. The pressure is estimated to reach thousands of atmospheres. This hot, dense state is thought to emit light through a process called thermal bremsstrahlung, where electrons decelerating within the plasma release energy as photons.
The exact mechanism of light emission is still researched, but the spectrum of the light is continuous, consistent with a very hot object emitting thermal radiation. The intensity of the light is sensitive to several factors, including the type of gas dissolved in the water. Adding a small amount of a noble gas like argon or xenon can dramatically increase the light's brightness. In fact, for stable single-bubble sonoluminescence in water, chemical reactions during the initial cycles remove the nitrogen and oxygen, leaving primarily the trace amount of argon from the air to fuel the light emission.
The physics of sonoluminescence is so extreme that for a time in the early 2000s, it was linked to a controversial field called "bubble fusion" or sonofusion. Experiments by other research groups claimed to have detected neutrons and tritium—byproducts of nuclear fusion—from collapsing bubbles in deuterated acetone. These claims, however, were met with widespread skepticism and have not been successfully replicated, with investigations uncovering evidence of academic fraud.