Sound into Light

Physicist Seth Putterman wishes the folks who made the movie “Chain Reaction” had consulted with him on the screenplay. “We could have given them a much better plot,” he said from his lab at UCLA. “Real life sonoluminescence is far more exciting.”

Sonoluminescence literally means “sound into light,” and the mysterious phenomenon serves as a central plot device in the film, in which Keanu Reeves plays a student who stumbles upon a limitless source of energy. It’s a completely unexpected alchemy that turns thunder into lightning.

Although physicists had known for 50 years that under some conditions concentrated sound could produce flashes of light, it wasn’t until a few years ago that Putterman’s group put the topic back in the scientific forefront. Injecting a tiny drop of air into pure water to produce a single bubble resonating with sound waves, they found to everyone’s surprise that the flashes lasted for less than a trillionth of a second and repeated with clockwork precision up to 30,000 times a second.

The result is analogous to blowing air over the top of a Coke bottle and producing a glow that rivals the sun. No other known phenomenon in nature creates such an intense concentration of energy.

Putterman’s experiment at UCLA is a small-scale version of the same general principle. Instead of a Coke bottle, a small globe filled with water sits on a tiny stage. Attached to its side is a dime-size ceramic vibrator that sends sound waves resonating through the globe. The whole hand-built contraption sits inside an open black box draped with wires.

Putterman turns on the electricity and turns off the room lights. Suddenly, in the center of the globe, you can just make out a steady blue light shining like a tiny star.

The researchers think they understand roughly what happens: A single bubble of gas gets trapped in the center of the flask, and the sound energy causes it to puff up and collapse in rhythmic cycles. At its maximum expansion, the gas is so rarefied inside the bubble that it’s almost a vacuum; when it collapses, the stuff gets so dense that it turns into a liquid--and maybe even a solid.

Like the Santa Ana winds dropping in toward the ocean from the High Desert, the compressed air inside the bubble gets extremely hot--up to 100,000 degrees Fahrenheit. The tiny bubble seesaws between this extreme, dense heat and the rarefied cold of the vacuum tens of thousands of times each second.

The question is: How does the transformation work? How does sound get turned into light?

“There are a lot of ideas out there,” said Putterman, who isn’t satisfied with any of them.

Not that transformations of energy from one form to another are unheard of in nature. People turn muscle energy into motion when they ride bikes, and power their cars with energy siphoned from the sun by prehistoric plants.

But light is a trillion times more energetic than sound. So, for sound to make light requires an almost unbelievable concentration of energy. Keith Weninger, who designed and built the apparatus, recently measured the bubble collapsing at four times the speed of sound.

“It’s such an incredible implosion,” said Putterman. But even that hasn’t helped the researchers understand what’s really going on. “It hasn’t helped us figure it out,” he said.

While some researchers think such focused energy might some day be tamed for everything from welding atoms together to attacking tumors, Putterman is moved by the basic mystery of how the puzzling transformation works.

“Mother Nature gave us some wonderful gifts, but now that we’re trying to get the details, she’s making things difficult,” he said.

One obstacle to understanding the phenomenon is that the flashes are too brief to capture even with the most sophisticated equipment. Without such a measurement, researchers cannot pin down the mechanism that makes sonoluminescence work. The flashes might even be shorter than a trillionth of a second--but at present, there isn’t a light detector around swift enough to pin them down.

Other stumbling blocks are even more basic. For example, the UCLA group has experimented with various gases inside the bubble. Air, which is about 1% argon, mixed with oxygen and nitrogen works quite well. Take out the argon, and the bubble hardly shines. Until the researchers find out why argon makes such a big difference, they cannot fully understand the process.

By fiddling with the recipe, the researchers found that any argon-type gas works extremely well at 1% concentration; other gases don’t.

What’s special about argon? Like helium and neon, it is what chemists call a “noble gas”--a single self-contained atom that keeps its distance from its neighbors. In contrast, atoms of oxygen travel in pairs, and generally like to glom on to anything in sight.

Just why single-atom gases are critical to transforming sound into light has the physicists completely in the dark.

“This whole wonderful phenomenon works with one [type of atom] but not the other,” Putterman said. “Why?”

A similar mystery shrouds the liquid surrounding the bubble. Sonoluminescence works best when the bubble floats in water. This is a funny coincidence, because water is omnipresent on Earth, and Earth’s atmosphere of 1% argon is the same needed to produce sonoluminescence. What’s more, the temperature and pressure inside the bubble in its resting state are about the same as normal pressures and temperatures on Earth.

Why should sound turn into light under exactly those conditions that are present on Earth? For now, no one knows.

Contrary to the promise implied in “Chain Reaction,” the phenomenon cannot yet be put to practical use because more energy goes into making sonoluminescence than comes out as light. But the process does have potential.

One group is working on a way to deliver concentrated energy to tumor sites via sonoluminescent bubbles, while others are investigating “sonochemistry,” which uses concentrated sound to destroy toxic chemicals.

Putterman thinks an understanding of how to focus energy so intensely will be useful in trying to build nuclear fusion reactors. “We might be able to tell people how to start with energy at a bigger scale and focus it to a smaller scale,” he said.

Indeed, anything that connects phenomena from the everyday world to the exotic microscopic world of atoms tends to be fertile territory for physicists to explore, he said. “Anything that lies at that intersection is exciting.”