Weird Phenomenon Seems To Make Light Of Laws Of Physics

The permanence of physical laws makes life comfortably predictable. But it's no comfort to physicists who are trying to explain sonoluminescence, a bizarre phenomenon in which sound waves generate light. Sonoluminescence seems to break the rules.

Physicists discovered sonoluminescence 60 years ago, when they found that sound waves propagating through water could generate an eerie blue glow in the liquid.

In 1988, that phenomenon became known as multiple-bubble sonoluminescence when scientists found that a more sophisticated setup can generate repeating flashes of light, each lasting about a trillionth of a second. This latter form is known as single-bubble sonoluminescence.

The 1988 discovery has generatednew sonoluminescence research.

Hollywood film on the topic

Sonoluminescence is so mysterious that it has even inspired a Hollywood film, due for release in August. In "Chain Reaction," actors Keanu Reeves and Morgan Freeman discover that sonoluminescence could solve the world's energy problems.

But contrary to the movie's implication, sonoluminescence is not really creating energy out of thin air; that would violate physical laws. Somehow, sonoluminescence concentrates the energy of sound waves so much that they can generate light.

Claudia Eberlein, a theoretical physicist at Cambridge University in England, and several others have proposed that some pretty far-out physics is behind sonoluminescence.

Expanding on the work of late Nobel laureate physicist Julian Schwinger, Eberlein has suggested that sonoluminescence is caused by the interactions of water bubbles with the subatomic realm of quantum physics.

Most sonoluminescence experts prefer a more prosaic explanation.

They say sonoluminescence likely is generated by gas in a tiny bubble that is alternately expanded and then crushed by passing sound waves.

Crushing the bubble compresses the gas in it. And compressing gas generates heat, as anybody who pumps up a bicycle tire may notice.

Tiny sonic boom?

That's not all. As the walls of the bubble collapse inward, they can move nearly as fast as, and maybe faster than, the speed of sound. That might generate a tiny sonic boom or similar shock wave that smashes into the bubble's center, heating the gas even more.

And really hot gas glows. Witness the sun, whose surface temperature is about 10,000 degrees Fahrenheit. Physicists estimate that sonoluminescence can involve temperatures even hotter than that.

"What I've done seems to be consistent with that," said William Moss, a physicist at the Lawrence Livermore National Laboratory in California.

Moss has modified supercomputer programs that were designed to simulate hydrogen-bomb explosions, using them to show how much energy can be packed into a small area by a collapsing water bubble. His calculations suggest that sonoluminescence might even pack enough energy into a tiny space to generate a nuclear-fusion reaction - but so far, Moss cautions, there's no real evidence for that.

But in a paper scheduled for publication this month in "Physical Review Letters," Eberlein dismisses such explanations, arguing that they cannot account for many of the characteristics of sonoluminescence.

Shock-wave heating would produce light flashes that would last longer and be of different wave lengths than those physicists see in sonoluminescence experiments, Eberlein says. There are other problems.

She says that instead of generating light by heating gas, the collapsing bubble wall and sonic boom might shake photons (light particles) out of something known as the "quantum vacuum."

The quantum vacuum sounds like something that would be empty, but it's not. It pervades space and is necessary for electromagnetic and other fields to exist. In fact, it is only through these fields that the quantum vacuum is observable. But the vacuum is still there, even if there is no field with it.

The vacuum occasionally generates "virtual particles," a bizarre creation of quantum physics. Virtual particles pop into existence for incredibly brief periods of time, then disappear again. The virtual particles cannot be measured because they are not real particles, but they really are there nonetheless.

From virtual to real

In the case of sonoluminescence, Eberlein suggests, virtual photons popping out of the quantum vacuum get converted into the real thing. According to quantum theory, virtual particles can be made real with the addition of a lot of energy.

It's like a cosmic game of whack-a-mole. The virtual particles are the moles, popping up for a few seconds. If a player can whack the mole before it pops down again, points are scored and the existence of the mole is recorded. But if not, it's like the mole never popped up at all.

In sonoluminescence, the bubble is the hammer-wielding whack-a-mole player. If the bubble wall or the shock wave it generates sweeps past a virtual photon at just the right time, wham!

"The photon has no choice but to become real" and thereby generate a flash of light, Eberlein said.

Pop enough virtual photons into existence fast enough, and you've got sonoluminescence, she says in her paper. The paper includes equations showing that the quantum vacuum would generate photons similar to the ones in sonoluminescent light.