Odd phenomenon seems to make light of physical rules

You can get away with crossing against the light, for example, but you can't go faster than it. Wrongdoers aren't always held accountable for what they do, but each of their actions requires an equal and opposite reaction.

The intractability of physical laws makes life comfortably predictable. But it's no comfort to physicists who are trying to explain sonoluminescence, a bizarre physical phenomenon in which sound waves generate light. Sonoluminescence just 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. That phenomenon became known as multiple-bubble sonoluminescence when five decades later, in 1988, 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 generated a renaissance of sonoluminescence research, all geared toward understanding a phenomenon that appears to deliver something for almost nothing.

Inspires a Hollywood movie

Sonoluminescence is so mysterious that it's even spawned a Hollywood movie, scheduled for release in August. In the film 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.

``That's something that's very startling,'' said Claudia Eberlein, a theoretical physicist at Cambridge University in England. ``It's really important to know what is going on there.''

Eberlein and several others have proposed that some pretty far-out physics is behind sonoluminescence. They suggest that what happens in a pint-sized sonoluminescence chamber is a result of physical processes that generally are thought to be important only near black holes and in other exotic locales.

Expanding on the work of the 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 are intrigued by the idea, but they don't think Eberlein's theory is right. They prefer a more prosaic explanation for the phenomenon.

`Have some doubts'

``I am not a quantum mechanic, but I have some doubts,'' said Kenneth Suslick, a chemist at the University of Illinois at Urbana-Champaign.

Most physicists say they believe that sonoluminescence 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 inside it. And compressing gas generates heat, as anybody who has ever pumped up a bicycle tire may have noticed.

But 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, where the 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 can't account for many of the characteristics of sonoluminescence.

Shock-wave heating would produce light flashes that would last longer and be of different wavelengths than those that physicists see in sonoluminescence experiments, Eberlein argues. And there are other problems as well.

``Of the explanations that have been suggested so far, nothing really fits,'' Eberlein said.

The quantum vacuum

She argues that instead of generating light by heating gas, the collapsing bubble wall and sonic boom might shake photons, or 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's only through these fields that the quantum vacuum is observable. But the vacuum is still there, even if there's 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 and then disappear again. The virtual particles can't be measured because they're not real particles, but they really are there nonetheless.

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 whole lot of energy.

It's like a cosmic game of whack-a-mole. The virtual particles are the moles, popping up for a few brief 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 in the first place.

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 argues in her paper. The paper includes equations showing that the quantum vacuum would generate photons similar to the ones in sonoluminescent light.

The paper also suggests experiments that might be done to support the quantum hypothesis. But it doesn't provide any supporting evidence.

``It's not clear that this mechanism actually works,'' said Alan Chodos, a physicist at Yale University in New Haven, Conn. Some physicists have calculated, for example, that the bubble walls would have to travel faster than light in order to shake photons out of the quantum vacuum.

And that's against the rules.