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The bubbles produced by ultrasound in water (sonoluminescence) reach extremely high temperatures and pressures for brief periods. Could these conditions initiate or facilitate nuclear fusion, as suggested in the recent movie "Chain Reaction"?

A detailed discussion of the physics of can be found in the article "Sonoluminescence: Sound into Light," by Seth J. Putterman (Scientific American, February 1995). In it, the author outlines a different interpretation of the phenomenon from the one given below, though he agrees that the likelihood of getting fusion to occur in sonoluminescence bubbles is insignificant.

Andrea Prosperetti in the department of mechanical engineering at the Johns Hopkins University has studied this question in detail. He responds:

"It must first of all be stressed that the 'extremely high temperatures' referred to are, at least for now, speculation. While many researchers would concede temperatures of up to, say, 10,000 kelvins (which is way too low for nuclear fusion), a much smaller number would feel comfortable with temperatures in the millions of degrees range. The computations that indicate such extreme conditions inside a pulsating bubble are based on rather extreme idealizations.

"The most fundamental one is the fact that the bubble remains absolutely spherical during its radial oscillations. On theoretical grounds, there are many reasons to doubt this premise: a collapsing sphere is highly unstable (which is the reason why attempts at producing fusion by causing the implosion of gas-filled micro-balloons with powerful pulses of laser light have so far failed), and liquid jets may develop that span the bubble.

"Furthermore, experiment suggests that the light emitted by a bubble has a weak directional asymmetry, which would be incompatible with perfect sphericity. Hence, while it is not absolutely possible to rule out the occurrence of nuclear reactions inside a pulsating bubble on the basis of the present knowledge, the actual occurrence of such reactions is, to say the least, doubtful."

The Johns Hopkins University has also provided this official statement on Dr. Prosperetti's work:

Sonoluminescence, the puzzling glow emitted by a bubble in a field of high-pitched sound waves, may be caused by a tiny jet of liquid that shoots across the interior of the bubble at supersonic speed and slams into the opposite side, a Johns Hopkins researcher has proposed. At the point where this powerful jet strikes the bubble wall, it "fractures" the liquid, releasing energy in the form of light, says Andrea Prosperetti, an internationally respected expert on the mechanical properties of bubbles.

Prosperetti's theory appears in the April 1997 issue of the Journal of the Acoustical Society of America. His paper offers an alternative to the widely held view that the bubble glows because of shock waves that concentrate energy in its center as it shrinks.

His theory also deflates the hope among some researchers that sonoluminescence generates enough pressure and heat to produce nuclear fusion, a potential source of cheap, clean energy. Some scientists have speculated that bubble temperatures during sonoluminescence exceed 2 million degrees Fahrenheit, near the levels needed for fusion. This idea became a key plot point in the motion picture "Chain Reaction," starring Keanu Reeves. But if Prosperetti's theory holds true, the heat inside the bubbles would peak at about 10,000 degrees F, the level found at the sun's surface. "It's enough to explain the chemical activity, but it's far below the amount needed to produce nuclear fusion," says Prosperetti, who is the Charles A. Miller, Jr. Distinguished Professor of Mechanical Engineering at Hopkins.

Sonoluminescence was discovered in 1934 by two German physicists who immersed powerful ultrasound generators in a vessel of water, creating a cloud of tiny bubbles that gave off a glow. Scientists were intrigued but found it was too difficult to study in detail the unwieldy mass of short-lived bubbles. In 1989, however, Lawrence Crum, then a professor at the University of Mississippi, and his graduate student, Felipe Gaitan, were able to induce sonoluminescence in a single bubble trapped within a sound field inside a cylinder of water.

Since then, scientists have been able to study the phenomenon more closely. Much to their surprise, they realized that this "single-bubble" luminescence was different from the massive "multiple bubble" phenomenon first observed 60 years earlier and -- as it turns out -- far more mysterious. For example, the flash of light lasts an incredibly short time, a few tens of trillionths of a second. Also, the phenomenon is extremely sensitive to the nature, purity and temperature of the liquid and to the presence of dissolved gases in it.

Sound waves passing through the liquid cause the bubble to compress and expand repeatedly. At its largest point, the bubble's diameter is about that of a human hair. Scientists believe the sound energy is concentrated during the bubble's compression phase, then is released as light near the point where its size is smallest. But the exact mechanism has remained a mystery.

In his new paper, Prosperetti says it is unlikely that shock waves within the shrinking bubble trigger sonoluminescence because the bubble would need to maintain a near-perfect spherical shape. "I think it is absolutely impossible for the bubble to remain spherical," he says. "In a sound field, there is a very well-defined mechanism that will prevent this from happening. The fluid wants to push a jet, a finger of liquid, through the bubble, hitting the other side. What you see in sonoluminescence is the initial result of this 'hammer of water.'" This jet, moving at perhaps 4,000 miles per hour, or more than five times the speed of sound in air, strikes so quickly that water molecules do not have time to flow away from the point of impact. Instead, the fluid fractures. "This is what happens with Silly Putty, for instance," Prosperetti says. "If you pull slowly, it just stretches or flows. But if you pull it really hard, it snaps, and you get a brittle fracture."

Ice and even Wint-O-Green Lifesavers candy sometimes give off light when they crack, and water molecules could produce the same effect, the Hopkins researcher suggests. His theory holds the promise of explaining many facets of the phenomenon. For example, bright light emission requires tiny amounts of a noble gas such as xenon, argon or helium dissolved in the liquid because, Prosperetti believes, these inert atoms create flaws or weaknesses in water's crystal-like structure that provide a foothold where the fracture begins. In his paper, Prosperetti urges other researchers to test his theory. He suggests several lab experiments for this purpose, including the firing of a hyperfast bullet or fluid jet at water in a controlled setting to see if it produces luminescence.

Lawrence A. Crum of the Applied Physics Laboratory at the University of Washington expands on the above response:

"If one is to consider the possibility of nuclear reactions produced by sonoluminescence, it is helpful first to consider some simple physics, particularly the energy levels associated with these various systems.

"When a sound wave propagates through a fluid, the amount of energy density in the wave is quite small. The reason we think the sound of a jet aircraft is really loud has more to do with the sensitivity of our remarkable auditory system than with the energy in the sound wave itself. Our ear is so sensitive that as newborn babies, we can hear molecular displacements on the order of angstroms--about the diameter of an atom. Even though we lose this sensitivity with age, our adult ears can still detect molecular displacements on the order of nanometers. Thus, if one considers the energy density in a sound field capable of producing sonoluminescence, one finds it to be quite small--on the order of 10-11 electron volt per molecule. The electron volt may seem a crazy unit, but we shall see later why it is a convenient one.

"When a sound field propagates through a liquid such as water, the molecules of the liquid are held together by molecular bonds that are relatively strong. Thus, it is very difficult for the negative pressures existing in a propagating sound field to tear apart the water--and it practically never happens. What does happen is that the sound field interacts with any small gas bubble that may exist in the water and causes the bubble to grow dramatically during the passage of the negative pressure portion of the sound field--the water essentially 'boils'--because the pressure is below the vapor pressure. During the negative pressure cycle, the bubble can grow to many times its original size--say a factor of 1,000 in volume.

"When the sound field eventually turns positive, the pressure is now above the vapor pressure; the vapor rapidly condenses, and all the energy that was given to the bubble during its growth process is available to be concentrated into a small region as the bubble is driven to an implosive collapse. This process is called acoustic cavitation. Because this implosive collapse is dominated by the inertia of the liquid surrounding the bubble, and there is little stiffness supplied by the condensing vapor (only the small amount of residual gas contained within the bubble), the energy density can become much larger than that originally present in the sound field itself. The energy concentration is now so high that the residual gas contained within the bubble is heated to incandescence temperatures and emits light. This process is called sonoluminescence. Because these electromagnetic emissions are on the order of an electron volt, and they probably come from a single molecule, or atom, or electron, we can now say that the energy concentration is now on the order of one electron volt per molecule--an increase of a factor of 1011 or so.

"Energies on the order of an electron volt are typical on an atomic basis and correspond to an effective temperature on the order of 10,000 kelvins. This is a pretty high temperature, of course, and can influence chemical reactions. Thus, sonoluminescence is often associated with 'sound chemistry'--or 'sonochemistry.' The fact that a rather benign mechanical mechanism such as a propagating sound field can produce atomic reactions is a quite remarkable and has attracted considerable scientific attention (see "The Chemical Effects of Ultrasound," by Kenneth S. Suslick in Scientific American, Vol. 260, No. 2, pages 8086 [or 62-68 for non-U.S. readers]; February 1989).

"Although energies on the order of an electron volt per molecule are relatively large for our macroscopic world, they are the typical energies of reaction in the atomic world. When we consider thermonuclear fusion, on the other hand, we need to move from the atomic to the nuclear scale. Because a proton or a neutron is on the order of a million times smaller than an atom, nuclear fission and fusion typically require energies on the order of millions of electron volts (MeV). The substance of the question posed by the reader is essentially: Can this benign mechanical sound field now interact at the nuclear level? Of course, our immediate response is that we are still six orders of magnitude too small in energy, and there is no possibility for nuclear fusion from sonoluminescence.

"Given that controlled fusion is such an attraction because of our nearly inexhaustible source of hydrogen as fusion fuel and that existing devices designed to harness this energy are of enormous dimensions and costs, it would seem desirable to see if there is some mechanism to boost the energy density by another six orders of magnitude. There has been a glimmer of hope in this direction when it was determined that there are strong indications that the collapsing bubble can generate an imploding shock wave within the gas contained within the interior of the collapsing bubble. This imploding shock wave can compress the interior of the bubble's contents even more; indeed, William C. Moss and his colleagues at Lawrence Livermore National Laboratory have obtained theoretical estimates of the temperatures achievable with an imploding shock wave, and these values approach those required for nuclear fusion.

"Is an imploding shock wave possible? Seth Putterman and his colleagues at the University of Califnornia at Los Angeles have measured the velocity of the bubble interface and have determined that it can reach values on the order of four to five times that of the velocity of sound in the undisturbed gas. These data seem very promising. Andrea Prosperetti--see his recent comments here in 'Ask the Experts'--has suggested, however, that the bubble must remain spherical for the shock wave to develop much strength--which he believes is not very likely. Tom Matula and his colleagues at the University of Washington have observed a shock wave in the liquid after bubble collapse, which might be a consequence of a shock wave in the gas. Values of the amplitude of this waterborne shock wave correspond to predicted values, assuming it arose in the interior of the gas, so there is additional evidence of the effect.

"The state of the art of sonoluminescence research at the moment is that investigators are trying to understand the bubble collapse process and look for any evidence of the shock wave within the bubble itself.

"It is difficult to understand just what the writers of the movie Chain Reaction intended in their screenplay. Certainly the science was so awful it turned off any serious scientist. Their hypothesis that sonoluminescence generated hydrogen (and no oxygen) was kind of silly. In the movie, a 'chain reaction' occurred, but it was difficult to determine if this was a nuclear chain reaction or just a big hydrogen explosion. And the fact that Keanu Reeves was able to outrun the shock wave from the explosion on a motorcycle suggests that it was actually pretty mild--even though it did demolish several city blocks.

"As one who is involved in sonoluminescence research, I was particularly disappointed that the writers really messed up the science. I think that they sold the public short and that a little more authentic science would have attracted an explosion of interest among young kids on the Internet and would have greatly improved the attendance at the theaters. It frustrates me that movies and TV shows that depict doctors and lawyers are made to look quite authentic, but when it comes to physics or chemistry, Hollywood seems not to have made it past the third grade!"

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