Shock wave simulations also indicate that temperatures inside the collapsing bubbles may reach up to 10 million degrees Kelvin, as hot as the center of the sun. Although the high temperatures and pressures within the bubbles would be sufficient to generate fusion, the overall results of the study only suggest, but do not confirm, nuclear fusion in the bubbles' collapse.

Nuclear fusion joins together light atoms, such as hydrogen, in a reaction that creates a third heavier atom and converts some of the original atoms' mass into energy. Nuclear fission, the type of reaction currently used in commercial power plants, splits heavy atoms like uranium and releases some of the excess energy stored as mass in the uranium atoms. Scientists have been eager to harness fusion as an energy source, because unlike fission, fusion uses readily available raw material as fuel and produces fewer radioactive waste products.

The experiments performed by the Science researchers suggest that nuclear fusion might occur in bubbles created by "acoustic cavitation," a phenomenon studied for nearly a century. In acoustic cavitation, the pressure of a sound wave creates and collapses bubbles in a liquid. The first part of the wave is a tension wave, which stretches the liquid and pulls apart a space for bubbles to form when the liquid is bombarded by energetic particles like neutrons. A second compression wave follows close behind, squeezing and bursting the bubbles, which then emit a brilliant but extremely brief flash of light called sonoluminescence.

Sonoluminescence's exact cause is still somewhat mysterious, but many researchers believe that the shock waves of the collapse generate high temperatures and pressures in the bubble's gas, which releases a burst of energy. Scientists have learned to trap single bubbles within a sound wave, causing them to swell and shrink and emit light in a regular fashion.

Temperatures inside these bubbles can be as high as 5000-7000 degrees Kelvin, about as hot as the sun's surface. But, recent experiments by a number of researchers suggest that bubble temperatures can reach even higher temperatures--closer to the heat needed for nuclear fusion--if the original bubbles are very small and allowed to grow rapidly before collapse.

Rusi P. Taleyarkhan of Oak Ridge National Laboratory and colleagues devised an experiment to produce these super-hot bubbles, trapping bubbles in deuterated acetone (acetone with its normal hydrogen atoms replaced by deuterium, a heavy hydrogen isotope that can undergo fusion reactions). The experiment's entire apparatus is well within the bounds of "table-top physics," about "the size of three coffee cups stacked one on top of the other," says Taleyarkhan.

Using a pulse of neutrons to first "seed" the tiny bubbles, each no bigger than the period at the end of this sentence, the Science authors then used a sound wave to grow the bubbles rapidly just before their implosion. The process produced stable bubbles that could expand to nearly a millimeter in radius before collapsing, a key part of producing very high pressures and temperatures.

The researchers then searched for signs that fusion might be taking place in the implosions. Deuterium-deuterium fusion reactions create two telltale products: neutrons of a characteristic energy and tritium, another hydrogen isotope. Using very sensitive detectors, Taleyarkhan and colleagues detected higher levels of tritium in samples with extensive bubble implosion. The researchers also observed the emission of neutrons with energy close to 2.5 million electron volts, which is the characteristic neutron energy associated with deuterium-deuterium fusion.

As a part of an elaborate series of control experiments conducted throughout the research, the authors prepared identical experiments in non-deuterated (normal) acetone, and observed no neutron emission or tritium production in these experiments.

Currently, the level of neutron emissions with the characteristic fusion energy appears to be lower than would be expected from the tritium signals observed in the experiment. Further tests are needed to account for this discrepancy, and to verify the observed relations between the neutron emissions, tritium production, and bubble collapse.

If fusion is confirmed in further tests, these bubbles would still have a long way to go before they could be considered as a possible energy source with any commercial value, says Science co-author Richard T. Lahey Jr. of Rensselaer Polytechnic Institute. First of all, the bubble reaction would have to demonstrate net energy gain--that is, it should produce more energy than the energy needed to drive the reaction itself. Second, scientists would have to find a way to make the reaction perpetuate itself in a chain reaction, without constant input from a neutron source.

In the short term, the research may provide a more convenient way for scientists to produce and study nuclear fusion processes in the laboratory, says Fred D. Becchetti of the University of Michigan, in an accompanying Perspective article.

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