Once-maligned claim rebounds.

By blowing bubbles and then crushing them with sound waves, a tabletop gadget appears to force atomic nuclei to fuse, a process usually requiring temperatures and pressures found in stars and thermonuclear explosions. "For the first time in history, we've been able to use simple mechanical force to initiate and control nuclear forces" says Rusi P. Taleyarkhan of Purdue University in West Lafayette, Ind.

If verified, this controversial claim by a U.S.-Russian team of university and government researchers might lead to new power sources and other payoffs, the experimenters say. Those could include compact neutron generators for probing materials and new techniques for studying hot plasmas like those of stars.

A report slated for the March Physical Review E describing the new experiments represents a comeback for Taleyarkhan and his coauthors. Two years ago, the same team reported in Science that imploding bubbles in small tanks of acetone yielded neutrons and tritium, a radioactive form of hydrogen, suggesting that fusion was taking place (SN: 3/9/02, p. 147). The report brought on a storm of criticism.

In both sets of experiments, the researchers used chilled acetone in which hydrogen atoms had been replaced by deuterium, or heavy hydrogen, which fuses more readily than ordinary hydrogen does. The team bombarded this liquid with sound waves tuned to ultrasonic frequencies that caused wide variations in pressure in the fluid. The experimenters seeded bubble formation by beaming pulses of neutrons from a neutron generator into the acetone.

During the low-pressure phase of each sound wave, bubbles expanded rapidly. Then, during the high-pressure phase, the bubbles violently imploded to the accompaniment of flashes of light, evidence of a well-known phenomenon called sonoluminescence.

Researchers have long explained those emissions as the result of extreme temperatures and pressures that build up during implosions. Some scientists had even proposed that these conditions might be ripe for fusion.

The new evidence of fusion products, which was obtained with far more sophisticated measuring equipment than had been used before, should quell previous concerns, the experimenters say.

"We took all the criticism seriously," says Richard T. Lahey Jr. of Rensselaer Polytechnic University in Troy, N.Y.

Unlike in the earlier experiments, the team recorded gamma rays, a revealing product of fusion reactions. They also monitored radiation levels around the acetone chamber more precisely and longer than they had before.

Nonetheless, critics abound. William C. Moss of Lawrence Livermore Laboratory says that the team failed to vary the conditions of the experiments sufficiently to adequately test such a remarkable claim. Aaron Galonsky of Michigan State University in East Lansing argues that the new report fails to present certain data needed to evaluate its findings.

On the other hand, Lawrence A. Crum of the University of Washington in Seattle, who panned the Science paper in 2002, says he's no longer such a doubter. "I wouldn't want to be quoted saying this is right," he says, "out [the claim] is more and more difficult to ignore."