By Associate Editor Erico Guizzo

Purdue University announced this week that an internal inquiry has cleared nuclear engineering professor Rusi P. Taleyarkhan of allegations of research misconduct made against him nearly a year ago. Taleyarkhan is the leading proponent of what is known as bubble fusion, or sonofusion, which uses high-frequency sound waves to implode bubbles in a liquid, producing thermonuclear fusion. If the method works and can be scaled up, it could yield sizable amounts of energy.

Officials of the university, in West Lafayette, Ind., started the inquiry last March after other nuclear engineering faculty members expressed concerns about Taleyarkhan's work. The faculty members, who were trying to replicate Taleyarkhan's bubble fusion experiment, complained that he was uncooperative and secretive.

The Purdue committee conducting the inquiry said in a statement "that the evidence does not support the allegations of research misconduct and that no further investigation of the allegations is warranted." However, Purdue released no details of the inquiry and said it wouldn't publish the report.

In a brief telephone interview, Taleyarkhan said he had had "a very difficult time, being under these kinds of allegations, with doubts arising in people's minds about the very integrity of a person." He added: "I'm very happy that this is behind us now."

Bubble fusion has been surrounded by controversy since 2002, when Science accepted a paper by Taleyarkhan and colleagues despite objections from some of the paper's reviewers and from other experts familiar with the experiments described.

A team led by Taleyarkhan, then at Oak Ridge National Laboratory, in Tennessee, reported that they had used high-frequency sound to blast a Pyrex flask filled with a liquid rich in deuterium, a hydrogen isotope, creating pressure oscillations that imploded tiny bubbles. They also argued that the bubbles' violent collapse had caused some of the deuterium to fuse. Thermonuclear fusion is the process that powers the Sun and hydrogen bombs. (In May 2005, IEEE Spectrum published a description of the team's work for the general reader.)

From the beginning, criticism has centered on whether Taleyarkhan's group ruled out all possible sources of error in the tricky business of detecting neutrons, the telltale sign of fusion. The controversy illustrates how science sometimes functions in a messy, non-black-and-white way. One critical question is, how much evidence is enough?

Taleyarkhan's group has since published peer-reviewed papers in other respected journals, and some scientists believe that their data is compelling. Others argue, though, that the group should have performed certain additional control experiments. Since it hasn't, they maintain that the possibility of false positives persists.

Without a doubt, the greatest mark against bubble fusion has been the failure of any group outside Purdue to replicate the experiment using its own equipment. How hard can this be? Again, there's disagreement. Some insist that the slightest variations in how you custom-make the finicky Pyrex flask or fine-tune the bubble-imploding sound waves could determine whether you get fusion. Others, however, say that the experiment is not all that esoteric, and that if it were possible to replicate it, someone would have done so already.

In the meantime, Taleyarkhan says he has allowed some outside groups and individuals to come to his lab to gather their own data using his bubble-fusion setup. Among those was a team at LeTourneau University, in Longview, Tex., which used a liquid scintillation detector as well as plastic fast-neutron detectors and reported positive results. Another visitor was William M. Bugg, an emeritus physics professor at the University of Tennessee, in Knoxville; he conducted his own measurements and controls using plastic neutron detectors. He also saw positive results.

But neither such quasi-independent verifications nor the exoneration by the Purdue committee seem likely to satisfy Taleyarkhan's critics. One of them is Seth Putterman, a physics professor at the University of California, Los Angeles. He led a major effort funded by the U.S. Defense Advanced Research Projects Agency to reproduce Taleyarkhan's results, but found no evidence of fusion.

"Although I still claim that Taleyarkhan's work is wrong, I have always maintained that the best outcome would be if his claims turned out to be true," Putterman said in an email. "Unfortunately, Purdue's expression of faith in Taleyarkhan's work doesn't help independent scientists to duplicate his claims." He added that Purdue should have interviewed Taleyarkhan's critics outside the university and also appointed external members to the review committee. "How do I respond to a secret investigation by a secret internal panel?"

Ross Tessien, president of Impulse Devices Inc., a company in Grass Valley, Calif., that wants to commercialize sonofusion, said in an email that the Purdue inquiry's outcome was "great news." Tessien hopes Taleyarkhan "can now move forward in productive fashion and that he is able to prove his claims to the satisfaction of the scientific community."

Impulse Devices hasn't been able to reproduce Taleyarkhan's results, despite what it calls "painstaking" attempts. Now, it has returned to its original approach of blasting sound waves into a spherical chamber filled with liquid metals containing hydrogen bubbles. The company believes that this setup is better suited to driving sustained thermonuclear fusion reactions.

As for Taleyarkhan, he says he's resuming his experiments and trying to obtain new sources of funding, which because of the Purdue inquiry "have gone down below zero, just like the weather over here in Indiana these days." Taleyarkhan says he wants to scale up his setup so it can eventually produce more energy than it consumes. "We just have to pick up the pieces and see what we can do now."

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