Fresh questions surround the claims that bubble fusion has been achieved, according to an investigation by Nature.

Reports by Rusi Taleyarkhan that he had achieved table-top fusion in collapsing bubbles caused a storm when they were published in 2002 (R. P. Taleyarkhan et al. Science 295, 1868–1873; 2002).

Taleyarkhan, a nuclear engineer now based at Purdue University in West Lafayette, Indiana, used sound waves to cause the formation and collapse of bubbles within a liquid. The conditions inside the collapsing bubbles are theoretically extreme enough to allow nuclear fusion to take place. Taleyarkhan claims to have achieved this — an effect that, if real, could one day provide an almost limitless source of energy.

Four years later, Taleyarkhan's work retains an almost magical ability to grab the headlines, most recently in January, when his latest results (R. P. Taleyarkhan et al. Phys. Rev. Lett. 96, 034301; 2006) were promoted in a press release by the American Physical Society. Millions of dollars are being spent trying to repeat the work, including $800,000 from the US Department of Defense.

But corroboration remains elusive. Now, an investigation into the circumstances surrounding Taleyarkhan's experiments is throwing up serious questions about the validity of the work.

Interviews with researchers who have worked closely with Taleyarkhan at Purdue reveal concerns about his actions since he arrived there full-time in 2004. The steps he has taken, they say, include claiming he obtained positive results from equipment on which they had seen only negative data, and removing the equipment from their lab altogether.

And physicist Brian Naranjo of the University of California, Los Angeles, has completed an analysis that he plans to post later this week on arXiv. It suggests that the spectrum reported in Taleyarkhan's latest paper as proof of nuclear fusion came instead from the radioactive decay of a standard lab material.

Taleyarkhan has declined to comment on events at Purdue, or on Naranjo's analysis, and he vigorously affirms that his results are valid and the effect is real. But the overall message from people close to this work is that there is little hope this particular approach will yield a viable fusion energy source.