MAKE a mistaken claim in any branch of science, and endeavours in that field may be tainted for years. Faced with two such claims, the field is definitely in trouble. And that now seems to be the case for so-called “tabletop fusion”.

In 1989 Stanley Pons and Martin Fleischmann claimed to have achieved “cold fusion”—nuclear fusion in apparatus built on a laboratory bench. Their work, however, was discredited and the field is now a no-go area for most physicists. This week, fresh doubts emerged about a related claim to have used bubbles generated by ultrasound to create fusion. If these are substantiated, the idea of tabletop fusion will, indeed, become a pariah.

For decades, physicists have sought to achieve nuclear fusion on the cheap. Fusion involves small atomic nuclei colliding to form larger ones, a process that releases energy. But, since atomic nuclei are positively charged, and like charges repel, it is a difficult stunt to pull off. It happens in the cores of stars, where high pressure squeezes nuclei together and high temperatures mean that they are travelling fast enough to overcome the repulsion. It can also be made to happen briefly on Earth, in huge machines that can create high temperatures and pressures. A small-scale version that operated at normal temperatures and pressures would be a real boon.

Such was the device that was published by Rusi Taleyarkhan, then of Oak Ridge National Laboratory in Tennessee, in Science in 2002. Dr Taleyarkhan and his colleagues used a technique called acoustic cavitation—the imploding of a bubble created by sound waves passing through a liquid—to generate the pressures and temperatures required for nuclear fusion.

Bubble trouble

The team created these bubbles in acetone, which has molecules containing six hydrogen atoms. They ran the experiment with normal acetone, as a control, and with acetone made with deuterium (a form of hydrogen that has a proton and a neutron in its nucleus, rather than the single proton of normal hydrogen). Deuterium is easier to fuse than normal hydrogen, and all fusion experiments use it.

As they ran each experiment, the researchers looked for the release of neutrons possessing the characteristic energy that comes from deuterium fusion. With normal hydrogen, they found none. With deuterium, they claimed to have found significant numbers. However, in a paper submitted this week to Physical Review Letters, Brian Naranjo of the University of California, Los Angeles, argued that the neutrons generated in Dr Taleyarkhan's experiments did not have the right energy to have been produced by fusion.

Dr Naranjo took his data from a paper published by Dr Taleyarkhan in January. This appeared in an electronic format that allowed him to deduce those data from the graphs it showed, even though the raw numbers were not published. He argues that the resulting neutron energies are consistent with the decay of a standard radioactive source called 2{+5}2californium. Dr Taleyarkhan's description of his method explicitly excludes the possibility of such a source being present. Moreover, if Dr Naranjo is correct, 2{+5}2californium would appear to be present only in the experimental runs using deuterated acetone and not in the control experiments using normal acetone, pointing to the possibility of direct human interference.

There is a certain amount of “history” between the two scientists. Dr Naranjo works in the laboratory of Seth Putterman, one of three researchers who peer-reviewed Dr Taleyarkhan's original Science paper and did not like it. When the journal published the paper anyway, Dr Putterman went public, arguing that Dr Taleyarkhan had not ruled out several potential sources of error in his paper.

But the problems now appear to be more than personal. In this week's Nature, colleagues of Dr Taleyarkhan criticise his methods and claim to have had no success in repeating his positive results. In 2004 Dr Taleyarkhan moved to Purdue University in Indiana, where Lefteri Tsoukalas, head of nuclear engineering at the university, had been trying to repeat his experiments. Dr Tsoukalas's team had completed several experimental runs, but had not seen any evidence for bubble fusion. Once Dr Taleyarkhan had arrived, members of the laboratory became increasingly concerned by his actions. Tatjana Jevremovic, an assistant professor of nuclear engineering at the university, told Nature that Dr Taleyarkhan would sometimes claim that the experiment was producing positive results when she could see no such thing. He then removed the equipment from a communal laboratory and took it to his own labs off-campus, preventing the team from continuing with its experiments.

Moreover, the American patent office has quietly but firmly rejected Dr Taleyarkhan's bubble-fusion device. An application for a patent was filed in 2003, when he was still at Oak Ridge, on behalf of the Department of Energy, which funded the work. On December 27th last year the department formally abandoned the claim. Ricardo Palabrica of the Patent Office had described the application as “no more than just an unproven concept”.

As The Economist went to press, Dr Taleyarkhan had made no public comment on the matter. But if the apparent problems are substantiated, they will cast a shadow over the entire endeavour. Unlike the work of Dr Pons and Dr Fleischmann, bubble fusion has a strong theoretical underpinning, and may yet work. The trouble is that eager, young researchers who want to make a name for themselves are unlikely to touch it with a barge-pole.