AFTER 16 years, it's back. In fact, cold fusion never really went away. Over a 10-year period from 1989, US navy labs ran more than 200 experiments to investigate whether nuclear reactions generating more energy than they consume - supposedly only possible inside stars - can occur at room temperature. Numerous researchers have since pronounced themselves believers.

With controllable cold fusion, many of the world's energy problems would melt away: no wonder the US Department of Energy is interested. In December, after a lengthy review of the evidence, it said it was open to receiving proposals for new cold fusion experiments.

That's quite a turnaround. The DoE's first report on the subject, published 15 years ago, concluded that the original cold fusion results, produced by Martin Fleischmann and Stanley Pons of the University of Utah and unveiled at a press conference in 1989, were impossible to reproduce, and thus probably false.

The basic claim of cold fusion is that dunking palladium electrodes into heavy water - in which oxygen is combined with the hydrogen isotope deuterium - can release a large amount of energy. Placing a voltage across the electrodes supposedly allows deuterium nuclei to move into palladium's molecular lattice, enabling them to overcome their natural repulsion and fuse together, releasing a blast of energy. The snag is that fusion at room temperature is deemed impossible by every accepted scientific theory.

That doesn't matter, according to David Nagel, an engineer at George Washington University in Washington DC. Superconductors took 40 years to explain, he points out, so there's no reason to dismiss cold fusion. "The experimental case is bulletproof," he says. "You can't make it go away."

From issue 2491 of New Scientist magazine, 19 March 2005, page 30