By Jeff Hecht

Though their work is dismissed by most physicists, a determined cadre of scientists is still chasing after what could be an energy jackpot--and their experiments are producing heat and nuclear byproducts that can't be otherwise explained.

Fifteen years after the first controversial claims hit the headlines, cold fusion refuses to die. A small cadre of die-hard advocates argues that experiments now produce consistent results. The physics establishment continues to scoff, but some scientists who have been watching the field carefully are convinced something real is happening. And now the U.S. Department of Energy has decided that recent results justify a fresh look at cold fusion.

Fusion of the nuclei of hydrogen atoms powers the sun, and promises nearly limitless energy on Earth. But fusion is extraordinarily difficult to tame because nuclei strongly repel each other. The tremendous heat and pressure inside the sun can overwhelm this repulsion, and thermonuclear bombs can attain those conditions, fleetingly, on Earth. But building a fusion reactor that can convert that tremendous heat into useful energy has posed an immense challenge. After decades of research, the conditions needed for fusion still can be attained only briefly, and these experimental fusion reactions produce less energy than is needed to ignite them.

Physicists were stunned when two University of Utah electrochemists, Stanley Pons and Martin Fleischmann, claimed in 1989 that they had achieved nuclear fusion at room temperature. Their experiment packed deuterium--the stable heavy isotope of hydrogen--into palladium electrodes. After many hours of operation, they reported that more heat was generated than a purely chemical reaction could have produced. At first it looked like Pons and Fleischman might have come up with a revolutionarily easy way to tap fusion energy, and laboratories around the world rushed to try the experiment for themselves. The simple-looking experiment proved virtually impossible to reproduce, however, and within weeks, most physicists wrote off cold fusion as a mistake--an experimental result that contradicted the known laws of physics.

Yet the potential of limitless energy lured a band of would-be revolutionaries who kept on working the problem. Often they found nothing. Sometimes, however, their experiments appeared to produce more energy than they expected from chemical reactions; at other times they detected traces of potential fusion reaction products, suggesting that some previously unknown physical effects may be at work.

The evidence for "new physics" has been building for years, says Peter Hagelstein, associate professor of electrical engineering and computer science at MIT, who chaired the tenth International Conference on Cold Fusion in Cambridge last August. Experiments performed under properly controlled conditions reliably produce more heat than standard theory predicts. Nuclear products show up in about the right amounts to account for this excess heat. Patterns have emerged that explain previous anomalies. When Hagelstein saw how pieces of the puzzle were fitting together at the August meeting, he urged the Department of Energy to reconsider a field that had been cast out of orthodox science soon after its birth.

Over the past 15 years, enthusiasts have generated some 3,000 manuscripts on cold fusion, but very few were ever published in scientific journals. Many results evaporated under outside examination, and promoters pushed "free energy" schemes that sounded more like perpetual motion than physics. Most of those manuscripts "are not helpful," says Hagelstein, a theorist with wide-ranging interests in optics, energy, and nuclear physics. But some 50 do show interesting, reproducible effects. "The heat effect has been replicated many times," Hagelstein. It works only when deuterium is loaded into palladium cells, and never when normal hydrogen is used instead of the heavy isotope. Exacting measurements with heat-measurement instruments have answered criticisms of the original experiments. Excess heat has been measured beyond what Hagelstein considers any reasonable doubt.

Experiments that produce excess heat also have yielded helium-4, one potential product of the fusion of two deuterium nuclei, in amounts that correlate with the excess heat. Theory predicts that the fusion reaction should generate 24 million electron volts (MeV) of energy per helium-4 nucleus. An analysis by Michael McKubre of SRI International detected energy of 31 MeV-- a match within the experimental uncertainty of plus or minus 13 MeV. Skeptics had doubted the reaction was possible, but Hagelstein says McKubre's analysis of the experiments, reported at last year's cold fusion meeting, shows that fusion of two deuterium to yield helium-4 "is not as nutty as it initially seemed."

McKubre has also found that the seeming inconsistency in experimental heat production arose from differences in the amount of deuterium packed into the palladium electrode. Whenever the number of deuterium atoms loaded into the metal matched or exceeded the number of palladium atoms, excess heat was generated. Palladium loaded with slightly less deuterium produced inconsistent results, and if the deuterium level was reduced by a great amount, then no excess heat at all was produced. Deuterium loading was hard to control and limited by the strength of the metal. Unfortunately, palladium strength is difficult to predict or control, and is not improved by purification; indeed, the purest palladium ruptured at lower loadings, and the highest strength was seen only in one impure batch.

The growing evidence has convinced fusion physicist George Miley of the University of Illinois at Urbana-Champaign that "there are important physical phenomena occurring." Skeptics aren't changing their minds, but he thinks that previously neutral observers are becoming more receptive to the possibility that a real phenomenon is occurring in these experiments. Yet while cold fusion researchers have gone from thinking they smell smoke to feeling warmth, it's still not clear what's really going on. "This field is led experimentally. We've got to get the theories up to where they start helping lead the experiments," Miley says.

The challenge for theorists like Hagelstein is to fill the yawning gap between traditional nuclear theory and cold fusion experiments. He suspects the difficulty lies with "a very powerful approximation" at the root of 70 years of nuclear physics--that all nuclear interactions occur between two particles in a vacuum. He thinks that assumption breaks down in cold fusion, where the interacting particles are tightly packed in a metal lattice. His idea is that the deuterium nuclei exchange vibrational energy, or "phonons," with the surrounding palladium atoms. That exchange could enhance nuclear interactions that would otherwise be vanishingly small, so that the reactions can occur at the rates implied by cold fusion experiments. Hagelstein's theory is still in development, but is reaching a point where he can start making testable predictions--a vital step toward making cold fusion a credible science. "In time, hopefully, we'll get more of the puzzle figured out," he says.

A positive Department of Energy review would open the door to badly needed research support, but big questions remain even if the reality of the physics can be established. Is the cold fusion effect strong enough to be used for practical energy production? If it is, it's not likely to compete directly with hot fusion, says Miley, who works on both. Cold fusion works on a small scale, so it might find a home in small distributed power units. Hot fusion's natural home is inside the sun; if it can be controlled on our planet, it would be inside large reactors feeding power into the grid.

But those goals are a long ways off. For now, the little community of cold-fusion researchers hopes it is on the threshold of validation after 15 years of struggle.