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…

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