The Woodward effect is a hypothesis proposed by James F. Woodward, a physicist at California State University, Fullerton, that energy-storing ions experience transient mass fluctuations when accelerated. While some have expressed doubt about this hypothesis, no respected theoreticians have yet disproved it. If these doubts are unfounded it is conceivable that the Woodward effect could be exploited in spacecraft engines that would not have to expel matter to accelerate. Several groups of researchers have therefore conducted experiments designed to prove or disprove the existence of the effect.

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