The Woodward effect, also referred to as a Mach effect, one of at least three predicted Mach effects, is part of a hypothesis proposed by James F. Woodward in 1990.The hypothesis states that transient mass fluctuations arise in any object that absorbs internal energy while undergoing a proper acceleration. Harnessing this effect could generate a thrust, which Woodward and others claim to measure in various experiments. If proven to exist, the Woodward effect could be used in the design of spacecraft engines of a field propulsion engine that would not have to expel matter to accelerate. Such an engine, called a Mach effect thruster (MET), would be a breakthrough in space travel. So far, no conclusive proof of the existence of this effect has been presented. Experiments to confirm and utilize this effect by Woodward and others continue.