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In page Tractor beam:

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A force field confined to a collimated beam with clean borders is one of the principal characteristics of tractor and repulsor beams.[1] Several theories that have predicted that repulsive effects do not fall within the category of tractor and repulsor beams because of the absence of field collimation.[citation needed] For example, Robert L. Forward of Hughes Research Laboratories showed that general relativity theory allowed the generation of a very brief impulse of a gravity-like repulsive force along the axis of a helical torus containing accelerated condensed matter.[2][3] The mainstream scientific community has accepted Forward's work.[citation needed]