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A tractor beam is a device with the ability to attract one object to another from a distance. The concept originates in fiction: the term was coined by E. E. Smith (an update of his earlier "attractor beam") in his novel Spacehounds of IPC (1931). Since the 1990s, technology and research has laboured to make it a reality, and have had some success on a microscopic level. Another method to realize tractor beams is based on the use of biaxial birefringent media. Less commonly, a similar beam that repels is called a pressor beam or repulsor beam. Gravity impulse and gravity propulsion beams are traditionally areas of research from fringe physics that coincide with the concepts of tractor and repulsor beams.
"Not very. Our own ships, using only the acceleration of gravity, and both plus and minus at that, make the better than four hundred million kilometers of the long route to Mars in five days. These birds are using almost that much acceleration, and I don't see how they do it. They must have a tractor ray. Brandon claimed that such a thing was theoretically possible, but Westfall and I couldn't see it. We ragged him about it a lot—and he was right. I thought, of course, they'd drift with us, but they are using power steadily. They've got some system!"