A tractor beam is a device with the ability to attract one object to another from a distance. It uses manipulated gravitational forces to push or pull objects. The concept was originated in fictions: the term was created by E. E. Smith in his novel Space hounds of IPC (1931). Since 1990s physicists have been working hard trying taking this technology from science fiction to 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.
Tractor beams are most commonly used on spaceships and space stations. They are generally used in three ways:
- As a device for securing or retrieving cargo, passengers, shuttle craft, etc. This is analogous to cranes on modern ships.
- As a device to harness objects that can then be used as impromptu weapons by the craft
- As a means of preventing an enemy from escaping, analogous to grappling hooks.
In the latter case, there are usually countermeasures that can be employed against tractor beams. These may include pressor beams (a stronger pressor beam will counteract a weaker tractor beam) or plane shears aka shearing planes (a device to “cut” the tractor beam and render it ineffective). In some fictional realities, shields can block tractor beams, or the generators can be disabled by sending a large amount of energy back up the beam to its source.
Two objects being brought together by a tractor beam are usually attracted toward their common center of gravity. This means that if a small spaceship applies a tractor beam to a large object such as a planet, the ship will be drawn towards the planet, rather than vice versa.