Tractor beam
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A tractor beam is a hypothetical device with the ability to attract one object to another from a distance. Tractor beams are frequently used in science-fiction books and movies.
Tractor beams can be compared to the gravitational attraction between two masses, or the attraction between two magnets. They differ from such natural attractive forces in that they operate in a particular direction or on a particular object. The theory of operation depends upon the story they are used in. In the Star Trek television show, for example, the tractor beam is explained as using gravitons, the elementary particles speculated to cause gravity. In some cases, the theory is not explained, and is left to the imagination of the reader or viewer.
Tractor beams are sometimes depicted as being reversible, so they repel the other object rather than attracting it. When used in this mode, they may be referred to as pressor or repulsor beams.
The most common places where tractor beams are used are on spaceships and space stations. They are generally used in two ways:
- As a device for securing or retrieving cargo, passengers, shuttlecraft, etc. This is analogous to cranes on modern ships.
- As a means of preventing an enemy from escaping, analogous to grappling irons.
In the latter case, there are usually countermeasures that can be employed against tractor beams. These countermeasures may include pressor beams (a stronger pressor beam will counteract a weaker tractor beam) or plane shears (a device to "cut" the tractor beam and render it ineffective). Also, shields can sometimes block tractor beams, although this seems to be more based on the plot of the episode than any overall physics, and the generators can be disabled by sending a large amount of energy back up the beam to its source.
Tractor beams and pressor beams can be used together as a weapon: by attracting one side of an enemy spaceship while repelling the other, one can create severely damaging shear effects in its hull.
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.
Tractor beams do not always work like this. In Star Trek, tractor beams are non-Newtonian. The target is always drawn toward the emitter, irrespective of mass, because the beam does not actually transmit a force across space. It functions similarly to the warp drive, manipulating space-time at the target's position.
The following works contain well-known appearances of tractor beams:
- Babylon 5 (TV series)
- The Honor Harrington books by David Weber
- The Lensman books by E. E. Smith (possibly the original appearance)
- The Sector General books by James White (source of the combined tractor/pressor beam weapon)
- Star Trek (TV series, movies, books, games)
- Star Wars (movies, books, games)
- The "gravity gun" in Half Life 2 by Valve Software (games)
- The "Grabber" in Doom 3: Resurrection of Evil (games).
In the real world, scientists are able to manipulate objects at the atomic and molecular scale using lasers (see optical tweezers), but no technology capable of manipulating bulk matter with the level of precision envisiaged in science-fiction has yet been devised.fr:Rayon tracteur