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. No real-world equivalents exist outside microscopic level. The exact specifics vary, but there are generalities.

Usage

Tractor beams are most commonly used 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 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). Also, shields can sometimes block tractor beams (though this seems to be more based on plot 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. Another mode of destructive use of such beams is rapid alternating between pressing and pulling force in order to cause structural damage to the ship as well as inflicting lethal forces on its crew.

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.

Appearances

Works containing well-known appearances of tractor beams include:

Babylon 5 (TV series)
Buck Rogers comic strip - originally just repulsor beams; tractors appeared by 1970s
Galaga - used by the Galagas to steal the player's ship.
The "Grabber" in (games)
The "Gravity Gun" in Half-Life 2 by Valve Software (games)
The Salvage Corvette's "Salvage Field" in Homeworld (games)
The Honor Harrington books by David Weber
The Lensman books by E. E. Smith (possibly the original appearance)
The Grapple Beam from the Metroid series (games)
The Sector General books by James White (source of the combined tractor/pressor beam weapon)
Starfire series - the combined tractor/pressor beam weapon
Starplex by Robert J. Sawyer
Star Trek (TV series, movies, books, games)
Star Wars (movies, books, games)
One Crystal Eres (magic spell) in Tales of Legendia and a magic spell in Tales of Phantasia (games) is named Tractor Beam, that launches the enemy in the air.
In Eve Online (game), tractor beams are modules that can pull cargo canisters towards the ship in space.

Real-world usage

Optical tweezer technology allows for the manipulation of objects at the atomic and molecular scale using lasers. No technology capable of manipulating bulk matter with the level of precision envisaged in science-fiction has yet been devised.

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