Tractor beam

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 place where tractor beams are used is on spaceships and space stations. They are generally used in two ways:

1. As a device for securing or retrieving cargo, passengers, shuttlecraft, etc. This is analogous to cranes on modern ships.
2. 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).

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 appear in the following works:

• Star Trek (TV series, movies, books, games)
• Star Wars (movies, books, games)
• The Lensman books by E. E. Smith
• The Sector General books by James White

In the real world, scientists are able to manipulate objects at the atomic and molecular scale using lasers, but no technology capable of manipulating bulk matter with the level of precision envisiaged in science-fiction has yet been devised.