In the Star Trek fictional universe, shields refer to a 23rd and 24th century technology that protects starships, space stations, and planets from damage by natural hazard or enemy attack. They are sometimes called deflectors, deflector shields, and screens (the last of which is used only during the original series) as synonyms for shields.
A frequently-used plot device holds that ships with shields raised may not use their transporters. This often prevents the protagonists from simply pulling endangered colleagues out of danger and running away, for to do so would require lowering shields, resulting in the possible destruction of the ship.
The term shields first appears in the Star Trek episode "Balance of Terror". They were used by a Federation outpost under attack by a Romulan bird of prey but proved ineffective. The first shown use by a starship appeared in the Star Trek episode "Arena", in which the USS Enterprise (NCC-1701) raises its shields after being attacked by an alien warship of unknown origin. Shields are not used by Starfleet at the time of Star Trek: Enterprise. Starfleet ships of that time period use "polarized hull plating", in effect an energy charge applied to a ship's hull to make it more resistant to weapons fire. However, some alien powers were known to use primitive shielding systems.
Like most technologies in the Star Trek fictional universe, the exact nature of how shields work is never directly cited. Characters discuss the existence and manipulation of these technologies while only superficially explaining the exact physics behind them. Essentially, some form of energy is projected from a ship or other body so as to deflect or disperse incoming projectiles. Energy based weaponry such as phasers, lasers and disruptors as well as solid ballistic weapons (i.e., torpedoes) and solid objects can be blocked. Shields are invisible until struck by an attacking force, and are then often shown briefly for dramatic effect as a translucent "field" of energy.
Visuals in LCARS displays and various accounts by characters imply that shields operate by emitting a layer of energy distortion containing a high concentration of gravitons around the object (such as a ship or city) to be protected. Shield energies can be emitted from a localized antenna or "dish" (such as a ship's navigational deflector) or from a network of "grid" emitters laid out on the object's surface (such as a ship's hull).
Shields can be lessened by repeated attacks, and can go offline completely. Prolonged exposure to hazards weakens the shields and may eventually cause them to collapse. Shields may not be effective against certain types of weapons. For example, until upgrades to Starfleet shield technology, shields were completely ineffective against the phased polaron weapons of Dominion ships.
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