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Dilithium is depicted as an extremely hard crystalline mineral that occurs naturally on some planets. When placed in a high-frequency electromagnetic field, magnetic eddies are induced in its structure which keep charged particles away from the crystal lattice. This prevents it from reacting with antimatter when so energized, because the antimatter particles never actually touch it. Therefore, it is used to contain and regulate the annihilation reaction of matter and antimatter in a starship's warp core, which otherwise would explode from the uncontrolled annihilation reaction. Though low-quality artificial crystals can be grown or replicated, they are limited in the power of the reaction they can regulate without fragmenting, and are therefore largely unsuitable for warp drive applications. Due to the need for natural dilithium crystals for interstellar travel, deposits of this material are, much like real-world equivalents such as oil, a highly contested resource, and as such, dilithium crystals have led to more interstellar conflict than all other reasons combined.
As shown on the series, the streams of matter and antimatter directed into crystallized dilithium are unbalanced: there is usually much more matter in the stream than antimatter. The energy generated in the annihilation reaction heats up the excess deuterium gas, producing a plasma that is used to power the warp drives that allow starships to travel faster than light. In addition, most starships use this plasma as a power source for the ship's systems; in the series Star Trek: Enterprise (2001-2005), this was referred to as an electro-plasma system, (a backronym of the term "EPS", which was used in all other series except the original (1966-1969) series) to refer to a ship's or station's power system. The specific details of this reaction were officially established in the Star Trek: The Next Generation (1987-1994) series and technical manual; in earlier works it is not clearly defined.
Dilithium's chemical symbol is Dt, its atomic weight is 87 and it is a member of the hypersonic series of elements, according to a periodic table graphic seen in episodes of The Next Generation and Star Trek: Deep Space Nine (1993-1999). The dilithium crystal structure is 2(5)6 dilithium 2(:)l diallosilicate 1:9:1 heptoferranide, according to the Star Trek: The Next Generation Technical Manual (1991).
In the original series, dilithium crystals were rare and occurred only naturally (could not be replicated), making the search for them a major plot element for a number of stories. In Star Trek IV: The Voyage Home (1986), Spock discovers a method of re-crystallizing dilithium that allows the crew to regenerate the crystals on board their Klingon Bird-of-prey. This involves using 20th century fission reactors that generate high-energy photons that could cause the crystals to regenerate. Presumably, this technique was not used earlier in the series because fission reactors had long been replaced by cleaner and more efficient technology.
Made familiar by the television series, the term has since appeared in other contexts, such as: