Mass lightening is a technology used by many races in Star Trek and may be a common technology in Star Wars as well. The effect of this technology is to reduce the effective mass and inertia of a large object like a starship so it can be accelerated quickly with a relatively small amount of thrust.
How it Works
Mass lightening technology is fictional, and its fictional principles have never been clearly explained in any Star Trek series or movie. The technology is known to involve subspace field effects, so a common hypothesis is that the technology somehow shifts the ship's mass from normal space to subspace, where it has no inertia. A rather small amount of thrust is sufficient to rapidly accelerate the mass that remains in normal space.
A consequence of using mass lightening technology is that Star Trek ships are highly susceptible to subspace phenomena, like the subspace shockwave from the Praxis explosion and "subspace weapons".
A 2007 Expanded Universe supplement, Starships of the Galaxy, explains how it works in Star Wars:
"Starships also have ion drives capable of incredible acceleration (thousands of times the force of gravity) due to a combination of exceptional thrust and manipulation of the starship's mass relative to its exhaust. In addition, repulsorlift drives are preferred for delicate maneuvering during takeoff and landing. In fact, the mass manipulation that makes ion drives so efficient in deep space is markedly less efficient in atmosphere, so almost all starships use both drives in conjunction during atmospheric flight, particularly when near the surface of a planet."
Mass effect technology can reduce the mass of vehicles and projectiles, in addition to causing gravitational field effects. Reducing a ship's mass allows it to avoid the limits of general relativity. For military applications, it is common to reduce the mass of a projectile while it is being accelerated from the weapon, then allow the projectile's mass to return to normal; how this doesn't violate conservation of energy isn't explained.
Mass Reduction Examples
How much the mass lightening system normally reduces the starship's mass isn't known, but the subspace fields have been used to reduce the mass of other objects on at least two occasions.
- In TNG "Deja Q", the Enterprise-D used its warp nacelles to generate a subspace field that reduced the mass of a moon from an estimated 6.5E13 kg to just 2.5E9 kg, reducing its mass by a factor of about 26,000.
- In DS9 "Emissary", the station generated a subspace field from its deflector shield emitters that reduced its mass enough for it to complete a 160 million km trip that would normally take approximately two months in less than a day. This would require reducing the station's mass by a factor of at least 4,000.
In Star Wars, specific examples of mass lightening are not readily found, but if the "Starships of the Galaxy" supplement's relation with repulsorlift technology is correct, then Luke's description of antigravity and repulsorlift in the Star Wars: A New Hope novelization would be a possible example.