Ordinary matter distorts the geometry of spacetime, causing the effects of gravity. The physicist Miguel Alcubierre proposed a method of stretching space in a wave, causing the space "ahead" of a spacecraft to contract along the axis the spacecraft wishes to travel in and the space "behind" it to expand. The ship would ride this wave inside a region of flat space. Since the ship is not actually moving within this region, but rather being carried along as the region itself moves, conventional relativistic effects do not apply. There is no way known to induce such a wave, however, or to leave it once started; the Alcubierre drive remains a theoretical concept at this time.
Note that the term "Warp drive" is used in science fiction to denote a wide variety of imaginary propulsion methods, most of which have nothing to do with the Alcubierre drive or any other physical theory.
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