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In page Alcubierre drive:

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Thus, because the energy density is negative, one needs exotic matter to travel more quickly than the speed of light.[1] The existence of exotic matter is not theoretically ruled out; however, generating and sustaining enough exotic matter to perform feats such as faster-than-light travel (and to keep open the "throat" of a wormhole) is thought to be impractical.[citation needed] According to writer Robert Low, within the context of general relativity it is impossible to construct a warp drive in the absence of exotic matter.[2]