Most of these galaxy-spanning domains depend on some form of transportation capable of quickly or instantly crossing vast cosmic distances, usually measured in light-years, many times faster than regular particles such as photons traveling at light speed. These, instantaneous or faster-than-light (FTL) technologies invariably require some type of propulsion or displacement technology forbidden by Albert Einstein's theories on relativity. Described methods often rely on theories that circumvent or supersede relativity. Examples include the hypothesis of a warp drive (such as, more specifically, an Alcubierre drive) that bends the fabric of space-time.