text: Background 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 familiar particles such as photons traveling at light speed. These, instantaneous or faster-than-light (FTL) (tachyonic) 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: for instance, the hypothesis of a warp drive (such as, more specifically, an Alcubierre drive) that bends the fabric of space-time.
  In many cases, the term "galactic empire" is misleading. First, the "galactic" in "galactic empire" suggests an organization encompassing far more star systems than is actually described. This may come about as a result of propaganda exaggerating the spread of an imperial entity in order to appear stronger than is actually the case — just as historical nation-states such as the 'Holy Roman Empire' presented themselves; being roughly twice the size of modern Germany. While some of the noted fictional empires encompass a large portion of the galaxy, many other empires may be classified as interplanetary or interstellar empires, since they encompass only a local group of star systems.