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Galactic empires are a science fiction setting trope, in which most or all of the habitable planets in the setting's galaxy are ruled by a single centralized political entity. Galactic empires most frequently appear in works in the sub-genres of science fantasy and space opera, although they may appear in other sub-genres as well. Works featuring galactic empires may have them as the story's focus, chronicling the empire's growth and/or decline. Alternatively, they may merely serve as a backdrop against which the events of the story play out.

While depictions of galactic empires can vary wildly from work to work, most galactic empires share several key tropes:

• The empire, or at least its wealthier and more powerful regions, has advanced considerably in science and technology compared to current human civilization.
• Among these advances are some form of faster-than-light travel, which enables transit and communication across the empire's extent to occur in a reasonable time frame.
• Habitable worlds are either directly administered by representatives of the centralized imperial government, or else their local leadership is ultimately subordinate to the central government.

Characterizations of these empires can vary wildly from work to work. They may be benevolent or malevolent; all-powerful or too weak or incompetent to exert power in anything but name; or anywhere in between.

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.^{[citation needed]}

When works feature galactic empires that truly do span most or all of their parent galaxy, they usually depict their capital as a "core world", a planet in a star system relatively close to a galaxy's supermassive black hole. Typically, "core worlds" will be portrayed as wealthier and more politically powerful than "rim worlds" closer to the edge of the galaxy. However, this portrayal, while common, is not universal; many works may invert this dynamic or reject it altogether.

Second, the "empire" in "galactic empire" is also misleading, as it suggests the empire's government is authoritarian to at least some degree, if not completely autocratic. While many galactic empires do correspond to this description, many do not: galactic empires may adhere to any real-world political philosophy, or a unique form of governance created by the work's author(s). A prominent example of a non-autocratic galactic empire is the Galactic Republic of the Star Wars prequel trilogy, which is a hybrid regime parliamentary democracy (albeit one that collapses into fascist dictatorship over the course of the story).

The best known such organization to the general public today is the Galactic Empire from Star Wars, which was formed in turn from the Galactic Republic.

Star Wars depicts an empire ruled autocratically by Darth Sidious, supported by a powerful space navy. It is stated in the original Star Wars film that there was an Imperial Senate that was later disbanded by the Emperor. There was a galactic empire called the Sith Empire founded by Darth Revan. This galactic power was close to overthrowing the Jedi's Republic during the Jedi Civil War.

As a military dictatorship based upon fear and terror, the Empire is an explicitly villainous force with linguistic and visual traits directly reminiscent of Nazi Germany. For example, their armored forces known as "stormtroopers" are named analogously to the Sturmabteilung (often known as the SA), a paramilitary entity created by the Nazis in 1920.^[1] Their best-known weapon is the iconic Death Star, a Moon-sized space platform that can destroy entire planets.

In the Star Wars universe, the fall of the Galactic Republic and its replacement by the Galactic Empire – as depicted in Revenge of the Sith – recall the historic fall of the Roman Republic and its replacement by the Roman Empire headed by Augustus.

The term "galactic empire" has, perhaps because of association with the Empire from Star Wars, gained a pejorative meaning.

In contrast to Star Wars, the galactic empires from the Foundation universe and the CoDominium universe are relatively benign organizations. Much of the plot of the Foundation series, authored by Isaac Asimov, revolves around who can best and most quickly revive the fallen galactic empire; it is taken for granted that this is a positive and worthy aim. In writer Jerry Pournelle's CoDominium series, members of the empire often work to maintain the best interests of humanity despite efforts by violent political extremists to pursue their own ends.

Galactic empires are in many cases consciously modeled on historical Earth-bound empires. Asimov stated explicitly that the Galactic Empire whose fall is depicted in his Foundation books is also modeled on the Roman Empire, with the author taking direct inspiration from the historical writings of Edward Gibbon, even to the point of basing some individual characters on historical figures. Specifically, Pebble in the Sky, which is set on Earth – a poor and backward province of the Galactic Empire – is modeled on Roman-ruled Judea in the 1st century AD. Asimov's Earth – like the historical Judea – is sharply polarized between those who accept the Imperial authority and the fanatic "Zealots" who hatch violent plots of bloody rebellion and are the book's clear villains.^[2]

Foundation is depicted as having started life as a Foundation of scholars, taking up just one city on one faraway planet and setting up a modest municipal government headed, naturally, by a Mayor. Through a centuries-long series of developments which are the main subject matter of the Foundation Series, the Foundation gains enormous power and territory and comes to rule virtually as many stellar systems as the earlier fallen Empire - but continues to call itself "Foundation" rather than "Empire", and its ruler - though wielding as much power as any Emperor - retains the title of "Mayor".

Writer Poul Anderson makes the point that the declining empire depicted in his Dominic Flandry series does not span the entire galaxy but only a fraction of one of its spiral arms. Nevertheless, the institution is vast beyond a regular human's ability to truly comprehend, and it is in the process of collapsing under its own weight.^{[citation needed]} This series also consciously compares its imperial organization with the Roman Empire, to the point of tracing out the space equivalents of the Roman 'Principate' and 'Dominate' phases.

The universe established in Frank Herbert's Dune once again recalls the Holy Roman Empire, as well as the Byzantine and Islamic empires, especially given the role of hitherto disregarded desert-dwellers who, due to a powerful new religion, expand to topple an old empire and build a new one. For example, the Egyptian-Canadian commentator Khalid M. Baheyeldin has enumerated the obviously Islamic concepts and references appearing in Dune to the level of finding multiple similarities between the career of Herbert's Paul Atreides and that of the Islamic prophet Muhammad.^[3]

In Dune, the empire's power is held within three organizations: the Imperial family; the Landsraad, representing the nobility; and the Spacing Guild, an interstellar travel monopoly.

Another notable example of a galactic empire is the Imperium of Man from the Warhammer 40,000 universe, which is a feudal theocratic industrial and militaristic totalitarian regime (nominally an absolute monarchy) that does in fact span almost the entirety of the Milky Way Galaxy. Despite massive strength, the institution's territories are constantly at risk due to unending conflicts with various alien races and rebel factions. The Imperium of Man is managed by a vast bureaucracy, ranging from the High Lords of Terra to various mostly-autonomous planetary governors (they can do whatever they like so long as they pay taxes), all of whom govern the Imperium's territories on behalf of the comatose God-Emperor. It is supported by several organisations, such as the Ecclesiarchy, its state church; and the Adeptus Mechanicus, which produces most of its military equipment, which also operate independently from each other and the central Terran government.

In the final arc of the Sailor Moon manga series by Naoko Takeuchi, a fictional organization called Shadow Galactica has established an empire all over the Milky Way. Shadow Galactica is stealing "starseeds", the essence of sentient life in the galaxy. Its members come from different Star Systems and Sailor Galaxia, the self-proclaimed "Golden Queen of Shadow Galactica", has built her palace around the Galaxy Cauldron, the birthplace of all life in the Milky Way located in Galactic Center.

Bertram Chandler wrote two interstellar series – one featuring a Galactic Empire ruled by a series of non-hereditary Empresses while the other has a Republican Galactic Federation. Chandler's Empire and Federation, both relatively benign, have much in common – both covering the same volume of space, having much the same kind of Space Navy and both having the same commercial spaceflight company called "The Dog Star Line", suggesting that these are two alternate history timelines which branched off from the same original space travelling culture.

In Ursula K. Le Guin's Hainish Cycle, the interstellar entity known as "The League of All Worlds" and later as "The Ekumen" is in possession of the 'ansible' (a tachyonic device). Technology makes possible instantaneous interstellar communications, and the ability to send instantaneous unmanned ships carrying bombs to another planet is exploited as well. However, living beings cannot survive such travel, and thus humans are limited to space exploration at relativistic speeds. Correspondingly, this organization, despite on occasion waging war across interstellar distances, ends up being looser than a true empire.

Author Orson Scott Card's "Starways Congress", an organization featured in the work Speaker for the Dead (the follow-up to Ender's Game), similarly relies on the ansible. Yet it is more authoritarian and less benevolent than Le Guin's creation. Much of the story-line of the book and its sequels involves attempts to avoid interstellar bloodshed despite difficult circumstances.

• Legend of Galactic Heroes

• Ash, Brian, ed. (1977). "Galactic Empires". The Visual Encyclopedia of Science Fiction. Harmony Books. pp. 110–115. ISBN 0-517-53174-7. OCLC 2984418.
• Kerslake, Patricia (2005). "Galactic Empire". In Westfahl, Gary (ed.). The Greenwood Encyclopedia of Science Fiction and Fantasy: Themes, Works, and Wonders. Greenwood Publishing Group. pp. 325–327. ISBN 978-0-313-32951-7.
• Stableford, Brian; Langford, David (2017). "Galactic Empires". In Clute, John; Langford, David; Sleight, Graham (eds.). The Encyclopedia of Science Fiction (4th ed.). Retrieved 18 April 2023.
• Westfahl, Gary (2021). "Galactic Empires". Science Fiction Literature through History: An Encyclopedia. ABC-CLIO. pp. 309–311. ISBN 978-1-4408-6617-3.