The Xeelee are a hyperadvanced species from Stephen Baxter's Xeelee Sequence. They were first remotely mentioned in the 1994 novel Timelike Infinity and were later central actors of several novels and a substantial number of short stories. Within the Xeelee Sequence, they are considered to be the most advanced of all Baryonic life-forms and posses technology and abilities far beyond other spacefaring civilization. They have demonstrated the ability to routinely construct Closed Timelike Curves and have exploited time travel to engineer their own evolution and history as far back as 13.5 billion years ago (within two hundred million years of the Big Bang). Their abilities in the series far exceed that of a Type III Civilization on the Kardashev scale.
Although the Xeelee seldom directly appear or take action in the stories their impact on the development of most civilizations and the universe as a whole is profound. The Xeelee frequently leave behind derelict ships, tools, and other materials giving the appearance of thoughtlessness or reckless abandon at what a junior species might do with it. Most junior species see no benefit in developing a technology on their own when they can simply steal it from the Xeelee debris. This has a profound influence on the various galactic economies and technological development of junior species.
In the Xeelee sequence life and intelligence are ubiquitous throughout the universe. The universe has a fundamental tendency toward complexity and autocatalystic systems are easily created. This is not limited to particle matter, but also applies more exotic structures such as dark matter, quarks, and flaws in space-time itself. Thus, life emerged when the universe was extremely young, to within Planck Time of the Big Bang, and there were intelligent civilizations that witnessed, and indeed consciously influenced, major events in the early universe such as inflationary expansion, symmetry breaking, and various phase changes. At the close of each of these epochs, as the universe took on a fundamentally different characteristic, there was a corresponding mass extinction, but small enclaves of the intelligences would survive in one manner or another, witnessing the universe becoming progressively colder and slower-reacting.
The novel Exultant describes the progenitor of the Xeelee, referred to as the proto-Xeelee, as one of the few survivors of the last phase change (when the universe had cooled enough for atoms to form). The proto-Xeelee established symbiotic relationships with other survivors as well as autocatalystic reactions in the new baryonic matter, thus emerging what would later be called the Xeelee. This potentially dates the Xeelee to when the universe was only a few hundred thousand years old, but with direct knowledge and observations that reaches back to within minutes of the Big Bang.
The Xeelee to use primordial black holes as a habitat, construction tool, and computing devices because, at the time they emerged, the event horizon was one of the few thing that survived from earlier epochs that they understood. More so, heavier atoms were exceedingly rare because stars had not yet had the time to create them through nuclear fusion. As the primordial black holes evaporated over time the Xeelee eventually migrated to using the supermassive black holes around which Galaxies were forming. Although the galactic black holes were trillions of times more massive than their ancestral homes, the Xeelee proved to be quite adept at using the complex physics around the event horizon for their own purposes. Over the billions of years since their migration, the Xeelee have become ubiquitous part of the complex ecology of the super massive black holes along with an unnamed number of other species that are also veterans of the same very early eras of the universe.
There is no explicit physical description of the Xeelee throughout the series. In the novel Exultant it is eluded that the over their exceedingly long history, the Xeelee have combined themselves with their technology and, as such, may not have a distinct individual presence that Humans would be familiar with.
War with the Photino BirdsEdit
The Photino Birds are a species of dark matter with that prefer a habitat in the gravity well of stars. In general, the photino birds are probably unaware of almost all other forms of life due to the fact that dark mater interacts extremely weakly with baryonic matter (normal matter that is composed of atoms). The Photino Birds recognize a risk to their habitats from supernovas and other consequences of normal stellar evolution and have set themselves to ceasing nuclear fusion in the cores of stars, thereby prematurely aging them into white dwarves. The resulting dwarves will have the same gravity well but be stable for billions of times longer than their progenitor. Because white dwarfs are extremely cold and dim compared to typical stars, they would not support life on nearby planets. The long project of the Photino Birds will ultimately make the universe hostile to baryonic life, and they have spread across the universe. Further, the activities of the Photino Birds will effectively halt formation of new black holes due to a lack of Type II (core-collapse) supernova. This effectively puts the Xeelee and the Photino Birds at irreconcilable odds because each species is fundamentally seeking to threaten the ideal habitat of the other.
The Xeelee became aware of the Photino Birds approximately 13.5 billion years ago. Seemingly as a unified species, the Xeelee have concerned themselves with defeating the Photino Birds for almost the entier history of the universe. Despite their state of high technological advancement, the Xeelee were ultimately unsuccessful.
War with the HumansEdit
While recovering from a brutal occupation by the Qax that enforced stagnation and flirted with extinction, Humanity adopted an extremely xenophobic imperative that aimed to ensure the future of the species (known as the Druz Doctrines after it's founder, Hama Druz). Eventually Humanity again began to expand into the galaxy, wiping out an uncounted other species during their advance. Eventually Humans became the second most advanced and widespread race in the galaxy, second only to the Xeelee. Humans had no knowledge of the Photino-Xeelee war or even what was at stake, making the Xeelee actions interpreted as being sinister. Building the Ring had the appearance of being destructive on a galactic scale because it's larger purpose was not understood until the very far future, and many smaller Xeelee projects, such as changing the orbit of Callisto to preserve the bacteria living in the ice, lacked an obvious explanation. Though there were several orders of magnitude fewer Xeelee than humans known to be in the Galaxy, their high degree of technical advancement made them a formidable enemy.
Through a bitter war of attrition, Humans eventually contained the Xeelee within the Galactic Core where the battle lines stagnated for at least three thousand years. Both humans and the Xeelee gained strategic intelligence through time travel by constructing Closed Timelike Curves, further contributing to the stagnation. Human casualties were estimated to exceed more than 10 billion per year and more than 30 trillion over the course of the assault. The imperative commitment to total war also stagnated human physical and intellectual evolution. The Druz Doctrines did not encourage exploration or research and there was little new development. Further,very little effort was directed toward projects that did not have an immediate and conspicuious benefit to maintaining the massic logistics tail necessary to continue the assault.
Eventually the Humans developed a means of hiding their actions from the Xeelee by making movable pocket universes that had no causal relationship to the universe as a whole, thereby eliminating any advantage of moving information into the past with time travel. Humans further developed a time-travel based computer that allowed humans arbitrary computer power to outhink the Xeelee by analyzing tactical problems (giving the pilot tactical information to defeat Xeelee maneuvers) and a dual black-hole cannon that would fire two singularities to merge at a known distance away, releasing enormous gravity waves in the process. Knowing that the ecology around the event horizon and the creatures that live within it would be damaged of destroyed if humans continued to batter the event horizon of the supermassive black hole, the Xeelee withdrew from the Milky Way within minutes after the humans scored the first successful hit.
No longer having the Xeelee as a unifying threat, humanity fragmented into multiple governments over the next several hundred years. The doctrinal stagnation collapsed and Humanity advanced technologically over the next 100,000 years. Humanity then unified again to begin directly attacking Xeelee concentration in the greater supercluser of galaxies across the local group. Humans proved only a slight annoyance or distraction to the Xeelee.
Interactions with other speciesEdit
As far as is known and aside from the Photino Birds, the Xeelee were seldom warlike and usually treated the junior species with disinterest or abandon. There are very few reported instances of them initiating interaction with another species. The Xeelee, however, demonstrated several instances of compassion and even charity toward other species, particularly when it involved escaping the Photino Bird's destruction of the habitable universe. In the conclusion to novel Ring, set 5 million years in the future, they did not interfere with the humans escaping to another universe despite hundreds of thousands of years of humanity's war on the Xeelee (some interpretations are that they even provided an armed escort for the humans in the final leg of the voyage). Even though they were thought to have been driven to extinction by the human, the Xeelee constructed a universe for the Silver Ghosts that was appealing to their mathematical nature. Before their own retreat from the universe, the Xeelee left ships as 'life boats' at various locations so that other species could use them to escape the Photino Birds through the Ring.
The Anti-Xeelee is a universe spanning quantum-wave consciousness constructed by the Xeelee to manage their projects. It was first briefly described in the conclusion of Timelike Infinity when it was encountered by Michael Poole who had been pushed several million years into the future. It was later developed somewhat more in the short story Vacuum Diagrams. Its name is derived from the particle/anti-particle naming convention to convey it's paired and complimentary existence with the Xeelee; it does not describe an opposition to the Xeelee. It further implied that the Anti-Xeelee, is traveling backwards in time like some anti-particles are thought to do.
The Xeelee represent the unattainable pinnacle of technological development. As a result, they are feared, envied, and hated by virtually every other starfaring power; the Xeelee themselves consider these species beneath their notice. The only known exception is humanity, whom the Xeelee eventually attacked and imprisoned after two long wars were fought between the two races.The Xeelee's primary adversary was the Photino birds whose reshaping of the stars was opposed (ultimately futilely) by the Xeelee, when the war was lost the Xeelee retreated through the ring and made some provision to save the younger races, providing (at least in the case of humanity) a lifeboat to reach the ring and escape the dying universe. In spite of their previous wars against the humans, the Xeelee did not molest the Great Northern when it escaped through the ring carrying the last of humanity to safety in a new universe.Nevertheless, Xeelee technology is highly coveted, especially by Man, who make extensive use of the Xeelee technology base to wage three futile wars against the god-like aliens. The first from 12,000-25,000 AD resulted in victory for humanity and control of the Milky Way. The second from 100,000 - 200,000 AD resulted in stalemate with man eventually giving up and returning to the Milky Way, while the third and longest began in 700,000 AD and resulted in a massive prolonged Xeelee attack named the Scourge as man was pushed back from the stars to Earth and finally into a dimensional prison around 1,000,000 AD.The Xeelee were masters of time and space. They crafted their starships, known as Nightfighters, out of a strange form of matter that seems to break Pauli's Exclusion Principle (Which states that no two electrons of the same atom can be in the same state at the same time). The material is incredibly sharp, hard, and transparent; Due to its unique spacetime construction, it has strange gravitational effects. The Xeelee have hyperdrive technology, enough to cross galaxies in a short amount of time.
The Xeelee are a Universe-spanning civilization with instantaneous FTL, time travel to any point in time in the universe, temporal duplication of ships, their entire race exists in a time loop that resets at the end of the universe and goes back to the beginning so their knowledge and technology continues accumulating, construction based on molded timespace (Xeelee Construction Material, or XCM) which is effectively immune to brute force attacks, since it survived in the first moments after the Big Bang and took hypernova - level energies at point blank without damage, perfect energy - to - matter conversion by way of the Xeelee Flower, an XCM device that converts energy hitting it into matter to allow it to grow, starbreakers, gravity-based weapons which have no recoil due to "anchoring themselves in timespace", they are capable of destroying anything they hit including XCM, and even a handheld pistol version can destabilize and destroy a star, almost perfectly efficient computing via using the central black holes of galaxies, capable of moving massive cosmic strings to destroy or mold galaxies, have deconstructed many galaxies for use as raw material to build The Ring, a portal to another universe, capable of creating dimensions and trapping other beings in them, said dimensions are configured to the Xeelee's own parameters so they can be virtually anything. The Anti-Xeelee, a component of their race which is responsible for initiating and maintaining the timeloop, exists as a non-corporeal quantum consciousness and can transform other beings into similar consciousnesses, instantaneous communication throughout the universe, and more.
The Xeelee use an exotic form of matter for most of their construction. Known by the generic name of "construction material", the substance is very dense, though it shares virtually no similarities to expected properties of degenerate matter such as neutronium. Instead, it appears as if the material violates the Pauli exclusion principle, allowing electrons to simultaneously share quantum states.Construction material has been observed to be synthesized or grown from a small flower-like object, known anecdotally as the Xeelee Flower (as seen in the short story The Xeelee Flower, found in Vacuum Diagrams). This device creates construction material when the control unit is removed in the presence of sunlight. It is unknown if the Xeelee have other methods of creating the substance. The only things known to be able to damage it are magnetic monopoles and gravitational singularities. During the third human/Xeelee war the Xeelee encased stars in this material, this prevented any radiation and heat escaping and so forcing humans to leave those systems or die.
The Xeelee hyperdrive functions by moving the ship from one point in space to another instantaneously. Though the travel time between points is zero, there is a short pause between each jump required to compute the parameters of the next jump. This pause was a couple seconds when a Xeelee Nightfighter was flown by Jim Bolder, however it is implied that Xeelee pilots operate their craft more efficiently. This hyperdrive can cross at least 100 parsecs in a single jump if required, however the Xeelee have been known to use much smaller jumps tactically, even in combat. The hyperdrive does not rely on the craft being entirely made of Xeelee construction material, as an instance of one jumping with another ship tied to its back was shown in The Ring.
The Displacement Drive used by Xeelee Nightfighters directly pushes on Spacetime. This is done by a pair of extremely large "wings" that can extend from the Sycamore seed shaped Nightfighter. These wings are not made of typical Xeelee construction material, but are constructs of pure spacetime and in this way they can directly exchange momentum with the fabric of the universe itself. The means by which the wings interface with the degenerate matter Nightfighter cockpit is unknown, but it is suggested that this may be a fundamental aspect of Xeelee physiology and/or technology. The wings appear jet black and seem to be at least as durable as the cockpit.
The displacement drive is capable of tremendous accelerations, and is observed to allow a Nightfighter some degree of maneuverability even near the surface of a Neutron Star. This suggests accelerations on the order of 10^12 - 10^13 m/s^2, allowing the Nighfighter to go from a dead stop to near the speed of light in about 100 microseconds. It is also suggested that the Xeelee ships perform better in regions of highly curved spacetime, so their accelerations in flat space may be a couple orders of magnitude lower. 
The starbreaker is the primary weapon used by the Xeelee. It has only been observed in use on the nightfighter starships. The starbreaker is a gravity wave laser that has the ability to destroy a star. The starbreaker was first observed in the short story Blue Shift (Vacuum Diagrams). In this story, the Qax (another alien race that has conquered Man) have found a damaged nightfighter. The only thing aboard is a small object that is similar to a handgun with delicate wire wrapped around the barrel, and controls described as being for hands smaller than a human's.
At low power settings, the device emits synchrotron radiation, but the Qax have been too scared to try the higher settings. It is not until later in the story when Jim Bolder, piloting the nightfighter for the Qax, discovers the true nature of the device when he witnesses a small fleet of nightfighters destroying a star with cherry-red beams. After hatching a plot to change his entry point into the Qax system, this theory is proven when the starbreaker is accidentally fired into the system's sun at full power in an attempt to destroy the nightfighter. The star begins destroying itself within four days.
The full capabilities of the starbreaker have never been clearly explained. For example, it is not known if the weapon makes use of direct energy transfer, or if the stellar disruption effect is due to some other mechanism. Furthermore, with the variable outputs of the device, it remains uncertain as to what magnitude of damage it can inflict upon "normal" targets. It would seem reasonable to assume that the device is formidable no matter what the mechanism, as the Xeelee have found it to be quite effective for their needs.
Presumably since it is capable of destroying stars the Starbreaker (like most Xeelee equipment) was designed with the Photino birds in mind.
Time travel is an integral component to the Xeelee civilization, as they have self-created as a closed timelike curve. White cube-shaped devices were found throughout Xeelee-controlled space as Man expanded. The function of these devices was unknown until the Xeelee activated them, and they vanished backwards in time. Led by the quantum-mechanical anti-Xeelee, this temporal migration had a specific goal: to engineer the Xeelee's early development, and to give them a leg up so that they could complete their great project.
The Xeelee have also been known to travel through time by using their Hyperdrive and very efficient Displacement Drive to assemble closed timelike curves. They have been known to use this tactic in the war against Humanity to carry information about a battle backwards in time so it can be used to their advantage during the battle. Future foreknowledge gained from time travel is consistently used by not only the Xeelee but also the Humans, preventing its use from immediately deciding the conflict.
Arguably the ultimate expression of the Xeelee's technological prowess, the object known as Bolder's Ring or simply the Ring was long a source of wonder and mystery.
When Jim Bolder flew to the Great Attractor in the Qax's captured nightfighter, what he found was astonishing. All matter in the universe was falling towards the object's massive gravity well. Even more astounding, this object and indeed the entire motion of the galaxies themselves appeared to be the work of the Xeelee, who were using the mass to create a ring of cosmic strings several million light years in diameter.
The purpose of the device was a mystery, but it was assumed the Xeelee's intent was hostile and that they were effectively destroying the universe for their own ends. Indeed, it was this assumption in part that led to Man's species imperative to wage a million-year war to defeat the Xeelee at all costs.
It was only after Man's ultimate defeat that the sad remnants of the species would discover the truth. The Ring was not a battleship, or a fortress, or a weapon. The Ring was an escape hatch, with the naked singularity at its center providing access to other universes. The Xeelee had been waging an eons-long war against an opponent that even they could not beat, not even with the ability to engineer time itself. Their last resort had been to build the Ring to escape before the ultimate death of the universe.
Only three groups of humans were known to have escaped through the Ring. The humans of the generational GUTship Great Northern, displaced five million years into the future to a universe of dying stars, and the last human refugees of the hypersphere prison that held Man after their defeat by the Xeelee. The Xeelee proved themselves to have at least some compassion towards their baryonic kindred in the latter case, by leaving a partially sentient starship to carry the humans away. During the war one ship crashed through the ring arriving in a universe with ultra high gravity. The crew survived and the novel Raft recounts some of their history.The Ring itself was ultimately destroyed by the Photino Birds. They had set in motion a radio-frequency amplification effect, a bomb of sorts that would disrupt the Ring after reaching critical levels in several million years. However this gradual destruction was vastly accelerated due to the Photino Birds being tricked into thinking the Xeelee had returned, and the retaliatory star-strike conducted by the Qax in an attempt to halt them. The cosmic strings that composed the ring would cut if they self-intersected, creating a chain reaction that would ultimately destroy the entire Ring.
Nearing the end of their work on the Ring, roughly 4 million years from the present, the Xeelee constructed several hundreds of enclosures to return to the beginning of their history 13.5 billion years ago. These enclosures, call Sugar Lumps, appeared as enormous glowing cubes varying in size from a few thousand to tens of thousands of kilometers on a side (capable of enclosing moons or planets). More than a simple Closed Timelike Curve, the Sugar Lumps are described as having a negative vector in time (i.e. traveling backwards in time via a time symmetry) and function as inverse time capsules. When the Sugar Lumps arrive at their destination in the distant past, the Xeelee would emerge as a fully developed race with their knowledge of future events and be able to immediately initiate their grandest projects against the Photino Birds.
The Sugar Lumps are guided both in management of their travel and coordination of their effort, by the Anti-Xeelee. Use of the Sugar Lumps enabled the Xeelee to consciously manage their own evolution as well as make any necessary edits to the history of the universe so that their projects are unopposed to the maximum extent possible. It is therefore likely that any species whice made a substantial rivaly with the Xeelee were prevented from evolving in the first place, and further that the Xeelee were unable to prevent the evolution of the Photino Birds.
Nightfighters are the main spacecraft utilised by the Xeelee in their multi-millennia long existence. Nightfighters are capable of intergalactic flight in seconds and this is done by a pair of extremely large "wings" that can extend from the Sycamore seed shaped Nightfighter. These wings are not made of typical Xeelee construction material, but are constructs of pure spacetime and in this way they can directly exchange momentum with the fabric of the universe itself. The means by which the wings interface with the degenerate matter Nightfighter cockpit is unknown, but it is suggested that this may be a fundamental aspect of Xeelee physiology and/or technology. The wings appear jet black and seem to be at least as durable as the cockpit. Nightfighters are traditionally armed with starbreakers.