Stephen Baxterin kuvailema Kardashev level IV-sivilisaatio/ jumalolento,
The Xeelee.
The Xeelee Sequence deals with many concepts stemming from the fringe of theoretical physics and futurology, such as exotic-matter physics, naked singularities, closed timelike curves, multiple universes, hyperadvanced computing and artificial intelligence, faster-than-light travel, and the upper echelons of the Kardashev scale. Thematically, the series deals heavily with certain existential and social philosophical issues, such as striving for survival and relevance in a harsh and unknowable universe and the effects of war and militarism on society.
The overarching plot of the Xeelee Sequence involves an intergalactic war between humanity and the Xeelee, and a cosmic war between the Xeelee and the Photino Birds, with the latter two being alien species that originated in the
early universe. The technologically advanced Xeelee primarily inhabit
supermassive black holes, manipulating their
event horizons to create preferable living environments, construction materials, tools, and computing devices. The Photino Birds are a
dark matter-based species that live in the
gravity wells of stars, who are likely not aware of
baryonic life forms due to dark matter's weak interactions with normal matter. Due to the inevitable risk of their habitats being destroyed by
supernovae and other consequences of
stellar evolution, the Photino Birds work to halt
nuclear fusionin the cores of stars, prematurely ageing them into stable
white dwarfs. The resulting dwarfs provide them with suitable habitats for billions of times longer than other types of stars could, but at the expense of other forms of life on nearby planets. The Photino Birds' activities also effectively stop the formation of new black holes due to a lack of
Type II supernovae, threatening the existence of the Xeelee and their cosmic projects.
After overcoming a series of brutal
occupations by extraterrestrial civilizations, humanity expands into the galaxy with an extremely
xenophobic and militaristic outlook, with aims to
exterminate other species they encounter. Humans eventually become the second-most advanced and widespread civilization in the
Milky Way galaxy, after the Xeelee. Unaware of the Photino–Xeelee war and the existential ramifications of what is at stake, humanity come to the (unwarranted) conclusion that the Xeelee are a sinister and destructive threat to their
hegemony and security. Through a bitter
war of attrition, humans end up containing the Xeelee to the
galactic core. Both humans and the Xeelee gain
strategic intelligence by using
time travel as a war tactic, through the use of
closed timelike curves, resulting in a stalemate for thousands of years. Eventually, humanity develops defensive, movable pocket universes to compartmentalize and process information, and an exotic weapon able to damage the ecological stability of the core's supermassive black hole. Minutes after the first successful strike, the Xeelee withdraw from the galaxy, effectively ceding the Milky Way to fully human control. Humanity continued to advance technologically for a hundred thousand years afterwards, then attacked the Xeelee across the
Local Group of galaxies. However, despite having annoyed the Xeelee enough to give up activities in the Milky Way, humans, having become an extremely powerful Type III civilization themselves at this point, prove only to be a minor distraction to the Xeelee on the whole, being ultimately unable to meaningfully challenge their dominance across the universe.
Although the Xeelee are masters of
space and time capable of influencing their own evolution, they are ultimately unsuccessful in stopping the Photino Birds. They instead utilize
cosmic strings to build an enormous ring-like structure (which comes to be known as Bolder's Ring, or simply the Ring) to permit easy travel to other universes, allowing them and other species to escape the Photino Birds' destruction of the universe. The Xeelee, despite their unapproachable aloofness and transcendent superiority, appear to be compassionate and charitable toward the younger and less advanced species that inhabit the universe, demonstrating this by doing such things as constructing a specially-made universe suited to the Silver Ghosts, who humans had nearly driven to extinction. Humans are likewise shown compassion by them and allowed to use the Ring to escape, despite their relentless long war against the Xeelee.
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