Schild's Ladder is a (very) hard Science Fiction novel by Greg Egan. Possibly his hardest ever, filled with non-trivial mathematics and physics.
Physics researchers at a lab in remote space accidentally unleash a runaway reaction that threatens to eat the universe. As the threat grows (at half the speed of light), various factions disagree on how to deal with it.
This novel contains examples of:
- A Form You Are Comfortable With: When Tchicaya and Mariama venture into the novo-vacuum, the interface they use presents it as a weird-looking but still comprehensible realm, and the objects within are rendered in false-color. In fact, there is no light, matter, or energy at all in there, everything is millions of times smaller than atoms, and the only reason Tchicaya and Mariama are able to enter at all is because they have encoded themselves as some sort of quantum graph software vehicle analogized as a submarine.
- Apocalypse How: Class X-4: the runaway reaction itself.
- Call a Smeerp a "Rabbit": Many of the xennobes are named after various terrestrial creatures on account of some vague resemblance, either in shape or ecological role. In fact, one species is specifically called "rabbits," despite being weird tube things a billion times smaller than an electron and made out of quantum graph pieces.
- Continuity Nod: Everyone, even embodied people, runs on Qusps rather than brains; this is never explained, and you have to read the short story "Singleton" to get why this matters. And when Tchicaya wakes up, he muses that the process of constructing a brain must create unrememberable experiences; this was explored in the short story "Transition Dreams".
- Diabolus ex Machina: The anachronauts, who show up in the novel's climax to blow up the research ship. They are only mentioned twice before, never shown to have any specific agenda, and their actions have no lasting consequences beyond isolating the two main characters to continue exploration on their own.
- Do Unto Others Before They Do Unto Us: This is one of the rationales presented for destroying the novo-vacuum after it's discovered to be inhabited. If the novo-vacuum creatures figure out that they are surrounded by hostile space with possibly hostile inhabitants, all they have to do is figure out a way to get the novo-vacuum's expansion up to the speed of light, and that will be the end of humanity and the universe. Therefore, destroying the novo-vacuum and the incomprehensible numbers of aliens within is the best decision. Fortunately, this outcome is averted, and the novo-vacuum creatures seem to be mostly benign.
- Eldritch Location: The novo-vacuum is a rapidly expanding sphere of altered spacetime that is over six hundred light-years across by the time the story really gets going. Its surface is very bright, but not hot at all, and only a few Planck lengths thick. Beneath that lies a seething mass of alien reality, technically a whole different universe, where the rules are so unfathomably different that mass and energy as we think of them don't exist. There are no consistent physics, and reality consists of Planck-scale patches of different physical laws. These patches form "vendeks," the novo-vacuum's simplest form of life. The entire region is a seething cauldron of life, from "microbes" to advanced civilizations, making the standard universe look like a sterile desert in comparison.
- Eternal Love: Before Tchicaya finally gets to have sex with his childhood lover Mariama he contemplates "Nothing could have lived up to four thousand years of waiting. Except perhaps an original theorem."
- First-Contact Math: The protagonists realize that there's intelligent life inside a pseudovacuum when they notice that a series of pulses coming from it represent consecutive prime numbers.
- Lightspeed Leapfrog: Played straight. The slower travelers are known by most of spacefaring humanity as "anachronauts". It's an interesting variant, though: By the time the story begins, most anachronauts choose to continue traveling at relativistic speeds instead of using FTL, so that they can "hop" forward through time and observe the development of civilization. It's Played for Laughs when entire planetary civilizations coordinate elaborate pranks on the intrepid time travelers, leading them to think the development of the human race is even more bizarre than it actually is - which is saying something.
- Minovsky Physics: The work pretty much runs on this trope, as it is based around the fictitious but believable (apparently it was based on loop quantum gravity) "quantum graph theory" and the "Sarumpaet rules" that govern all fundamental interactions. It's been described as the "hardest science-fiction ever" with good reason.
- No Biological Sex: Features essentially genderless posthumans but retains both male and female pronouns. When embodied people couple up, their bodies grow custom genitals over a period of six months that will only work with each other—it's considered quite romantic.
- Speculative Fiction LGBT: Most of the characters are gender-neutral, as most of them exist as software and were created as such, having never been in an actual homo sapiens body.
- Sphere of Destruction: The novo-vacuum, the expanding and destructive sphere of altered spacetime that is the focus of the story.
- Starfish Aliens: the apparent life inside the "eaten" region, which ranges from non-sentient fauna to sentient beings with their own civilization and technology, exists at the Planck scale. (Mind-bogglingly tiny, for those not familiar with the term—by analogy, quantum graph edges are to electrons as electrons are to 100 km or so.) If that wasn't enough, the narrative suggests that the creatures are physically composed entirely of discrete regions of space that follow unique laws of physics—no actual "matter" as we understand it. It's the biological equivalent of a naturally-occurring computer made out of rocks and streams of water... only without the rocks or streams of water.
- The Singularity: Everyone runs on computers; some people are embodied (their processors located inside their heads), and some aren't. Nobody's actually died (apart from very rare suicides) in millennia. But one of the central ideas of Singularity thinking is averted; it turns out that strong superintelligence is impossible. Sort of like all Turing-complete machines have the same power; they're just sometimes faster.It was a rigorous result in information theory that once you could learn in a sufficiently flexible manner—something humanity had achieved in the Bronze Age—the only limits you faced were speed and storage; any other structural changes were just a matter of style.
- Twinmaker: People make backup copies of themselves with remarkable regularity, but generally only run one at a time. When someone dies a "local death" (which is pretty difficult, given that everyone runs on Qusps), they restore from the latest backup. People seem to fork themselves only rarely. Transport over great distances is accomplished by transmitting the contents of your mind and having it downloaded into a new body (or just run on local hardware).