Literature / Orthogonal

aka: The Arrows Of Time

"We need an age, and we don't have it. So we find the time elsewhere. We make a rocket powerful enough to leave the world behind.... When the rocket's history is orthogonal to the history of the world, no time at all passes back home. So for the travelers, the journey can last as long as it needs to. If they require more time to complete their task, they can prolong the flight for another era, another age; it won't delay their return by one flicker."

— Eusebio, The Clockwork Rocket

A characteristically Hard Science Fiction Trilogy by Australian author Greg Egan. Consists of:

• The Clockwork Rocket (2011)
• The Eternal Flame (2012)
• The Arrows of Time (2013)

Orthogonal is set in a universe where a single minus sign, deep in an equation governing the rules of spacetime, has been changed to a plus sign. Through the story, Egan explores the higher-order ramifications that change has on the laws of physics (and, by extension, on nearly every other discipline of science). Light has no universal speed (colors of light with shorter wavelengths, such as violet, literally travel faster than colors like red, which have longer wavelengths, meaning that distant stars appear as rainbow-colored streaks instead of white specks), and the generation of light creates heat and energy; reverse Time Dilation is in effect; and entropy works more or less backward.

The plot follows a group of Shapeshifting Starfish Aliens faced with an impending apocalypse caused by incredibly destructive Antimatter meteorites. They realize they can't do anything to save themselves with their current technology, so they build a Generation Ship to fly into that alternate dimension, where they will have all the time they need to advance their technology to the point where they will be able to return and Save the World.

In true Greg Egan fashion, the trilogy mostly alternates between chapters that advance the plot and chapters that explore the scientific implications of the One Big Lie that governs the rules of the universe. The latter is mostly accomplished (again, in Egan's signature style) by having a few scientifically-inclined Mr. Expositions bounce Techno Babble off each other.

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This Trilogy contains examples of:

• Aerith and Bob: Throughout the trilogy, you have names like Clara, Tamara, and Ramiro... alongside names like Yalda, Tarquinia, and Eusebio.
• Alien Geometries: Time is fundamentally the same as space, meaning that there are technically four spatial dimensions and no such thing as time. The trope is even discussed in-universe when Yalda hypothesizes "four-space" and Giorgio points out how batshit insane it sounds.
  Giorgio: So according to your theory, an object could have a trajectory entirely orthogonal to our own?
  Yalda: Yes.
  Giorgio: It could move with infinite velocity?
  Yalda: Yes, that's how we'd describe it. But that's no stranger than saying that a vertical pole has an 'infinite slope': unlike a mountain road, it gets where it's going vertically without bothering to go anywhere horizontally. An object that gets where it's going without bothering to move across what we call time isn't doing anything pathological; in reality, there's nothing 'infinite' about it.
• Aliens Never Invented the Wheel: Because of this universe's physics, basic electronics are not completely impossible to construct, but it is highly unlikely that sentient beings would ever discover the principles that would make it work without a lot of luck or help. Between the second and third books, though, they do invent "photonics", which serves pretty much the same function as electronics, except that it works by using photons instead of electrons.
• All There in the Manual: Egan has posted multiple essays on his website that delve even further into the workings of Orthogonal's universe — and yet, here they are.
• Alternative Number System: Numbers in the trilogy are always shown in decimal thanks to a Translation Convention, but it's strongly implied that the aliens use a duodecimal/dozenal (base-12) number system.
• Antimatter: Occasionally referred to by name, but usually just called "orthogonal matter". When interacting with matter that originated in a cluster traveling in a different direction through the time-dimension, it's important to take into account exactly which direction it was traveling. Depending on which "time direction" you approach it from, it could either be ordinary harmless matter with Merlin Sickness (i.e., experiencing time backwards from your perspective), or it could be Made of Explodium and annihilate you in a massive explosion as soon as you touch it.
• Artificial Gravity: The first part of the Peerless' journey involves accelerating at a rate of one-G for about a year, generating the predictable and familiar downward gravity. When they stop accelerating, the lack of gravity unexpectedly causes their crops to fail, so they spin the ship to compensate. Centrifugal Gravity is partially deconstructed and reconstructed, as the characters are forced to reconfigure most of the ship's layout to accomodate the fact that the new gravity just put all their furniture on the walls.
• Big Damn Heroes: The Eternal Flame's climax is a textbook Big Damn Heroes moment. One of the two halves of the Ensemble Cast is kidnapped and held under the threat of death by extremists who believe their reproductive experiments are an affront to nature. They are rescued in a classic Big Damn Heroes scene by the other half.
• Bizarre Alien Psychology:
  • Since Out with a Bang and Death by Childbirth always applies (for the women), men typically raise their children alone or with the help of their own fathers. Thus, women are widely (but not universally) considered unfit for childrearing.
  • Sex is a very complicated and touchy subject. It's obviously a biological imperative, but for a woman, it's suicide, and for a man, it's murdering your co. Despite this, most men have no qualms about their duty to reproduce, to the extent of shaming women who don't look forward to Death by Childbirth. Predictably, this strikes many of the female characters as an in-universe example.
• Bizarre Alien Reproduction: Let me count the ways...
  • Sex immediately induces reproduction; there is no gestation period.
  • Sex consists of two of the Shapeshifting aliens melding the flesh of their chests, through which the male transmits a light-based signal into the female's body to induce the reproductive process. No actual matter or bodily substance is exchanged; genetic diversity is ensured through the infrared transmission of "influences" by every individual to everyone around them — which, incidentally, is also how diseases spread.
  • Females reproduce by splitting into four children; two sets of male-female twins respectively referred to as cos, who are genetically ideal mates for each other. The children who are not cos to each other are referred to as brothers and sisters, but the same terms are generally not used to refer to cos, probably to avoid implying that the relationship between cos is Brother–Sister Incest.
  • In the rare event that a woman is born without a co ^note known as a solo; all solos are female, and solos are generally much larger than the average individual, leading to the scientifically discredited but pervasive idea that solos eat/absorb their cos during childbirth or is otherwise unable or unwilling to mate with her co, she can instead reproduce with another willing male, known as a co-stead. These relationships are treated more as a husband/wife relationship than the relationship between cos. It's considered a kind of reproductive Undesirable Prize, generally resorted to only when one co is dead or nonexistant, but there are occasional exceptions.
  • Under the right circumstances, primarily old age or spending too much time away from their co or co-stead, spontaneous reproduction is common enough to necessitate women taking a drug to prevent it.
  • In The Eternal Flame, things get even Weirder Than Usual: A woman fasting to the point of near-starvation helps to both stave off spontaneous reproduction as well as to ensure that, when it happens, it will result in only one pair of cos instead of two, for Population Control.
  • And even weirder ^note or, depending on your perspective, completely and utterly normal: Near the end of The Eternal Flame, the biologist half of the Ensemble Cast develops a way to induce childbirth that produces only one child, does not kill the mother, renders her sterile for the purposes of traditional (fission) childbirth, and is shown to be repeatable in the final book, and can produce male children. In other words, they can more or less reproduce just like us.
• Bizarre Sexual Dimorphism: Even when they are not solos, women are generally larger than men, because the extra mass is needed to successfully split into four healthy children. As a result, women are generally stronger than men and thus do most of the manual labor. ^note Because of this, for most of The Clockwork Rocket, before the launch of the Peerless and the subsequent retooling of societal norms, men tend to take center stage on intellectual pursuits. Educating women is widely considered a waste of time, since they have much shorter lifespans and their knowledge is not passed on when they fission. It's mentioned in passing that these gender roles were reversed at some point in the past, when it was believed that a woman's knowledge did get passed on to her children, while knowledge taught by men only went skin deep.
  • Without exception, every solo is female, including the ones born as a result of the induced survivable births, "shedding", in the latter part of The Eternal Flame. Later on they learn how to "shed" male children, so the trope is no longer universal.
• Body Horror: Played straight several times, occasionally without even using any visual cues.
  • In The Clockwork Rocket, Yalda is sent to jail, where she is "shackled" by use of a resin that causes two of her limbs to be melded together, creating a loop that can be chained to a wall. This is described as a highly disturbing experience—and that's before they literally cut her loose.
  • One experiment by the biologists involves recording the light signals used by the brain to induce muscle movement, and playing it back into the body to try and trigger the movements artificially. It doesn't quite work as planned, and the resulting uncontrollable hand spasms terrify the subject so badly that he tells his associates to cut off the hand. The effect is slightly lessened by remembering that he is a Shapeshifting Starfish Alien who can reabsorb the damaged flesh to heal the wound, but his brief Inner Monologue during the muscle spasms makes it clear that this is his reaction.
  • During the biologists' experiments on the arborines, Carlo views the surgical implements as being this.
  • Many characters have this reaction to the idea of any kind of deliberate interference with natural childbirth, but especially childbirth that would allow the mother to survive.
• Call a Smeerp a "Rabbit": Orthogonal takes place in a universe with entirely different laws of physics from our own, and thus predictably Bizarre Alien Biology. It still uses common words like "plant" and "vole" to describe things that are roughly analogous. ^note Never mind that plants gain energy by emitting light rather than absorbing it, and voles are just a smaller species of Shapeshifting Starfish Alien than the protagonists.
• Call a Rabbit a "Smeerp": "Luxagens" are analogous to electrons and fulfill the same role as atoms in Real Life, and "photonics" in The Arrows of Time, which are analogous to electronics. They function fundamentally differently in several ways, but in this universe, what doesn't? Space, as in, outer space, is never referred to as such. It is always referred to as "the void".
• Clarke's Third Law: The entire point of the Peerless' journey is to have an indefinite amount of time to develop sufficiently advanced technology that will let them Save the World from the impending Cluster-Shattering Kaboom, even though they have no idea what kind of technology it will be, or even if the people that return will have evolved enough to no longer be recognizable as the same species as those who left. See also The Singularity, below.
• Collision Damage: Whenever matter that originated in clusters traveling orthogonally to each other comes into contact, if care is not taken to ensure that they are traveling in the right "direction" through time, it creates an immediate and massively explosive reaction. Combine that with the fact that, under ordinary circumstances, such clusters technically have infinite velocity respective to each other, and even a pebble of orthogonal matter can — and, in a combination Mass "Oh, Crap!" and Portent of Doom moment, does — literally Set A Planet On Fire.
• Conveniently Close Planet: A large part of The Eternal Flame's plot revolves around a Big Dumb Object known as the Object, a large chunk of orthogonal matter that drifts close enough to the Peerless for the protagonists to travel to it. The characters realize that the Object is one of the greatest opportunities they've had since the launch, because if they lose it, they will probably never come this close to another.
• Covers Always Lie: The cover art for each book in the trilogy depicts the Generation Ship Peerless as a sleek and sexy Cool Starship, while the actual books describe it as looking exactly like what it is: A mountain that was hollowed out, reinforced, fitted with rockets, and blasted into space. The cover of the first book is particularly egregious, as it depicts no fewer than five such spacecraft. In fact, a second, much smaller craft is constructed part of the way through The Eternal Flame, but that's it for the first two books.
• Crapsack World: Discussed in The Clockwork Rocket by Nino, who quotes an earlier offscreen conversation with an opponent of the project.
  Nino: He said that if the mountain crashed into the ground, it would be a mercy. He said the whole idea of a city in the void was insane. One by one, things would go wrong — things that couldn't be fixed without help from outside. Within a generation you'd all be starving. Eating the soil. Begging for death.
• Death by Childbirth: Universally, natural childbirth consists of the mother fissioning into four children.
  Yalda: I wish I could have met [my mother].
  Vito: That's like wishing you could fly.
• Doomed Hometown: The entire trilogy revolves around an attempt to preemptively defy this trope and save their Doomed Home Planet.
• Ensemble Cast: The Eternal Flame actually features two sub-ensembles, each with their own separate story arcs, though they frequently interact with each other:
  • The "Physicist" Ensemble: Carla, Tamara, Patrizia, and Ada, whose arc focuses mostly on the study of light and matter. They end up being Big Damn Heroes in the climax by rescuing the "Biologist" Ensemble from their kidnappers, and are generally more focused on the Peerless' ultimate, overall mission to Save the World.
  • The "Biologist" Ensemble: Carlo, Macaria, and Amanda make up the other ensemble, whose arc focuses more on biology, especially reproductive functions. They are generally more focused on solving the immediate problems faced by the crew of the Peerless, namely the overpopulation and resultant famine.
• Exposed Extraterrestrials: Not usually significant to the plot, but the only instance of anything like clothing being mentioned is the "cooling bags" that must be worn during EVAs to avoid hyperthermia.
• Exposition Diagram: Even for Egan, Orthogonal has an unusual amount of these. On average, at least two or three appear in almost every chapter, especially in the second book. The alien physics would be even more difficult to follow without them.
• Expo Speak: The Book! Particularly in the first and second books, nearly all of the scientific exploration of the universe occurs in the form of characters explaining it to each other. This is made even more noticeable by the instances in which characters who already have an understanding of the subject matter feel the need to explain it aloud. Egan tones it down in the third book, though, which focuses much more on action and events.
• Farmboy: Yalda in The Clockwork Rocket grew up on a farm and went into academia against her family's wishes. Several characters in The Eternal Flame are also characterized as farmers, Tamara probably being the most notable. The Generation Ship wouldn't be able to subsist without a great deal of highly-specialized agriculture, so the science of space farming is vital to the ship's inhabitants.
• Faster-Than-Light Travel: Thanks to the way spacetime works, there is no universal speed limit, and achieving FTL speeds is a surprisingly simple matter. Of course, you stop moving forward through time and interacting with other matter becomes slightly disastrous, but that's the breaks.
• Fling a Light into the Future: The Generation Ship's mission is to accelerate into an orthogonal arrow of time, so that, from their perspective, time will stop for the homeworld. This will allow the crew all the time they need to develop technology and science to the point where they'll be able to return and Save the World. Thus, the ship may drift through space for an indefinite amount of time, but no matter how long they have, when they return, only four years will have passed on the homeworld.
• For Science!: The characters initially have no idea what they need to study in order to Save the World, so they pretty much just research anything and everything that comes to mind. The hope is that if they study enough subjects, something will lead to a revelatory breakthrough. This includes light, agriculture, biology, the fundamental building blocks of matter, and more. ^note Granted, some of these studies are geared more toward ensuring that they don't all starve to death due to overpopulation before accomplishing anything meaningful.
• Gendercide: Discussed by opponents of the reproduction-inducing experiments, who hypothesize that if survivable single childbirth always results in female children, it will lead to the eradication of the male gender.
• Generation Ship: This trope is the focal point for the entire trilogy. The Clockwork Rocket focuses mostly on the realization of the oncoming disaster, the idea for the Generation Ship Peerless, the project to construct/prepare the ship, the launch, and the deceleration once they reach orthogonality with the homeworld. The second and third books take place almost entirely aboard the Peerless, not counting a few EVAs and a brief trip (narratively speaking; In-Universe, the trip takes about twelve years for the travelers, four for the residents of the Peerless) to the orthogonal planet Esilio.
• Genre Shift: The second book is a highly-technical romp through the science of the trilogy's universe, in which not much actually happens aside from a brief kidnapping plot for the climax. In contrast, the first and third book are much more plot-oriented, with the third book in particular being fairly action-packed. Whereas the first book has little action aside from the launch, and the second book's climax is over almost as soon as it starts, the third book is rife with sabotage, justified Timey-Wimey-ness, questionably Well-Intentioned Terrorists, and more than a few explosions (none of which are accidental, or have anything to do with Antimatter, unlike the previous books).
• Homeworld Evacuation: Discussed a few times as one of the possible solutions to the Generation Ship's mission.
• Human Popsicle: Because of the universe's Minovsky Physics, reversed Time Dilation is in effect: Traveling fast enough will essentially cause Time to Stand Still from your perspective. The Generation Ship uses this to effectively suspend their entire Doomed Home Planet while they figure out a way to Save the World.
• Hyperspace Is a Scary Place: Not least because Everything Is Trying To Kill You, including dust and even air thanks to the fact that orthogonal matter in this universe is essentially Antimatter, and therefore causes catastrophic Collision Damage if it touches normal matter under all but the most carefully-calculated circumstances.
• Infodump: Every other chapter, complete with Expo Speak, Techno Babble, and Exposition Diagrams. Less prevalent in the final book, which focuses less on discussing the science and more on observing its practical effects.
• Insufficiently Advanced Alien: Despite having almost no understanding of their own biology, no vehicles more advanced than a truck (and no understanding of what makes their fuel work except that it does), a social structure that regards women as walking wombs whose reproductive decisions (which inherently include dying) can and should be dictated by her male relatives, and no concept of basic electronics, the aliens still manage to build an indefinitely self-sustaining ship that can travel faster than the speed of light and (hopefully) come back with Sufficiently Advanced Technology to Save the World.
• Just Before the End: The Generation Ship's mission is to suspend their Doomed Home Planet Just Before The End, and unsuspend it once they are able to figure out a way to avert the disaster.
• Kill Him Already!: The opinion of most of the Peerless' crew towards Yalda's humane treatment of Nino, the would-be saboteur during the launch.
• Lecture as Exposition: Because so many of the characters throughout the trilogy are deeply-entrenched in academic pursuits, many of the Infodumps in the first two books consist of a professor explaining something important about the science of the trilogy's universe to his or her students.
• Long Game: Much of the trilogy's drama and tension comes from juxtaposing the Peerless's actual mission, the Long Game of Saving the World, against the very real and immediate concerns of overpopulation and limited food supplies. Later in the trilogy, some even advocate abandoning the original mission and just looking for a new homeworld for themselves.
• Made of Explodium: Whenever matter that originated in clusters traveling orthogonally to each other comes into contact, if care is not taken to ensure that they are traveling in the right "direction" through time, it creates an immediate and massively explosive reaction. Combine that with the fact that, under ordinary circumstances, such clusters technically have infinite velocity respective to each other, and even a pebble of orthogonal matter can — and, in a combination Mass "Oh, Crap!" and Portent of Doom moment, does — literally Set A Planet On Fire.
• Minovsky Physics: Where to begin? Egan starts with a Real Life equation governing the structure of spacetime, and changes a minus sign to a plus sign. He then posits a universe where that equation is the correct one, and proceeds to extrapolate all the laws of physics from the quantum scale up to the structure of the universe itself. The trilogy is both a Save the World plot and an academic exploration of this hypothetical universe.
• Mood Whiplash: Can be caused by the alternation between "plot" chapters and "Infodump" chapters for the first two books, but The Arrows of Time largely avoids it.
• More than Three Dimensions: Egan rewrites the laws of physics to create an internally-consistent universe where there really are four spatial dimensions, one of which is perceived by the protagonists as time. An (oversimplified) explanation for why time seems so different from space is that the protagonists' momentum through the dimension of time is so great that it's impossible to change trajectory without technological assistance.
• Mr. Exposition: Virtually the entire cast, who frequently spend entire chapters Infodumping Techno Babble at each other.
• Narrative Shapeshifting: One application of the Starfish Aliens' Shapeshifting ability is that they write by raising shaped ridges on their skin, and then dusting their skin with dye and pressing it to paper for posterity. But when characters can't hear each other, either because the environment is too noisy or because they are in the silent vacuum of space, they can also communicate by "writing" notes on their skin for others to read.
• One-Gender Race: Discussed as the inevitable result of a hypothetical Gendercide. Shown to have happened at the end of the trilogy; they don't consider themselves either male or female, but they are certainly one gender.
• One True Love: A set of cos is biologically optimized to mate with each other, and most such couples spend their lives together as a socially acceptable cross between brother/sister and husband/wife. Co-steads, non-related couples who have decided to mate, are also examples to a lesser degree. Averted on several occasions, though, including Tamaro's imprisonment and near-attempted-murder of Tamara, which results in Tamara's estrangement from the entire family and Tamara's new co-stead explicitly telling her that he will completely abandon her if he believes that she never intends to mate with him, even if she has children by another (survivable) means.
• One-Word Title: Orthogonal.
• Parental Abandonment: Death by Childbirth is universal for women. Discussed a few times, and ultimately defied by the development of a procedure that allows women to survive childbirth.
• Perpetual Motion Machine: The titular Eternal Flame of the second book is a hypothetical chemical reaction that never exhausts itself, or at least one that continues for an absurdly long period of time, AND can be controlled and put to practical use, such as powering the ship.
• Planet Spaceship: When they develop their Perpetual Motion Machine engines that are powered by light and consume (almost) no fuel, some characters discuss building massive engines on the Doomed Home Planet and "flying" the planet to safety like a straightforward Planet Spaceship. At/after the end of the trilogy, though, they instead put out the sun and build engines on its surface, so that they can move it and let its gravity take care of pulling the planet along, which will cause fewer siesmic disturbances on the planet. Bizarrely justified in that, In-Universe, the sun actually is just a big ball of flammable rock that is on fire, not a ball of gas undergoing a continuous fusion reaction.
• Population Control: A major theme in the second book. Since every generation inherently doubles in size, crop yield and overpopulation on the Generation Ship quickly become a problem that puts the whole crew at risk. The trope occurs in the form of "the famine", the practice of women starving themselves so that when they reproduce, they will have only two children instead of four, and later in the form of a procedure that lets women give birth to only one child at a time.
• Professor Guinea Pig:
  • Despite extensive preparations and tons of redundant precautions, the construction and launch of the Peerless is still rushed. The crew has every reason to fear that the launch of the Generation Ship might be an utter disaster and kill everyone on board before they even get off the ground.
  • Carlo during the light experiment, in which he uses light signals to attempt to artificially induce muscle movements in his hand. It doesn't go well.
  • In a desperate gambit to keep research on reproductive biology from being banned, Tamara volunteers to have the arborine reproductive signal used on her. It goes surprisingly well.
• Puppeteer Parasite: Discussed almost by name during the light signal experiment, when Carlo loses control of his hand. Worth noting that there aren't actually any parasites present; artificial light signals induce an uncontrollable runaway reaction in the subject's muscles.
  He tried to clench his fist, but his body had news for him: the burrowing parasites owned that flesh, and they weren't taking instructions from him.
• Rape Is a Special Kind of Evil: Sex immediately induces reproduction, and the females reproduce by fissioning into four children. Logically, this means that it is impossible to commit rape without also committing murder.
• Save the World: The mission of the Peerless is to find a way to prevent an imminent collision between their world and a bunch of Antimatter that will utterly annihilate it.
• Sex Is Evil: Suggested and occasionally discussed throughout the trilogy, but ironically not discussed in depth until the third book, by which time sex no longer equals murder. Ramiro and Tarquinia discover the joys of recreational sex, and even though Tarquinia has already shed children and is therefore at no risk of fission, Ramiro still has serious reservations until Tarquinia talks him around. From his perspective, even though she's at no risk, he's still acting on a primal instinct that, if biology was allowed to take its course, would be tantamount to murdering Tarquinia.
• Shapeshifter Mode Lock: When Carlo's hand starts spasming uncontrollably during the light experiment, he tries to reabsorb it into his body, but can't. It's implied that he was just so viscerally repulsed by the phenomenon that he couldn't make himself absorb the hand, rather than actually being physically incapable of doing it, but it still qualifies.
  Carlo began drawing the flesh in at his shoulder. He managed to shorten his arm by about a third before his body rebelled and halted the process. The prospect of bringing the afflicted hand any closer felt like ingesting something rotting and poisoned. And for all he knew, his body was right. What if it couldn't reorganize this flesh, any more than it could subdue a virulent parasite?
  "I can't do it," he said finally. "It has to come off."
• Shapeshifting: One of the more notable traits of the protagonist Starfish Aliens is their ability to freely reshape their bodies, although they're limited by the fact that they can only redistribute mass at will, not increase or decrease it.
• Shoot the Shaggy Dog: Everyone on the ship starving to death due to overpopulation and insufficient crop yields (and therefore rendering the majority of the trilogy, if not the whole thing, pointless) is outright stated to be a possible ending to the second book, which provides the motivation for the biologists and horticulturists in their attempts to defy it.
• Shown Their Work: The entire story could be considered a vehicle for Egan to show his work in extrapolating how physics would function in this universe.
• The Singularity: The entire point of the Peerless' journey is to have an indefinite amount of time to develop sufficiently advanced technology that will let them Save the World from the impending Cluster-Shattering Kaboom. Several characters wonder aloud just how much time that will actually be, and whether the returning travelers will bear any resemblance to their ancestors when they return.
• The Slow Path: A very unusual example that depends on your perspective in this universe's spacetime. The Generation Ship is essentially taking The Even Slower Path, deliberately (and literally) taking a roundabout path to the near future, so that they can have more time to figure out how to handle the future before it arrives.
• Space Age Stasis: In The Arrows of Time, when the inhabitants of the Peerless construct a messaging system that allows them to send messages back in time, it results in the inability of anyone to come up with any new technological or scientific advances. This is pretty significant when you consider that up to this point, they have progressed from technology based on clockwork and combustion engines (or the equivalent thereof) to photonics (read: electronics) and nearly-Perpetual Motion Machines that are powered directly by light — all within six generations of the launch. ^{SPOILERIFFIC note} The lengthy explanation given for this is as follows: In this universe, time travel is relatively easy, because acceleration in space changes the direction through which you travel through time. Accelerate a lot, and you'll go sideways; accelerate even more, and you'll go backwards. Continue accelerating and you'll eventually come full circle and be traveling "normally" again. But since the universe is a closed loop and time travel is just a matter of course, obviously everything has to be mutually consistent. Stable Time Loops are really no big deal — but a Stable Time Loop containing an ontological paradox (at least, one more complex than a basic exchange of obvious or useless information), while not patently impossible, is astronomically unlikely, for the same reason that dumping a truckload of pebbles onto the ground is astronomically unlikely to produce the complete works of Shakespeare in Braille. Basically, anything that is created entirely inside of such a loop is an example of ordered complexity arising from chaos. It's physically possible, it's just unbelievably improbable. Likewise, if a Stable Time Loop involved, say, a physicist receiving a message from herself in the future that explained a revelation about the curvature of spacetime, it would mean that no one had actually come up with the idea; the completed thesis simply coalesced out of the chaotic interactions of matter and energy and forces in the universe. So, since there's no reliable way to ensure that any new discoveries wouldn't be sent back in time with the messaging system (or rather, it's extremely improbable that people would not find a way to send those messages — notice a pattern?), thus creating an ontological paradox and a Stable Time Loop, the simplest way for the universe to keep itself internally consistent is to render innovation impossible as long as the system exists.
• Speculative Fiction LGBT: Most is unusually silent on the subject of LGBT issues and characters (the trilogy instead examines issues of gender equality and women's rights), but by the end of the trilogy, the protagonists' initially two-gendered, male/female species has undergone a singularity that has resulted in, among other things, a single-gender race that is explicitly considered neither male nor female.
• Spontaneous Human Combustion: Life in this universe relies on chemical reactions that occasionally get out of control, resulting in dangerous and spectacular instances of this trope. In fact, the trilogy opens with one.
• Stable Time Loop:
  • The theory of four-space states that the dimension of time is literally identical to any of the dimensions of space and that the universe is a closed loop in all dimensions, implies that this is the natural state of the entire universe.
  • In The Clockwork Rocket, a character wonders if free will is just an illusion in such a universe, and Yalda explains her opinion of why it's not.
  • In The Arrows of Time, the main plot is driven by the idea of a messaging system that will allow people to send messages back in time. Cue extensive discussions about why this could not possibly lead to a Temporal Paradox.
  • At one point in The Arrows of Time, while en route to a planet that is essentially traveling backward through time, a few travelers have an interesting conversation about leaving a spyglass on the surface to decay: From the planet's "perspective", it will be as if a rock eroded into sand, which then gradually and autonomously formed itself into a spyglass, which would then be picked up by the visitors and taken off with them into space. Things get weirder when someone wonders what they may have on the ship with them that came from the planet that nobody has ever set foot on yet.
• Standard Time Units: Time (and mass, and distance) are measured in units invented for the story. However, two of those time units are days and years. It's also implied that a "flicker" is analogous to a second.
• Starfish Aliens: What other kind of life would you expect to evolve in a universe where the laws of physics are fundamentally different?
• Storyboarding the Apocalypse: In The Clockwork Rocket, once Yalda predicts that a Hurtler will destroy the planet, her student Eusebio explicitly asks her to do this. ^note What he really wants is a timeframe, so that he can judge whether the idea of the Generation Ship is worth pursuing, or if they're all going to die too soon for it to matter.
  Eusebio: I want you to imagine the worst, and then tell me how we can survive it.
  Yalda: The worst? The Hurtlers will keep coming, ever larger and in ever-greater numbers, until the odds that we're struck approach a certainty. If we survive that, we'll probably collide with an orthogonal clump of gas — turning the world itself into something like a giant Hurtler. Somewhere along the way, there will be gravitational disruption, maybe ripping us free from the sun completely — or maybe tossing us into it. And if none of these things sound sufficiently fearsome, the encounter might scramble our arrow of time completely, leaving us with no past and no future. The world will end as a lifeless mass of thermal fluctuations in a state of maximum entropy.
  Eusebio: So how can we survive that?
  Yalda: We can't.
• Synthetic Plague: Discussed by opponents of the experiments on survivable reproduction, when rumors begin spreading that the procedure has created an "influence" that can spread like any other disease, inducing the same phenomenon in anyone exposed to it.
• Techno Babble: The science-talk is extrapolated from Real Life physics, but taken out of context, wew:
  Carla: Let's work with a stationary luxagen, to keep things simple. Then its energy-momentum vector points straight into our future. Suppose the luxagen field has a leftor of Up; its rightor will be the same, because Up divided by Up is Future. Suppose we rotate this luxagen in the horizontal plane: the North-East plane. Any such rotation will come from multiplying on the left and dividing on the right by a vector in the Future-Up plane — which will move our leftor and rightor from Up to some new position in the Future-Up plane. But the Future-Up plane is one we're treating as a single complex number, so if the luxagen field remains within that plane, it hasn't really undergone any physical change. And if you can rotate a luxagen in the horizontal plane without changing it, it must be vertically polarized.
• Theme Twin Naming: Cos are named with masculine and feminine forms of the same name, always ending in 'o' for males and 'a' for females. Examples include Carlo/Carla, Addo/Ada, Tamaro/Tamara, and so on.
• Time Crash: Played literally, if bizarrely. Since spacetime is a closed loop in all dimensions, and time is fundamentally identical to space, clusters of matter that were thrown in all directions in time as well as space by the Big Bang occasionally collide with other clusters, many of which are traveling in different directions or even orthogonally to each other (again, in time as well as space), which means they literally have infinite velocity relative to each other. The plot is driven by one such impending collision, which threatens to annihilate the cluster in which the trilogy's cast originates.
• Time Dilation: Traveling fast enough will cause you to be traveling through space without moving through time, which causes time to stop for the starting location. It drives the plot: By taking advantage of this phenomenon, the characters have all the time they need to figure out how to Save the World.
• Title Drop: All over the place in each book, with the exception of The Clockwork Rocket, which is never stated in those words.
  • The "Eternal Flame" is a hypothetical inexhaustible energy source that could be used to fuel a Perpetual Motion Machine.
  • Any given object has an "Arrow of Time", a four-dimensional vector that describes its path through spacetime.
  • The trilogy's title is dropped far more frequently than any of the individual book titles; "Orthogonal", which is basically a fancy word for "perpendicular", is constantly used to refer to situations where something is traveling orthogonally through spacetime with respect to something else — "something else" usually being the Doomed Home Planet. ^note In other words, if two space rocks have arrows of time that are perpendicular to each other (each one's movement through space is perceived as movement through time by the other, and vice versa), they are said to be orthogonal to each other. The phrase "orthogonal matter", which is this universe's equivalent to Antimatter, is by far the most frequently used Title Drop.
• Translation Convention: Presumably in constant effect, given the setting. Most noticeable in the lack of an obvious Alternative Number System despite virtually everything being based on powers of twelve.
• Unto Us a Son and Daughter Are Born: With highly unusual exceptions, this trope is universally played straight with every birth. When women reproduce, they fission into two pairs of male and female twins. Rarely, one of the pairs might instead be a single female, but males are never born solo, and the solo females are, again, very rare. There are no instances in the book of both pairs being born as solo females instead, so every natural birth results in at least one pair of twins.
• Viewers Are Geniuses: Par for Greg Egan's course, though he does make truly heroic efforts to avert it by explaining everything from the ground up.
• Weapons-Grade Vocabulary: The poem, if written on body then pressed on other being body, would be deadly in short time was briefly discussed as myth. They are very different so they form writing with their bodies, then dye skin and press paper on it. So, basically written weapons-grade poem.
• Wrap Around: The universe is a four-dimensional sphere or possibly a torus, a donut-like shape. This is the explanation for why orthogonal matter from this universe's version of the Big Bang can be expected to occasionally collide. ^note If the universe didn't Wrap Around, orthogonal matter would just perpetually travel farther from the origin point, and never come close. But in the case of a torus, matter eventually loops back around and there's a chance of it colliding with other clusters going in the opposite direction.
• The X of Y: The Arrows of Time.
• You Already Changed the Past: On Esilio, a planet with Merlin Sickness, the crew of the Surveyor blow up a rock. After the explosion, they find what looks like writing etched into a newly-exposed part of the rock, which seems to be a message from the ancestors (the inhabitants of the homeworld). Because of the planet's Merlin Sickness, the message must have been carved at some point in the future. The obvious assumption is that the message means the journey is successful; the Peerless makes it home, and at some point the ancestors visit Esilio and carve the message as encouragement to the travelers. But Ramiro decides that he wants to have a hand in fate, so he plans to go out and "carve" the message himself. Tarquinia prevents him from doing so — and he then realizes that she is going to carve the message. He spends most of the rest of the book under the impression that she did — only to discover after the climax that she tried to carve it, but no matter what she tried, the message stayed there, which means that she didn't do it either. The book ends with a subtle implication that a minor character who returns to the homeworld in the epilogue is the one to go back to Esilio and carve the message.
  • The Surveyor returns to the Peerless after a long absence to find that the inverted Time Capsule messaging system (which essentially lets people send email back in time) has been built, but also mysteriously stops working all at once at a known point in the future. But since no one actually knows what causes the disruption, the crew of the Surveyor realize that if nobody does anything, they are most likely consigning themselves to being hit by a meteor. On the other hand, if they attempt to sabotage the system, they are raising the probability that they will cause the disruption, which means no one will be harmed. In other words, they know they Can't Fight Fate since the universe is an absolutely Stable Time Loop, but if nobody tries to cause the disruption, then it's almost guaranteed that it's caused by a disaster such as a meteor strike; but as long as no one knows what causes the disruption and someone is trying to cause it, they are increasing the odds that the disruption has a harmless cause.

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