This book is quite complicated, so bear with me...
The book begins describing how Yatima, an embryonic software mind (the book's term for this is "psychoblast"), is initially created. Typically, psychoblasts are created by one or more parents, in a process that's basically the software version of physical reproduction—some traits are inherited from the parents, some are chosen by the parents, and some are randomly selected. However, Yatima is a special case, a so-called "orphan". This means Yatima has no parents, and is created via a structured yet randomized process.
As a newborn, Yatima doesn't understand the world, and doesn't even have a sense of self. She meets some friends in "scapes" (the software equivalent of places), and overtime develops into a "citizen" (a fully fledged software person). Basically, she grows up.
Yatima has a propensity for "truth mining"—in other words, she wants to discover the nature of the universe and how things work. In the Konish polis (the software community Yatima lives in), axioms are represented as parts of a large mine, and those who seek to further humanity's knowledge attempt to unearth new areas of the mines.
Inoshiro, one of Yatima's friends, has different inclinations, and cares much more about the physical world. At his request, Yatima and Inoshiro visit the "fleshers" (regular humans like you and me) on Earth by inhabitating the bodies of two abandoned robots. They only stay for 24 hours, although this is a considerable amount of time since citizens experience things 800 times faster than fleshers (this is called "rushing"). During the trip, they meet two fleshers named Orlando and Liana.
A gleisner (conscious software that runs on a robot, as opposed to citizens which run in polises) named Karpal observes a disturbing event. Two neutron stars have smashed into each other (for unknown reasons), creaeting a gamma ray that will kill everything on Earth. News of this reaches Konishi polis, and Yatima and Inoshiro go back to Earth in an attempt to warn the fleshers.
They only have 24 hours until the gamma ray strikes earth, and urgently attempt to communicate the imminent danger to the fleshers. Some fleshers believe them, and some do not (fleshers are biased against citizens), but ultimately it doesn't matter. The gamma ray wrecks the Earth. Yatima and Inoshiro head towards Orlando and Liana, only to find that Liana has been killed. Orlando is wounded, and Yatima converts him from a flesher to a citizen (basically, she shoots him with something that uploads his brain to a polis).
As the years pass, the Earth gets worse and worse. Yatima and others occasionally visit to convert more fleshers into citizens, until eventually everyone has either died or been converted.
The gleisners send out 63 ships to explore the universe, and learn more about why the neutron stars smashed into each other—humanity's physics has no explanation. The Carter-Zimmerman (CZ for short) polis intends to do something similar, but by using wormholes instead of spaceships. Yatima tries to convince Inoshiro to join her, but he has forced his software mind to permanently have a certain "outlook" on life, which makes him believe that striving is futile. Yatima joins the CZ polis.
Gabriel and Blanca (well, mainly Gabriel) research how to build a traversable wormhole. After some time, and with the help of others, they build a giant particle accelerator called the Forge. They use the Forge to make a traversable wormhole based on Kozuch theory, but it turns out that the wormhole is useless—traveling through it takes just as long as traveling outside of it.
Finally, we get to the Diaspora! Since the wormhole turned out to be useless for traversing space, the CZ polis decides to create 1000 copies of itself and send them in different directions. Specifically, each polis is sent to some distant star. Most of the citizens decide to freeze themselves until they reach their destination.
One of Blanca's copies decides not to freeze herself. Instead, she spends her time trying to figure out why Kozuch theory did not predict the wormhole's length. After some time, she arrives at a modified theory of Kozuch theory which is consistent with the wormhole's length, and sends this theory back to the original polis which stayed near Earth.
Paolo, Orlando's citizen son, arrives at a planet named Orpheus. There, they discover these strange, apathetic creatures whose surface is composed of Wang tiles. Because of this, they are nicknamed "Wang's Carpets".
These tiles form a computer capable of running simulations—in other words, each carpet is basically a biologically computer. Not only that, it turns out that these computers are running a 16-dimensional universe in which there are sentient beings. Don't ask me to explain any further, it's quite confusing.
Ultimately, this part is not that critical to the overarching story. It is more of an interesting side story about how the Diaspora first discovered alien life inside of a biological computer—not what they expected!
Now the story turns to one of Orlando's clones. Orlando's polis has arrived at a planet named Swift, which is distinguished by the presence of heavy isotopes in its atmosphere. The citizens eventually discover that neutrons in these isotopes contain a hidden message, which they uncover by doing... some science thing... to the neutrons.
The first message they discover is a video of the universe, which shows that in the not-too-distant future, a HUGE gamma ray burst will occur that will wipe out all life in the universe.
The second message they discover is instructions on how to use neutrons to make traversable wormholes to the "macrosphere" (i.e. a different universe).
Paolo, Orlanda, Yatima, and others travel to the macrosphere. Their goals are twofold. First, they want to check if the macrosphere is a safe place to hide in when the huge gamma ray burst occurs. Second, they want to discover more about the Transmuters.
The macrosphere has 6 dimensions (including time), which makes it very hard to comprehend.
In the macrosphere, they start exploring a planet called Poincaré, which is teeming with life. They discover creatures dubbed Hermits, which are insulated from all other life on the planet. Orlando communicates with them by making multiple copies of himself, each one progressively closer to the Hermits. This forms a line of communication, where Orlanda can communicate with the clone most similar to himself, that clone can communicate with the next clone, and so on. By doing this, Orlando discovers that the Transmuters stopped at Poincaré, but ended up leaving to a second macrosphere.
Paolo and co travel to the second macrosphere, where they meet a non-sentient program called a Contingency Handler. Albeit non-sentient, the Contingency Handler knows everything about the citizens, since it hacked the system. It tells them a few important things. The first revelation is that each universe interacts with an infinite number of adjacent universes. This is what caused the neutron stars to collapse (which in turn caused the first gamma ray burst that caused the Diaspora). The second revelation is that the huge gamma ray burst will annihilate all life in the universe.
While everyone else chills out in the first and second macrospheres, Paolo and Yatima continue to chase the Transmuters. They travel through trillions and trillions of other universes, only to find that the Transmuters all committed suicide after experiencing basically everything. Satisfied with this conclusion, Paolo also commits suicide. Yatima, however, decides to spend the rest of her life in the truth mines.
I'll admit, I had some trouble getting into this book—I found the first chapter quite difficult to fully parse (it's been more enjoyable on subsequent re-reads). I suspect this is fairly common, given how Greg Egan is known for how "hard" his books are. However, once the story got rolling (for me, this was when they first visit the fleshers), I was hooked. The point I'm trying to get across is that you shouldn't be intimidated to read this book, even though certain parts are extremely complicated and mathy. You can kinda just skim over those parts without losing much. That's why I did when Egan started going in-depth on manifolds and 6 dimensions and yadda yadda yadda.
The main question this book raises is the following: if an introdus actually happened, which group would you join? The fleshers, the gleisners, or the citizens? Personally, I'd join the citizens—sounds pretty awesome to be a piece of conscious software. Here are the top 3 things I'd look forward to as a citizen:
- No physical constraints/diseases—I could fly around, I could "eat" whenever I want, etc.
- I could speed up time to see all the cool stuff that's happening in the physical world
- I could slow down time to spend more time doing leisure activities (e.g. reading) That being said, maybe the limitless opportunities would actually throw me into a deep depression. Whatever, seems worth the risk.
I want to double click on this though, because the idea of having unlimited time and freedom is so interesting. Currently, so much of life is defined by death. There is a biological timeline that undergirds everyone's lives, and although it's flexible, it is ultimately inescapable. From 0 to 18 you mature from a kid to an adult, from 18-40 you have kids (or choose not to), from 30-50 you take care of your kids, then you chill, and then you die. This timeline affects every part of our lives. It affects what jobs we take (perhaps choosing to optimize for money over satisfaction, so you can raise kids in a nice environment), the people we meet, the hobbies we pursue. So what would life look like if it diverged from physical and biological constraints? Would it make people happier? Or would people be lost in the limitless possibilities? The book's main character, Yatima, offers one possibility—Yatima throws herself into math, making it her life's goal to figure out the rules that underly the universe. I suspect this would be quite an uncommon goal, just as it is in the book.
Another interesting idea is the idea of software clones (see also Digital People Would Be An Even Bigger Deal). Obviously, this would be good for productivity. But it also raises a bunch of questions—if you create multiple clones, who is the "real" you (shoutout to Jin Bubaigawara from My Hero Academia)? Would cloning be good or bad for society? How much would the clones diverge (nature vs. nurture)? If you create clones to do grunt work for you, will they get mad and rebel?
This book contains lots more interesting ideas, many of which are captured by the quotes below. My final opinion—go read this book!
If ve ever wanted to be a miner in vis own right - making and testing vis own conjectures at the coal face, like Gauss and Euler, Riemann and Levi-Civita, de Rham and Cartan, Radiya and Blanca - then Yatima knew there were no shortcuts, no alternatives to exploring the Mines firsthand. Ve couldn't hope to strike out in a fresh direction, a route no one had ever chosen before, without a new take on the old results. Only once ve'd constructed vis own map of the Mines - idosyncratically crumpled and stained, adorned and annotated like no one else's - could ve begin to guess where the next rich vein of undiscovered truths lay buried. (Page 41)
Yatima knew that Radiya, and most other miners, used outlooks to keep themselves focused on their work, gigatau after gigatau. Any citizen with a mind broadly modeled on a flesher's was vulnerable to drift: the decay over time of even the most cherished goals and values. Flexibility was an essential part of the flesher legacy, but after a dozen computational equivalents of the pre-Introdus lifespan, even the most robust personality was liable to unwind into an entropic mess. None of the polises' founders ahd chosen to build predetermined stabilizing mechanisms into their basic designs, though, less the entire species ossify into tribes of self-perpetuating monomaniacs, parasitized by a handful of memes. It was judged far safer for each citizen to be free to choose from a wide variety of outlooks: software that could run inside your exoself and reinforce the qualities you valued most, if and when you felt the need for such an anchor. The possibilities for shoft-term cross-cultural experimentation were almost incidental.
Each outlook offered a slightly different package of values and esthetics, often built up from the ancestral reasons-to-be-cheerful that still lingered to some degree in most citizens' minds: Regularities and periodicities - rhythms like days and seasons. Harmonies and elaborations, in sounds and images, and in ideas. Novelty. Reminiscence and anticipation. Gossip, companionship, empathy, compassion. Solitude and silence. There was a continuum which stretched all the way from trivial esthetic preferences to emotional associations to the cornerstones of morality and identity. (44)
No wonder most fleshers had stampeded into the polises, once they had the chance: if disease and aging weren't reason enough, there was gravity, friction, and inertia. The physical world was one vast, tangled obstacle course of pointless, arbitrary restrictions. (Page 55)
The bridgers, he explained, had tailored themselves to the point where any individual could rewrite parts of vis own genome by injecting the new sequence into the bloodstream, bracketed by suitable primers for substitution enzymes, wrapped in a lipid capsule with surface proteins keyed to the appropriate cell types. If the precursors of gametes were targeted, the modification was made heritable. Female bridgers no longer generated all their ova while still fetuses, like statics did, but grew each one as reuired, and sperm and ova production - let alone the preparation of the womb for implantation of a fertilized egg - only occurred if the right hormones, available from specially-tailored plants, were ingested. About two-thirds of the bridgers were single-gendered; the rest were hermaphroditic or parthenogenetic-asexual, in the manner of certain species of exuberants. (67)
They kissed. Yatima wondered if Blanca and Gabriel ever did that - if Blanca had modified verself to make it possible, and pleasant. No wonder Blanca's parents disapproved. Gabriel being gendered wasn't such a big deal, as an abstract question of self-definition - but almost everyone in Carter-Zimmerman also pretended to have a tangible body. In Konishi, the whole idea of solidity, of atavistic delusions of corporeality, was generally equated with obstruction and coercion. Once your icon could so much as block another's path in a public scape, autonomy was violated. Reconnecting the pleasures of love to concepts like force and friction was simply barbaric. (68)
"There's already talk of cloning a thousand copies of Carter-Zimmerman and dispatching them all in different directions, to help us catch up with the gleisners. If the wormholes had been instantly traversable they would have bound the whole galaxy together; we could have moved from star to star as easily as we jump from scape to scape. But now we're destined for fragmentation. A few clones of C-Z will fly off to the stars, centuries will pass... and by the time any news comes back the other polises will be past caring. We'll all drift apart." He scooped a handful of dust forward, speeding its fall over the precipice. "I was going to build a network spanning the universe. That's who I was: the citizen who'd put it all in the palms of our hands. Who am I now?"
"Instigtator of the next scientific revolution."
"No." He shook his head slowly. "I can't turn that corner. I can live with failure. I can live with humiliation. I can meekly follow the gleisners into space, slower than light, accepting that there's no better way after all. But don't expect me to take the thing that's poisoned my dreams and embrace it as some kind of triumphant revelation."
Blanca watched him staring morosely into the distance. Ve'd been wrong, for all these centuries: the elegance of Kozuch Theory had never been enough for Gabriel. So the chance to uncover and remove its flaws was no consoloation to him at all. (156-157)
Ve ran vis Kozuch avatar; an image of the long-dead flesher appeared in the scape beside ver. Kozuch had been a dark-haired woman, shorter than most, sixty-two years old when she'd published her masterpiece - an anomalous age for spectacular achievement in the sciences, in that era. The avatar wasn't sentient, let alone a faithful recreation of Kozuch's mind; she'd died in the early years of the Introdus, and no one really knew why she'd declined to be scanned. But the software had access to her published views on a wide range of topics, and it could read between the lines to some degree and extract a limited amount of implicit information. (165)
^ This is like ChatGPT!
"I keep asking myself, though: where do we go from here? History can't guide us. Evolution can't guide us. The C-Z charter says understand and respect the universe ... but in what form? On what scale? With what kind of senses, what kind of minds? We can become anything at all - and that space of possible futures dwarfs the galaxy. Can we explore it without losing our way? Fleshers used to spin fantasies about aliens arriving to 'conquer' Earth, to steal their 'precious' physical resources, to wipe them out for fear of 'competition' ... as if a specifes capable of making the journey wouldn't have had the power, or the wit, or the imagination, to rid itself of obsolete biological imperatives. Conquering the galaxy is what bacteria with spaceships would do - know no better, having no choice.
"Our condition is the opposite of that: we have no end of choices. That's why we need to find another spacefaring civilization. Understanding Lacerta is important, the astrophysics of survival is important, but we also need to speak to others who've faced the same decisions, and discovered how to live, what to become. We need to understand what it means to inhabit the universe." (190)
Paolo thought: We might as well have run a pure simulation ... and pretended to follow the capsules down. Elena gave him a guilty/admonishing look. Yea - and then why bother actually launching them at all? Why not just simulate a plausible Orphean ocean full of plausible Orphean lifeforms? Why not simulate the whole Diaspora? There was no crime of heresy in C-Z; the polis charter was just a statement of the founders' values, not some doctrine to be accepted under threat of exile. At times it still felt like a tightrope walk, though, trying to classify every act of simulation into those which contributed to an understanding of the physical universe (good), those which were merely convenient, recreational, esthetic (acceptable) ... and those which constituted a denial of the primacy of real phenomena (time to think about emigration). (194)
Orlando's celebration of the microprobe discoveries was very much a carnevale-refugee affair. The scape was an endless sunlit garden strewn with tables covered in food, and the invitation had politely suggested attendance in strict ancestral form. Paolo politely faked it, simulating most of the physiology but running the body as a puppet, leaving his mind unshackled. (201)
"It won't be death." Paolo seemed calm now, perfectly resolved. "The Transmuters didn't die; they played out every possibility within themselves. And I belive I've done the same, back in U-double-star... or maybe I'm still doing it, somewhere. But I've found what I came to find, here. There's nothing more for me. That's not death. It's completion." (320)