Seth is a surveyor, along with his friend Theo, a leech-like creature running through his skull who tells Seth what lies to his left and right. Theo, in turn, relies on Seth for mobility, and for ordinary vision looking forwards and backwards. Like everyone else in their world, they are symbionts, depending on each other to survive.
In the universe containing Seth's world, light cannot travel in all directions: there is a “dark cone” to the north and south. Seth can only face to the east (or the west, if he tips his head backwards). If he starts to turn to the north or south, his body stretches out across the landscape, and to rotate as far as north-north-east is every bit as impossible as accelerating to the speed of light.
Every living thing in Seth’s world is in a state of perpetual migration as they follow the sun’s shifting orbit and the narrow habitable zone it creates. Cities are being constantly disassembled at one edge and rebuilt at the other, with surveyors mapping safe routes ahead.
But when Seth and Theo join an expedition to the edge of the habitable zone, they discover a terrifying threat: a fissure in the surface of the world, so deep and wide that no one can perceive its limits. As the habitable zone continues to move, the migration will soon be blocked by this unbridgeable void, and the expedition has only one option to save its city from annihilation: descend into the unknown.
There are so many ideas that shape this world into being different from ours, that when I first started reading it seemed like any one of these premises would have been enough for a perfectly interesting story. However, having finished the book, I can see how all the weirdness, for lack of a better word, is interlinked. No one key premise would have worked rigorously without the other elements holding it together. The geometry of the world requires the migration and begets the alien configuration of the beings we follow in the story. The not-human beings can only see in two directions (forwards and backwards, though they don't refer to them like that) and have a symbiotic relationship with intelligent leech like beings. The Walkers have holes in their heads and the Siders live inside these holes and, through supersonic "pinging" can see sideways and share that information with their Walkers. The Walkers and Siders can also silently communicate, and Siders can speak in the Walker language and also among themselves in their own higher-pitched language that Walkers can't hear. And I've barley scraped the surface of the geometry aspect.
The story follows the investigation of a looming crisis. The river our protagonists' city follows is going to run out as the city continues migrating. Seth and Theo are sent to survey a new region and hopefully find a new river or other means for the city to sustain itself in the future. As they make various discoveries, the situation escalates and they learn more about their world than they bargained for.
This book was a little baffling to read, in a way not foreign to a reader who has read a few Greg Egan books before (ie me). I have a degree in pure maths and one in physics and while some of the geometric ideas were a little familiar to me, I found it difficult to picture some of what happened or predict the effects of some actions. Ultimately, I found the book enjoyable enough if I just went with the flow and didn't overthink it. When I got to the end and read the afterword on geometry (not a spoiler if you want to jump ahead to it), I discovered that I hadn't been thinking of things in quite the right way, which explained some of my confusion. I don't think that knowledge would have helped me all that much if I'd read it earlier, however, since I think a proper representation would be more or less impossible without writing out some equations.
Maths aside, I was also put in mind of Flatland (but without he weird chauvinism), for the exploration of a world with foreign geometrical properties. The main story was more a exploration and discovery yarn, albeit not an exploration of Earth or anything in our universe.
I recommend Dichronauts to fans of Greg Egan and people who are not afraid of unconventional mathematics. Readers who enjoy exploration stories and don't mind not understanding exactly how the physics of the world works should also enjoy this book.
4 / 5 stars
First published: June 2017, Nightshade Books
Format read: eARC
Source: Publisher via Edelweiss
Challenges: Aussie Science Fiction Reading Challenge