Dichronauts

by Greg Egan

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.

Genres: Science Fiction, Fiction, Audiobook, Hard Science Fiction, Science Fiction Fantasy

312 pages, Hardcover

First published July 11, 2017

About the author

Greg Egan specialises in hard science fiction stories with mathematical and quantum ontology themes, including the nature of consciousness. Other themes include genetics, simulated reality, posthumanism, mind transfer, sexuality, artificial intelligence, and the superiority of rational naturalism over religion.

He is a Hugo Award winner (and has been shortlisted for the Hugos three other times), and has also won the John W Campbell Memorial Award for Best Novel. Some of his earlier short stories feature strong elements of supernatural horror, while due to his more popular science fiction he is known within the genre for his tendency to deal with complex and highly technical material (including inventive new physics and epistemology) in an unapologetically thorough manner.

Egan is a famously reclusive author when it comes to public appearances, he doesn't attend science fiction conventions, doesn't sign books and there are no photos available of him on the web.

Excerpted from Wikipedia.

Reviews

[Claudia · Rating 3 out of 5]

This was one tough book to comprehend. I read all I could find on Greg Egan’ site and some more about differential geometry and I still did not understand how the world imagined by the author works.

Because this is not an action or character driven story, but one in which geometry makes the rules and the reader tries to visualize how this world is constructed. Instead of a 3-dimensional space and 1 dimension of time, we have 2 of each.

After playing with this simulation here, I could imagine a bit how one can move into this two-dimensional space, but I’m very far in imagining exactly how or how the inhabitants look like. They are east or west facers, have axial fingers, northern and southern ones, the east facers can move and see only east and when moving backwards to west they tip their heads backwards to see where they are moving, to north and south they can only slide and rely on their symbiont companion’s echolocation system to ‘see’ north and south, as they cannot rotate.

Sounds insane, doesn’t it? It really does. And is. It is a mind-bending book, not resembling anything else I read so far, the exact definition for sci-fi of ideas. Unfortunately, by not having enough background in math, I can’t say I enjoyed it to the fullest. However, I liked the idea, the relationship between the symbionts, their system of values, the way world is constructed. It’s not the author’s fault that my neurons didn’t live up to the challenge.

But I would recommend this book to all those who welcome a challenge and are open minded, because it is a one of a kind experience.

Note: before you start it, take a look at the material related to this story on Greg Egan’ site:

http://www.gregegan.net/DICHRONAUTS/D...

And engrave this image into your brain, because this is the world of the dichronauts:


*ARC received via NetGalley thanks to Skyhorse Publishing

[Bradley · Rating 3 out of 5]

For fans of multiple world-time-lines and especially for fans of Christopher Priest's The Inverted World, welcome to Dichronoauts.

Walking one way brings you to the future, the other, to the past. But space is still space and time is still time. Thanks to the little creature attached to the people here, we've got a cultural/exploratory thing going on that is the spiritual godchild of Priest's classic novel. Just look at the cover to get an idea. That's a picture of the Earth. As in, the Earth, to these people, is shaped like an hourglass. That makes EVERYTHING pretty messed up. :)

So let's explore! And the characters barely know anything more than us, so we're introduced to many theories that may sound absurd, but the Earth is ALREADY absurd... because time and alternate worldlines are as easily traversed as walking across a field.

As a LITERAL worldbuilding novel, almost all the fun is in exploring and visualizing the world they inhabit.

The rest... well... as okay. Not overly special. The chasm was pretty cool. Bits and pieces elsewhere. But overall, I was not overly invested in any character.

Win some, lose some. The cool aspects are VERY cool, however. :)

[Cathy · Rating 4 out of 5]

Symbionts on a planet with a different set of dimensions and shaped as an hyperboloid.

A somewhat human body and a leech-like creature, cohabiting and sharing their life. How do you decide the direction of your life, how do you choose your profession, your partners.... who gives in, who takes precedence, what happens in case of irreconcilable differences? Fascinating.

The book is split in several parts, each part of the book dealing with a distinct section of plot. Part One did a nice job of explaining the tensions between the cities competing for resources and the possible societal conflicts, caused by the symbiotic relationships of its inhabitants. And the next parts... well, you‘ll have to find out for yourself.

The tricky part in this book is the planet and its physics. Because of the way it‘s shaped, nothing works as you would expect. I really struggled to picture this world and its inhabitants. I am not sure if it‘s me lacking imagination or if the author did a poor job of explaining it. Or did he do a deliberate job of leaving blanks, to challenge the readers to figure it out by themselves?

It took me over half of the book to even realize that the walkers do not have symmetrical bodies (I think?). I understood that they were shaped differently, with restricted options of movement and a different visual system, but I could not for the life of me picture them for most of the book. Not sure I have the right visual even now, after finishing.

Very, very imginative and pretty much a mystery to me. Without the book blurb I would have been lost in the beginning. Minor degrees in physics, math, astronomy and mechanical engineering might have helped. At one point I started reading up on orbital planes, torque and Euler‘s theorem of rotation. And I looked up Egan‘s explanations as well: http://www.gregegan.net/DICHRONAUTS/0...
He also explains the physics at the end of this book.

I got so occupied with trying to understand the physics of this world, that the plot took a backseat until the last few chapters.

Explain this to me... I know it‘s a play on words and letters, but I can‘t figure it out:

Theo said, „Flerdibyll graznisniff?“
Seth responded effortlessly:
„Mulpeneresh, sockulee!“

Translation into plain English, please!?

This book was hard work! Fascinating stuff and deeply weird. I am docking one star for the sheer incomprehensibility (is that a word?).

I received this free e-copy from the publisher/author via NetGalley, in exchange for an honest review, thank you! Sorry it took me so long....

[Erik · Rating 3 out of 5]

Dichronauts’s back cover has the perfect blurb: “Impressively bizarre… Egan may have out-Eganed himself with this one.” Yeah, pretty much. Dichronauts gives us a world in which water flowing uphill is not even its most bizarre feature. Nor that the world is an infinite hyperboloid. Nor that light cannot travel in certain directions. Nope. For me, the most bizarre feature of Dichronaut’s world is that its denizens cannot safely rotate.

It’s all so bizarre, in fact, that if you want to become a Dichronaut, you really need to first read Egan’s ”Gentle” Introduction to Dichronauts. I got roughly half of the way through the book without doing so. And I felt incredibly dumb that entire time.

Now that feeling is rare enough for me that I can enjoy it, but the book was still better once it was more comprehensible. So let me also provide my own primer:

So our universe has 3 (macro) dimensions of space and 1 (macro) dimension of time. To create Dichronauts’s world, Egan swapped one of these, so that it has 2 space-like dimensions and 2 time-like dimensions. Hence, the DICHRON - “two time” - of the title.

The result, as you probably cannot imagine (but can calculate), is a world with many strange features. For example, you’ll find that light does not radiate spherically (that is, in all directions). Instead, we must subtract out two “dark cones” from our sphere, representing places light CANNOT travel from a given source.

Thus, sunlight only allows the characters in Dichronauts to see east and west. In fact, they can really only see in one direction but are able to “tilt” their heads (I always pictured it as a full 180 degree backwards tilt, like something from a horror film) to see in the opposite direction. So in this world, people are BORN as “eastward-facing” or “westward-facing.” Really. Born into permanent facing directions.

Because that’s another strange feature of replacing one space-like dimension with one time-like dimension: rotating “into” the time-like dimension is quite problematic. Note: In Dichronauts’s cardinal directions, east/west and up/down are the space-like dimensions, while north/south is the new time-like dimension. So objects/characters can only manage partial, incremental rotations to the north/south.

This leads us to the second meaning of the title, the DI__NAUTS, “two travelers.” Because of the dark cones and this lack of rotation, every “person” is actually made up of two creatures: the large Walker, who possesses East/West vision, and a lizard-esque symbiote, the Sider, who lives burrowed into the Walker’s skull and possesses North/South sonar, via two “pingers.” Because, yes, while LIGHT cannot travel north/south, sound waves are mechanical waves and do not have to adhere to the forced speed of light. Thus they actually can travel north/south.

So that’s the people, and that’s why they are the way they are. One major conflict of the novel is that some Walkers really dislike this relationship with the Siders.

The two main characters - Seth the Walker and Theo the Sider - spend the majority of the novel as surveyors, so you’ll also need a good grasp of the terrain these people inhabit. Start here with this picture, their “planet:”



It is NOT a sphere orbiting a star. Rather it’s an infinite, hollow hyperboloid, which is orbited by a star. This picture is also inaccurate in one important way: it depicts a fixed, unchanging orbit of the star. But in the novel, the star’s orbit is actually tilting, which causes the habitable zone to be slowly moving, thus requiring a constant migration of the book’s peoples.

Okay, all that was just providing background, but here’s the real question: How’s the reading experience?

Honestly, it’s dissatisfying.

And I don’t want to write something like, “The imagination is great, but the human/character elements of the story are lacking” because I don’t think that’s it. You’ll find reviewers writing stuff like that on every hard sci-fi book ever written because it’s a convenient cliche to explain an apparent disconnect with the story. But I’ve never found Egan to struggle with capturing the human element.

Rather, here’s an experience to which most people can relate: I get really grumpy when I’m doing some sort of challenging technical work - building furniture whose pieces don’t fit well, for example. Whenever someone tries to interact with me when I’m in such a state, I get snappish and terse. I wouldn’t say there’s anything wrong with the “human element” part of me there. It’s just that, distressed and distracted by a technical problem, I don’t really want to have a conversation or other social experience.

That’s how I feel about Dichronauts: it has frustrating technical problems that can distract from the plot / character relationships.

For example, this book talks a lot about water and rivers and such. It’s one of the core parts of the setting, that Seth and Theo’s city migrates along a river that is drying up. But water does not exist in this book’s world. Egan says so himself. But because he’s using all these familiar words, these ideas FEEL like they should be familiar. Except then he throws in some of this world’s exotic geometry and suddenly these “rivers” are flowing uphill - or just outright shooting straight up into the air and coming back down as rain.

The disconnect creates an unpleasant friction in the mind, this attempt to reconcile what should be familiar with what is clearly unfamiliar and exotic.

In other cases, scenes are practically impossible to grasp without having done the mathematical work (or reading Egan’s primer). For example, there’s an early scene in which the main character and his fellow surveyor students use the “grain” of the dirt to help determine their orientation. It will make no sense because on OUR planet in OUR universe, dirt has no particular grain. The notion is simply ludicrous. Dirt particles are just chaos. They don’t NATURALLY line up. But in Dichronauts’s world - because of the restrictions on rotation - they do.

I call these “problems,” but I don’t really mean them as criticisms. What was Egan to do? Use a neologism instead of “water” and “river”? Would that have improved the clarity?

In a few instances, yes the wording is bad. He uses the phrase “absolute summer” to describe parts of the "planet" so close to the sun (because of the weird geometry) that they receive lethal doses of heat and radiation. But it’s not seasonal, so why not call it “absolute desert” or something like that?

Overall, though, I doubt there’s much he can do. This is just a ludicrously ambitious setting. That's my real criticism: it's TOO ambitious.

And it doesn’t help there are substantial problems with Egan’s Physics work that make me question much of his conclusions. For example, photons aren’t just used for visible light - they mediate the entire electromagnetic force. So if there’s ‘dark cones’ caused by light sources… there will be ‘dark cones’ for electrons and protons, causing strange geometry restrictions within atoms and molecules. And how about entropy, the “arrow of time”? How does that work with two time dimensions?

I have some criticisms about the story too, which ends prematurely without having resolved some of the larger themes and conflicts. But those criticisms are trivial by comparison with the overall awkwardness of the setting and how it’s presented.

Ultimately this is one of those books that I’m glad exists. As an act of ambitious creation, it’s great. It’s just… not a great read.

[Rachel (TheShadesofOrange) · Rating 4 out of 5]

3.5 Stars
I loved the mathematical and geometric ideas explored in this novel. However, in terms of story and characters, I found this one a bit thin. I loved the ideas explored, but wished the storytelling had been stronger.

[Liviu · Rating 3 out of 5]

finally finished this one and after a great beginning which was very intriguing and another maybe 100 pages that kept me really interested, the book became sort of a slog as the difficulty of reconciling the strangeness of the Dichronauts universe and beings with a sort of human-like behavior started showing more and more and the book became "it sounded a cool idea at the time but it cannot sustain itself beyond an initial introduction" as on the world building side things got stranger and occasionally quite cool, but on the characters side they devolved into a mixture of banality and impossibility of sustaining suspension of disbelief (regarding actions and thought processes of these really strange beings translated into human terms for us)


(first thoughts on getting the book) read a few pages and it's excellent (G. Egan is one of the few truly mind boggling sf authors of today) so far - really worth checking the author's page about it and thinking a little about the implications of the universe metric

http://www.gregegan.net/DICHRONAUTS/D...

[Tim Hicks · Rating 3 out of 5]

If this is your first Greg Egan, just put it down slowly and back away quietly. No one needs to get hurt here. Go find an earlier work as an introduction.

This is hard SF to make Hannu Rajaniemi blush. This is hard SF made of adamantium, or impossibilium. I'm guessing there aren't 200 people in the world who could read this and smile and say, oh, yes, haha, of COURSE it would be like that, wouldn't it?" and maybe ten who could say "ahem, a little error there on page 212 with the rotational forces out of balance with the shear forces, heh heh."

The plot is negligible, and not terribly important. The characters are barely sketched, and the setting less so. There is a Quest, because how else do we get a tour of the world? There's a river that runs uphill, unless you want to go down, in which case look just over there for one that runs the other way.

If I understood it right, which I probably didn't, our heroes are exploring their bowl-shaped part of the world when they find a hole that goes through to the hyperboloid side. The effect of this is intriguing, because it leaves our heroes and the reader equally confused.

The book that this one makes you anticipate is this world's equivalent of Darwin's The Origin of Species. Here, you take the physics and produce a creature that could *possibly* cope in such a setting. Could it evolve from something else? How? Can we imagine that millions of generations of primitive creatures died off except for those that learned not to try to turn around? How did the Siders start out?

I certainly can't fault Egan for not including any of that here.

I didn't enjoy this book, but I'm glad I read it. Someone has to go to the edge of "hey, fellas, what if ..." and by golly, Egan hasn't just gone to the edge here -- he's moved the edge.

[Gary · Rating 3 out of 5]

The world of Greg Egan’s Dichronauts contains two spatial dimensions and two temporal ones. The people of this world are symbiotes, each comprised of a “walker” and a “sider” – siders are parasites who leech nourishment from their walkers; walkers can only see one side of the world on their own and need their siders to see the other.
The sun revolves around the earth in this world, so its people are constantly migrating to stay within its habitable zones. The walker Seth and his sider, Theo, are surveyors who scout the migration paths for their home city. Together, they make a discovery, and embark on a journey, that quite literally turns their world upside down.
That journey is not lacking for interesting turns and revelations, though these are only stimulating to the intellect, and to the base desire of the sci-fi reader to explore and map out new frontiers. The physical properties of this world are explored in detail, but its society has no real culture to speak of; they seem to exist only to rationalize their environment, which seems to be all the author is concerned with as well. Characters and relationships are crudely fashioned and bear no emotional fruit to engage the reader.
In other words, if you love geometry as much as Greg Egan does, Dichronauts is your kind of novel. I can’t make a higher recommendation without a more complete experience.

Thanks to Netgalley and the publisher for providing me with this ARC.

[David Swanson · Rating 3 out of 5]

Fascinating but a bit too clever

As with most novels by Egan, I'm very glad I took the time to read it, and equally as glad that I never have to read it again.

Egan has a knack for introducing us to genuinely alien worlds and viewpoints, and this novel is no different. Introducing exactly how things differ from our viewpoint is possibly a spoiler, but suffice to say that the afterword explains things extremely clearly... if you can grasp the mathematics.

The downside to all this is that the plotting is rather elastic, with no consistent sense of pace, and I couldn't really connect with any of the characters.

I feel almost bad rating this at three stars, but it's not a book I can recommend to everyone. In this case, a three star rating means you should pick it up if hard but abstract sci-fi is your thing, or possibly if any of the above has made you curious. Otherwise, I think you'll find it a hard book to enjoy.

[Gendou · Rating 5 out of 5]

This is another one of Egan's marvelous alternative universes with different physical laws from our own. It's an adventure story following some young denizens of a nomadic civilization. Through the course of their adventure the reader slowly (and with a lot of hard work!) comes to understand the bizarre world they inhabit.

In this fictional universe there are two dimensions of time. I know this sounds a lot like Egan's orthogonal trilogy, but it's very different. This creates some very bizarre physics where things fall up 45° slopes and animals are stuck looking in one orientation their entire life. The protagonists are a symbiotic organism of north/south facing + east/west facing species. This leads to interesting social problems when one or the other species tries to dominate.

One challenge reading this book is that the written word is insufficient to communicate some of the mathematical concepts necessary to fully grasp what's going on. These descriptions are metered out slowly over the course of the book, too. So it's really hard to be fully engaged. It's the kind of book you have to read more than once to fully understand what's going on. I managed by doing some research after reading the first chapter, which spoiled some of the book, but allowed me to better follow the book starting over from page 1.

It's possible to just enjoy the story without trying to understand the science, but that would leave you with a mediocre experience at best. This book isn't for the casual reader. It's for fans hard science fiction. Hard as fuck.

[Tomislav · Rating 4 out of 5]

I read Greg Egan’s new hard-sf novel “Dichronauts” in kindle format. I have previously read and liked a half dozen of his novels, and so I was well prepared that this would be a challenging read.

Dichronauts physics - http://gregegan.net/DICHRONAUTS/00/DP...
Dichronauts world - http://gregegan.net/DICHRONAUTS/01/Wo...

The setting is the outstanding feature in this work, and it is described as a world in a universe with 2 dimensions of space and 2 dimensions of time, rather than the 3 and 1 of our familiar space-time universe. It turns out that is not really an accurate description, as the geometry is not 2-dimensional in the way of Flatland: A Romance of Many Dimensions. And the characters do not live in 2-dimensional field of time, but along a single linear time line. I think it would be more accurate to say the third spatial dimension is time-like, and able to combine with our regular dimension of time at an arbitrary angle. This leads to all kinds of non-intuitive and weird consequences – such as the surface of a world being hyperboloid rather than spherical. All readers should be aware that there is an Afterword in the book, that gives a brief overview of the physics of the Dichronauts universe and the resultant geometry of their world. But the information at the two links above is much better and includes dynamic illustrations. There is more yet at Greg Egan's website, but I found these two the most useful, and referred to them repeatedly during my reading of the novel.

The narrative never stops to explain the geometry of the Dichronaut world, but it is exposed through events, as the characters travel about, exploring. You see, there is a problem in this world. The hyperboloid world is revolving off axis with regard to the rotation of its hyperboloid sun (see the dynamic illustrations). The areas near and beyond the curve of intersection are uninhabitable, and the observable advance of that curve is driving civilization continually across the world towards a crisis of geometry. The main characters are the symbiotic pair Seth and Theo, who have become surveyors for their city, sent out to search for routes of future migration. The symbiosis is another logical consequence of the world’s geometry, as Seth can only see in one orthogonal direction and Theo senses the other (see the explanation of light cones). During a mission of “conventional” exploration, in which the strange and wonderful surface of the world of the Dichronauts is exposed, the small team of Seth/Theo and a few others discover a flaw of geological scale in their world. The second half of the book then steps the reader up to an even more extreme level of imagination, about which I will not say more here.

The challenge, of course, is understanding the setting. It is a world I had never imagined before reading this novel, but it seems to be entirely a thought experiment. Egan glosses over the physical consequences on particle physics – describing the world and the beings as if they were solid objects, when they would really be arrays of coordinated and intersecting probability fields (yikes!). And I feel there may be dynamic consequences not fully integrated into the way of life of the characters – such as taking advantage of extremely fast travel in diagonal surface directions.
Beyond understanding the setting, it would have been easy to populate the world of Dichronauts with stick-figure characters (metaphorically). There is some of that, as it is not really possible to empathize with beings so alien. But they do have feelings, some of them easily understood, and some of them speculative. After a while, when the characters get beyond the world regions of their own understanding, the adventure takes on a sense of fun reminiscent of Journey to the Center of the Earth.

It would be possible to read this without understanding the setting, just as an adventure in a crazy unpredictable world – but that would miss out on the premise that it is a predictable world. It is an accomplishment just to describe and write a coherent story in this world, and I enjoyed the ride. But it takes work, and that is not for everyone.

[Craig · Rating 3 out of 5]

This review was originally published at Book Learning.

Who knew geometry could be so confusing?

According to the biography on the inside cover, Greg Egan is a computer programmer. For those who are familiar with his work, if not his biography, that won’t come as a great surprise. Egan writes hard sci-fi, the sort of hard sci-fi that doesn’t take any prisoners and doesn’t bother to go slowly to let stragglers keep up. There is a certain clockwork logic to all his books that makes them feel almost digital in a strange sort of way, with plots that roll along with mathematical precision as if guided by some sort of literary computer program.

What I like about Egan though is that between all of the maths and physics, he occasionally manages to slip a punch under your guard and hit you right in the feels. Yes, The Safe Deposit Box, I’m looking at you. Over the years, his work has become more and more esoteric, detailing universes with entirely different physics to our own and looking at the ramifications of what those changes would mean for those living there. Dichronauts is the latest in this sequence, positing a world that’s even more bizarre than that portrayed in his “Orthogonal” series.

You see, in this universe, light can only travel in two dimensions, and the cosmic topology makes the act of rotation seemingly impossible, leaving the intelligent ‘Walkers’ locked into only being able to look east or west, the job of sensing to the north and the sound being taken by the ‘Pingers’, symbiotic creatures who live in the skulls of the Walkers and use a sonar-like ability to determine what lies outside of the light cone. The mathematics of it are, as far as my limited ability can go, impeccable enough, although I have a few problems with the plausibility of the scenario. How life, let alone complex intelligent life, could evolve in such a restrictive environment baffles me. The structure of space in this universe seems to offer no advantage, but imposes a whole lot of limitations on the characters that make even simple tasks an ordeal.

The book takes some time in world-building before it gets to the main “problem,” a seemingly impassable chasm in the path of the protagonists’ migrating city that will block their progress and lead to certain disaster for their society. From there the main characters engage on a Vernean journey to explore the abyss in the hopes of finding a way around or through it. The specifics of the story aren’t as important as Egan’s exploration of how the world’s geometry impacts on the characters though, and there are significant passages of infodump where the protagonists discuss the mathematics of their situation to one another.

So is it a good book? On balance, yes it is, but it’s not Egan’s best work. Part of the issue is that it’s pretty difficult to get your head around how to visualise the story, and this is not helped by Egan’s decision to use cardinal compass points to refer to dimensions in space and time that bear little relation to our own universe. I’m not sure how I would have written this, but using plain old “north” and “south” gives the impression of a relatively simple three-dimensional Cartesian space that just happens to be ‘dark’ in one of those dimensions, an understanding that breaks down completely after events in the second half of the book.

Egan also passes up some interesting opportunities to explore the biology and societal ramifications of the walker/pinger symbiosis. Early on in the book Egan teases what sounds like a fascinating story involving the Elena/Irina pair, and then completely abandons this plot line. For the most part when Egan starts a thread he eventually weaves it into the end, but I was particularly disappointed that we don’t find out anything about how that particular plotline ends.

Those whinges out of the way, Egan has managed to construct yet another strange yet mostly plausible universe and set a story in it that doesn’t sacrifice things like ‘plot’ or ‘story’ in favour of mathematics and world building. This doesn’t rise to the heights of Diaspora, but it’s yet another solid entry in Egan’s impressive body of work.

[Simona · Rating 5 out of 5]

I have tried to read this 3 times. Twice I have stopped after 3 pages utterly confused. Few weeks ago I read his science notes on it, started my third attempt and was hooked. (I would tip my nonexistent hat to anyone who read it without studying those notes first).

This book imagines a world of index (2,2). I know a little about differential geometry and the combination of (2,2) and DI-chronauts scared me. Really. A lot. The author really wants to create consciousness and beings in a universe which has no business existing at all (planets requiring infinite mass, two objects having zero distance, but not being at the same place) and even if it did the beings would face horrible obstacles (not being able to rotate 90 degrees, sawing fabrics where no thread can rotate more then few degrees, tripping in the wrong direction is a certain death).

Moving right along-like the author does- this book is certainly very weird even for Greg Egan, but it is definitely in the vein of his other works. The obligatory moral dilemma where the author clearly favors one side. Also few illogical choices of the characters that I really want them to make to play with the ideas of the book some more.

I would, traditionally, want to repeat my distaste for reviews which say that Egan should focus more on character development then on the ideas. Sorry people, I wish he does not listen to that advice :)

[Quinn Dougherty · Rating 4 out of 5]

very impressive! i thought it would've been nice as two books-- in one, a story about the physics of an infinite hyperboloid planet (and the edge cases where that model breaks down), and in another, a story about humanoids who rely on slug-oid parasites for more than half of their sensory input. But, for the sheer joy of being able to tell you this book exists, I'm ultimately glad both stories were fused into one.

it's not even that weird, he makes it relatable i swear.

Of course i could spend time getting to know the ficto-physics better (he wrote a companion for just this purpose), i feel like i know it well enough to follow the story but not well enough to be the characters and approach their decisions like i would reading a ratfic.

and yes, if you're wondering, he also has time for the social antagonisms and political economy of a humanoids-with-slugs-living-in-their-skulls civilization.

[Anna · Rating 4 out of 5]

The book was definitely an interesting read, from the point of "actual hard science fiction" instead of "a mix of romance and humans fighting each other in an unusual setting". If you are into the first type (which not necessarily means completely disdaining the second :D), and willing to invest some mental efforts into imagining the world based on different geometry rules than our own due to having two space-like and two time-like coordinates, and therefore different constraints and consequences from that, consider giving this book a try. And if you get weary of straining your geometrical imagination, there is still an option of getting along with the story relying on the description of impact which the story's events made on the protagonist :)

Some visuals on the author's site were immensely helpful for me to understand what he was trying to describe: http://www.gregegan.net/DICHRONAUTS/0... - might be useful to look at that before reading the book itself. The author tries very hard to be true to his model up to the tiniest detail, though sometimes his narration goes a bit too schematic, as if the characters are living within some elaborate computer game where they have to progress from one level to another.

Also, there is an interesting take on close symbiosis of two different sentient forms and the impact it could have on the ordinary lives and the society in general, including musings of "what could possibly go wrong there".

I wish the author would put a bit more thought into describing how the inhabitants of that world actually look (he gives hints, but not too many), because that's one of the reasons they look so sketchy: they differ from each other only by their names. (But that's where the other type of science fiction does a better job ;-) )

[S.J. Higbee · Rating 5 out of 5]

In order to be better able to visualise this world, my firm advice is to visit Greg Egan’s site at the link:
http://www.gregegan.net/DICHRONAUTS/D...
where you can discover how he came up with this intriguing creation and the inhabitants. Alongside all the maths, the world is also more fully explained before you plunge into this one. Inevitably, I discovered the site after I had completed the book and although I had picked up the gist of what was going on, it would have been helpful to have understood more of the complexities of the world and this remarkable indigenous species as I was reading it. However, if this story was simply about an enjoyably weird world and creature with little story or dry-as-dust descriptions bulking up the book – while I would doubtless have something enthusiastic and polite to say about Egan’s extraordinary imagination, I wouldn’t be nearly as excited about this one as I am.

For not only does Egan offer a unique world and alien race – he also provides a cracking adventure story full of tension and excitement right from the start through to the climactic ending. I quickly bonded with Seth and his parasitic companion Theo and enjoyed the tensions and teamwork evident in their linked partnership. What happens if the Walker host has a major quarrel with his Sider? This premise is also explored within the story. I stayed up far later than I should to find out what happens to this embattled species as Seth and Theo struggle to discover a river big enough to support the large city where he was born and bred.

I love this one. Brilliant and inventive, this book reminds me all over again just why I love science fiction so much…
10/10

[Ryan · Rating 2 out of 5]

Greg Egan might be my favorite sci-fi author (top-5 for sure), but this is both his worst book to date and a bad book overall. It is built around some completely novel physical geometry and universe dimensionality, poorly explained in the book (to have any hope of understanding, you'll need a guide on Egan's website), but that's fine. The problem is that is really the only positive part of the book; the rest is a stupid/weird morality story about different groups and coexistence (4000/800 had a simplified form of this as well; I hope Egan hasn't gone full SJW) -- the characters themselves are really shallow and unremarkable.

There is a moderate amount of focus required to understand the geometry of the world, but there is no payoff. Skip.

[Pavel Lishin · Rating 4 out of 5]

Gonna have to brush up on my Dichronaut cosmology, and re-read this one, I think. I'm still pretty unclear how Walkers' hands and fingers work.

There's some interesting parallels here with Peter Watts' Blindsight: The Gang's accusation of what 20th century psychiatry was up to, and Thanton.

[Sherron Wahrheit · Rating 3 out of 5]

This book reminds me a bit of Flatland, except the difficult science fiction world building is waaaay more difficult—in fact I felt like a Flatlander trying to comprehend the fifth dimension. I wasn’t very impressed by the story part where we understand their trials and tribulations. The plot in Flatland on the other hand was clever and socially relevant.

I guess I’d sum it up as too hard AND too easy.

[Chris Moorhead · Rating 3 out of 5]

As the book tag says, truly the strangest world in science fiction. The mathematical concepts are super interesting to me, but a lot of the events are hard to visualise because of the unusual geometry of the world. I had to take a pause half-way through because I just couldn't see it. Greg Egan's website to the rescue! The mathematical notes and diagram should probably be included with the audiobook and required reading before embarking on this bizarre journey! It gets marks for imagination, but the ending is kind of unsatisfying as they only achieve a partial solution to the problem they face throughout the novel. I was warned in advance that this was not his best work, but nobody can keep me away from non-Euclidean geometry. I will try some more of Egan's works, but definitely read the notes on his website first!!

[Paul · Rating 4 out of 5]

I really enjoyed this book, but your experience will be made much better by taking the time to read through the supplementary materials about the physics of this world, because it is very hard to understand from just reading the novel. This book is basically an intense, modern incarnation of the genre pioneered by Hal Clement's Mission of Gravity , and the tradition that (to a lesser extent) Larry Niven continued in books like Ringworld and The Integral Trees . An exploration travelogue set on a world very much unlike our own.

Greg Egan is all about ideas, so he really thrives with short stories; it's no surprise that a lot of Egan's books read like fix-ups of a few of his short stories, woven into a novel. There's definitely something of that in this book - he's not just setting a story on a world with weird physics, he also adds in an idea of a symbiotic relationship between two sentient species (each optimized for different axes), and throws in a first contact story in there. Still, to me it seems like he's doing a better job of interweaving the stories and making them feel like a single seamless novel.

It was very fun to inhabit this world for a while. I still don't totally understand all the aspects of the world here, but with a combination of the novel and the supplementary materials (particularly the interactive simulation), I think I mostly have a handle on it. I'm still not entirely sure that it makes sense that only light is affected by the "dark cone" phenomenon, or why it seems the characters perceive a single timelike dimension like space (in that they can move forwards and backwards in it), but one of them still acts like time in that it can only move in one direction. Hopefully I'll get more time to think about this when I'm reading a sequel (hint, hint) or playing through a video game version of this (a video game set in this world seems like it would be really fun).

[Lavinia · Rating 5 out of 5]

This is mind-strainingly weird sci-fi. No one I have described it to will read it, but I loved almost every minute: it's one of the few books I'd like to reread. The e-book I bought has notes at the end (which I didn't realize until I got there), but I was baffled by the physics of the mundane tasks described in the first chapter. The notes on Egan's website are very helpful and cleared up all the confusion. I think one gets more out of it if they've done the math themselves and internalized the physical rules.

There is a great deal of action, all described with the abundance of a martial arts thriller — but it isn't enough. Not every fan of lurid hand-to-hand combat knows how to fight, but on some intuitive level they do know how geometry works. I'll quote,

"What use is all that academic nonsense? Every child knows in their bones how geometry works!" And no doubt they did, until they found themselves in a land where it was geometrically impossible to place both feet on the ground."

The land(s) of Dichronauts very rarely allows one to feel grounded in anything.

The plot feels like it could be allegories for real-world problems, but the comparison very quickly breaks down. I really love sci-fi stories that come up with problems unique to that world instead of putting rayguns and nanobots in our world. On top of that, the main plot is of discovery: the story follows a team of surveyors as they explore the limits of their world.

I can't explain the physics of it better than Egan can, but suffice it to say that a theme of the book is climate change, or something like it. The way their "sun" orbits their hyperbolic planet, a habitable patch moves relatively quickly across the land. Buildings are built on wheels because they must be moved every so often; there's no argument that the local climate is changing. Their ability to do anything about it is limited, though: people don't even know whether the sun orbits their planet or vice versa, or whether their planet is infinite. Instead, the city-states of the book are constantly on the move, sending out surveyors to recon the land ahead and look for places to settle with enough water. While it sounds at first like a thinly-veiled metaphor, I assure you that it's much more foreign a problem.

The POV character Seth is a Walker, implied to have somewhat human anatomy (2 arms, 2 legs, caudal, 2 eyes on one side of head). His constant companion is Theo, a leech-like Sider who lives in a natural tunnel through Seth's skull. Theo and Seth can share the images Seth gets through his eyes and the images formed by Theo's pings (detection and ranging through a wide variety of sound frequencies). Together, they can navigate a world where light cannot move north or south and no one can turn to the side. There is significant development of the social ramifications of this dual life. Siders cannot move on their own and literally suck the blood of their Walker. Early in the book is a microcosm of the ills a Walker-Sider relationship can potentially suffer: any given pair tends to mate with another pair at the same time (or at least in coordination; gestation periods aren't mentioned) to ensure that every Walker baby has a Sider, and vice versa. Seth's sister decides to become pregnant by a Walker whose Sider her own Sider dislikes. She responds to this by attacking the symbiote in her own head: her Sider is rescued with Theo hears her (the gender identities of the pairs seem to always match up; it's possible that one or both species are hermaphroditic and the neuter one is assigned the gender identity of the other) screaming in a frequency Walkers cannot hear.

In this little segment, Egan foreshadows much of the rest of the book's social pains:
1) Siders and Walkers, despite sharing thoughts, may not necessarily act as one. In fact, many have different occupations, and it's common for a Sider to have an intellectual job for which their Walker is simply a manual assistant (like teaching, or anything that requires reading print).
2) The Siders of Seth's community have full personhood, at least on an intellectual level. Even so, it hasn't lost sight of old beliefs about Siders stealing Walkers' autonomy, nor the individual discomfort of realizing that they cannot be physical (and often intellectual) equals. Siders can also speak a Sider-only language in a Walker-inaudible frequency, which sometimes breeds distrust. Despite this, the idea of taking away a Sider's right to express opinions is considered abhorrent.
3) Some Walkers hate the company of their Siders: enough to try to remove them from their bodies. This doesn't kill either, but the Sider cannot live alone long without being fed blood. This, however, is crippling: in their often unpaved world, being side-blind can be fatal. An isolated community that Seth and Theo find in their duties as surveyors has found a solution: they eat a neurotoxic fruit, which provides just low enough levels of toxin to put Siders in something like a coma. In this state, they ping constantly, but never talk or intellectually engage. When this community joins with Seth's to explore a possible permanent solution for their migration, they're reviled — and hailed as liberators by those who hated their Siders. At long last, these Walkers are free of the tyranny of this inescapable second voice.
4) Siders can share an internal voice and sight with their Walkers, but they have no control over their Walkers' bodies and don't share their sense of touch. Yet where their Walkers go, so they must go. Ada, having spent her life without regard for her Sider, finds herself unable to walk and depending on its intellectual capacity. She must decide if she would prefer to explain to her now-intelligent Sider that she knowingly enslaved it for decades, or to spend the rest of her life in pain, carried around by people who value only her Sider.

Is this a metaphor for women's rights? Abortion rights? Illegal immigration? I thought about it, and I don't think that's it. Perhaps Egan meant to make some grander statement and missed. I prefer it this way, where the world is rich and bizarre enough to have problems that are wholly its own. Once I've done the math, I hope I remember to come back and read it again.

[Shhhhh Ahhhhh · Rating 5 out of 5]

I have mixed feelings about this book. It's a striking, novel concept. Describing the physics of a novel universe where tipping over means stretching out into infinity and dissolution, is necessarily novel. Wrapping my mind around the story happening in that strange space was a new and interesting sensation while reading. It definitely raised the difficulty level for me. Nevertheless, I found the basic, and archetypal, story to be fairly entertaining. In fact, the elements of their world, while necessitated by the constraint of the physics, by themselves would have made for a fascinating story. Kind of want to know how it would have ended.

[Stacey Kondla · Rating 3 out of 5]

I found this to be a challenging read and quite different from the Greg Egan books I have read previously. The math and geometry in this was a bit beyond me, but it was still an entertaining story with an interesting sociological story thread. I wouldn't recommend this as a starter to Greg Egan, but I'm glad I read it and will be interested to read whatever he comes out with next. Without a doubt this was the strangest world with a very strange society and strange life forms.

[Eric · Rating 4 out of 5]

Wow!! I don't think I will ever fully understand how the physics of these characters and their world works but I found this book to be very readable. It would be hard to recommend to people who are not into Science/Speculative fiction to a fair degree but if people are willing to take a leap into the unknown this is an interesting and mind bending world to experience.
My first Greg Egan read and I will be exploring more of his work.

[Odo · Rating 3 out of 5]

2.5/5.0

[Julie · Rating 5 out of 5]

Wow, now those are some weird aliens!

[Lindsay · Rating 4 out of 5]

This one gave me weird dreams.

[Baal Of · Rating 3 out of 5]

Like all of Egan's work, this novel is based on some mind-twisting speculative premises, in this case what would it be like to be in a world with 2 spacial and 2 time dimensions. I found it absolutely necessary to read the supplemental materials on his web site to help with comprehension. From that perspective, this book is good, however the story itself was unfortunately boring, and the characters seemed too human despite the fact they were symbiotic pairs, one of which lived in a hole in the skull of the other, with direct brain connections allowing them to "inspeak". This book reminded me a lot of Hal Clement's "Mission Of Gravity".

[Guilherme · Rating 3 out of 5]

This really should have been a comic or videogame, or anything with more of a visual component to it. I hadn't even realized it was possible to get myself this mixed up in regard to cardinal directions.