Sunday, September 17



By Greg Egan

This is perhaps the strangest book I have ever read.  The premise is simple enough: Seth and Theo are newly graduated surveyors from the town of Baharabad.  Their town has a problem - the River Orico is drying up, leaving them without a reliable water supply, and the next-closest river is already heavily settled and unlikely to welcome a new town.  So it's up to the surveyors to head out and find a new place for the people of the town to settle.

First strange thing: Theo is a brain slug.

Second strange thing: Seth is blind to the north and south.  He relies on Theo's use of sonar to ping things in those directions.  In fact, everyone is blind to the north and south - they can see east and west, and up and down, but can't see anything in the so-called "dark cone".

Third strange thing: This is because light doesn't travel in those directions.

Fourth strange thing: Seth and Theo live on one side of a hyperboloid, infinite in all directions but with finite surface gravity.  The Sun "orbits" that hyperboloid, slowly baking everything behind its orbital path to ash, so that all the world's inhabitants - even the vegetation - must continually migrate to new and more hospitable lands.

Then it starts getting weird.

If you read Flatland or The Planiverse, the authors make it clear that they're describing a universe with only two spatial dimensions.  With Dichronauts, it's more complicated than that, because their universe is four-dimensional just as ours is; it's just that instead of three spatial dimensions and time, it has two spatial dimensions and two temporal ones.

That is, north and south are a timeline just as future and past are.

And that means, for example, that the people in the book can't turn around - they can't rotate left-to-right, though they can flip upside down and stand on their hands - any more than you or I can turn pastwards and walk into yesterday.

And in the entire book, no-one turns around.

The rest of it pales into comparison beside that, at least for me.  Water flowing uphill, our heroes falling off the edge of the world, the sex life of brain slugs...  In the entire book, no-one turns around.  Everyone in the story is facing east the entire time.

Egan is a mathematician, and has created a web site explaining in detail the geometry involved.  Little of this is given directly to you in the story, though the characters do know they live on a hyperboloid, and assumed it was infinite right up until they fell off it.  They don't discuss north and south in terms we'd understand, only in terms they understand, and you're left to figure it out.

Oops, spoiled that.  Sorry.

Still, recommended if you like weird hypotheticals that play out as relatable stories and not just as mental exercises.  If you're new to Greg Egan I'd suggest perhaps starting with Permutation City or Schild's Ladder instead; they're not necessarily better but are more accessible.  (The middle part of Schild's Ladder is a bit dry, all research and politics, but the third part where the heroes finally enter the alien universe is a pure delight.)

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Apple pies are delicious. But never mind apple pies. What colour is a green orange?

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