Nearly two years after first starting the series, we now come to the final book of the trilogy by Greg Egan. This brings to an end the journey of the Peerless and its inhabitants across many generations as they look forward to reuniting with the homeworld. I believe that this volume has the least mathematics and physics of the three but makes up for it with a conception of free will that is philosophically very mind-bending.

Six generations have lived and died on the Peerless since its launching and the time has come for the turnaround, when the mountain makes a U-turn to return to the homeworld. Its inhabitants don’t yet know how to save the homeworld from the Hurtlers but are confident that they can find a solution within the next six generations. However a faction argues that they have diverged too far and that they owe nothing to the ancestors. Attempting to return would be dangerous as their arrow of time would be against the arrow of the orthogonal cluster. They propose instead that they explore the possibility of colonizing one of the orthogonal worlds instead. The conflict threatens to become violent while at the same time, the scientists realize that they can create time-reversed cameras that allow them to receive messages from the future. As a compromise a four-person mission is launched on a small ship to study the feasibility of living on one of the orthogonal planets while on the Peerless, the system of sending messages from the future to the past is constructed.

The book’s blurb makes it sound like the central conflict here is between those who still feel an allegiance with the homeworld and those who believe that the Peerless should make its own and look out for themselves. One might also reasonably suppose that figuring out  how to actually save the homeworld upon their return would be one of the major challenges covered here. Unfortunately it turns out to be a bit of a bait and switch as the vast majority of the book is concerned about another kind of conflict: those who want to build the messaging system that allows the Peerless to receive news from the future because it will help to avoid certain dangers and those who fear that the system will rob them of any agency as the future knowledge will trap them into inevitable decisions. This may sound like a mere philosophical debate to some of us but the travellers take it seriously, moreso even than the conflict over the changes to how they reproduce as covered in the previous book. It begins with a bomb attack that kills off the engineering team working on the time-reversed cameras and threatens to escalate into an all out civil war that will tear the mountain apart.

I won’t pretend to fully understand the mathematics behind it all but the same phenomenon that allows this messaging system to work also makes it so that the orthogonal worlds no longer behave as if they were made of antimatter on their altered trajectory but instead experience time in reverse. Again this is no mere theoretical construct but a physical phenomenon with real effects. When the Surveyor ship arrives on the orthogonal planet Esilio, from the planet’s perspective, they are actually leaving. When objects with two opposing arrows of time interact, strange things happen. As the ship lands on the planet for example, one character notes an unusual amount of dust in the airlock and realizes that it must have come from the planet as from its point of view the airlock has just closed and the ship is departing while from the character’s point of view, the ship is arriving and the airlock is just about to open.

There are fewer grand scientific discoveries in this novel and so there is less technobabble and exposition. It’s pretty cool however to note all of the new technology they now possess including light-driven engines and photonics in place of our electronics which enable computing, automation and all kinds of fancy sensors. Instead there’s a lot of discussion on how future knowledge creates a stable time loop and its effects on individuals and society. There are also more action scenes which makes this book an easier read. Unfortunately I find Egan’s choices on which scenes to spend precious pages on describing in detail to be rather questionable. Two action scenes in particular get a lot of attention: one in which they need to intercept a rogue gnat and another in which they need to repair a gash in the Surveyor after it has been grazed by a Hurtler. These may be important events but nothing new is learned during these scenes and they aren’t terribly exciting. I mean Egan has a whole page on his website describing the mathematics of soft interception but it doesn’t feel like it matters much in the grand scheme of things. On the other hand, I was dismayed to find the pages slipping by and they have not gotten to the long awaited reunion with the homeworld yet. It turns out that it is relegated to an epilogue of about ten pages.

Overall I can’t deny that this trilogy has been an epic journey. The description of the reunion may be short but the frisson of thrill as the first traveller to return meets Eusebio and the ancestors come to terms with the immense debt that they owe the generations who have known only life on the Peerless is priceless. Still I think this is the weakest of the three books. The action scenes are a poor substitute for the scientific discoveries of the previous volumes and the time travel stuff is a bit too confusing and ambiguous for me. As usual, there is so much potential for powerful emotions but Egan’s choices actually diminish the power of the narrative highs. I don’t regret spending my time on the trilogy but I would recommend it only to those who fully appreciate the physics and mathematics underlying this fictional universe.