The generation ship Peerless is suffering from a population explosion, and the only way to reduce the number of children is by drastically limiting the females' food intake. So population control consists of two barbaric choices: starvation, or suicide.
Trying to find a better way, a biologist starts experimenting with animals, and stumbles on a technique that radically alters the reproductive cycle. But while the advantages are obvious, there's a major drawback: while it spares women from their old role - reproduction without hope of survival - it will essentially wipe out an entire sex.
Amid the turmoil created by this new possibility, physicists on the ship are working to develop the technology they will need to complete the mission of the Peerless. One of the expedition's founders dreamed of discovering the Eternal Flame: a way to generate thrust without consuming any fuel at all.
The inhabitants on board the Peerless have some hard choices to make - and the wrong one could spell extinction for their entire race.
Readers unfamiliar with 2011's The Clockwork Rocket, vector mathematics, and the physics of light are likely to stumble through this dense sequel, which puts the "hard" in hard SF. The series' intriguing underlying premise is apparently that light in the "Orthogonal universe" can travel at different speeds even in a vacuum, but this is never spelled out in the text. The natural laws governing this universe are detailed only in an afterword (which itself follows three appendices) that's laden with sentences like "Because there is an upper limit on the frequency of light there is no ultraviolet catastrophe, and the spectrum predicted by classical physics is only slightly different from the true, quantum-mechanical version." Pages of clunky scientific discussion within the book proper also stifle the plot about an alien race's struggle to avoid the threat of meteorlike objects. Egan's tendency to bludgeon readers with esoteric detail will be a turnoff even for those who like fiction that assigns homework.