Let me borrow a page from Greg Egan's playbook and include a diagram with this review:

This lays out exactly who needs to avoid Egan's Orthogonal trilogy based on the hard alternate cosmology physics contained therein: Everyone should be fine, unless you're the sort of person who absolutely hates math and science and also isn't willing to skim while reading science fiction. If you enjoy the math, you'll have a great time with this book, and if you don't enjoy the math but are OK with skimming, there is also plenty here to keep you engaged. This is a generation starship story, and the characters wrestle with their biological imperatives and limited resources as much as with fundamental physics. The physics is mind-blowing: in this rigorously worked out alternate cosmology universe, the characters gain a more thorough understanding of the quantum mechanics of their universe, and how these could allow them to make a coherent light engine (a bit like a laser) that might enable them to turn their mountain-sized generation starship (in fact, an entire mountain launched into space) and go home.

But what else is going on here? This is the second book of a trilogy. In the first book, The Clockwork Rocket (2011), we learn about the basic physics of the Orthogonal universe, the biology of the aliens that live there, the societal and political milieu in which the characters operate, and about an existential threat that may annihilate their planet. The answer to the threat is to launch the mountain, christened the Peerless, to take a journey that could last for generations for those on board, but will be over in less than five years for the observers back on the planet (the reverse of relativistic time dilation in our own universe). We see the struggles of the characters on board the spaceship to overcome the initial challenges and ensure that the ship has a viable future.

The Eternal Flame begins two generations later. The most crucial problem facing the crew of the Peerless is curtailing population growth. In normal circumstance, this species reproduces by the male triggering the female to split into four parts, each of which becomes a new person. The female ceases to exist, and the male raises the children. Women can try to stave off the partition as long as possible, by avoiding triggering with their co (the male they were raised with) and by taking a drug to prevent spontaneous fissuring. However, having four children would mean that the population would double (or close to it) each generation, and there isn't enough food for that. Instead women have been intentionally starving themselves, hoping that when their time comes to fissure that they will only have two children instead of four—this starvation is a constant background distraction that makes it harder for them to focus on their professional tasks.

There are two main plot threads here: in one, "hard" scientists (physicists, astronomers, and chemists) are trying to solve the fuel problem and turn the ship around. They find and stop an asteroid worth of Orthogonal matter (think of it almost like anti-matter in our physics), and then make breakthroughs in quantum physics that may allow them to harness the power of light. In the other thread, the "soft" scientists, biologists, learn about how their own species exchanges information—with infrared light signals instead of with chemicals such as DNA. They use this information to potentially change how they reproduce, and chaos ensues as people worry about their future when something so fundamental is changed.

Egan develops almost all of the ideas in the story through dialogue. Some people may say that when the dialogue occurs the action grinds to a halt. However, it's clear that in these novels, the dialogue is the action. It would make more sense to say that the occasional action set-pieces (running away from a matter/anti-matter collision, capturing an arborine racing through a zero-gravity forest, rescuing a hostage from kidnappers on the exterior of a spinning mountain) interrupt the important conversations that push scientific understanding forward. Towards the end of the book, one chronicler complains that people’s recollections of their conversations have been tidied up, that it’s better to record them as it happens, with all the messy bits intact:

"Does it really matter?" Patrizia wondered. "The techniques that work will be repeated, the results we proved will be taught and retaught. Does anyone need to know how much we blundered about, getting there?"

Onesto said, "Imagine the time, a dozen generations from now, when wave mechanics powers every machine and everyone takes it for granted. Do you really want them thinking that it fell from the sky, fully formed, when the truth is that they owe their good fortune to the most powerful engine of change in history: people arguing about science." (p. 361)

In generation starship stories going back to Heinlein's "Universe" in 1941, a common feature has been the stagnant society that has been isolated so long that they've forgotten where they came from, or that the ship is not the world. There is no hint of that trope here: everyone on board knows where they came from, where they're going, and what has to happen in order for them to get back to their home world. It's not an easy road, and even within one volume there are many false starts and places where the science gets more confusing before it gets simpler. But no one has any doubt about their goals.

More than any Egan story to date, the books of the Orthogonal trilogy place science in a broader social context. For this alien species, men and women have drastically different biological fates, and that has significant consequences for their societal roles. In The Clockwork Rocket, the contraceptive drug that delayed spontaneous fission was officially banned. In The Eternal Flame, several iterations of the female/male dynamic are played out. The astronomer Tamara is kidnapped by her co, who is afraid that by traveling to the Orthogonal asteroid she will die and he'll lose the chance to have children borne of their mother’s flesh. He emphasizes that she is only "borrowing" her flesh from her mother and from their children. Carla and Carlo, on the other hand, are trying to prolong Carla's life as long as possible, with Carla being one of the key physicists of her generation and Carlo a biologist working on the reproduction issue. Although they both want children eventually, Carlo is stricken at the thought of losing her. After Tamara escapes and her co is locked up, she starts a relationship with a man looking for a co-stead after his co has died. He's not opposed to the new reproductive technology in principle, but flatly says that if she decides to go that way that their relationship will no longer be worthwhile to him—he wants traditional children, and if that's not what she's going to do, he'll find someone else. Along the way there are Councilors who seek election and want to promote or hide scientific achievements. This same Council keeps control of scarce resources, and must be convinced to support various research avenues, both physical and biological. Larger decisions are made by popular vote of the town hall style, with speeches from the different sides being presented to the crowd. (This approach to collective decision making is also seen in Schild's Ladder (2001), but there it takes place in a self-selected community of scientists.)

Compare this with earlier novels such as Distress (1995), which is set at a scientific conference on a remote island in the Pacific. In that story the protagonist learns a lot about the anarchic government of the island, but that has little bearing on the primary plot. The conference itself is surrounded by various protesting factions, but they are largely caricatures of intellectual fads that Egan presumably found frustrating in the 1990s. The main physicist in that story is Violet Mosala, a black woman and mother from South Africa, but her race and national origin likewise have little bearing on the story, and her status as a mother balancing work and career isn't really mentioned at all. Egan has come a long way in understanding and portraying the different forces that affect women in the sciences, and the need of science to operate and argue for itself in a broader social context.

Since he wrote Diaspora and "The Plank Dive" in the late 1990s, we can see Egan's work as attempting to make a statement that the passion for and understanding of science can be every bit as intense, meaningful, and exciting as an action-packed space opera or introspective character drama. In this quest he is swimming upstream against the constant tide of Western culture that appreciates what technology has to offer but regards pure science as uniformly dull and difficult. As seen in the diagram at the top, this book has a lot to offer those who are willing to buck that tide and invest some time in opening themselves to what pure science, placed into a social context where it is of critical importance, has to offer.

Karen Burnham is vocationally an engineer at NASA's Johnson Space Center, and avocationally a speculative fiction reviewer. She edits the Locus Roundtable blog, and she can be emailed at karen.burnham@gmail.com.