If you were to hand “Schild’s Ladder” to someone who had never read SF, they would likely run screaming from the room, possibly chucking the book at your head as they went. It’s the sort of book that has all the good and bad of SF all wrapped up in one really interesting package. It may seem at first glance that you need a Ph.D. in quantum physics to really appreciate this story, but if you have any familiarity with the genre, the “Sense of Wonder” will come through strongly even if you never went past Astronomy 101.

Much of science fiction deals in various, usually metaphorical ways with piercing the boundary between the known and the unknown – understanding the alien, going boldly, lifting the veil of the future. This book presents one of the most explicit realizations of that “boundary” concept you’ll ever see: an accidentally created region of space-time with not only different laws of physics, but no stable laws of physics at all. And it’s swallowing up our universe around it, expanding at half a light-year per year.

Our intrepid heroes, all of whom are post-human super-physicists, are tracking that boundary, trying to examine the space-time structure behind it. The debate rages: once we figure out what it is, shall we try to destroy it? Try to stop it in its tracks? Try to accommodate it? The characters are all basically mouthpieces for these different points of view. Given the nature of science, and of science fiction in the pre- and post-New Wave eras, the answer that the author supports won’t come as a surprise to anyone. If you’re not down with embracing the unknown, understanding it no matter what, then why are you reading SF? And once the barrier is breached, you’ll be treated to some of the most mind-bending descriptions of alternate space-time that you can imagine, embodying the concept: “To see the universe in a grain of sand.”

Egan’s post-humanity differs markedly from other depictions of the far future in the genre. The one similarity with your average post-human future is that we’re effectively immortal, and that has led to some serious conservative tendencies. On the other hand, we aren’t disembodied brains, we aren’t all living in virtuality (although we can if we want to), we haven’t converted all the matter in the galaxy to run our computer-selves (we don’t need to thanks to quantum computing), and we’ve completely given up gender self-identification (although Egan continues to use gendered pronouns for convenience). Everyone spends most of their time gender-neutral. When you meet someone you want to have sex with your bodies get together and decide what sort of mutually compatible genitalia to grow. It can be different every time, which keeps things from getting boring. Heinlein would be disappointed though, as Egan never explains how group sex would work.

I’m afraid that all this may make the book seem dry, but it isn’t. There are some scenes of real humor and warmth here, especially some dealing with sex between people used to being computer programs and people used to being embodied. Through the characters of the anachronauts Egan has some choice words for those who would imagine the gender wars projected out to the far future, and has some real fun at their expense. This book is by and for the science fiction aficionado. The plot is about ideas, and the characters are embodiments of ideas. But with his wit and gift for description, he brings us through even the most awkward multi-page infodumps to the fantastic stuff on the other side of the border with the unknown.