Imagine one of Greg Bear's hard physics Science Fiction books, set in the far future, that doesn't take itself quite so seriously. If this is your idea of heaven, then you will love Greg Egan's Schild's Ladder.

Admittedly, it is not an easy read. Despite my B.S. in physics, his quantum mechanics is way beyond me. I have no way of knowing if he's even in the ballpark with his ideas. The Big Idea in this book plays with QM the way it should be played with, however. Not as some wimpy, literary "Oh, everything is uncertain, so Western determinism is wrong and evil and we can never know anything for sure," interpretation of Heisenberg's Uncertainty principle. Egan is positing a whole new type of space-time at the quantum level.

In the first few chapters of the book, you learn about the underlying theory of quantum mechanics that has held true in this universe for 20,000 years. Already Egan is operating beyond the speculative fringes of QM, positing a Quantum Graph Theory that underlies the structure of space-time. Quantum Graph Theory is real -- I found a few references to it on the Internet -- but Egan mentions in an endnote that his version is fictitious. [If you're interested in Egan's own explanations of his QM, check out his Web site --ed.]

A physicist named Cass gets research funding and time on the best equipment to try to create a type of Quantum Graph that the theory says should be stable. As is usually the case in this kind of physics, "stable" on this scale means: "it could exist for a few picoseconds." All the initial experiments go as expected. When the culminating Graph is created, however, it is stable on much longer timescales. It is actually a "novo-vacuum" that starts expanding at 1/2 light speed, swallowing and converting ordinary vacuum and matter as it goes.

600 years later it has swallowed many world systems (although only a few lives have been lost). Tchicaya, our protagonist, is on a research station, the Rindler, that is keeping pace with the expanding border. He wants to study this unknown form of space, to try to truly understand it. In their initial investigations, the researchers have not yet been able to probe beyond the borders of the novo-vacuum. While Tchicaya and a like-minded group (called the Yielders) want to study the novo-vacuum, there are others, the Preservationists, who understandably want to destroy it before it eats more worlds. The Yielders want to know whether or not there is life beyond its border before taking actions to destroy it. On board the Rindler relations between Yielders and Preservationists become ever more strained as the story progresses.

Despite these growing tensions, the researchers can afford to have a leisurely pace, since they are effectively immortal. Their brains are contained on quantum computers (Qusps) that can be backed up and stored. By setting the book 20,000 years in the future, Egan has given himself a lot of latitude. With such a timespan to work from, Egan could think of something he would like to do and decide that there's no reason why humanity wouldn't have figured out how to do that by then. And yet, he never actually breaks the laws of physics as we know them. For instance, these humans are still bound by relativity. By their system of immortality, however, they can get around these limitations. They can beam the information in their Qusps to another Qusp at the other end, and have that data implanted in a new body. Through this, they can travel at the speed of light. The Qusps can also be recovered from back-ups, or even a dead body. They are the human equivalent of an airplane's "black box." Death is not a thing to be feared in this universe, and the result is a pretty laid-back society with a lot of tolerance.

The day-to-day functions of this society are prominently displayed through Tchicaya's youth. On Tchicaya's home planet, many people live their whole lives in one place. Travelers like Cass and Tchicaya are somewhat rare. They miss so much in transit between worlds, even at the speed of light, that it can be hard to adjust. Cass was reluctant to do so, but was driven by her need to seek out the best research equipment available. Tchicaya, however, has embraced this lifestyle, but not without some regrets about what his life could have been like.

There is another type of post-modern human in this book, exemplified by the character Yann. These are acorporeals, who live their whole lives in computer systems, putting their Qusp into a body only rarely, if ever. I'm not entirely sure that I buy the exchange of original theorems as a form of courting, but the existence of such humans is a logical extrapolation of the Qusp system. Why bother with the body when your entire being is really only a computer anyway? Yann spends part of his time on the Rindler embodied, mostly for the sheer novelty of it, and partly to make it easier to interact with his embodied colleagues. When bodies get a little short on the station, he gives his up and goes back into the computer network. Let me add here that Egan's sex scenes are (a) hilarious, (b) not obtrusive and (c) far beyond anything that mainstream authors who talk about "post-modern sex" could ever dream of.

Those academic theorists who speak of post-modern humanity should really read this book. Their conceptions of how humanity can change are so limited! They think of a time when cosmetic surgery can make anyone look however they want, when sex will be destigmatized and merely physical. Some of them look ahead and see a race of supermen coming, or stultifying conformity, or complete anarchy. All of them are much too constrained. In Egan's world, new bodies are grown or modified to the specifications of the owner. The Qusps, it is implied, have more or less replaced the brain entirely. Biology has always been one of the most flexible aspects of science fiction, being that the field has never developed firm, mathematical laws in the ways physics has. Egan uses this vagueness to play with what we can define as human. If your body can change overnight according to your sexual preference of the moment, what does that imply? Are you still human if you choose to renounce the flesh altogether? Egan paints a future where one can revel in the possibilities of infinite human variety, yet all are still recognizably human. Let me say that in a book that is relatively short compared to many novels on the shelves today, he has painted the broad strokes that suggest, to the active imagination, a future that would be a ton of fun to live in.

Egan has written a good plot, with strong pacing, fun characters, and an atmosphere of lightness that makes it an easier read for those of us who don't have a combined Ph.D. in Math and Physics. The dialog he writes flows easily, even when it is used for long blocks of math and science explication. He never falls into the trap of having one side of the conflict be simply "bad guys." Here both sides have well reasoned and sympathetic positions. Rational actors are almost always more fun to read about than 2-D good vs. evil types. For example, at the start of the novel when the novo-vacuum is expanding rapidly, my sympathies were firmly with the Preservationist camp. Although Tchicaya was clearly the progatonist, I couldn't relate to his arguments immediately. However, as the story progressed, the Yielder argument, presented by Tchicaya, became more persuasive:

". . . if it's possible, I'll fight to preserve all the same things as you. But if all we're going to do with our precious embodiment is cling to a few warm, familiar places for the next ten billion years, we might as well lock ourselves into perfect scapes of those planets and throw away the key to the outside world."

Although Egan treats characters on both sides of the conflict sympathetically, he evinces somewhat less sympathy for his own contemporaries. He enjoys making fun of the limited humanity of the 21st and 22nd centuries, and how unenlightened we are compared to people living in a future like his. I think that that is a little unfair, considering that he's enlightened enough to write about it, but I can accept it. Humans always look back on their ancestors as little more than rock bangers, and wonder how they could possibly have survived. That is one of the very human traits that his far-future characters have retained.

In sum, although this book is not for those who grow woozy at the sight of hard physics, for those who are willing to open their minds to the universe Egan has drawn, it should be a very rewarding and enjoyable experience. His sense of humor and fun more than offset the difficult concepts that he asks you to accept. I really loved this book. I look forward to going back and reading his previous works as well as any that come out in the future.

Karen Burnham has a B.S. in physics from Northern Arizona University. She has somehow managed to harness this for two job endeavors, both as science fiction reviewer and as a radar analyst. She lives in Long Beach, California, with husband Curtis and cat Godzilla.