Books by Egan, Greg

Egan, Greg. Dichronauts. New York: Night Shade Books, 2017. ISBN 978-1-59780-892-7.
One of the more fascinating sub-genres of science fiction is “world building”: creating the setting in which a story takes place by imagining an environment radically different from any in the human experience. This can run the gamut from life in the atmosphere of a gas giant planet (Saturn Rukh), on the surface of a neutron star (Dragon's Egg), or on an enormous alien-engineered wheel surrounding a star (Ringworld). When done well, the environment becomes an integral part of the tale, shaping the characters and driving the plot. Greg Egan is one of the most accomplished of world builders. His fiction includes numerous examples of alien environments, with the consequences worked out and woven into the story.

The present novel may be his most ambitious yet: a world in which the fundamental properties of spacetime are different from those in our universe. Unfortunately, for this reader, the execution was unequal to the ambition and the result disappointing. I'll explain this in more detail, but let's start with the basics.

We inhabit a spacetime which is well-approximated by Minkowski space. (In regions where gravity is strong, spacetime curvature must be taken into account, but this can be neglected in most circumstances including those in this novel.) Minkowski space is a flat four-dimensional space where each point is identified by three space and one time coordinate. It is thus spoken of as a 3+1 dimensional space. The space and time dimensions are not interchangeable: when computing the spacetime separation of two events, their distance or spacetime interval is given by the quantity −t²+x²+y²+z². Minkowski space is said to have a metric signature of (−,+,+,+), from the signs of the four coordinates in the distance (metric) equation.

Why does our universe have a dimensionality of 3+1? Nobody knows—string theorists who argue for a landscape of universes in an infinite multiverse speculate that the very dimensionality of a universe may be set randomly when the baby universe is created in its own big bang bubble. Max Tegmark has argued that universes with other dimensionalities would not permit the existence of observers such as us, so we shouldn't be surprised to find ourselves in one of the universes which is compatible with our own existence, nor should we rule out a multitude of other universes with different dimensionalities, all of which may be devoid of observers.

But need they necessarily be barren? The premise of this novel is, “not necessarily so”, and Egan has created a universe with a metric signature of (−,−,+,+), a 2+2 dimensional spacetime with two spacelike dimensions and two timelike dimensions. Note that “timelike” refers to the sign of the dimension in the distance equation, and the presence of two timelike dimensions is not equivalent to two time dimensions. There is still a single dimension of time, t, in which events occur in a linear order just as in our universe. The second timelike dimension, which we'll call u, behaves like a spatial dimension in that objects can move within it as they can along the other x and y spacelike dimensions, but its contribution in the distance equation is negative: −t²−u²+x²+y². This results in a seriously weird, if not bizarre world.

From this point on, just about everything I'm going to say can be considered a spoiler if your intention is to read the book from front to back and not consult the extensive background information on the author's Web site. Conversely, I shall give away nothing regarding the plot or ending which is not disclosed in the background information or the technical afterword of the novel. I do not consider this material as spoilers; in fact, I believe that many readers who do not first understand the universe in which the story is set are likely to abandon the book as simply incomprehensible. Some of the masters of world building science fiction introduce the reader to the world as an ongoing puzzle as the story unfolds but, for whatever reason, Egan did not choose to do that here, or else he did so sufficiently poorly that this reader didn't even notice the attempt. I think the publisher made a serious mistake in not alerting the reader to the existence of the technical afterword, the reading of which I consider a barely sufficient prerequisite for understanding the setting in which the novel takes place.

In the Dichronauts universe, there is a “world” around which a smaller ”star” orbits (or maybe the other way around; it's just a coordinate transformation). The geometry of the spacetime dominates everything. While in our universe we're free to move in any of the three spatial dimensions, in this spacetime motion in the x and y dimensions is as for us, but if you're facing in the positive x dimension—let's call it east—you cannot rotate outside the wedge from northeast to southeast, and as you rotate the distance equation causes a stretching to occur, like the distortions in relativistic motion in special relativity. It is no more possible to turn all the way to the northeast than it is to attain the speed of light in our universe. If you were born east-facing, the only way you can see to the west is to bend over and look between your legs. The beings who inhabit this world seem to be born randomly east- or west-facing.

Light only propagates within the cone defined by the spacelike dimensions. Any light source has a “dark cone” defined by a 45° angle around the timelike u dimension. In this region, vision does not work, so beings are blind to their sides. The creatures who inhabit the world are symbionts of bipeds who call themselves “walkers” and slug-like creatures, “siders”, who live inside their skulls and receive their nutrients from the walker's bloodstream. Siders are equipped with “pingers”, which use echolocation like terrestrial bats to sense within the dark cone. While light cannot propagate there, physical objects can move in that direction, including the density waves which carry sound. Walkers and siders are linked at the brain level and can directly perceive each other's views of the world and communicate without speaking aloud. Both symbiotes are independently conscious, bonded at a young age, and can, like married couples, have acrimonious disputes. While walkers cannot turn outside the 90° cone, they can move in the timelike north-south direction by “sidling”, relying upon their siders to detect obstacles within their cone of blindness.

Due to details of the structure of their world, the walker/sider society, which seems to be at a pre-industrial level (perhaps due to the fact that many machines would not work in the weird geometry they inhabit), is forced to permanently migrate to stay within the habitable zone between latitudes which are seared by the rays of the star and those too cold for agriculture. For many generations, the town of Baharabad has migrated along a river, but now the river appears to be drying up, creating a crisis. Seth (walker) and Theo (sider), are surveyors, charged with charting the course of their community's migration. Now they are faced with the challenge of finding a new river to follow, one which has not already been claimed by another community. On an expedition to the limits of the habitable zone, they encounter what seems to be the edge of the world. Is it truly the edge, and if not what lies beyond? They join a small group of explorers who probe regions of their world never before seen, and discover clues to the origin of their species.

This didn't work for me. If you read all of the background information first (which, if you're going to dig into this novel, I strongly encourage you to do), you'll appreciate the effort the author went to in order to create a mathematically consistent universe with two timelike dimensions, and to work out the implications of this for a world within it and the beings who live there. But there is a tremendous amount of arm waving behind the curtain which, if you peek, subverts the plausibility of everything. For example, the walker/sider creatures are described as having what seems to be a relatively normal metabolism: they eat fruit, grow crops, breathe, eat, and drink, urinate and defecate, and otherwise behave as biological organisms. But biology as we know it, and all of these biological functions, requires the complex stereochemistry of the organic molecules upon which organisms are built. If the motion of molecules were constrained to a cone, and their shape stretched with rotation, the operation of enzymes and other biochemistry wouldn't work. And yet that doesn't seem to be a problem for these beings.

Finally, the story simply stops in the middle, with the great adventure and resolution of the central crisis unresolved. There will probably be a sequel. I shall not read it.

August 2017 Permalink