Dichronauts

Lore & Background

In the world of Dichronauts, two types of beings live in symbiosis: Walkers, who can only see to the west or east, provide mobility; Siders, leech-like creatures running through their skulls, provide sight to the north and south. Every city is in permanent migration to follow the sun's shifting orbit and the narrow habitable zone it creates. The Walker Seth and his Sider Theo from the city of Baharabad join an expedition to the edge of the habitable zone to map safe routes. They encounter a river with the city of Thanton, where Walkers seemed to have used poison against their Siders. Seth's sister Elena got pregnant pushing her Sider Irina to abandon her, leaving her side-blind with a hole in her head. Theo calls through Thanton in the Sider language and suspects Sleepsiders, prompting him to ask Seth about Sleepwalkers. Both return to the expedition, where a vote decides against returning to Baharabad and for more explorations. The expedition reaches a cliff without another side or bottom visible, suspecting the end of the world.

Reader's Guide

Dichronauts is significant as a hard science-fiction work that rigorously explores the consequences of a single sign change in the metric signature of flat spacetime—from (-,+,+,+) to (-,-,+,+). This alteration leads to a universe where rotations in one chronological direction are precluded, objects can roll uphill or not fall over when oriented suitably, negative kinetic energy exists, a fourth state of matter appears, and planets are hyperbolic with three separate surfaces. The novel is the cosmological inverse of Egan's earlier Orthogonal trilogy, where characters perceive a space dimension as time; here, they perceive a time dimension as space. The mathematical basis, including the effect of two dark cones in opposite directions, is detailed on Egan's website, along with an interactive applet. The work stands as a thought experiment in speculative physics, demonstrating how fundamental laws can shift with a single mathematical change.

Did You Know?

• The novel describes a universe with two time dimensions, one influencing spatial perception.
• Planets in the Dichronauts universe are hyperbolic and have three separate surfaces.
• The mathematical difference from our universe is a single sign switched in the metric signature.

The Architecture of a Two-Time Universe

The foundational premise of Dichronauts rests on a universe containing two temporal dimensions rather than one. One of these dimensions governs the characters' experience of time as we understand it, while the second operates as a spatial axis that shapes how they perceive and navigate their environment. This arrangement carries a profound constraint: rotation is impossible along the second temporal axis, meaning inhabitants cannot simply turn to face a new direction. This limitation forces an extraordinary biological solution—two species must exist in symbiosis to achieve even basic omnidirectional awareness. The altered physics extend far beyond perception. Objects can roll uphill when oriented correctly, and the usual tendency to topple over disappears under certain alignments. The universe also harbors negative kinetic energy and a fourth state of matter, concepts with no analogue in our own physics. Even planetary geometry is transformed: worlds are not spheres but hyperbolic shapes, each possessing three distinct surfaces rather than one continuous one. Egan has elaborated on these worldbuilding details on his personal website.

Symbiosis as Survival — Walkers, Siders, and the Wandering Cities

In the world Egan constructs, no single organism can perceive its surroundings fully. Walkers, the mobile species, see only toward the west—or the east if they reverse their orientation—while Siders, leech-like creatures dwelling in the Walkers' skulls, supply vision to the north and south. This interdependence is not optional; it is the biological prerequisite for navigating a world where rotation is forbidden. The social structures that emerge are equally remarkable: every city is locked into perpetual migration, tracking the sun's shifting orbit and the narrow band of habitable conditions it produces. The novel follows Walker Seth and his Sider Theo, residents of Baharabad on the river Zirona, as they join an expedition to chart safe passage along the edge of the habitable zone. Their journey brings them to the city of Thanton, where evidence suggests Walkers once turned poison against their own Siders, and introduces the concepts of Sleepsiders and Sleepwalkers—beings whose symbiotic bond has been severed. The expedition ultimately votes to press deeper into uncharted territory rather than return home, only to confront a cliff whose far side and bottom are both invisible, hinting at the edge of their world.

A Single Sign Change — The Mathematics Behind the Fiction

The entire alternate physics of Dichronauts rests on what is, mathematically, an extraordinarily modest alteration. In our universe, the signature of flat spacetime is (−,+,+,+). In Egan's world, that signature becomes (−,−,+,+). One sign has been flipped, and from that single change the entire edifice of different physics follows. Egan illustrates the principle by restricting the discussion to two dimensions. In our familiar Euclidean plane, the scalar product of two vectors (x₁, y₁) and (x₂, y₂) is simply x₁x₂ + y₁y₂, corresponding to a (+,+) signature. Switch to a (+,−) signature and the same operation becomes x₁x₂ − y₁y₂. That one minus sign is enough to reshape geometry, to make hyperbolic rather than spherical shapes the natural form of planets, and to make rotations impossible along one axis. The elegance of the construction is that Egan does not add new forces or new particles; he changes the metric and lets the consequences cascade. The result is a universe where objects can roll uphill, where kinetic energy can be negative, and where a fourth state of matter exists—all derived from the same one-sign shift that governs the Walkers' inability to turn.

The Cosmological Mirror of the Orthogonal Trilogy

Dichronauts is best understood as the cosmological inverse of Greg Egan's earlier Orthogonal trilogy, which comprises The Clockwork Rocket (2011), The Eternal Flame (2012), and The Arrows of Time (2013). In the Orthogonal universe, the characters perceive a spatial dimension as though it were time, and the strange physics that result—time-like behavior along one axis of space—drive the plot and the worldbuilding. In Dichronauts, the relationship is reversed: the characters perceive a temporal dimension as though it were space. In both cases, Egan alters only a single metric signature and then works out the full implications with rigorous consistency. The two works thus form a pair of mirror images, each exploring what happens when the boundary between time and space is shifted by exactly one dimension. This structural symmetry is not merely a clever literary device; it underscores Egan's broader project of demonstrating how deeply the geometry of spacetime is woven into every aspect of physics, biology, and culture. A reader who has followed the Orthogonal trilogy will recognize the same methodological discipline in Dichronauts, applied to the opposite sign change, producing an equally alien but equally coherent universe.