his theoretical work and Robert Millikan for his painstaking experiments—though Millikan appears to have been trying to refute the theory! The photoelectric effect refers to the release of electrons from a metal surface in a vacuum when the surface is struck by light of various frequencies; the electrons can be collected and their rate of ejection from the surface measured as a current flowing along a wire. The abrupt cessation of this photoelectric current when the frequency of the light striking the metal falls below a critical value supported the idea that light could only be absorbed and emitted in discrete amounts whose energy was proportional to the light’s frequency. Since it required a certain amount of energy to tear each electron away from the metal surface, unless each individual quantum of light, or photon, carried that minimum energy the light could not produce a current.

The Orthogonal version works somewhat differently: rather than absorbing light to gain energy, the surface is stimulated by the incoming light to radiate light itself, which is accompanied by the production of conventional energy. Also, more than one quantum of light is needed to bridge the energy gap between bound and free luxagens, since a smaller gap would make the material unstable.

With no electronics at her disposal, Carla can only observe the tarnishing itself, along with the scattering of light by the luxagens released into the vacuum. The unexpected way in which free luxagens interact with light echoes another landmark experiment in our universe, in which the X-rays scattered by free electrons in graphite were found by Arthur Compton to betray distinctively particle-like behavior.

Despite all the difficulties they face, the scientists of the Orthogonal universe do have one advantage: the mathematics of quantum mechanical spin fits into a beautiful geometrical framework that they would have had good reason to explore, long before the discovery of quantum mechanics itself. In their universe, four-dimensional vectors are naturally identified with the number system that we call quaternions (see Appendix 3 for more details). Remarkably, quaternions can also be used to describe entities known to us as spinors, which correspond to particles such as electrons in our universe, or luxagens in the novel. Having a ready-made mathematical system that can encompass both vectors and spinors offers a powerful short-cut to insights that took many years to achieve in our own history of quantum mechanics. For this insight I’m indebted to John Baez, who explained to me how spinors can be viewed as quaternions.

Although the word “magnetism” appears nowhere in the novel, most readers will recognize Patrizia’s ideas about aligning the spins of luxagens in a solid as something closely analogous to the creation of a permanent magnet. Just as an electrostatic force that pulls in one direction over macroscopic distances is impossible in the Orthogonal universe, the same is true of magnetism, so there is no long historical tradition of familiarity with this phenomenon. But amazingly enough, the quantum subtleties that Patrizia discovers to be dictating the alignment of spins are even more crucial to the existence of permanent magnets in our own universe than in hers! Under our rules, the magnetic force between spinning electrons encourages them to adopt opposite spins and cancel each other’s magnetic fields, and it’s only the quantum effect that we call the “exchange interaction”—which relies on the way different combinations of spin affect the average distance between electrons, and hence the average electrostatic repulsion between them—that allows a substance like iron to hold a powerful magnetic field.

The “optical solids” described in the novel might sound reminiscent of the “optical lattices” that are used by real-world researchers to trap and study atoms at extremely low temperatures—but in fact these are very different systems. In the Orthogonal universe, the hills and valleys of light’s electric field can be made to move slowly enough that charged particles can be trapped in the valleys and carried along with the light. A combination of three light beams can sculpt this “energy landscape” so that the valleys confine the trapped particles in all three dimensions.

In our own universe this is impossible: charged particles could never keep up with a traveling light wave, and in a standing wave—where the intensity of the light forms a fixed pattern in space—the electric field is still oscillating in time, with each valley becoming a hill, and vice versa, hundreds of trillions of times per second. But while an optical lattice can’t trap charged particles in its ever-changing electric field, it can nonetheless exert subtler kinds of forces. These forces relate to the intensity of the light rather than the direction of the electric field, so they retain a consistent direction over time and can be used to confine electrically neutral atoms.

Supplementary material for this novel can be found at www.gregegan.net.

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