Agata dragged herself back to her cabin and harnessed herself to her desk. She looked up at the pictures she’d brought: Medoro, Serena, Gineto, Vala and Arianna, scattered among the colourful childish sketches that Azelio was sharing with her. If she’d commandeered the Surveyor she could have flown in a loop right back to Ancestors’ Day on the Peerless. So far as she could tell there was nothing in the physics that would forbid it – so long as she didn’t try to cut corners and make do with a semicircular route, arriving as antimatter and spoiling the party. But she hadn’t seen herself anywhere else in the crowd that day, staring longingly at her friends – and if a visitor from the future really had joined them in her absence, Medoro had done a very good job of keeping it a secret.

She looked away. Nostalgia passed the time, but it needed to be rationed. And if no one else was celebrating the Surveyor’s parallelism with the home world, she might as well forget it and focus on her work. Though Lila had given the vacuum-energy problem to one of her students, Pelagia, Agata had decided to pursue it independently, in the hope of preventing her brain ossifying from disuse. With a wildly unfair eight-year advantage over her rival it wasn’t impossible that she’d return with a worthwhile contribution of some kind, but she hadn’t told Lila about her plans, sparing herself the weight of any expectations.

So far, she was still grappling with the notion of the vacuum. She’d read the definitive treatment by Romolo and Assunto, who’d adapted Carla’s wave mechanics to the study of fields, but all they’d really cared about was predicting the results of particle collisions. They’d deliberately sidestepped all the distracting cosmological issues, and – apart from Yalda’s insight that the cosmos had to be finite in order to prevent exponential surges in the light field – it did make sense that none of the results of any small-scale experiment should depend on whether the cosmos was a torus, a sphere, or some four-dimensional analogue of a thrice-knotted pastry.

Since all of the old-school field theorists’ measurements depended on changes in energy, rather than any absolute scale, Romolo and Assunto had been free to set the vacuum energy to zero by decree. They’d certainly understood that the true value was a difficult quantity to pin down – so they’d vaguely sketched its origins, and then subtracted it out of all their other formulas so they could concentrate on the remaining parts that were more mathematically tractable and contributed to nice, tangible things like the rate at which positive and negative luxagens annihilated each other in their experiments at the Object.