Incandescence by Greg Egan

when there were no people at all.
    Early in the third shift of her journey, Roi stopped near a fork in the tunnel and took a weight measurement. The navigation sign told her the lengths of the tunnel's three branches, and where they led. Her measurement told her that she was slightly more than halfway to the Null Line.
    At first she thought this meant that she was roughly on schedule, but on reflection she realized that the news was far better: by Zak's count of levels crossed, she was well ahead. She reviewed the maps Zak had given her to guide her to their meeting place. She'd wandered away from his suggested route, but she wasn't far off course. She set off again with her energy redoubled.
    Though her eyes were adapting to the ever softer light, they couldn't render the change imperceptible. Roi was used to the rock around her being noticeably brighter when she looked in the garm-sharq direction, with a characteristic drop in intensity as she turned away from that all-pervading beacon. Here, the distinction had become much subtler, and every contrast that depended on it was equally diminished. It was not that people or plants had ceased coloring and complicating the light, but many of the cues she had grown accustomed to were missing. When her team-mates working in the crops came to the edge of a field and changed direction, she could see patches of brightness temporarily imprinted on their carapaces, a record of their earlier orientation that took a few heartbeats to fade.
    The vegetation was unmistakably sparser now, but so were the people. Roi could see none of the signs of a serious food shortage, such as unpalatable plants lying around half-eaten; desperate people would chew on anything, but most weeds tasted so bad that it was impossible to swallow them. She had to stay alert to spot enough food for herself, but that was only to be expected when she was moving through unfamiliar territory, away from the abundance she was used to.
    As the shift wore on, the pleasant buoyancy she'd felt since the halfway mark began to mutate into something disconcerting. It was no longer just implausibly easy to ascend a steep tunnel; she noticed herself beginning to grip the floor by making her claws adhere slightly, in the same way she would grip the ceiling in order to walk upside-down. Sometimes her lightness even felt like the product of an upward force, the pull of an invisible assailant opposing her weight and attempting to dislodge her. When this happened she'd freeze on the spot, waiting for the bizarre tugging sensation to subside.
    She was not yet in the Calm as cartographers defined it, but the air was still and silent to the limits of her senses. The tunnels were not quite barren or deserted, but the flatness of the light magnified the sense of solitude and rarefaction.
    As Roi encountered strangers, usually in twos and threes, she greeted them and tried to guess their purpose. Few people stayed long in the Calm. Couriers and other travelers passed through by necessity, and the sick and injured sometimes spent time close to the Null Line in the hope that the conditions there would speed their recovery. The couriers crossing between the quarters were easy to identify, and she met one male limping along with a gash across his side, but many of the travelers remained difficult to categorize. No doubt some were briefly escaping their own work teams and had decided to experience the restorative powers of the region, or even just its novelty. In one small chamber she saw a trio of youths simply playing, clambering around on the ceiling and then releasing their hold and slowly drifting down.
    As she approached the rendezvous point, Roi began to wonder how she might locate Zak, given that she'd be arriving a full shift earlier than he was expecting her. Despite having started the journey doubting the motives behind his invitation, she was eager to see him now, and she couldn't bear the thought of merely killing time while she