Wu Zheng found him by smell.

  Not in person — the Chen Xi Line was absolute. Little Abacus enforced it with the zealotry of a boy who had decided the most important job in the world was protecting his teacher from himself.

  The old man had prepared breakfast. Not the inn's standard fare. A meal cooked on his portable stove, from ingredients sourced at the river market the afternoon before.

  A soup. Clear broth. Noodles. Two eggs. A handful of green onions.

  A single drop of chilli oil that sat on the surface like a tiny sun.

  He placed it on the midpoint table and walked back to his end.

  "Eat," he called.

  Chen Xi retrieved the bowl. Carried it thirty-one metres. Sat on his floor.

  The broth was the colour of pale amber. It tasted of ginger, star anise, and the warmth of someone who gave a damn.

  "It's good," he called back.

  "Of course it's good. I have been cooking for four hundred and fifty years, and I have never once served a bad meal. Not in the graveyard. Not when the ingredients were spiritual moss and the carcass of a creature I could not identify.

  Cooking is the one thing that has never failed me."

  Stolen story; please report.

  A pause.

  "You threw your notebook."

  "How do you know?"

  "Sound carries. Also, Little Abacus measured the impact frequency and determined it was consistent with a bound notebook striking plaster at eleven metres per second.

  He says this indicates significant emotional distress."

  "The boy is too perceptive."

  "The boy is exactly perceptive enough. And he is worried. And so am I. And so is Su Yiran, who has not slept properly in a week because she is working on something she won't tell me about."

  The hallway was quiet.

  "I can't fix it," Chen Xi said.

  The words were the hardest he had ever spoken. Harder than the divorce papers. Harder than telling his graduate students the seven-year project produced nothing.

  "The constant I built the filter on isn't a constant. The ambient Qi is too structured. I can't distinguish what's mine from what's everyone else's. The mathematics I have aren't sufficient."

  "Then get better mathematics."

  "It's not that simple—"

  "Is it not? You are a man who came from a world with better mathematics. Xu Ling's manual proved this world once had them too, before they were destroyed. The mathematics exist. They can exist again."

  Chen Xi sat with this.

  "The Vortex Core was elegant," he said.

  "Yes."

  "The filter won't be. It'll be ugly. Probabilistic instead of deterministic. Approximate instead of exact."

  "And?"

  "And I hate that. I hate approximate solutions. I became a physicist because physics is the study of certainty."

  "And now you are in a world where the constants change and the equations depend on context."

  Wu Zheng's voice carried something Chen Xi had not heard in it before. Not sympathy. Recognition.

  "Welcome to everyone else's experience of reality."

  Chen Xi laughed.

  It surprised him — the sound came from somewhere behind the equations, somewhere Elena might have recognised. A wet, broken, genuine laugh echoing down a hallway.

  "You're right. I need to stop being a physicist and start being an engineer."

  "What is the difference?"

  "A physicist discovers the perfect equation. An engineer builds something that works."

  The Newcomer Assessment was twenty-three days away. He had a filter to build — ugly, approximate, and good enough.