The Henderson lab at eleven on a Friday night was exactly the way Sicheng preferred it.

He had stayed later than necessary, which was not unusual. The temporal variance extension had been resisting him all week — not in the way that meant the approach was wrong, but in the way that meant the approach was right and the implementation wasn't there yet, which was the more demanding kind of problem because it required patience rather than revision. He had learned, over four years, to tell the difference. He stayed late when the problem was the second kind. The quiet helped.

He had been staying later than necessary since Thursday.

He was not examining this fact. He was aware of it in the peripheral way he was aware of most things he had decided not to examine directly — present, noted, set aside. Thursday's methodology discussion had been productive. The phase transition argument had been finished, finally, two weeks after the seminar corridor and the half-completed sentence of *another time.* She had come. She had not replied to the note in the shared document. She had simply appeared at the conference room at the agreed time and opened her notebook and said: *the phase transition argument — where did you want to start?*

As if the note had been purely professional. As if *another time* had been exactly what it said.

He had matched her register, because that was the correct thing to do. They had worked for two hours. It had been, as he had predicted and simultaneously not fully prepared for, the most intellectually engaged he had been in a seminar context in longer than he wanted to calculate. She had been right about three things he had been wrong about and wrong about two things he was right about, and the ratio had felt, somehow, like exactly the right proportion for a conversation he would be thinking about for the rest of the week.

He was not examining why.

He had been in Edinburgh long enough to know that the city had a way of stripping things back — the long winters, the rain, the particular isolation of being far from wherever you had come from. People became more themselves here, or less defended versions of themselves, which amounted to the same thing viewed from different angles. He had arrived four years ago already defended and had built the defenses higher since, and had believed this was what the work required. He was finding, with increasing frequency, that he was less certain than he had been.

He looked at the temporal variance draft on his screen. The cursor blinked. Outside the lab's single window, Edinburgh was doing its late-October thing: rain on the glass, the orange of streetlamps diffused through wet air, the particular darkness of a city that went to bed earlier than it appeared. He had worked through enough Edinburgh nights to have stopped seeing them as atmosphere and started seeing them as conditions — temperature, light level, the likelihood of the building's heating system making its eleven o'clock sound.

The lab at this hour had a specific texture he had come to know well: the faint hum of the equipment that ran continuously, the building's ambient settling, the particular quality of artificial light against dark windows. He had spent more hours in this room after midnight than in most places he could name, and it had stopped being a place he came to and started being a place he simply was, which was either a sign that he had found his work or a sign that he had stopped distinguishing between work and the absence of everything else.

He was, as a rule, not inclined to examine which.

The heating system made its eleven o'clock sound.

He heard the floor door open.

---

Footsteps in the hallway were not unusual at this hour. The building had researchers who kept stranger hours than he did, and the night-time cleaning staff came through on a schedule he had memorized without intending to. He registered the footsteps peripherally and returned to the draft.

They stopped.

He looked up.

Through the Henderson lab's glass door panel, in the hallway with its after-hours half-lighting, Song Yutong stood with her bag on one shoulder and an expression he recognized as the particular stillness of someone who had arrived somewhere and was deciding what to do about it. She was looking at the lab door. At him, through the lab door. At the light that meant someone was still here.

He stood up. He was not sure he had decided to stand up. He opened the lab door.

"Whitmore lab is locked," he said. It was eleven-fifteen on a Friday. The Whitmore lab was not locked — her keycard would open it any time before midnight. This was a statement of fact in the way that certain statements of fact are really something else.

She looked at him. "I left my notebook."

He stepped back from the door.

She came in.

---

The notebook was in her bag. He did not know this with certainty but he knew it the way you know things that are true and unspoken and beside the point. She set the bag on the edge of his desk — not the visitor's chair, his desk, the particular claim of a space that said something about the decision she had made before walking through the door — and took the notebook out and set it down. Object acknowledged. Reason established.

She looked at his screen.

"Temporal variance extension," she said. Not a question.

"First draft. It's not—" he paused, which he rarely did "—cooperating."

She looked at it for a moment. Then she pulled out the other chair — not the visitor's chair on the other side of the desk but the second workstation chair, the one that belonged to whichever lab member sat closest to his desk — and sat down. She did this the way she did everything: without asking, because she had decided, and asking would have introduced a variable she had already resolved.

He sat back down.

"Show me where it's not cooperating," she said.

He showed her.

---

They worked for an hour. He had not planned this. He suspected she had not planned this either, or had planned it in the way you plan things you intend to be able to say you hadn't planned. The distinction mattered less as the hour passed.

She found the problem in the implementation seventeen minutes in — a boundary condition he had been treating as fixed that needed to be dynamic, which was, he noted without saying so, the same class of error she had caught in his seminar assumption. She did not say so either. She simply pointed at the relevant section of code and said: *this is where it's breaking,* and she was right, and he spent the next twenty minutes revising while she read through the theoretical framework document and asked questions that were useful in the specific way of questions from someone who understood enough to know where the gaps were.

Edinburgh outside the window. The heating system had gone quiet after its eleven o'clock noise, leaving the lab in the particular silence of a space that was old enough to have its own ambient sound — the faint hum of equipment, the building settling, the rain. He was aware of all of it with the peripheral precision that his mind allocated to things that didn't require active attention, which meant his active attention was on the work and on her, in proportions he wasn't calculating.

She said: "The phase transition modeling — if you use the dynamic boundary condition here, it resolves the instability in the third chapter predictions."

He looked at where she was pointing. She was right. It also meant the stability predictions he had been defending in seminars for eight months required revision, which was not comfortable, and which he was going to have to think about seriously, and which he would think about later because right now he was looking at the solution she had found in his work that he had not found himself and feeling something adjacent to relief.

"Yes," he said. "It does."

She nodded. Made a note in her own notebook — her actual notebook, the one she had produced from her bag alongside the stated reason for being here. He watched her write. Her handwriting was small and dense and moved fast, the handwriting of someone who had always needed to record more than they had space for.

He had not seen her handwriting in four years. It was the same.

He looked back at his screen.

They worked until midnight in the particular companionable silence that he had not allowed himself to remember was possible — the silence of two people doing parallel work in the same space without needing to fill it. He had forgotten what it felt like to not perform. To simply think, and have someone nearby who was also simply thinking, and have that be sufficient. There was a version of this he had known before, in a different city, in a different configuration of circumstances, and he had put it somewhere he did not go and left it there and told himself the leaving was the right thing.

Tonight it did not feel like something he had left. It felt like something that had been waiting.

He had forgotten, or had worked to forget. The distinction was becoming harder to maintain.

He was aware, somewhere in the middle of this, that he was not performing. It was the specific awareness of an absence — the weight he usually carried in professional interactions, the slight monitoring of how things sounded, the gap between what he thought and what he said. Tonight the gap was smaller than it usually was. He could not determine whether this was because work simplified things, or because it was midnight and the building was empty, or because of something more specific to the person sitting two feet to his left. He decided not to determine which.

At some point she said: "Your supervisor — Henderson — does he know about the boundary condition issue?"

"Not yet."

"Will you tell him?"

He considered the question with more seriousness than it probably warranted from a purely technical standpoint. "When I have the revision ready. Not before."

She looked at him sideways — the same look she'd given him in the conference room, the one that said she had noticed something she wasn't going to name. "You don't like showing him unfinished things."

It was not a question, so he did not answer it. She returned to her notes. He returned to the code. The rain continued.

---

"I should go," she said, at midnight.

"Okay," he said.

He did not say *you don't have to.* She did not say *I know.* These were the correct responses and he was aware, precisely and with some effort, that they were the correct responses.

She closed her notebook. Reached for her bag. Stood up, and reached at the same time for her jacket — the one she had taken off at some point in the hour and draped over the back of the chair without remarking on it, the small domestic gesture of someone who had settled in — and he stood up at the same moment, reflex, the same reflex that made him hold doors and step back from entrances, and the distance between them in the narrow space between his desk and the workstation was not eighteen inches.

Neither of them moved.

He was aware of the specifics with the involuntary precision that his mind reserved for things it had decided were important without consulting him: the distance, approximately twelve inches; her jacket in her hand; the rain still audible on the window; the fact that he had not touched his watch strap once all evening and his left wrist felt, in a way he was not going to examine, like that absence meant something.

He reached out and took the jacket from her hand. Not dramatically. Not slowly. Just — took it, the way you take something someone is trying to put on when you are standing close enough that it is easier for you to hold it than for them to navigate the logistics alone. She was still for a moment. Then she turned slightly, and he held the jacket while she slid her arms into it, and his hands were at her shoulders for exactly as long as it took to settle the jacket into place, and then they weren't.

She didn't turn back around immediately.

He didn't step back immediately.

The rain on the window. The lab's ambient hum. Twelve inches and a jacket and four years and whatever tonight was, which he did not have a name for yet. It was a small thing. It was not a small thing.

Then she moved forward and he stepped back and the space recalibrated and she picked up her bag from the desk.

"Thursday," she said. It was not a question. It was a confirmation of the standing collaboration meeting. It was also, he understood, something else.

"Thursday," he said.

She walked to the lab door. He followed to hold it. In the hallway, she went toward the lifts without looking back — the same as baggage claim, the same as every other exit she had made, except that this time he knew she would be back Thursday and the knowing was different from hoping.

He stood in the lab doorway until the lift doors closed.

---

He came back to his desk.

Her notebook was still on it. She had taken her bag, her jacket, her reason for being here. She had left the notebook. The stated object of the visit, sitting at the corner of his desk in the position she had set it down, exactly where he would see it first thing when he came in on Monday morning.

He looked at it for a moment.

He did not open it. He had not touched it. He had not needed to. He set it precisely at the corner of the desk, squared to the edge, where it would be visible from the door and where she could retrieve it without having to ask. It was a small, careful gesture — the kind he made for people he was protecting in ways they hadn't requested and might not want acknowledged. He was aware that he was doing it. He was aware that he had been making these gestures all evening in one form or another.

He sat down. Looked at the screen — the temporal variance extension, the revised boundary condition, the solution that had arrived in his code from a conversation he hadn't planned to have. He scrolled through the notes he had typed in the last hour.

Two of the ideas were not his.

He could identify the exact moments they had entered the draft: the first when she had said *this is where it's breaking* and he had started revising; the second during a tangent in the theoretical discussion that he had flagged as interesting and then apparently used without tracking the attribution. Her ideas, in his work, in the way that working closely with someone sometimes meant their thinking started to live in the edges of your own.

He looked at the two ideas.

Outside, Edinburgh was still raining. The castle would be lit above the Old Town — he couldn't see it from this window but he knew it was there, the same way he knew certain things that were outside his line of sight but present nonetheless. He had been in this city long enough that its geography lived in him at a level below navigation — a felt sense of where things were, even the things he couldn't see.

He thought: *dangerous.*

Not about her, exactly. About the specificity of it — the way her ideas had found their way into his draft without a clear moment of entry, the way the hour had passed without the weight of performance he carried through most interactions, the way his left wrist felt like the absence of something he had been using as a barrier without fully acknowledging it was a barrier. He had built his Edinburgh carefully. He had built it to function without exactly this. He had been very good at the building, and the building had held for four years, and he was becoming aware that what he had thought was a structure was actually a habit, and habits were not the same thing.

He saved the draft.

He closed his laptop.

He sat for another few minutes in the lab that still had her bag's indentation on the desk corner and the second workstation chair pulled slightly out of alignment and the notebook at the corner of his desk that she would come back for. He thought about the word *dangerous* and whether it was the right word for something that had felt, the entire evening, like the opposite of danger.

Then he turned off the light and went home through the Edinburgh rain, and did not resolve the question, and suspected he was not going to.

The notebook was still there when he turned off the light. He knew it would be there on Monday. He walked out knowing this, which was its own kind of answer, even if it wasn't the question he had been asking. The rain would still be falling in the morning. Edinburgh always was. He pulled his coat tighter against the wet and walked, and let himself not think about anything except the next step, and the one after that, all the way home.