Translator: Dreamscribe
These days, Su-jeong rarely went home.
She spent most of her time in the lab and lecture halls, and whatever time remained was usually passed sitting in the library or a café.
Today was another one of those ordinary days.
A café overlooking the Charles River,
Su-jeong sat in a corner seat, staring down at her notebook.
A beautiful East Asian girl with silk-smooth hair draped over her shoulders, lost in thought, drew the glances of those around her.
Stage two,
Causal Inference.
The patterns were already more than enough.
Which intersections had accidents, which time slots saw delays spike, which variables moved in sync with them. Statistics didn't lie.
The problem was what came next.
'Why?'
Su-jeong spun her pen in a quick circle, then stopped.
There was a wide gap between correlation and causation. Especially so in a city-scale system. It was rare for a single cause to map neatly onto a single effect.
And it wasn't a change that appeared gradually, either. In most cases, the nature of things shifted abruptly, all at once.
'Step-like.'
Su-jeong jotted a small note in the margin of her notebook.
Up to a certain point nothing happens, and then the moment one condition is met, the entire system changes.
A signal difference of just a few seconds, a response delay of just one minute, one hour of daylight saving time.
These trivial differences stacked up until, at some moment, they pushed the city into an entirely different phase.
"This is tricky. Can I get it done by the end of this week?"
She had made a big show of promising to have it ready by the next meeting.
Su-jeong lifted her head and stretched.
Her body, stiff from sitting still for so long, cracked and loosened.
'Differentiation isn't going to give me an answer here.'
She finally understood why Seo-ha had chosen this approach.
There was no way to express this with continuous functions.
The shape of a city expressed in numbers was a massive staircase structure, carved up by laws and rules, by standards and thresholds.
'Still, it's doable.'
No matter how complex it got, she felt certain that if she got the structure right, she'd arrive at the answer.
Su-jeong closed her eyes and tried to organize the thoughts she'd had so far.
"Excuse me, are you majoring in math, by any chance?"
At the unfamiliar voice, Su-jeong turned her head.
Brown curly hair and gold-rimmed glasses. A man with an intelligent-looking face. As if to flaunt the fact that he was a Harvard student, a vivid red badge was pinned to both his notebook and his bag.
Su-jeong nodded.
"Yes, I'm studying math."
He flashed a bright smile.
"What a coincidence. I'm a math major too. The assignment looks tough, would it be all right if I helped you out?"
Su-jeong looked him straight in the eye.
Tall, a handsome face, confidence radiating from his gestures and his way of speaking. He probably hadn't even considered the possibility of being turned down.
"There's no need."
Perhaps because being refused was a novelty, he became even more interested in Su-jeong.
Glancing at his wristwatch, he said,
"Then how about this. It just so happens to be dinnertime. Come out with me now and we'll have a meal together. While we eat, we can discuss the problem you're working on. It's bound to be better than studying alone.
Or else..."
"Or else?"
"Does the assignment have a deadline?"
"Yes."
"If it's not too forward of me, I'll solve it for you right here. You only have to give me as much time as I saved you."
Harvard might not have a strong STEM image, but its math department was among the very best in the world.
If MIT was strong in applied math, she'd heard that Harvard placed greater weight on theory and philosophical depth. When Su-jeong showed a flicker of interest, the man took it as a sign of agreement.
"May I see the problem?"
He strode over and sat down in the chair across from her.
A mischievous thought began to creep up inside Su-jeong.
'What kind of face will he make when he cries? If I give him a problem he can't even touch, will he run away first out of embarrassment?'
On any normal day this would have been a little incident she'd laugh off, but days of sleep deprivation had summoned back the version of her who once struck fear as the Demon King.
Back then, crushing smug boys with math had been part of her daily routine.
'Good thoughts, good thoughts.'
Su-jeong shook her head, casting off the temptation.
"Thanks for the offer, but I'll have to decline that one too. An assignment only has meaning if you solve it yourself."
Screech.
As Su-jeong pushed back her chair and stood, he put on an admiring expression.
"Are you an MIT student?"
"Yes."
"I figured. My name is Ted. I have a feeling we'll be seeing each other again soon."
"Su-jeong."
She stated her name curtly, grabbed her bag, and walked away.
She was busy congratulating herself on holding back when a grating remark reached her from behind.
"Wouldn't you like to take my number, at least? Are you really sure you don't need my help?"
That same smug expression.
The last thread of her patience, which she'd been barely holding on to, finally snapped.
Su-jeong marched back and sat down across from him. Ted wore a look of expectation.
"I can't show you the assignment. There's a confidentiality agreement on it, so don't take it the wrong way."
A question flickered across his face.
"Confidentiality agreement?"
"Ted, if you're a math student, you must remember a few problems that gave you a really hard time when you were solving them. Am I right?"
"Of course. There's one I solved a few days ago. It was genuinely difficult, but I finally cracked it. Took me a whole week, though."
"That problem. Give it to me, right now."
"What?"
"Go on. If I can't solve it, I'll do whatever you want, dinner, a date, you name it."
Ted swallowed hard.
"You promise?"
He pulled a notebook from his bag and began writing out the problem.
Inwardly, Ted gave a small smile.
To think she'd react to a casual little joke like this. It was a charm he'd never encountered in other women.
Scratch scratch.
What he wrote down was a matrix problem.
Diagonalization, spectra, and iterated operations all bundled together in a fairly high-difficulty question.
'This is probably around undergraduate level.'
For her own college-freshman self, this would have been quite a struggle. But now? Not a chance. Did this person realize that setting a problem revealed one's own level far more starkly than solving one?
Su-jeong picked up her pen and wrote out the equations in a single stroke.
His expression turned to one of shock.
"Done. No need to check. I already know it's correct."
Before five minutes had even passed, Su-jeong put down her pen.
"Ah..."
He sat there with his mouth hanging open, unable to say a word.
"Were you trying to earn my respect through math? I won't deny it, I do like people who are good at math. Unfortunately, that doesn't seem to be you."
He stared intently at Su-jeong's face.
"...Now I remember. LOGIA Ranker Lim Su-jeong. A member of the MIT team that proved Minimum Energy Theory."
This was Cambridge.
Everywhere she turned, she saw students from Harvard and MIT. Most of them wore the same confident expression as Ted.
They considered themselves special and were itching for a chance to prove it.
Just like...
'The way I was before I met Seo-ha.'
Su-jeong decided to give him some sincere advice.
"I know one truly remarkable genius.
When I stand next to him, I feel like I'm nothing at all. All the knowledge I spent a lifetime building up crumbles under a single casual question he tosses my way. At times like that, it still makes me cry from sheer frustration, you know?
But at the same time, I find myself wanting to see the world he sees. This might be a little different from just wanting to be good at math. It means I want to become part of that lofty, fiercely demanding world he loves."
Ted gazed at Su-jeong in a daze.
"One thing I know for sure now."
"I'm glad."
Did my message get through?
Su-jeong nodded with a satisfied expression. There was always a higher sky above the sky, and math was the most elegant language for interpreting this world. So don't let your attention wander, and work hard.
This was the advice she'd wanted to give her own younger self, and it would surely help Ted too.
But what came out of his mouth was something she hadn't expected at all.
"What I know for sure is that I've been turned down. Just who is this enviable man?"
Su-jeong's face flared red in an instant.
"No, that's not what I meant!"
Su-jeong waved her hands, trying to hide her flustered reaction, but there was no concealing her face, flushed crimson like a ripe apple.
"Anyway, don't forget what I said!"
Su-jeong hurried out of the café as if fleeing.
A night breeze blew in from the river. The cool air seemed to soothe the heat on her face.
'Was that really how I'd been thinking of him?'
When they first met, he had been a wall she could never cross. After that, he'd become her most reliable colleague, and at the same time, the ideal she wanted to emulate.
'Since when, exactly?'
She looked back at what she'd just said to Ted.
And she was stunned.
She had as good as confessed that every effort she'd made up to now had, in the end, been a means to reach him.
The moment she had seen Gyeo-ul's name flash on Seo-ha's phone screen, her chest had ached with a sharp pang.
Su-jeong shook her head fiercely.
"It doesn't matter. Start with causal inference!"
If she could become an irreplaceable resource to him, she thought, everything would sort itself out.
Because then they'd end up spending the rest of their lives together anyway.
Her head was spinning.
Su-jeong walked quickly through the night streets. Equations jumbled together in her mind and swirled like a storm.
'Step structure, discontinuity.'
As she breathed in the cold air, the tangled threads seemed to unravel all at once.
Think of the city as a set of gears. Several small cogwheels turn, and in the end they drive a large gear. And the moment that large gear advances one tooth, every gear connected to it shifts phase simultaneously.
"Found it."
A smile spread across Su-jeong's lips.
Days of sleep deprivation, and the absurd little scene with the man named Ted, all evaporated from her mind.
Only one goal remained, the yearning to solve the problem Seo-ha had given her, and it took over her every thought.
Su-jeong ran.
It was the first time she'd ever felt such an urge to solve a problem.
Bang.
When she shoved open the lab door and rushed in, the familiar smell of coffee and paper greeted her. And, as always, Seo-ha was standing in front of the blackboard.
Without even turning his head, he continued writing equations densely across the board.
Breathing heavily, Su-jeong approached him from behind. Seo-ha paused, his hand stopping.
"What brings you here at this hour?"
The same indifferent voice as always. But Su-jeong knew. Within that indifference was mixed a trust and an expectation directed at her.
"I found the method."
Without even thinking to set her bag down, Su-jeong picked up a piece of chalk. And at a pace many times faster than when she'd solved Ted's problem, she began writing out equations.
"After partitioning the state space, we substitute the probabilistic transition at each level into the threshold function."
Scratch scratch.
The sound of Su-jeong's chalk echoed through the quiet lab.
Seo-ha watched her back with an intrigued expression. Logic unfurling without hesitation, a clean and decisive development.
After losing herself in it for a while, Su-jeong put a final period on the equations and turned around.
"Haa, haa."
She was breathing hard.
Su-jeong studied Seo-ha's face, gauging his reaction.
"So? How is it?"
Seo-ha's stiff expression slowly softened. And then, at last, he smiled brightly.
"It couldn't be better."
Su-jeong felt as though every bit of effort she'd put in up to now had been rewarded.
*****
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