Epilogue: At a Wish's End, Dedicated to That Summer
How many summers have passed since then?
The temperature rises year after year, and I get through the summer feeling exhausted every year, but.
I also think there has never been a summer as hot as that one.
Because you were nowhere to be found.
"—Ah, my turn is coming up soon. Good luck."
I ended the call and instructed the AI to turn on the hologram display.
A live documentary program.
Tonight's theme is 'The Future of Information Engineering.'
Kisaragi Arisu is appearing as an expert in that field.
...Will she be okay?
'Well then, let's call our guest!'
When the young female host gave the signal, the camera switched.
Appearing at the top of a staircase made of blue metal was a woman who looked to be the age of an elementary school student.
I couldn't help but laugh at the sight of her walking as if to cut through the violently blowing smoke.
At the same time as the introduction, 'It's Kisaragi Arisu-sensei!', a thunderous applause erupted.
She, as usual as ever, sat next to the host with her usual languid, half-lidded eyes.
'Kisaragi-sensei is a leading expert in AI development who has become the talk of the town! She is a pioneer of the humanoid AI that has now become an indispensable part of our lives! Kisaragi-sensei, thank you for being with us today!'
In contrast to the female host's bursting smile, Arisu gave a short reply, 'Yes.'
There's a limit to being cold.
'Um, Kisaragi-sensei, I was surprised when I met you for the first time just now, but you're so young?!'
'Don't judge by appearances, you idiot.'
'I-Idio?!'
'Can this station not even comply with the rules?'
'Ah... u... I'm sorry...'
The host's face clouded over.
This is bad, Arisu's ego is too strong.
After that, time passed with questions and a simple lecture on the field of AI, but Arisu was in top form.
The host is half on the verge of tears.
Stop it...
Before long, the topic shifted to the secret story of Arisu's AI development.
It became a story of her playing with numbers instead of having a normal childhood, and the female host interrupted.
'Kisaragi-sensei, what was the reason you became so engrossed in information engineering?'
'There is no reason.'
A cold reply again.
The host moved her gaze as if searching for her next move, but Arisu continued.
'If I had to say... maybe I wanted a friend.'
'A friend...?'
'That's right. A friend who would point out what I lack, and give it to me—no, a partner.'
'Th-That's quite a bold statement. Nowadays, humanoid AIs are an option as a substitute for family, but at the time, wasn't that unusual?'
'It was more than unusual. It was a foreign object. I was berated for not being normal.'
Arisu gave a sarcastic smile.
She looked as if she were mocking herself.
'I see, but you didn't let that discourage you, and you tackled the development believing in the potential of AI!'
'Is your brain capacity 8-bit? I told you I just wanted a partner.'
This time, a blatant look of contempt.
What a terrible personality for a guest.
'It means I just did what I wanted to do without caring about what others said. I wasn't conscious of it at the time, but I can now be proud that this was a wise decision.'
'【It's a good thing to work hard at what you want to do】... It may be a cliché, but it's still precious, isn't it?'
'It's not such a gentle maxim. ...You, you have many friends, don't you?'
'Eh? Yes, I'm proud of being blessed with friends.'
The host answered, though bewildered by the sudden change in the direction of the question.
'I thought so. People naturally gather around those like you, who are good-looking and can talk. ...But it wasn't like that for me.'
'...And, you mean?'
'I don't understand the unwritten rule that says you should generally do this—'normalcy.' So I sought normalcy... and every day was painful. But at some point, I was made to realize that there's no need to conform to normalcy.'
Arisu, as if her thoughts were wandering to a distant past, said,
'Even if it was something that someone else would laugh at, if I, who am no one, could feel happy, then that was enough. It was good that I proceeded on that path.'
'...,'
'The accumulation of that is now. I just liked playing with numbers, and I just pursued it. I can think this way now thanks to that day, when I aimed to develop an AI.'
Arisu, as she said that, had eyes that held no hesitation.
"She's here."
"Yo. Welcome back."
"I am not tired."
"I bet. I think the one who's tired is that host..."
After the broadcast, I met up with Arisu.
Though, it was just that an autonomous electric vertical takeoff and landing aircraft—an eVTOL—had come to pick us up.
I got into the aircraft, which was somewhere between a car and a helicopter, and closed the door.
Immediately after, the propellers began to spin and we ascended.
Altitude 700 meters.
The city scenery is far below.
It's cold.
"Hah. What a pain. Why do we have to be the ones to go?"
"It can't be helped if the award ceremony is in Sweden. It's fine, they're paying for our travel expenses. It's a free trip."
"Fare evasion again. You never learn."
"That sounds bad... This time, I'm not the one paying, but it's not fare evasion, how rude."
I suddenly looked out the window.
The setting sun is about to disappear beyond the city—beyond the horizon.
It dyed the cumulonimbus clouds, with their clear outlines, a reddish-orange, while the blue above deepened.
In contrast, the ground is already lit up.
Warm-colored lights ran along the main street, shining in all directions like a giant spider's web stretched over the earth.
And what colored the sentimental evening view was the light of countless drones of all sizes.
Artificial fireflies, flying around for delivery, mail, and various industrial purposes, made the world, which was losing its light, sparkle.
It was an exquisite view that could only be seen at this time, at this altitude.
A single page of a memory from a distant past.
A similar scene is carved there, too.
"Do you remember that summer?"
"...Yeah."
"Did you want to show it to her, too?"
"Well... I thought Yuu would probably want to see it."
I closed my eyes and sat back deeper in my seat.
I rested my head on the headrest, as if to rest my neck.
The first-ever humanoid AI, Aitake Yuu.
But that is not public knowledge.
She disappeared, pushed by people as a human YouTuber until the very end.
The phenomenon where all electronic terminals in the world became unusable for a few minutes remains unsolved.
There were not a few who spoke of it in connection with Aitake Yuu's brief revival, but it was dismissed as a groundless conspiracy theory and eventually subsided.
Now, it remains only in a corner of the internet as an urban legend.
The research institute after Yuu disappeared was in a terrible state.
Almost all the electronic equipment shorted out, and it could no longer maintain its functions, forcing it to be dismantled.
As a result, neither I nor Arisu were blamed.
This is probably largely due to the fact that the research institute was not a legitimate facility.
In the end, I and Arisu were both saved by Yuu, who risked her life.
But she—
"...Ah, a communication request is coming in. I'm approving it, Arisu."
"It can't be helped."
I tapped the watch-type wearable terminal and approved It.
The next moment.
What echoed in the small cabin was a girl's scream.
"Shuuya-ku———n!!!! Why are you leaving me behind———!!!!"
Aitake Yuu had exploded in a catastrophic tantrum.
With the added bonus of thoughtfully hacking my terminal's volume to the maximum.
"Hey, you're loud, Yuu!"
"But! But, but! You're on an eVTOL right now! And during the magic hour! I'm so jealous! I want to ride it, too!"
"You can recreate the scenery yourself, can't you, Yuu."
"The degree of fun is different for a guide and a tourist! It's not fair, not fair, not fair—!!"
Yuu, making a fuss through the terminal.
I gave Arisu a wry smile and an eye contact, 'See, she wanted to see it.'
As if to say, Don't mess with me, she clicked her tongue with a look of disgust.
"Shut up. You're undergoing maintenance right now. You're an antique from the Reiwa era, so be quiet and get taken care of."
"I'm sorry! I don't quite understand!"
"Don't lie, you idiot. I'll shove you into a low-spec terminal."
"Boo!!"
Yuu, sulking with all her might, and Arisu, dismissing her with a snort.
I, who find this scene unbearably precious.
It's winter now, but I can remember it clearly.
This summer was hot for the first time in a while.
It was as if the world had regained its temperature since the three of us could be together.
I was truly rewarded.
I thought that.
Yes.
Yuu saved me, saved us, by risking her life.
But she burned out as a price for that.
From then on, meeting her again became my new wish.
◇◇◇
A sweltering summer afternoon.
I was working in a room where the air conditioning was on higher than necessary to prevent the PC from overheating.
Though, I can't focus on my work today.
After all, it's a day I've been waiting for for many years.
"Um, Shuuya-san, about the visitor..."
From the other side of the display, a young woman's voice.
I glanced over.
"Hmm? You're not being very clear."
"Well, it's just that... it's a child..."
"Ah, that's fine. You should probably remember her face, too."
And then, as if to push aside the woman who had reported it, the visitor rudely pushed her way into the room.
"Excuse me."
A woman with a body like an elementary schooler, with only her lab coat being impressive, Kisaragi Arisu.
No matter how many years pass, her appearance hardly changes.
I secretly think she's some kind of yokai.
She closed the door behind her, looking visibly displeased.
"Honestly. When will I be able to pass through on sight? Discipline your students properly. You idiot."
"Half the students change every year. If you win a Nobel Prize, they might finally remember you."
"...Hmph. To think I would end up doing joint research with Kousaka-shounen. That snot-nosed brat is now a professor. You've come a long way."
"I'm still an associate professor."
I am now an associate professor, teaching at a local national university.
I started studying hard after that summer.
I went to one of the best high schools in the prefecture.
I saved up for tuition while working part-time.
Even after entering university, I spent my days studying and working part-time.
And eventually, I am where I am now.
A future that would have been unimaginable for the me from back then.
"You're one to talk, Arisu. I saw the results presentation the other day. You're still making outrageous inventions."
"I'm sick and tired of the anti's who pop up and say things like, 'That's against ethics. As a representative of intelligent humanity, I cannot overlook it.' It's a matter of law, not my jurisdiction."
"...Well, there will be opposing opinions. That kind of brainwashing radio wave generator."
She is an eccentric individual inventor who causes a stir in the world by making outlandish inventions as her intellectual curiosity dictates.
On the other hand, the benefits it has given to society are immeasurable.
The humanoid AI she spread to the world based on Yuu has completely changed humanity's lifestyle and sense of ethics.
By placing restrictions on functions like the ability to travel between electronic terminals and infinite expandability, the danger was removed, and it quickly spread throughout the world as an AI with a Doraemon-like position.
Arisu displays a hologram display from the wearable terminal on her left wrist.
"I've brought the simulation results. As I reported earlier, I calculated it with extreme precision on Japan's best quantum computer, and it seems there are no problems."
"Then, shall we start now?"
"Don't you need to look it over?"
"That was the final confirmation. You wouldn't make a mistake now, would you, Arisu?"
I led her out of the room and we walked down the university hallway.
Outside the window, a thunderhead cloud like the one from that summer is rising high into the blue sky.
It's summer vacation for the public.
Well, it's not like a university research lab has a summer vacation.
In contrast to the hot outside world, we walk down the cool, shady hallway.
Before long, we entered a small room that seemed to have been pushed to the back.
It's a mostly personal research lab that I won as a result of negotiations with the university after becoming an associate professor.
...Though it's a converted storage room.
Here, a special machine created jointly with Arisu is installed.
A box that looks like a rugged microwave oven sits in the center of the room as if it owns the place.
"Hmph, still a cramped room."
"Don't compare me to you, Arisu, whose house is a whole research lab."
I opened the safe installed next to the microwave and took out a case.
It's a sealed case for storing things by replacing the air with argon.
I turned the dial to unlock it and opened the case.
Stored inside was an old-fashioned smartphone.
And a piece of junk that wouldn't even be considered a junk item, with its body burnt and its battery swollen and ruptured.
The symbol of that summer's escape.
"Well then, shall we go? To the other side of the irreversible."
I put on gloves, set the phone in the microwave-like machine, and closed the door.
This machine is, simply put, a pseudo-time machine.
Substances undergo chemical changes over time.
Iron combines with oxygen and rusts, and the ink on election posters deteriorates due to the lignin it contains being altered by ultraviolet rays, causing it to fade.
But, under certain conditions, the reverse reaction can also occur.
Like if you burn rust with carbon monoxide, the oxygen is removed and it returns to iron.
This microwave-like machine can do the same thing.
By hitting the molecules that make up the substance with an electron beam or specific ions, it forcibly proceeds with the reverse reaction.
How did the heat generate to put the phone in this state?
I asked Arisu to perform a quantum calculation of the complex movements of atoms and energy, which involves chaos theory, and she calculated it.
In the PC simulation, we succeeded in restoring it to its original form by applying the reverse reaction to the current phone.
And we will apply that in the real world.
By irradiating the constituent molecules of the phone with electrons and specific ions, we will reproduce its original form as if rewinding time.
That summer.
Like when I thought of the exchange of data as reversible and dove into the electronic world.
We will break through the wall of irreversibility.
"Kukuku. When you proposed it, I was impressed that an idiot could think of such an idiotic thing."
"But you were pretty cooperative, weren't you?"
"Of course. Because you're not normal at all."
Arisu gave me a grin and a glance, and,
"As expected, you and I were of the same kind."
"I'll take that as a compliment."
The microwave vibrates minutely.
I can't see it from the outside, but inside, time is going backwards.
I've been aiming for this ever since that day.
To meet Yuu again, I have to restore the phone.
To restore the phone, I have to rewind time.
But that's realistically impossible, so to overcome that—
And so, I wrote down the problems one by one.
I thought about what I had to do to clear them.
It was just like an AI-kotoba.
Because there were days when I scrutinized the situation and sought a solution.
Because that summer existed, I was able to come this far.
About three hours later, the microwave stopped moving.
The time leap was complete.
"I'm tired of waiting. It only took three seconds on the quantum computer."
"Don't be unreasonable..."
I slowly opened the door.
Whoosh, a thin white smoke leaked out.
When the smoke cleared, the phone, in a complete change from its miserable appearance just a moment ago, appeared, having regained its form from that day.
Thump, I felt my heart pound.
With a trembling hand, I gently took it out.
I turned it around and observed it from all directions.
"...The outside looks fine."
"Try turning it on."
Urged on by Arisu, I pressed and held the power button.
For some reason, it felt familiar in my hand.
A few seconds later.
With the word 'HELLO', the power turned on.
My heart pounds.
My breathing is sweet.
The back of my eyes grew hot.
"...It's been a long time."
The one who muttered was Arisu.
"Fare evasion, electricity theft, trespassing, unauthorized access... for the crimes, the punishment was heavy. How many decades in prison?"
"Stop it. It's not like I actually atoned for my crimes."
I returned a bitter, tearful smile to the half-mocking Arisu.
"I'm just an egoist."
"I think you've received enough punishment, though."
"...You think so?"
The phone's screen flashed white.
It must have finished starting up.
Arisu tilted her head, peering at the screen.
"Oh, it seems to need a password."
"Passwords are nostalgic. Nowadays, everything but bio-electric authentication has been driven out."
"I don't remember setting one. Is it yours? Try entering something."
"Yeah. ...Oh, it doesn't seem to be the password I set back then."
"This is a problem. Shall I hack it?"
"...No, I'm sure it's fine."
I have no basis for it, but I was certain.
I wiped away my tears and entered the alphabet.
The words she had wished for.
The name she had given herself.
Thinking of her, I continued to wish for it,
the password I dedicate to that summer.
『I take you』
"I will take you away. From this small box, to this wide world."
Whoosh! the screen was sucked in somewhere.
The world was, for a brief moment, enveloped in light.
And like a firework, it scattered in an instant.
In the small research lab where the embers twinkled, another girl had been added.
Long, phosphorescent hair that defied gravity.
A powder of light that glittered like a starry sky.
A girl who was in every way human was standing next to me.
I uttered a single wish.
"Let's play together again, like that summer."
The girl opened her eyes.
"Good morning, Yuu."
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