This is a story I heard from Kyosuke-san.
It happened in December of our first year in high school.
Right around the time the second-term final exams were about to begin. Unwillingly, I was studying at home. I knew I wouldn't get a good score anyway, but I also knew that if I didn't study at all and got a terrible grade, endless hassles would inevitably pile up later on.
As I compared my math textbook with the notes I'd taken myself, trying hard to decipher what they meant, the noise from the next room—which usually didn't bother me—became irritatingly noticeable.
Pop music was leaking through at quite a high volume.
"Hey, keep it down."
Next door was my sister's room. When I opened the door and yelled, my sister, who was sprawled out on her bed, said "Yeah, yeah" and plugged her earphones into the stereo.
Though we were twins, for some reason my sister was much smarter than me and attended a local public high school known as a prep school. Her school should have been in the middle of final exams too, yet she was showing off so much composure. It was genuinely sickening.
I pulled myself together and went back to my desk. Finally getting into the groove, I managed to focus on my studying for about an hour.
"Hah…"
My eyes were getting tired, so to take a break, I stretched and drank some coffee that had gone cold.
Suddenly, a small bottle sitting in the corner of my desk caught my eye. It was the little vial I'd gotten from a man wearing a strange mask in Shiramachi last night. I had gotten interested when my homeroom teacher, Xavier, told me it was a drug popular among high schoolers, but now that I actually had the real thing in front of me, my impression was just, *Oh, is that all it is?*
It was colorless and transparent, with a white cap. There was no label on the bottle either. Once, I timidly took off the cap and sniffed it, but it was odorless. Since smoking was already enough for me, I wasn't really interested in drugs, and even if someone told me it was good stuff, I didn't feel any urge to try it. It was just that I'd heard a rumor about "weird things happening," and my occult-loving blood had started acting up.
However, given the circumstances under which I'd acquired it, I felt a sense of caution. Mazaki Kyoko, who was with me, had talked to this masked figure we met in an alley about various things, and before I knew it, I ended up joining a gathering with a way-too-suspicious name: the "Poison Drinking Club," right along with Kyoko. There was no way I'd feel like trying a drug that this kind of guy was selling for a thousand yen. It was way too dangerous.
"Tomorrow, huh…"
I let out a deep sigh. Tomorrow at 8:00 PM was the gathering time. I had exam prep to do and even thought about blowing it off, but whenever I remembered the look in Mazaki Kyoko's eyes as she gazed at me and pleaded, it completely threw off my rhythm.
*If anything happens, you'd better take responsibility.*
Thinking such thoughts, and reluctantly deciding I should push ahead with tomorrow's studying as well, I opened my textbook once more.
The next day—meaning the day of the "Poison Drinking Club"—I headed toward the biology club room at the end of school hours.
Aside from a few athletic clubs that took things very seriously, club activities were suspended for everyone during this period.
"See you later~"
Leaving behind the stream of classmates heading home early, I walked down the deserted school hallway, feeling enveloped by a comforting sense of alienation. The realization that this solitude of walking a different path than everyone else was my true place of belonging soaked deep into my chest. But the person waiting ahead of me might share that exact same feeling—she was that kind of girl. That thought made me slightly unstable.
"Kyoko, are you in?"
When I opened the door to the classroom used as the biology club room, Mazaki Kyoko was sitting at a desk, reading a book by herself. We had made plans to meet up and discuss where and what time we'd rendezvous tonight.
Kyoko looked up and said, "Welcome." I sat down on the opposite side.
"Are you really going?"
"Yes. You'll come with me, right?"
"Well… I made a promise, but…"
There were many things I wanted to say, but naturally, seeing her right in front of me, the words wouldn't come out. I let out a sigh and rested my cheek in my hand.
"If it looks dangerous, I'm running away, you know."
"Right. Shall we run for it?"
For some reason, Kyoko seemed to be in a good mood, finding something amusing.
"That guy… he mentioned it, didn't he? About Cetus."
At my words, Kyoko's good-natured smile vanished. In its place emerged a faint smile, as if harboring some deep awareness within.
"A pitiful monster that fell from the ecliptic, he said."
"Yes."
"That story you told me about when you were a child. It's similar to that."
On the day I was invited to Kyoko's birthday party, I had heard about an incomprehensible event that had attacked her when she was little. It was an unbelievable story about how, along with an earthquake, all the constellations in the sky had changed.
For example, the current Zodiac constellations that everyone knows are different from the ones Kyoko used to know. That was why Kyoko said fortune-telling by stars was nonsense. The constellation for November 20th, the day she was born, was Cetus.
When she confessed such a secret to me, her eyes were not lying. At least, that was how it felt to me. That was the source of the fear, sadness, and isolation she harbored, something she could share with no one—even if it was merely the product of her delusions.
Yet, what on earth was the meaning of that masked man's cryptic words? Was it just some cold-reading trick used by fortune-tellers? No, that wasn't it. The Cetus Theatre had indeed existed for a while.
That masked bastard was dangerous. My intuition told me so.
Kyoko closed the book she was holding and placed it on the desk. It was a book titled *The World of Quantum Theory*.
"Yamanaka-san. Do you happen to know about Schrödinger's cat?"
I wondered what on earth she was going to bring up.
"I've heard of it. It's that thing where you don't know if the cat inside the box is alive or dead until you open it, right?"
"Well, something like that. But phrasing it that way is a conventional way of thinking. It assumes that the cat is definitely in one of those two states, and you just don't know which one it is inside the box. However, in the microscopic world of quanta, it's possible for a truly dead cat and a living cat to be in a state of superposition."
"Superposition?"
"Before that, there's a famous experiment in quantum theory called the double-slit experiment. You shoot electrons from an electron gun at a plate with vertical slits. Electrons are tiny particles—that is, matter. So when they're fired, they fly in a straight line. If they miss the plate and pass through a slits, white spots appear on a screen placed behind it where the electrons hit. If you fire them randomly many times, a cluster of elongated white spots shaped just like the slits appears on the screen. This is only natural. Electrons are particles, after all, so they fly straight."
"So what? Is this physics? Weren't you in biology?"
"I studied it because I was interested," Kyoko said with a composed face.
"Next, you do the same thing with a plate containing two slits. It's called the double-slit experiment."
Kyoko stood up and drew a diagram on the chalkboard.
"The electrons fired from the electron gun still pass through the two slits at a certain rate and head toward the screen. Now, I wonder what kind of pattern appears on the screen?"
Kyoko handed me a piece of chalk.
To be honest, I didn't want to accept it because I hated that scratchy feeling chalk leaves on the skin.
With no other choice, I tapped the white chalk onto the square enclosure representing the screen, making dots. I drew a cluster of dots shaped like the two slits. That was completely obvious.
"Right. That's the conventional answer. But the result of this double-slit experiment was eerie, as if it shattered the common sense humanity had taken for granted."
Saying this, Kyoko wiped away what I had drawn with a chalkboard eraser and instead drew five identical shapes at equal intervals. Since each was vertically elongated, it looked just like striped patterns.
"This is what appeared on the screen."
"That doesn't make sense. Aside from the number, there are dots right in the dead center. Isn't that the solid part of the board between the two slits? If they flew straight, they shouldn't hit there."
"No. This is the result of an actual experiment."
Feeling unsatisfied, I thought about it anyway.
"Then was it wrong to say those electrons fly straight?"
"That's wonderful. Great thinking. Were your physics grades good, Yamanaka-san?"
"Shut up."
"These striped patterns are called interference fringes. It's known that the same phenomenon occurs in experiments using waves."
"Waves? You mean ocean waves?"
"Yes. Waves are less about the substance itself and more about the way energy is transmitted. It's not that the same water keeps moving forward, but rather that the energy of moving water is passed on to the next water, pushing the shape of a wave forward. In the process of propagating a spatial distribution pattern, simply put, waves try to spread out radially. Therefore…"
Kyoko picked up the chalk again.
"After passing through the slits, the waves don't move straight ahead; they spread out radially. The waves that emerge from the two slits advance radially like this and collide with each other. Right around the middle in front of the board, the two waves crash, creating a high wave there. And the crest of the wave from the left and one of the subsequent crests from the wave on the right also collide, creating a high wave crest to the right of the center. The same goes if you reverse left and right. Furthermore, a similar thing happens outside that on the second subsequent crest. The way these high wave crests strike the screen is what creates these five interference fringes. Five is just an example, though."
I groaned while looking at the diagram. It was quite easy to understand. I briefly thought that she might actually be surprisingly well-suited for teaching.
"So, electrons were waves?"
"Well. Thinking that way seems to explain this experiment."
"What's so eerie about it? You're making a big deal out of nothing."
Kyoko smiled.
"However, electrons are not waves; they are particles. Just a single fired particle is flying through space. This is a fact. That's why, with a single slit, a single elongated line shaped like the slit appeared on the screen."
"I don't get it at all."
"This is a case where many electrons are fired rapidly from an electron gun. So, even if electrons aren't waves, perhaps when they pass through the left and right slits simultaneously, they interfere with each other through some property humanity doesn't yet know, creating striped patterns. So, what do you think happens if you fire electrons one by one, leaving some time between each?"
"One by one? Well… wouldn't the same thing happen as when there's only one slit? They don't interfere with each other."
Since she handed me the chalk again, I drew two lines on the chalkboard. Seeing that, Kyoko wore a suspicious smile.
"And yet, what actually appeared in the experiment was this."
Kyoko drew the same five striped patterns as before.
I felt a slight chill. I felt like I finally understood a bit of what Kyoko meant when she called it eerie.
"A single entity passes through both slits simultaneously and causes interference. In other words, the moment they are fired from the electron gun, things that were particles start behaving like waves. This is bizarre. Furthermore, an experiment was conducted where sensors were attached to each of the two slits to observe whether electrons fired one by one passed through them. Since an electron is a single entity called a particle, if one sensor reacts, it means the other one wasn't passed through. It's impossible for a single particle to pass through two slits at the same time. The results showed just that. One of the sensors registered a reaction of a particle passing through, and neither sensor reacted simultaneously. The particles apparently hadn't turned into waves."
"I feel kind of relieved."
I meant it as a joke, but it might have been mostly my honest feeling.
"But strange, isn't it? If so, what on earth were those five striped patterns that looked like they were drawn by waves? That's the question. However, the moment they started observing the slits with those sensors, the stripe patterns appearing on the screen changed. The two lines you drew earlier—the ones that appear when electrons are treated as particles rather than waves—started showing up."
"What is that supposed to mean?"
"Because they were observed, the electrons lost the possibility of 'passing through both slits simultaneously and potentially interfering with each other from left and right.' In other words, what was flying through the air was the very possibility of where the electron currently was."
Kyoko's explanation now sounded like something from a world completely detached from the field of science.
Yes. It was just like the occult.
"The common sense of the world we know doesn't apply in the microscopic world of quanta, which includes electrons. It's just like the world of the occult we love," Kyoko laughed, as if reading my mind.
"In the microscopic world, quanta are nothing more than possibilities until they are observed. You can only predict whether the probability of them being there is high or not. In other words, until observed, quanta exist in multiple places simultaneously as possibilities. This isn't wordplay; it's a fact derived from experiments."
"It's a fact claimed by this thing called quantum theory, right?"
Thinking it was a rather clever remark on my part, I felt like I had struck back. Kyoko nodded honestly, saying, "That's right."
"Schrödinger's cat thought experiment is what brings this microscopic-world fact of quantum theory into the real world. To skip the details, simply put, you put some radium, a Geiger counter, poison gas, and a cat inside a box."
"And I'll also add sex, lies, videotapes, a room, a dress shirt, and myself."
Ignoring my joke as if nothing had happened, Kyoko continued.
"Suppose the probability that a certain amount of radium undergoes alpha decay after a certain time and emits an alpha particle is 50%. The device is set up so that if an alpha particle is emitted and the Geiger counter reacts, poison gas is released, killing the cat. You are watching over this situation from outside the box. The box, the cat, and the poison gas are all things in the real world. This isn't a story about a mysterious microscopic world where common sense doesn't apply. Therefore, you think like this: after a certain amount of time, the probability that the cat is dead is 50%. You won't know until you open the box, but it's one of two things: it's either alive or dead."
"That's why I said that at the beginning. That you won't know until you open it."
"No. Inside the box, the alpha particle is a tiny quantum. Therefore, it follows the laws of quantum theory. Quanta behave as probabilities until observed. If the probability that an alpha particle exists at a position where the Geiger counter reacts after a certain time is 50%, it means that the possibility of it not being there and the possibility of it being there coexist simultaneously at equal concentrations. It's not one or the other. The life or death of the cat, determined based on that result, is the same. Inside the sealed box where you cannot observe it, a living cat and a dead cat coexist in a one-to-one superposition."
"Come on, that's a story about the microscopic world."
"As a thought experiment, Schrödinger devised a device where the laws of the microscopic world affect the macroscopic world, taking a stance critical of quantum theory. He thought it would be absurd if something like that happened in reality. After all, when you actually open the box, the cat is either alive or dead, one or the other. However, as experiments based on quantum theory progressed through the hands of many later physicists, the laws of that microscopic world could no longer be ignored. Einstein protested, saying, 'God does not play dice,' but electron spin 'quantum entanglement' was also confirmed repeatedly through experiments. Even if we call it the microscopic world, the matter making up our bodies, if you increase the magnification of a microscope and peer closer, is ultimately made of quanta like atoms. Everything in this universe is like that. We exist within the laws of quantum theory. We all exist in a superposition of multiple possibilities of what might be there. Therefore…"
Kyoko walked over and placed both hands on my shoulders. Her beautiful face drew close right in front of me, making my heart skip a beat. Her lips parted seductively.
"By observing me, right now, I am able to remain myself."
Sensing her face coming even closer, I instinctively stepped back.
Swept away by the mood, I felt like I was about to fall into a strange state.
"So what does that even mean?" I spat out. I wanted to escape from this bizarre atmosphere.
Kyoko let out a "fufu" laugh, one I couldn't possibly imitate, and sat right up on top of the desk, crossing her long, slender legs.
"A man named Hugh Everett at Princeton University developed this interpretation of the microscopic world and argued that quantum theory should also be applied to humans. After all, the observer is also an entity entirely composed of quanta. Schrödinger's cat in the box is definitively either dead or alive when you actually open the lid. But you, the one opening the box and looking at the cat, can be thought of as simultaneously coexisting as 'you watching a dead cat' and 'you watching a living cat' until someone else observes you. No, you have to think that way—as an actual, solid fact."
At Kyoko's words, I couldn't retort with "Isn't that just a fact of quantum theory?" Because somehow, her words felt like the truth.
"This is Everett's many-worlds interpretation. For you, this world is made of parallel worlds with countless possibilities that are determined every time you observe them. They coexist in superposition. And as time passes, those parallel worlds branch out into branches and leaves of 'possibilities that ended in different observation results,' diverging into countless parallel worlds that the you on this side can no longer interact with. That world, including you, likewise constitutes parallel worlds with countless possibilities, in superposition, until observed by someone other than you. This entire universe exists in superposition with countless other universes that are slightly different. A world that branches due to the coincidence of whether a single quantum happened to be at a certain point in the vast universe at a given moment, or not. Or an astronomical number of universes born from the possibility that this universe might have had different physical constants at the moment of its birth. This is Multiverse in theoretical physics—the theory of multiple universes."
"There's no way something like that exists."
"That which cannot be perceived is not synonymous with not existing. You can never absolutely prove that a multiverse overlapping with our universe does not exist."
"…Are you saying you came from there?"
Kyoko gave neither a "yes" nor a "no."
"A parallel world where the constellations are different is a world of possibilities lying at the end of a branch different from this observation world of mine now. Now that it has been observed and its possibilities closed off, it is a place where physical interference of any kind, such as the retention of memories, no longer reaches. And yet, I remember. Those stars. How do you explain—"