The source of Tatsuya's headache could be traced back half a day, to late last night. The phone rang almost immediately after Tatsuya and Miyuki had finished dinner. For the receiver, it wasn't a strange time to get a call.
However, on the American West Coast, it was the dead of night—just past the turn of the new day. It was no wonder Tatsuya braced himself, wondering what kind of emergency had occurred.
"Hello, Shizuku? Did something happen?"
As expected, Shizuku appeared on the display. Her appearance, however, was entirely unexpected. Shizuku was in her sleepwear. Furthermore, it was a fashion-forward negligee, and she wasn't wearing a gown over it.
"Shizuku?! What are you wearing!"
Miyuki, watching the screen alongside Tatsuya, flushed red. Barely, just barely, the critical areas were obscured, but judging by the video feed, Shizuku wasn't wearing any underwear. It was a state of dress so loose and unreserved that a single slip would reveal everything.
"Ah, Miyuki. Good evening."
"Never mind the greetings! At least put on a gown!"
"...Is it okay if I don't?"
(Taking this call in the living room might have been a mistake.)
It wasn't that Tatsuya harbored any lewd desire to ogle Shizuku's disheveled state. However, the moment her outfit appeared on screen, he had anticipated Miyuki might react this way. It was only natural that he wanted to avoid a complicated situation.
Shizuku, for her part, looked puzzled, but she sluggishly draped a gown over herself as instructed.
"Sorry for calling so late at night."
"It's not particularly late here... but are you drinking?"
"Drinking what?"
"No... never mind. More importantly, what's wrong?"
Her cognitive faculties seemed slightly impaired, but since she could still hold a conversation, Tatsuya decided to cancel the unnecessary line of questioning.
"Nn. I thought I should let you know as soon as possible."
"You found out already? That's impressive."
"Praise me more."
Being solicited for praise in such a flat, monotonous tone caused Tatsuya to feel a sudden, intense wave of exhaustion.
(Who was the one who got Shizuku drunk? ...No, she really is impressive. So, what did she find?)
He didn't want to rush someone who had gone out of their way to call him in the middle of the night, but he judged that wrapping this up quickly would be best for both of them.
"About the cause of the Vampire outbreak... it looks like it was an experiment with a surplus... what was it? A surplus-something black hole."
"Black hole? Shizuku, what are you talking about?"
The news was far more sensational than expected, but because it was followed by a sentence of gibberish, a swarm of question marks danced above Miyuki's head. Yes—only above Miyuki's head.
"I don't know. I was planning to ask you about it, Tatsuya-san."
"A micro-black hole generation and evaporation experiment based on Extra Dimension Theory, isn't it?"
"Yeah, that."
Tatsuya confirmed it with a low, stiff voice. Shizuku didn't seem to notice the change in his tone, but Miyuki looked fearfully at her brother's expression.
"So they actually went through with it..."
His voice was calm as always. No, it was even more composed than usual. Yet, Miyuki alone understood that Tatsuya had received a massive shock.
"What is that?"
"Explaining it in detail would be difficult, so to put it simply..."
At that point, Miyuki had intended to end the call, thinking to say, "It's late, so let's stop here." But Shizuku asked a short question before she could, and Tatsuya answered it.
"It is an experiment to artificially create a microscopic black hole and extract energy from it. It is predicted that as the generated black hole evaporates, its mass converts into thermal energy."
"Is that Extra Dimension Theory? Extracting energy from another dimension?"
"No, Extra Dimension Theory itself isn't directly related to the process of extracting energy. Small black holes are predicted to evaporate regardless of their formation process. Extra Dimension Theory is a hypothesis stating that our world is like a thin membrane—a 'brane'—trapped within a higher-dimensional space. Among physical forces, only gravity can cross the dimensional wall. In other words, because most of gravity's force leaks into other dimensions, the force we observe in this dimension appears much weaker than it actually is. However, at the subatomic scale—extremely short distances—gravity acts between objects in this dimension before it leaks out, meaning the attraction is far stronger than what is observed at normal scales. Therefore, compared to calculations that don't account for Extra Dimension Theory, it becomes possible to generate black holes with exponentially less energy. That is the theoretical foundation for black hole generation experiments based on Extra Dimension Theory."
"...Miyuki, did you get that?"
"Unfortunately, I didn't understand much of it."
Shizuku asked while swaying her head unsteadily from side to side; Miyuki shook her head with a strained smile.
"But Brother, how does what you just said connect to the appearance of Vampires?"
Tatsuya tried to answer Miyuki's question, but Shizuku cut him off through the screen.
"Tachunya-shan, I'm curioush too."
"Shizuku? Are you alright?"
"I'm fwine. Sho, what's the cause of the Vampire outbreak?"
"Assuming Extra Dimension Theory is correct, the fact that gravity is the only physical force that crosses the dimensional wall must have some significance. In other words, the energy required for magic is likely being supplied from another dimension. From here on, this is a hypothesis with no evidence—little more than fantasy—but perhaps the gravity acting on other dimensions is what supports the dimensional wall. Perhaps magic extracts energy from other dimensions without breaking that wall. It's true that magic is a phenomenon that doesn't require an external power supply, but that doesn't mean it's unrelated to energy balance. Even within the observable range, magic where the total energy balance is close to zero tends to have a lower rate of activation failure."
Tatsuya's explanation continued after that, and both Miyuki and Shizuku simply listened quietly. Even if they had wanted to interject, the theory was so advanced that even these two honor students lacked the knowledge to cut in.
"Brother, what happens if the dimensional wall wavers?"
"Magical energy uncontrolled by a Magic Shequence... leaks through...?"
"Energy spontaneously structures itself to form information bodies. If it didn't, the universe would have homogenized long ago into a world of nothingness. Magical energy from other dimensions must structure itself in the same way. And the moment the dimensional wall wavers... I believe the probability of information bodies formed from magical energy in another dimension invading this world is not zero."
At Tatsuya's explanation, Shizuku gave a visible shudder on the other side of the screen. On this side, Miyuki clung tightly to Tatsuya's arm, as if depending on him for support.
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