Author's Note: I usually don't write author's notes about the contents of the story, but I felt I should give you all some fair warning about the science-babble in this chapter (and, frankly, throughout the story).

First and foremost—yes, there will be discussion of the science behind time travel, the existence of numerous timelines, the fundamentals of space travel, etc. I will try to make it as accessible as possible, but this is where your feedback really helps. Please let me know if the science-talk makes sense in the context of the storyline.

Second—yes, I am playing fast and loose with relativistic physics. Some of the stuff in here is scientifically accurate. For instance, Trunks and Bulma have discussed that time isn't linear, but is actually just another dimension of space. That much is true (at least according to that eccentric German fellow known as Albert Einstein). It's also what makes time travel theoretically possible out here in the real world.

As another example: the best scientific evidence indicates that approximately 25% of the universe is made of dark matter, which is non-baryonic (i.e., not made of atoms—yeah, try to wrap your head around that). Astrophysicists have, however, hypothesized the existence of a small amount of baryonic dark matter, which can interact with "regular" matter (and antimatter) via electromagnetic forces. I'm oversimplifying a bit here, but that's the basic idea.

That being said, I am taking a fair amount of artistic license. That means that a lot of the science in here is not 100% accurate. I'm not really going for real-life accuracy here so much as internal consistency. I'm okay with contradicting Stephen Hawking. I'm less okay with contradicting earlier parts of the story. If there are any astrophysicists out there in the audience, please forgive the inaccuracies.

But let's think about it here. This is Dragonball Z. Spaceships travel faster than the speed of light, people come back from the dead with regularity, and five-year-olds can launch world-destroying ki blasts from their hands. Conservation of energy clearly does not exist in the world of Akira Toriyama. Neither the series itself nor this tale, then, are bound by the precise mechanics of relativity.

Apologies for the long note, but since I'd gotten a number of questions on this issue, I thought I'd try to clear things up here. Thanks for reading!

Percussion
Chapter 10

Fundamentals

Gohan blearily rubbed his eyes and let out a loud yawn. He ignored the mild pounding in his head as he sat up, catching a slight movement in the corner of his eye. He turned to see Trunks seated up on the couch, quietly flipping through a book on the coffee table.

"Morning," Gohan said around another yawn.

The younger boy turned to him. "You're awake."

"Yeah," Gohan said as he stood from the bed. He stepped over toward the couch, looking at the book resting on his coffee table. He raised an eyebrow at the other demi-Saiyan. "Are you reading my quantum mechanics textbook?"

"It was either that or Fundamentals of Statistics, Third Edition," Trunks said with a small smile. "You don't keep much in the way of reading material around, do you?"

"Not really," Gohan agreed. "How long have you been up, anyway?"

"A couple of hours."

Gohan frowned as he looked to the clock on his wall. It was nearly 11 a.m. "Why didn't you wake me?"

Trunks shrugged. "I thought you could use the sleep."

"I guess," Gohan said. "Looks like I'm going to be late, though."

"For what?"

"I promised to join my parents for lunch."

Trunks smiled as he shut Gohan's physics textbook and stood from his seat on the couch. "You'd better get going then. I'll head back to West City."

"You don't want to come with?"

"I don't want to impose," Trunks said with another shrug.

"It isn't an imposition!" Gohan rolled his eyes at the other demi-Saiyan. "Look, if it makes you feel better, I can call ahead and check with my mom. You'll be sure not to 'impose' then." Gohan rendered air-quotes with his fingers at the word "impose."

"You really don't need to."

"Come on, my parents would love to see you."

"I don't know."

"Do you have anything better to do?" After a moment's silence passed, Gohan continued, "I'm going to take that as a no."

"I do need to train," Trunks insisted.

"So we'll get in a spar after lunch. It's better than being cooped up in the Gravity Room all day, right?"

Trunks' face fell somewhat. "I'm never going to win an argument with you, am I?"

"Nope!" Gohan grinned. "I'll check in with my mom, then we can head out."

Much as Gohan had expected, his mother had been more than welcoming when he'd suggesting bringing Trunks over. The flight had been uneventful, and Chichi had ordered the teenagers out of the kitchen as she made last-minute preparations. Goku was, predictably, wrapping up a few training exercises in the woods of Mount Paozu as his wife prepared their lunch.

Goten, meanwhile, had apparently gotten over the awkwardness of meeting an adult version of his best friend, and was peppering the teenaged time-traveler with questions about the future. Trunks was answering as diplomatically as possible. He obviously didn't want to lie to the young demi-Saiyan, but neither did he wish to delve into the details of the androids' twenty-year reign and their eradication of half the human population of his earth. Gohan interjected where he could, but his younger brother was persistent in his questioning.

Goten nodded excitedly as Trunks explained, as simply as he could, the mechanics of time travel. It was clear enough to Gohan that Trunks didn't quite know how to handle his energetic little brother.

"And what about me?" Goten asked, bouncing on one foot as he interrupted Trunks mid-sentence.

Trunks frowned at the boy. "What do you mean?"

"What am I like as a grown up?" Goten asked, his eyes wide with wonder. "Am I your best friend in your time too?"

Gohan and Trunks glanced askance at one another, unsure how to address that question. Neither teen knew quite how blunt to be with the ten-year-old. Goten looked from one teenager to the other, clearly anxious for an answer.

"History has changed a lot, Goten," Trunks finally said, kneeling to eye level with the child. "In my time, I never met you."

"Oh." Goten looked down and shuffled his feet, the excitement instantly falling from his eyes. "That's kind of sad."

"Yeah," Trunks agreed. "It is." He shared another look with Gohan as a heavy silence fell over the three of them. Gohan shrugged one shoulder, unsure what, if anything, he could add to the time-traveler's answer.

"Chichi?" a familiar voice called out loudly as the door opened, breaking the uncomfortable silence. "Is Gohan here yet? I'm starving!"

Gohan turned to face his father, grateful for the interruption. "Hey, Dad."

"Trunks!" Goku said with a grin. "I didn't know you'd be coming too."

Trunks stood up straight, turning to the Saiyan. "I didn't either, until about an hour ago."

"Well, it's good to see you." His grin broadened as his wife brought in the first of what were likely to be several platters of food. "Alright, lunch!"

The woman glowered at her husband as she set the platter on the dining table. "Don't touch that! It's not ready yet."

Goku's face fell slightly as he looked at the platter. "But it looks ready . . ."

"There's more coming, and the table's not even set yet!" She slapped her husband's hand away from the platter. "And we have guests. Don't you have any manners at all?"

"It's Gohan and Trunks," Goku responded, somewhat dejectedly. "They're not guests."

Gohan heard a soft, almost choked sound to his left. He turned to see Trunks doing his best to stifle a laugh.

"Thanks again for having me over, Chichi," Trunks said, suppressing a chuckle.

"It's no problem," she said, folding her arms. "Just sit down, and make sure Goku doesn't clear the plate before I get back." With that, the woman stepped back into the kitchen, leaving the boys to take their seats around the table.

"So," Goku said as he pulled out one of the chairs and got seated. "Any word on what might be the problem with your timeline?"

Gohan and Trunks shared another glance. "Well," Trunks said, "sort of."

"What do you mean, sort of?" Goku reached for one skewer of grilled chicken. "Are you going back to your time soon?"

"I think I've got to get in a fair bit of training first."

"Why's that?"

"Because," Gohan interjected, "Trunks is planning on fighting Majin Buu."

Trunks frowned at the older demi-Saiyan. "That's not entirely accurate."

Goku nearly choked on the bite in his mouth. "Is Buu even around in your timeline?"

"Well, he was buried thousands of years before our timelines split. I'm guessing that he's still buried there, and never woke up."

"So you want to blow up his shell?"

"That's the plan."

"And what if you wake him up?"

Goten looked from his brother to his father, then back to Trunks. "Majin Buu is really strong," the boy added.

"That's why I need to train first," Trunks agreed. "I'm hoping that if I can ascend, and Buu's not at full strength, that'll be enough to take him out."

Goku furrowed his eyebrows, a thoughtful expression coming over his face as he absentmindedly reached for another skewer. "Do you want me to train you?" Goku asked before popping another chunk of chicken into his mouth.

Trunks' eyes widened at Goku's suggestion. He looked at the older man anxiously. "Are you serious?"

"Sure!" Goku said, swallowing the massive bite in his mouth. "I mean, if you're really going to take Buu alone, you're going to need all the help you can get. Even if he's not at full power."

"Sounds like a great idea," Gohan agreed. "At least you'll be prepared this way."

"Goku, that would be incredible." He nodded gratefully at the Saiyan, his eyes still wide at the prospect of having Goku take on the role of his master. "Thank you."

"Great! Then we'll start first thing tomorrow morning."

"Yes, of course," Trunks said, nodding excitedly. "Is there anything I should do beforehand?"

Before Goku could respond, a loud female voice interrupted the conversation. "Goku!" Gohan and Trunks both craned their heads around to see a very irate-looking Chichi with her hands on her hips. "What did I just tell you about touching that before it's ready?"

"I only took a few pieces," Goku replied, the pout on his face making him appear much younger than his thirty-eight years.

"A few pieces?" There was a blur of movement before the boys saw Chichi standing above Goku, menacingly waving a thick, cast-iron frying pan above her husband's head.

Goku raised his hands up defensively. "It just looked so good!"

"Uh . . ." Trunks blinked at the display before him. It might have been comical had it not been so bizarre. He watched the argument between Goku and Chichi unfold for a few moments before turning back to Gohan. "Where does your mother keep that frying pan?"

Gohan lowered his eyes and chuckled. "Trunks, I've been to outer space. I've been dead and back. I've known the guardian of the Earth since I was a kid, and I've met the supreme overseers of the universe. But that is one mystery I don't think I'll ever solve."

It was late afternoon before Trunks returned to the Capsule Corp compound. Lunch had been pleasant enough, and although Trunks didn't think he would ever get used to the voracious appetites all three Son men displayed, that wasn't what dominated the young time-traveler's thoughts.

Trunks looked toward the stack of files on his desk. On the one hand, Goku was probably the single greatest warrior he had ever met, and the man's offer to train him was an exciting one. On the other, Trunks was painfully aware that his plan to hunt down Majin Buu's shell was not only a dangerous one, but was speculative at best.

Trunks took a seat at his desk, pulling one stack of papers toward him. He had pulled out several of what he believed were the most relevant files, hoping that his mother's research would make more sense in light of his realization of Buu's likely role in damaging the timestream.

Trunks spent several minutes sorting the papers into some semblance of order, divided them by topic. His eyes fall upon one particular sentence listed below the bolded subject heading "Indicators."

Radiation—Terahertz. May be present in detectable amounts. Note any fluctuations greater than 0.7 mSv.

Trunks furrowed his brow, rereading the sentence. He vaguely recalled terahertz radiation being mentioned at least once since he'd arrived in this timeline. He frowned, struggling to remember where it was he'd first heard the term. A few seconds passed before his first training session with his younger self came to mind.

He stood from his chair, setting aside the sheet. He and his young counterpart had, he remembered, managed to set off the alarms of the Gravity Room during their spar. When he had asked Bulma about the cause, she had explained that the gravity generators were prone to releasing excessive terahertz radiation when overused. Trunks' frown deepened as he wondered whether there was a connection.

He only pondered the question for a few moments before resolving to find Bulma and ask if she had any insights. He hurriedly made his way down the stairs and into the basement, only to find her laboratory empty. He dashed back up to the main section of the house and stalked down the corridor toward her home office. The door was ajar, and though he couldn't see into the office, he could hear the whirr of Bulma's computer inside.

Trunks knocked, gingerly tapping on the door with his knuckles. He heard his mother's voice call him to come in before he pushed the door open.

His mother turned to him, still seated in her desk chair. "Hey, sweetheart. Where were you last night?"

"Out with Gohan's college friends, actually."

Bulma laughed in response. "Sounds like fun."

"Something like that." Trunks took in the stacks of paperwork on his mother's desk, and the numerous programs running and visible on her computer's monitor. "Don't you ever take a day off?"

"Days off don't really have much meaning when you run your own company." She smiled at the teenager. "So what's going on, honey?"

"I've actually got some questions. About terahertz radiation."

"What about it?"

"You said that the gravity generators can start giving off large amounts of terahertz radiation if they're malfunctioning," Trunks continued.

"Uh huh?"

"That kind of radiation also comes up in the files I brought over from my timeline."

"Yes, it does," Bulma nodded. "That's not exactly surprising."

"Why isn't it surprising? What's the connection?"

Bulma sighed and gestured for Trunks to take a seat in one of the empty chairs. "Do you really want a physics lesson now?"

"I want to understand what's going on."

"Trunks," she said after a moment's pause, "do you know what gravity is?"

Trunks frowned in confusion. "What kind of question is that?"

"Just answer me."

"The . . . the force that keeps us from floating out into space? What are you getting at?"

"Trunks, gravity is space."

Trunks raised an eyebrow in response. "What do you mean?"

"Gravity isn't like some magnetic field that grabs onto our bodies. It's quite literally the bending of space caused by massive objects. Mass bends space. The more mass an object has, the more it bends space. That's why, say, Jupiter has higher gravity than the Earth."

"I follow," Trunks confirmed. "But what does that have to do with the timeline instabilities?"

"I'm getting there," Bulma said. "Now, there are two ways to generate gravity. If you want, say, 300 G's, you can either go find something with the mass of ten suns, or you find a way to bend space in a limited area."

"The gravity generators actually bend space?"

"Exactly. It's a very precise, very involved process."

"So where does the radiation come in?" Trunks asked.

"Terahertz radiation is everywhere, all the time, in undetectable amounts," Bulma explained. "If something actually begins emitting detectable amounts, that means its spatial integrity might be compromised."

"Isn't that dangerous?"

"Potentially," Bulma agreed. "That's why the gravity generator has that auto shut-off feature. If detectable amounts of terahertz radiation are being generated, that means that something is wrong. Usually I just need to recalibrate the machines, but generating 500 G's in an enclosed space isn't exactly child's play."

Trunks frowned for a moment. "Is that why it's so hard to sense peoples' ki if they're in the Gravity Room? Because you're bending the space they're in?"

Bulma grinned at Trunks' question. "You're catching on."

"But that still doesn't explain what terahertz radiation has to do with my timeline."

"Actually, it does," Bulma said. "Remember how I said that time is just another dimension of space?"

"Yes?"

"Well, if terahertz radiation can be a sign that something is losing its spatial integrity . . ." Bulma trailed off, inviting the teenager to complete the thought.

"And if time is just another element of space . . ." Trunks continued with a thoughtful nod.

"Exactly. If your timeline has really become destabilized, then your entire Earth might be generating unusual amounts of terahertz radiation."

"Alright," Trunks said. "Alright, that makes sense. So are you going to try to measure the radiation from my timeline's earth?"

"Trunks," Bulma said gently, "I have absolutely no way to get those readings. I'm getting energy signatures from your timeline via the dark matter, but I can't very well set up radiation sensors for your entire planet. I don't have access."

Trunks' face fell. "So that's another dead end."

"I'm sorry," Bulma said with a soft smile. "But the more I think about it, the more I believe you're right about Majin Buu being the problem." Bulma sighed, taking a sip of coffee from the mug on her desk. "I hate to say it, but it looks like you really will have to go back to your home timeline and take Buu out."

"That's the plan. I'll stay here and train until I'm ready to take on Majin Buu."

"How will you know when you're ready?"

"I'm not sure," Trunks admitted. "I'm hoping Goku will help me figure that out."

Bulma raised an eyebrow at that. "Goku's training you? When did that happen?"

"A couple of hours ago, actually. He offered to start tomorrow morning."

"And you'd be stupid to turn him down." Bulma let out a soft laugh. "This actually makes me feel a little better about you fighting that monster."

"Yeah," Trunks said, rising from his chair. "Me too."

The sun had barely risen by the time Trunks arrived at Mount Paozu. He wasn't sure what time Goku intended to meet him, but the man had told him that they would begin their training "first thing" in the morning, and Trunks knew the older man was an early riser.

It wasn't long before Trunks realized that his instincts had been right. He could feel Goku's ki quickly traveling in his direction as he landed on the grass. Moments later, the older man was standing in front of him, a broad smile on his face.

"Great, you're here," Goku said, not bothering with any further greeting. "Let's get started."

"Of course," Trunks said. He put his hands together and gave the Saiyan a quick bow. "Where should we begin?"

Goku chuckled softly at the formality. "Okay, first, get in sparring form." Trunks complied immediately. When Goku was satisfied with the boy's stance, he continued, "Now attack me."

Trunks' posture stiffened, his mind immediately returning to the way his ill-fated training session with Vegeta had begin. Though he said nothing, Goku seemed to sense the teenager's discomfort.

"What's the matter?" Goku asked. "I told you to come at me."

Trunks took a deep breath before responding, still posed in sparring form. "I won't pretend that I'm a match for you, Goku."

"I'm not going to attack you. I just want to see what you can do."

"Alright," Trunks said with no small degree of hesitation. He took a moment to gather his energy before launching himself at Goku.

Trunks threw a punch as soon as he was within striking distance. The older warrior dodged, tilting his upper body mere inches to one side. Trunks countered with a roundhouse kick aimed at the man's head. Again, Goku avoided it effortlessly.

This exercise went on for several minutes. Trunks tried not to become frustrated with his own performance as he failed to land a single blow on his new master. Though Trunks was becoming winded, it was clear that Goku was nowhere close to breaking a sweat.

Goku suddenly pulled back from Trunks, landing on the grass several feet in front of the youth. "Okay, stop."

"Right," Trunks said, taking a moment to catch his breath. "So what now?"

Goku looked the teenager up and down, ignoring the teen's question. "Well," he said after several seconds, "you're strong."

Trunks creased his brow at the unfinished thought. "But?"

"But," Goku said, "you've forgotten the basics."

"The basics?" Trunks repeated. He knew that he had not progressed much since his last trip to this timeline, but he was not expecting that particular critique.

"It happens," Goku said casually. "It's easy to lose sight of the fundamentals when you advance as fast as you have." The man slowly raised both his hands, palms facing out. "Punch my hand. I'll catch your fist."

Trunks obeyed, pulling his arm back and making a fist with his right hand. A split second later, his hand landed Goku's open palm. He ignored the slight pain that shot through his wrist as he made contact.

"Hold on a second," Goku said, pulling his hand away. "You are way too stiff. C'mon, loosen your wrist. Otherwise you're just going to hurt yourself." Trunks nodded, shaking his hand out before pulling back and punching into Goku's palm once again.

"Better," Goku said with an approving nod. "Now, left fist." When Trunks complied, the older man immediately ordered, "Right fist."

Trunks quirked one eyebrow at the Saiyan. "Do you just want me to keep throwing punches for two hours?"

"'Course not," Goku responded with a smile. "This'll take three, at least."

Trunks laughed at the gleeful look on his new teacher's face. "What have I gotten myself into?"

Goku's grin widened. "Right fist," he repeated.

Trunks sighed, planted his feet, and threw another punch.

Unbeknownst to the time-traveler, his younger self was also engaged in an early-morning training session, albeit with a very different master. The eleven-year-old Trunks had not trained with his father since before his trip to Pepper City, and Vegeta apparently thought that a week-long hiatus was more than enough rest.

Trunks had barely spoken to either his father or his older self since he'd returned to West City. Though his reaction hadn't been nearly as loud or as angry as his mother's, he had also been shocked when he'd seen the elder Trunks' appearance upon his return. The teenager had tried to brush off his wounds as a routine training injury, but the younger boy had known better.

In six years of training with Vegeta, Trunks had never been treated so harshly.

Trunks was abruptly pulled out of his reverie as his father kicked his legs out from under him, causing him to land unceremoniously on the Gravity Room floor. He looked up to see his father glaring down at him, looking thoroughly unamused.

"You're distracted," the man said, gesturing for Trunks to stand up.

"Sorry," Trunks mumbled, brushing himself up and taking sparring form again.

"Stop muttering," Vegeta ordered. "Tell me what has you so preoccupied."

"Nothing," Trunks insisted.

"Don't lie to me," Vegeta said. "This whole session is going to be a complete waste of time if your mind is elsewhere. Now tell me."

"Fine," Trunks said, slipping out of stance and crossing his arms. "It's Trunks. Older Trunks."

Vegeta raised one black eyebrow at his son. "What about him?"

"He was kind of a mess when we got back from Pepper City, Dad."

Vegeta rolled his eyes. "Not you too. Your mother has already given me several earfuls over that."

"I mean, you beat him up pretty bad."

"You've seen him. He's fine."

Trunks looked away, his arms still folded. When he didn't respond to Vegeta's statement, the man continued.

"I've fought with him before," Vegeta explained. "I know his limits. It's his choice how far he wants to push those limits."

"I guess," Trunks said quietly, still staring off toward the wall of the room.

"I told you to stop muttering," Vegeta said firmly. "Now get back in sparring form. We aren't done here."