Twilight stayed with me at the lab while I wrote up our observations, and then she headed to the castle to alert the Royal Guard. I was eager to go back to her place and start my own research. I took the dress with me, just in case I thought of any tests to perform on it.
After arriving at Twilight's place, I spent several hours poring over textbooks in magic field theory and quantum mechanics. It was starting to get dark outside when the front door opened up. Twilight Sparkle was standing there, wearing a fancy set of saddlebags. Accompanying her was a nightwing guardstallion.
"Thank you, Nightcap," Twilight said to him. He silently nodded to us, then turned and shut the door. I could see him through the window. He remained stationed at the entrance to guard us.
"I've never seen a nightwing up close before," I said. "Interesting."
"They're cute, don't you think?" said Twilight, with a giggle. "Especially the ears."
"Cute? Ah, I guess," I responded, with a bit of a blush. "I don't really have much of an eye for ponies."
"I don't have, like, a thing for them, or anything," Twilight nervously insisted. "I used to have a bit of a thing for guards in general, but I grew out of it. I doubt I'd have enough in common with a guard to make it a realistic fantasy."
"I have nothing to contribute to this conversation. I've never even had a special somepony," I said, with a sardonic chuckle.
"Me either!" said Twilight in a hushed but energetic voice. "It's so weird! Here I am: a princess, a polymath, I go on weekly adventures, and I have an active social life. And yet, I've never been kissed."
A painfully awkward silence followed.
"Anyway," I said, mercifully breaking the quiet, "we should probably touch base on our research. I've been studying the physics since I got here. I'm low on insights, though, so maybe you should start?"
"Sure. I brought some books with me. This probably goes without saying, but these are not to leave this building unless they're being transported directly back to the library," she said. Twilight set her saddlebags down and pulled out several ancient-looking tomes.
"Holy Moon above," I gasped, feeling my pulse quicken. "As long as those are here, I probably won't leave either!"
Twilight laughed brightly. "I know the feeling! I'm afraid they're not as interesting as you might hope, though. I brought them from the Star Swirl Wing because each one contained offhoof references to temporal effects of thaumaturgy, but none of them are directly related to the topic. So far I've only found one interesting passage," she said. Twilight trotted over to a couch with a levitated book in tow, then sat and patted the cushion beside her with a hoof.
For a moment I felt like I was being beckoned like a dog, and it irritated me. I'm not sure what it was about other ponies' behavior that always seemed to set me off. I knew Twilight wasn't trying to be rude, but I felt very uncomfortable. I frowned for a moment, then shook off the feeling and walked over to sit beside her. It was clear from her face that she noticed my discomfort, but she said nothing.
"Um, so, here's what I wanted to show you," she said, and flipped to a left-hoof page in the book. A strip at the bottom of the page had been ripped clean off, and several pages which followed had been torn from the book entirely. The page read:
geodesics of spacetime curved by relativistic mass charge. Spatial transport via wormhole, such as standard teleportation magic, limits velocity to lightspeed. These three facts imply that causality violations do not generally arise in nature without magical intervention. There are two edge cases where causality violations might inadvertently occur: when teleportation wormholes are cross-linked, or when enormous gravitational masses are moving at close proximity and at relativistic velocities. As yet, there is no known way to cross-link the effects of teleportation magic, nor do we have access to gravitational masses anywhere near the magnitude required to create a causality violation. Because of this, the only known way for matter or information to travel backwards through time (hereafter referred to as 'time travel') is by means of magical spells designed specifically for that purpose.
Star Swirl the Bearded has identified three forms of time travel which are potentially accessible by thaumaturgical means. These are referred to as time travel of the first form, the second form, and the third form.
Time travel of the first form includes any kind of time travel that is forced to obey Neighvikov's Principle of Temporal Self-Consistency (as discussed in elementary texts). It is the only kind of time travel which is not proscribed by law. Spells which allow time travel of the first form are reasonably complex, but a practiced mage should have little difficulty. Since the magic follows Neighvikov's Principle, it does not allow causality violation. Consequently, such spells cannot be used to change the past or create any sort of temporal disjunction. The clever reader will, however, recognize the inherent potential of such magic to create beneficial ontological paradoxes.
Time travel of the second form involves a second dimension of time which passes through and intersects the first via the inertial frame of reference of the traveller (more technically, through all travellers who will ever exist). This allows a single 'master' timestream be altered probabilistically. Changes to this timestream affect the likelihood of all future events across Equestria, and in fact, across our entire universe (i.e. 'world dimension', to the laypony). This kind of time travel is proscribed except by Royal Decree, and to date no ponies other than Star Swirl the Bearded have ever been granted permission.
It is known by a few scholars close to Star Swirl that he has made extensive use of this form of time travel in order to secure a bright future for Equestria. In the parlance of the common pony, this form of time travel gives rise to the phenomenon known as 'prophecy'. Most prophecy is necessarily vague in order to achieve self-consistency. If not vague enough, prophecy leads to a contradiction that spontaneously alters the timestream, falsifying itself. The nature of determining how to make a prophecy self-consistent is well-known to Star Swirl the Bearded, but his methodology is a carefully guarded secret. However, it is generally assumed to require the use of time travel of the first form in conjunction with scrying magics.
Spells allowing for time travel of the second form are no more inherently complex than those allowing time travel of the first form; but unlike spells of the first form, they require a gargantuan source of mana. Centuries after Star Swirl's disappearance from Equestria, the only current source capable of channeling a sufficient amount of mana is the Tree of Harmony, and communing with the Tree is a task beyond the skill of all but the most talented and practiced mages. This is highly fortunate. Spells which allow time travel of the first form can easily be altered to allow time travel of the second form, and without such a natural restriction, all time travel spells would need to be proscribed by law. Were that the case, I would not be writing this chapter (naturally). Even knowledge of the existence of time travel of the second form is tightly restricted to those with Celestial approval, and so any books which bear this text must likewise be restricted.
Time travel of the third form is proscribed absolutely, as per Equestrian law on universal
I furrowed my brow as I attempted to parse the words. "I'm not sure I follow what 'second dimension of time' implies."
"I'm not entirely certain myself, but I suspect it just means casters like Star Swirl can manipulate the dimension of time that we're familiar with while moving through time in a different way. As Star Swirl manipulates events from outside the timestream, our future changes. This would match what Starlight Glimmer was trying to do."
"Wait, I think I get it. The only thing that matters in the end are the last changes that occurred from the traveller's perspective," I said. "The fact that the final action Starlight Glimmer took from her frame of reference was to leave the past intact, meant that Equestria's future would return to normal. It effectively erased all the meddling she had done in her previous attempts."
"Exactly. Put another way, her attempts to change the present occurred in a definite sequence that she experienced, even though each individual step of that sequence took place at the exact same time from Equestria's frame of reference," said Twilight. "So it's kind of like there's a second flow of time she's travelling through, even though I'm not sure 'dimension' is an accurate characterization for the idea."
"Right, but that doesn't explain how she was able to cast the modified spell. You said she cast it using a device called the Cutie Map, but according to this the spell would have required the Tree of Harmony," I pointed out. "This may be an old text, but I doubt any mana source more powerful than the Tree of Harmony exists today, either."
"That's easy enough to explain. The Cutie Map is an extension of the Tree of Harmony," said Twilight.
"That makes sense, then. But what's up with the missing pages? I'd assume it was some kind of accidental damage if it weren't for the missing strip at the bottom of this page," I said, pointing with a hoof. "It looks like the book has been intentionally damaged."
Twilight took a deep breath. "I think this information was willfully destroyed or censored, Moondancer."
"By who? Starlight Glimmer?" I asked.
Twilight looked away for a moment.
"Twilight, are you okay? What's going on?" I tried to say it gently. I hoped it sounded like I was concerned. I wasn't particularly good at this kind of pony relations thing, and I felt completely out of my element.
Twilight turned toward me and took my hoof in hers. "Look, you've shared some amazing research with me, and that took a lot of trust," she said. "Can I trust you to keep some things in confidence in return?"
She sounded so vulnerable all of a sudden. I lost my words, so I simply nodded.
Twilight sighed. "I'm pretty sure it came from the top down."
"You mean Princess Celestia," I said. "Okay. Why would that need to be some kind of secret?"
Twilight looked a bit taken aback. "Well, it just seems wrong to me. For Celestia to make a form of magic illegal is fine, but to cover up knowledge? What's next, blocking the road to scientific inquiry? I don't want to believe she would do something like this."
I bit my lower lip. "I don't mean this in a bad way, but maybe it's not so wrong for her to censor something that might destroy Equestria, Twilight."
"I agree," said Twilight. "But to censor it from academic texts that are already under lock and key? Somepony out there has to be qualified to understand it."
"She probably has unabridged copies somewhere."
"She never told me about them!" said Twilight. She seemed very upset, and then she sighed again and buried her face in her hooves.
It all seemed rather self-centered, but I didn't really know what her deal was, and she was clearly hurting. I realized I needed to be a friend, and I figured that meant not judging her. So I scooted over a little and put my foreleg around her.
"I must sound so full of myself," Twilight murmured into her legs.
"Twi, it's okay," I said, momentarily distracted in wondering whether 'Twi' was an appropriate moniker after it accidentally slipped past my muzzle. I gently stroked her back in a circular motion. "I don't know why this is important to you, but Celestia probably has her reasons."
Twilight lifted her face up. She wasn't crying or anything, but her face was a mask of stress and worry. "You don't understand what it's been like," she said.
"Don't? I probably can't understand. But why not tell me anyway? I already agreed to keep this in confidence," I said. "What's Celestia really like? Is she hard on you all the time, or something?"
Twilight shook her head. "No, not at all. She isn't even my mentor anymore, now that I'm a princess. But more and more my talks with her are so... heavy," she said.
"You talked to her while you were setting up the guard, didn't you?" I asked.
Twilight nodded. "I didn't reveal details of your research, I swear," she said. "She only knows what you've told the Court, and the fact that you might be a target."
"I guess it'd be okay if you did," I lied. "Some of what I'm doing is supposed to be a secret, but... it's complicated. Anyway, that doesn't matter right now. You were saying something about talking with Celestia?"
Twilight paused for a few moments as she stared at the floor. Then she sat up a little and turned to face me.
"Are you familiar with the ethical trolley problem?" she asked me.
"Of course. A Manehattan trolley is going to kill five ponies unless you throw a switch, in which case it will kill one. So you throw the switch unless you're an idiot," I say. "I don't understand where the ethical conundrum part lies, to be honest. Seems cut and dried."
Twilight chuckled, and it felt good to see her crack a smile. "Well, yeah. I'm largely a utilitarian and I agree with you, but for some ponies the 'active' thing makes it bad. But there are much harder variants, like, where you have to actually murder somepony to save other ponies, like shoving a heavily-armored pony in the way of the trolley to stop it."
I shrugged. "In that case I'm going to be guilty of murder, but I'll still do it as long as it's a sure thing, which is fortunately next to impossible. So the cost goes up to one life lost and one pony in jail, which is me, and that has less weight than the five lives I'm saving."
"Logical, and for the most part I agree in principle. But when the time comes to react in a real-world situation?"
"Well, I'd probably freeze up. I've never had to face an unwinnable scenario of that magnitude, and I know I'd feel guilty even if I did the right thing," I admitted.
Twilight nodded. "I've had to face those kinds of scenarios in real life, Moondancer. My friends have had to face them with me. If you're my friend, you might be forced to someday, too."
I huffed. "If you think I'm afraid to be your friend, you're crazy. I mean, I don't really want to get drawn into these insane adventures I've heard about where I'll have to murder some random pony by tossing them in front of a rail car or whatever, but I'd do it if I had to. Now is that what this is all about? Are you afraid of cursing me with your friendship or something? I mean, earlier today I was worried about the perks of being your friend..."
Twilight shook her head. "It's not that. It's Princess Celestia," she said. "Ever since my coronation, she's been... kind of intense."
"Is she trying to interfere with your friendships or something?"
"No, nothing like that. She's just so dead serious when we're alone, now. Please don't ever repeat this, but... now that I'm a princess, Celestia says I need to be ready and willing to do anything for 'my little ponies'. Emphasis hers."
"You mean, die for them, or something?" I asked.
"I wish that was all she meant—that would be easy!" said Twilight, shaking her head. "Of course I'd die for my friends, not to mention all of Equestria. The problem is just the opposite. She's been trying to convince me that I need to start valuing my life above the lives of other ponies. Someday, I might need to order somepony to their death in the name of the greater good. Moondancer, I'm barely twenty-five years old! I only discovered true friendship less than three years ago. I can't deal with this level of responsibility."
I didn't really know what to say, so I just leaned in and hugged her. "Well, you shouldn't have to," I said. "Princess Celestia got by without you or her sister for a thousand years. I'm pretty sure her paranoia isn't justified."
Twilight leaned into the hug. "She's been delaying things for a thousand years, Moondancer. She's been waiting for Luna to return and for me to appear," she whispered. "At Star Swirl's direction, all sorts of major catastrophes have been delayed until now. That's why we've had all these challenges lately: Sombra's return, Discord's Seeds of Disharmony, the changeling invasion..."
"Look, Twilight. If there's anything I know about you, it's that you have friends that will always be there when you need them," I said. "And I can't believe I'm saying this, but I'm one of them, and even though I'm new to this whole 'friendship' business I would probably take a spear to the heart for you, or for all of Equestria if it came to that. We're all on the same side, together. Princess Celestia probably doesn't remember what it's like to be young and confused. She needs to lighten up and lay off."
Twilight nodded and sniffed, wiping a wet eye. "Yeah. You know, I still see her like she's some perfect goddess or something, but I know she's a pony just like you or me," she said. "I need to tell her that this has been bothering me. Thanks for helping me see that."
I smiled. Did I just help somepony with a friendship problem, I wondered? Interesting. "No problem, Twilight. That's what friends are for. Or, so I've been told," I joked, poking her in the ribs and forcing a giggle out of her.
"Okay, okay. I'll need to ask Celestia about time travel of the third form sometime soon, so I'll talk to her about the pressure then. Just... not for a few days. I need more time to relax before facing her again. Besides, whatever the third form is, it isn't likely related to the first two, so it won't provide us with any data on how Starlight Glimmer may have been duplicated," she said.
"I agree. It's clear she modified that first form spell to a second form spell. If Star Swirl was the only pony permitted to use that kind of magic, we're not going to find other examples in the archives," I said. "How well do you remember of the spell?"
"Not well enough, I'm afraid. It was too complicated to cast by memory since there wasn't nearly enough time to memorize it, so I had to cast it from the same scroll Starlight Glimmer used, and the Cutie Map destroyed the scroll after we finally fixed everything. I'm pretty sure I could modify the original spell in the same way she did, but I have no desire to muck around with Equestria's past. Star Swirl undoubtedly set everything up as perfectly as possible, so any changes I might make would create a mess," said Twilight. "Anyway, that's all I've found so far. Do you have any new ideas?"
"Not many," I said. "How familiar with quantum theory are you?"
"I'm a little rusty," said Twilight. "I know the basics, but feel free to fill me in on the details."
"Okay. Well, I was thinking about how the dress seemed to pass through itself when I peeled it out of that shape it was stuck in," I said. "So I've been reviewing work related to CPT symmetry. You know what that means, yes?"
"I think it means that if you flip the electric Charge by going from matter to antimatter, everything stays the same as long as you also flip the Parity—meaning everything gets flipped to its mirror-image in space—and reverse the direction of Time. Is that right?"
"More or less. Essentially, the combined symmetries of electric charge, mirror-image parity, and the flow of time, are always preserved," I explained. "Antimatter has opposite electric charges: anti-protons are negative and anti-electrons (also called positrons) are positive. Since CPT symmetry is always preserved, if you do an experiment with antimatter that moves counter-clockwise, it will look exactly like a time-reversed movie of an experiment using matter moving clockwise."
"But it isn't actually like the antimatter is going backwards through time, is it?" asked Twilight.
I shook my head. "No. That's only true at the quantum level with virtual particles. When particle-antiparticle pairs spontaneously pop into being and annihilate each other, it literally is like a single particle moving forward in time, then turning into its antiparticle and moving backwards through time to the origin. It's a true closed timelike curve where no causality violation can occur," I said. "It works just like Neighvikov's Principle, in other words. If a virtual particle ever interacts with another particle, it isn't virtual anymore, and so the loop through time can't happen in the first place."
"It's like nature self-censoring by only allowing single-particle time loops that are stable," said Twilight. "But you're saying that doesn't happen if the particles interact?"
"Right. As you know, entropy—the flow of energy from ordered forms to chaotic forms—is the arrow of time. It's why time has a direction: why we can remember the past, but not the future. Entropy is a result of quantum randomness naturally producing more disordered states than ordered states."
"So, anything macroscopic only moves through time in a single direction, regardless as to whether or not it's made from matter or antimatter, yes?" asked Twilight.
"Exactly. But CPT symmetry also has implications that affect experiments beyond virtual particles. For one, because CPT symmetry is mathematically inviolable, this means anything that seems to break combined CP symmetry must have a matching violation in T symmetry. For example, some particles can transform into their corresponding antiparticles, but the probability is not symmetrical," I continued. "This is part of the underlying reason the universe is filled with lots of matter and very little antimatter. Time moving forward is more likely to have matter in it, while time moving backwards is more likely to have antimatter. Well, that's not exactly true, but it's the general idea."
"Oh! That would explain why CP violations are often called violations of T symmetry, because they're effectively the same thing," beamed Twilight. "If you flip charge and parity, everything runs backwards—except for entropy, of course. So what's the parity violation that accounts for the fact that antimatter favors turning into normal matter as time moves forwards?"
"It has to do with the spin orientation of individual particles. It's kind of complicated, though, and not directly related to our problem. I was curious if there might have been a parity violation given the odd behavior of the dress when I tugged on it, because it was antimatter before we opened the door, which might count as a charge violation," I explained. "But there wasn't a charge violation when the phenomenon occurred. It was normal matter when I was pulling on it, so there would need to be a violation of time symmetry somewhere. And it's clearly a normal dress moving through time like any other hunk of matter."
Twilight narrowed her eyes in thought, then stood up and walked over to where the dress was resting over a chair. "I'd like to ask Rarity some questions, but she's busy in Ponyville this week. However, her Canterlot store manager Sassy Saddles might know if this particular dress came from the Canterlot Boutique," she said. "Hay, would you like to come along? It's a really nice store, and I can introduce you to Sassy."
I blushed a little and tugged at my ratty sweater. "I'm not exactly a fashion horse, so I'd never really set hoof in a place like that by myself," I said, then pressed my lips tightly together for a moment.
"Oh. Well, it's okay if you don't want to," said Twilight, and she headed for the door. She sounded a little disappointed, I thought.
This felt like an opportunity passing me by. I didn't want to go to a stupid dress store, but maybe with Twilight... maybe it would be interesting just because of her. It would be a challenge to not screw up in public in front of one of Twilight's other friends, assuming that's who this Sassy pony was. Turned out I was up for a challenge.
"Hold up, let me come with you," I shouted, just before Twilight shut the door.