"WHAT!?!" Cadance shouted loud enough to be heard throughout the entire wing of the Castle before putting her hooves over her muzzle in embarrassment.
Sunset frowned at her. "I know you have this theory that Princess Twilight purposely kept me from returning to Equestria... but she's a good pony. She was my best friend in that timeline. I really miss her. I don't know for sure why she didn't respond to me during the Friendship Games, but I trust her. She wouldn't have done that to me."
Cadance raised an eyebrow dubiously then shook her head. "Our Twilight is a wonderful filly, but..." she sighed and shook her head. "No... never mind. I know you won't accept it without proof. Maybe talking to your Princess Twilight Sparkle IS a good idea. I'd like to hear her explanation for abandoning you like that for myself."
"She didn't abandon...!" Sunset started to protest.
"I don't want to fight with you about this, Sunset." Cadance interrupted with a sigh. "In any case, I'll help you with this, if I can. What do you need to do exactly?"
"Girls? What's going on?" Shining Armor's voice was as sleepy as the colt himself looked, still a bit mussed and dishelved from sleep as he opened the door to their bedroom.
Cadance winced. "Shiny... I'm sorry, I didn't mean to wake you."
Shining Armor closed his eyes, took a deep breath, and seemed to gather himself, and when he opened his eyes, he was looking much more alert and awake. "Well, something's obviously bothering you. Is there anything I can do to help?"
Sunset sighed. "Honestly, I think I'm on my own with this one. The only thing I still need is mom's journal. The one she used to communicate with me. Mine isn't around anymore... but I have a pretty strong theory as to WHY."
Shining Armor blinked. "Princess Celestia's journal?" He shook his head, as if shaking away his lethargy. "Right... you said something about that. You and Princess Celestia had linked journals, if either of you wrote in one, the writing would appear in the other. But what do you need that for?"
Cadance hesitated a moment, then made a decision. "Sunset wants to talk to Princess Twilight Sparkle. The one that she met in the other timeline."
Shining Armor blinked. "Princess?" His eyes widened as memory returned. "Right, right... while Sunset was on the human world, my sister became Princess Celestia's student, and then an Alicorn. Right?"
Sunset nodded. "When I came back to Equestria the first time, I stole her crown, and took it back to Earth, and tried to use its power to raise an army to come back and conquer Equestria. But Twilight followed me through the portal and stopped me, saved those poor kids from me...."
Cadance cut in. "You said yourself that the magic in that crown turned you into a monster... you weren't in control of yourself."
Sunset snorted. "That's being generous. I put it in motion, and I was the one who turned herself into a demon. It was my anger, my jealousy, my skill with magic that made me into her. Maybe I wasn't thinking clearly with all that magic going through me, but it would have been my fault. I nearly killed..." she choked. "I nearly killed her Shining. I nearly killed Twilight. If it wasn't for her friends... my friends, she'd have died. They saved her... the magic of friendship saved her."
Shining Armor blinked. "The magic of friendship?"
Sunset shook her head. "I still don't fully understand it. But the magic of friendship is the most powerful magic I've ever known. Honestly, Generosity, Laughter, Loyalty, and Kindness create a magic stronger than any other. Twilight and those five girls formed a bond in just a few days strong enough to stop me, to return me from my demon form, undo all the damage I did..." she chuckled. "Well, it DID leave a big hole in the front lawn of the high school... but otherwise..."
Shining Armor shook his head. "You told us about this before, but I still can't believe that MY little sister made friends with five strangers that fast. And that they somehow created magic on a world where there wasn't any?"
Sunset shrugged. "Well, Twilight's crown was the initial source of the magic in that world, I think, although, now that I think about it, she said that the magic of friendship was in their world AND ours. I don't think they'll be able to do magic without the trigger of the crown, but..."
She shook her head. "No, I felt it when I left. Applejack, Rainbow Dash, Fluttershy, Rarity and Pinkie Pie... when I took their magic, I could feel it. I took every bit, even the spark of magic left by their interaction with Twilight's crown. Without that spark, they'll never get magic working in that world again." She sighed. "I'm still not sure I made the right choice there. Magic was dangerous on Earth, sure. And it was really unpredictable... and if the wrong people there found out about it, it could have been really bad. But was it really my call to make? I just don't know."
Cadance moved over and gave her a hug. "I know you did the best you could, Sunset. You cared about those girls. You still do." Not that they deserved it.
Sunset sighed heavily. "Actually, that's kind of one reason I want to do this. If I can talk to Princess Twilight, maybe she can get a message to my friends as well. I obviously left my journal in their world when I left..."
Shining Armor shook his head. "Wait, this is the part I don't get. How do you talk to Princess Twilight? I mean, even if you could talk to my sister in the future... which I'm not saying you couldn't figure out, didn't you change the timeline? Isn't the fact that you changed history the reason you wanted to go back to... err... Earth in the first place?"
Cadance nodded. "Right. That Twilight Sparkle, that future, never happened. You changed all that."
Sunset Shimmer wrinkled her nose. "Well... yes and no. You see, that night I tried to go back. The night I went down into the vault to try to go back to Earth... and our Twilight accidentally broke the mirror? We saw Canterlot High School... but not the version that I ended up in when I first went through the portal, but Canterlot High School only minutes after I left for the last time. Mom said it. Time isn't a constant across dimensional borders. That's part of what I've been working on for the past week, working on Star Swirl's time and dimensional equations. When you go to a different dimension, you can, in theory, emerge at any particular point in their timeline."
Shining Armor looked surprised, then slowly started to nod. "OK, that actually makes sense. I mean, you said you were in that world for almost three years, but the Twilight you met was only a couple years younger than you, right? So while you were in the human world, at least ten years must have passed here in Equestria. Right?"
Sunset looked surprised, then pleased. "Right! Nice bit of deduction there, Shiny."
Cadance shook her head. "Well, I don't get it myself, but I trust you when it comes to magic, Sunset. So the mirror can go back and forward in time?"
Sunset looked thoughtful. "In theory. In practice, not so easily. It uses a lot of the same equations that Star Swirl used for his Time Travel spell... and that only takes you back one week, and creates a closed temporal loop. No paradox. You can't rewrite history with Star Swirl's spells. Or at least, I thought you couldn't. But somepony else DID."
Cadance blinked. "What do you mean 'somepony?' Wasn't it you that changed history by coming back in time?"
Sunset shook her head. "I simply went through the portal with the intention of coming home. At the time, I thought I felt a weird temporal signature affecting the portal, and obviously, that was the case, and we have a few clues regarding it. We know that SOMEPONY rewound time in all of Equestria while I was on the other side of the portal. The reason there weren't two of me in the room was because when time was rewound, I wasn't in Equestria at all. The spell didn't include me. If I hadn't traveled through the mirror when I did, the other me would have simply vanished. I'm honestly not sure WHAT would have happened... but that's the most likely scenario that I can come up with."
Shining Armor raised a hoof. "Wait... who cast the time spell that rewound time in all of Equestria? I mean... not even you've ever done anything that powerful."
Sunset frowned. "I don't know. It's a problem, because whoever did it could do it again. I've put up warning spells and a failsafe just in case somepony tries to rewind time again, but I'm not sure they'd even work. I've never even heard of a spell that could do what must have been done. Still, it's theoretically possible. You'd need an insane amount of magic... probably even more than I have right now, but it's possible." She wrinkled her nose. "That's one thing I want to talk to Princess Twilight about, actually. She's easily as smart as I was at her age, probably smarter, and Princess Twilight is from at least ten years in our future. She might know more about temporal and dimensional magic than I do, and at the least, she might be able to come up with something to help."
Cadance raised a hoof. "Wait... I don't understand. Maybe things haven't changed on the other side of the mirror portal... it's still right after you left over there... probably. But, Equestria was still rewound, right? Princess Twilight would have been rewound along with everypony else."
Sunset nodded. "I thought of that. In fact, when I first started working on this problem, I was sure that's why she never responded to me when I sent her messages through the journal during the Friendship games. Because she'd been rewound. She didn't respond, because she wasn't there anymore. But I've been working on the equations, and I think there's a chance, a very good chance, that that reality... the one where Twilight Sparkle is the Princess of Friendship, still exists. You see, there's a theory I heard about in the human world... multidimensional theory, actually. It suggests that every time you go back in time and make a change, you create a new dimension, splitting it off from the original. Well, it's more complicated than that... and believe it or not, Starswirl's time spells actually take that theory into account, or at least, the possibility. Starswirl's time-traveling spell specifically prevents you from changing time. If you use his spell to travel back in time, it turns out everything you did was destined to happen. You can't change anything."
Cadance blinked. "But things DID change, right? You remember a history that never happened here, so history can be rewritten."
Sunset nodded slowly. "Which means, it's possible that the spell split off another dimension when it rewound time. So Princess Twilight could still exist... in an alternate dimension."
Cadance frowned. "But it's also possible that the spell just reset everything, and that timeline is simply gone, right?"
Sunset wrinkled her nose. "Maybe... but just the existence of the human world proves that other dimensions exist. In fact, from my research, I know that Starswirl found more than one. Some of the dimensions he found were apparently pretty similar to our own! The problem was, the more similar they were, the thinner the walls between worlds. So going back and forth to a dimension really similar to our own could, in theory, collapse the walls between worlds and cause them to merge."
Shining Armor grimaced. "That sounds bad..."
Sunset shrugged. "Well, actually, I think that's one reason time isn't really messed up already. I mean, if the theory is sound, then there could be infinite alternate dimensions, where everything that COULD have happened, DID happen. But Starswirl only was able to travel to dimensions where the differences were pretty major. My working theory is that unless the difference is pretty big... like, say, a new Alicorn spontaneously being created where one didn't exist before, then the dimensions can merge really easily... and they probably do exactly that... so minor changes... we'll call them tears, minor tears in time just... fix themselves. The dimensions merge and small divergences just work themselves out so that time stays on track."
Cadance blinked. "But you coming back as an Alicorn was a BIG change, right?"
Sunset nodded eagerly. "Right... so if I'm right, the Princess of Friendship, Twilight Sparkle, still exists, and so does that version of Equestria."
Shining Armor shook his head. "I can't believe that Twilight becomes the Princess of Friendship. I mean... Twilight? Princess of Books, or Magic, or something... but Friendship?"
Sunset smiled softly. "Twilight taught me what friendship means. Well, her and my friends on Earth. I'm not saying she was perfect... she was scared sometimes. She didn't always know what to do. But she understood the power of friendship like nopony I've ever met. She believed in me, and gave me a second chance when I never deserved one. You'd be proud of her, Shiny."
Shining Armor smiled. "I am proud of her. I always have been."
Cadance was silent. What do I say? That I think Sunset's Twilight Sparkle was jealous of Sunset Shimmer's connection to Celestia and purposely left her there? Sunset doesn't believe that, and I CAN'T say that to Shiny. It'd kill him to hear that his cute little baby sister did something like that.
Sunset took a deep breath into the silence that followed. "Anyway, we already have a connection to the world where I lived as a human. In fact, it's the same one Princess Twilight used in order to open the mirror early when we had to deal with the Sirens. Princess Twilight used the connection between the journals, Mom's and mine, that is, to open the mirror early. It shouldn't be TOO hard to replicate that. After all, if I'm right, my journal is still on the other side."
She scratched her head sheepishly with one hoof. "Honestly, I can't believe I forgot about it. I meant to take ALL the magic out of that world, but I left one bit behind. That journal. It wasn't anywhere near when I closed all the breaches and took the magic from the girls and Spike."
Cadance caught on at that point. "And that journal is still linked to Celestia's journal here, right?"
Sunset nodded. "It should be. Mom showed me it the night I had decided to leave. For some reason, it took nearly a week, but it updated with everything that Twilight and I had written to each other since I used it to call for help against the sirens."
Shining Armor looked confused. "Wait. Even if you COULD send a message to the alternate world where Princess Twilight Sparkle is, she wouldn't be a princess there yet, right? She's still just a filly. It'll be years and years before she's a princess, there OR here."
Sunset shook her head. "Don't you remember? Time isn't a constant when traveling through dimensions. The only reason that time seemed to move forward at the same pace for me and Twilight after I first stole her crown was that there was a constant back and forth of magical energy after that point. My friends, Applejack, Rainbow Dash and the others, they had a connection to Twilight's crown... which was on the Equestrian side of the dimensions. Just like the Journals had a connection despite being in separate worlds. Now, when mom and I stopped using our journals, time started to split apart. Ten years passed here where less than 3 passed there, but after I stole Twilight's crown, the amount of energy that must have been passing through the portal constantly... must have kept the dimensions temporally aligned, so time for me and Princess Twilight passed about the same. Until I took all that magic away, anyway."
Shining Armor slowly nodded. "OK... so, time isn't a constant between dimensions unless magic makes it... er... constant?"
Sunset Shimmer nodded enthusiastically. "Right! But once I took the magic that connects our dimensions, time started passing differently again. I figure that time passes a LOT slower in the human world. That would explain how the Celestia there is only in her thirties, instead of over a thousand."
Cadance blinked. "Wait, what?"
Sunset went on, ignoring Cadance's confusion. "So, in theory, while the connection between this world and the world of the humans is now down to just Starswirl's mirror and my mom's journal, time's going differently here and there. The portal is closed now, and we aren't using mom's journal as a connection either. So we're temporally unlocked from each other." She waited for one of her lovers to respond, but this time she just got puzzled stares, so she continued. "Anyway, if Princess Twilight Sparkle is in a dimension that got split off from this one, that means she has a copy of Mom's journal as well. So we're both connected through that journal... in theory."
Cadance blinked. "What... do you mean you could just write in Celestia's journal and Princess Twilight will be able to see it?"
Sunset looked thoughtful. "Maybe... but even if my splitting the timelines... well, me and the mystery pony that rewound our Equestria, even with that, I'm pretty confident that I should be able to establish a connection between the journals even if it's failed for some reason. Assuming my theory is correct, anyway."
Cadance sighed. "So that's why you need Celestia's copy of the journal."
Sunset nodded. "Yup."
Shining Armor shook his head. "Wow. I can't believe you worked all this out in just a few weeks."
Sunset looked a bit guilty. "Well, technically I may have cheated a bit. You see, I didn't want to stay out all night, since I knew you'd worry, so whenever I went to the Starswirl wing to study, I cast that time dilation spell I showed you to give myself more time to study. And... er... some extra time to nap and stuff. It's been so crazy recently, I needed the extra sleep, to be honest. So I've actually been working on this for close to a month... relatively speaking."
Cadance glared at her, then sighed, shaking her head. "I really wish you wouldn't do stuff like that without telling us, Sunset. We'd have helped if you told us first."
Sunset smiled sheepishly. "Well, I had no idea if it'd even work... and no offense, but neither one of you has the background in temporal mechanics and dimensional magic that I do. Mom might have been able to help, but..."
Cadance laughed. "Yeah, trapped in a time bubble with your mom for days at a time? Probably wouldn't have been pretty."
Sunset grimaced. "We're getting better."
Cadance sobered, then nodded. "I know. Still, I guess you had your reasons. You WERE planning on telling us this once you'd worked it out, right?"
Sunset hesitated. "Yes. I wanted to give you a chance to talk to Princess Twilight anyway, Cadance. You'll see that you were wrong about her. She really only wanted the best for me."
Cadance glanced at Shining Armor uncertainly, then shrugged. "I'll try and keep an open mind. But... I'm guessing you could use some help with the next part?"
Sunset sighed. "Yeah, convincing mom to let me borrow her journal." All three of them winced.