It's just a thought experiment, honest!
“Heya, Twilight! Sunset! What’cha doin’?”
“Oh! Hello, Pinkie Pie! Is it one o’clock already?? I’m so sorry! I forgot we were going to exchange cupcakes here in the castle with Rarity and Fluttershy. Sunset stopped by for more enchanted paper for her journal, and while we were catching up, we got to talking about this thought experiment from the human world. I completely lost track of time. My chocolate coconut cupcakes are ready, though!”
“Science and magic do seem to be your proverbial catnip, Twilight. And hi, Pinkie! Long time no see. Oooh, those cupcakes look amazing.”
“Help yourself, Sunset!”
“Thanks! Mmmmmm… Yum. It’s delicious!”
“If you want to head into the Cutie Map Room, Pinkie, I’ll be there in just a sec—”
“No problemo, Twilight! But hey, what’s a thought experiment? Sounds kinda fun!”
“It’s an experiment carried out only in the mind. Back when I was still Celestia’s full-time student, we did these all the time. Mostly regarding philosophy. Y’know, like the nature of reality, what constitutes identity, questions of ethics—aaaaaaand I lost her...”
“Ha! I wouldn’t take it personally, Twilight. I didn’t have much better luck on the other side of the mirror.”
“Well, even I normally have better luck than that! Pinkie, are you okay? Your eyes don’t usually stay crossed that long.”
“Waugh… Sorry, Twi. That stuff made my head kinda spinny. That and I might be having a sugar crash. I’ve been taste-testing a new recipe for our cupcake date and it took me about… hmmm… a hundred tries?”
“A hundred—?!”
“Take it easy, lemon squeezy! I didn’t let a single cupcake go to waste! None of ‘em were baked bads, but I was chasing baking perfection. After using up all of my recipes, I had to wow everypony with something new!”
“Still, a hundred cupcakes, Pinkie… That’s quite a bit.”
“Oh pfft, silly. I didn’t eat it all, Twilight.”
“Oh?”
“Yep! I gave a cupcake to each of the Cakes.”
“But… Hold on, Pinkie, out of a hundred that’s just four less—”
“Mmm! Pinkie—sorry for interrupting, Twilight, I just gotta know—what is this I’m tasting? It’s really good, but totally weird, too. But in a good way!”
“Maple syrup and parsnip. Bonkers, right? You’d think with how many veggies us ponies eat, we’d try more baked goods with veggies! I wanna try zucchini next. Anyway, you guys were talkin’ about philosophication stuff?”
“It’s okay, Pinkie. Remind me to talk to you about your sugar intake later. And no, we weren’t discussing philosophy. Maybe Sunset could explain it better than I can. I’m still trying to wrap my brain around this, myself.”
“Mmmph—hmmm? Ahem. What, me? Um. Did I mention that Human Pinkie’s eyes crossed when I tried to—no? Ah, fine. Okay, I’ll try. But look, if she starts frothing at the mouth and leaking syrup out of her ears, I’m not being held accountable, all right? Especially after she just finished a massive sugar bender!”
“Whaaaat? Human Pinkie has syrup coming out of her ears?? No fair! Mine only leaks cake batter!”
“You have cake batter coming out of your—? No. Wait. Even in the human world, I’ve had enough Pinkie experience to recognize a rabbit hole when I see one.”
“Ha! Very astute of you, Sunset. Even after so many years I still find myself falling into those. So… You were saying?”
“Right. Schleswig’s Cat.”
“Whose cat?”
“Schleswig. He was an Austrian physicist who helped build the foundation of what we call quantum mechanics in the human world. It’s kind of funny, he ultimately got creeped out by his own work enough that he switched fields to study biology instead—”
“Psst. It’s a relatively new concept to us ponies, Pinkie, but we’ve been fairly close to making the same connections ourselves! I feel kind of guilty… Like I’m cheating out all of our researchers with insider information.”
“—But his work was super important for us to be able to have a lot of the technology humans have today. Like computers, and cellphones, and—”
“Ooh, is this spy stuff, Twilight?”
“Sort of? Since the majority of humans don’t know about our world yet, but—”
“—Though, I was kinda telling Twilight that maaaaybe ponies don’t really need all that stuff? This world gets by just fine with what it has, so why complicate things? I mean, after you’ve seen half the stuff I have on the Internet… Hoo boy—!”
“Special agent, Twilight Sparkle!”
“Yeeeah, I don’t think the spy goggles will work for me.”
“Hm! You’re right. Probably would clash with the crown.”
“—Don’t get me wrong, there’s plenty of good about the Internet. It makes research much faster and connects people worldwide, but ironically, it can be super divisive too. Algorithms are the worst. Though memes are kinda fun, I guess. Like there was this one involving Vice Principal Luna and a penguin that… Uh, hello?”
“Pinkie we really need to work on your whispering.”
“Darn. I’m not really ‘Fluttershy’ quiet, huh? Is this better??”
“Forcing your voice into a breathy hiss doesn’t actually—”
“Er, guys?”
“Oops! Ooh. Ah. Heh. Heh, heh. Yes, um… very sorry to interrupt, Sunset. I was just trying to give Pinkie a little context, but that was totally inconsiderate of me to do while you were talking. Please, do go on!”
“Ahem. Right. So, the quick and dirty explanation of quantum mechanics is that it’s the study of how light and matter particles behave and interact at, like… almost unimaginably tiny levels. They call it atomic and subatomic. Still with me, Pinks?”
“Yep! Quantum mechanics studies how light and stuff and things act on teeny tiny levels. Got it!”
“That’s… actually an okay way to phrase it. Sorta. Well, Schleswig came up with this thought experiment that sought to explain how two particles—these teeny tiny bits of matter—could be in two states at once until they’re observed.”
“Two states? You mean, like other kingdoms?”
“Uh, actually being in radically different locations is something humans have discovered some particles being capable of doing, but no. At least, not in terms of this experiment. You see, the idea went like this: If you put a cat into a sealed box with a device that had a fifty percent chance of killing it within an hour, the question most of us would ask is… is the cat alive or dead? But see, in quantum mechanics, until the box is opened and the cat’s state is observed and fixed into one outcome, it is both alive and dead. Both things are true. We, as the observer, collapse the cat’s quantum state into whatever outcome we see.”
“Huh.”
“Yep. Cool, huh? Stuff like this is what I’m going to study in college after my summer break ends.”
“...Huh?”
“Ha, ha! That’s what I said too, Pinkie. But then Sunset started explaining superposition to me, and—”
“HUH?!”
“Er, Pinkie? Did Sunset’s explanation upset you? You’re breathing really heavy.”
“The cat is dead? AND alive?? Twilight, they turned a cat into a zombie?!”
“Woah, wait, I didn’t mean that! Twilight, I didn’t— There isn’t really a cat at all, Pinkie!”
“Oh. Good! So humans don’t have zombie cats over there? Wait, then how did they come up with this wackadoodle thought experiment?”
“No, of course there aren’t zombie cats. Just normal cats! The Rarity I go to school with even has a cat, just like yours does. I just meant that Schleswig never did this with a real cat. Hence: a thought experiment.”
“...”
“...”
“...”
“Twilight, d-did I break her? She’s not blinking.”
“Mmm, no. I can see the cogs turning in her eyes. Just give her a sec.”
“All right, but is Pinkie going to move, or…?”
“OHHHH!”
“AAAAH!”
“I told you her cogs were just turning! Why’d you get in so close?”
“I was afraid she wasn’t breathing!”
“So there is no zombie cat. Nothing is dead. Or alive. Cuz it’s not real. Humans aren’t giant evil meanies. Gotcha, gotcha. So, superposition? What’s that?”
“Twilight, do you mind taking over this part? I think my heart is still trying to vibrate right out of my body.”
“Sure thing, Sunset. Right... So, superposition. Pinkie, one thing you need to understand is that our world, everything, is made up of many, many tiny particles. That includes you and me. Superposition is a consequence of dual particles and the wave nature of everything. For objects to have a wavelength, it has to extend over some region of space. Basically, superposition means that tiny particles can exist in multiple states or places at once until they are measured or observed. Like in the experiment. You couldn’t see multiples of me or even that table over there, because wavelength decreases as momentum increases. In other words, the bigger the object, the smaller the wavelength and possible instances of states.”
“Uh, Twilight, that may be a bit too much for her. Maybe?”
“Hmm… You’re right. I feel like I only have a surface understanding of it myself. Pinkie, from the even broader explanations Sunset gave me earlier, it sounds like this is a far deeper topic than we can manage right now.”
“No, no, I think I got it! So like… A super teeny meeny itty bitty particle thing can exist in several places at once if it shows it has a wavelength, right?”
“Wow! Ah, yes. Exactly right. Geez, Pinkie, maybe you missed your calling as a quantum physics student. Dang, maybe I should have tried explaining all of this again to my Pinkie. I tried going the electron particle route, and her brain just melted on me. I tried telling some of my classmates at school about superposition too, and they just asked if this was ‘another magical pony thing’.”
“There’s still a lot about magic we don’t understand, though, Sunset. It seems entirely possible to me that quantum mechanics could factor into some of our spellwork, particularly our temporal and spatial spells! Huh… Actually, I’ve seen Pinkie do some wild things. Materializing out of nowhere. Pulling objects out of her mane that otherwise wouldn’t fit. Sunset, do you think Pinkie could have a wavelength?”
“Pinkie Pie? No way. That’s impossible.”
“Ooh, ooh! I bet my wavelength’s all kooky and fun! Like a noodle!”
“It’s impossible. Pinkie’s too large!”
“Hmm… Although… With magic…”
“I mean… Maybe? Aw, shoot. Now I feel silly for never asking the question before! Even Human Pinkie seems to do weird spatial shenanigans. But could it really be possible?”
“Sunset, back when I was studying Pinkie’s Pinkie Sense, I used a special machine I’d constructed that measured levels of magic. I think with your help I could reconfigure it to measure her wavelength instead.”
“I guess I am on summer break. I could spare staying on this side of the mirror for a few days. I’d be happy to help you chase down answers. Who knows, maybe this will help me come up with a good research project for college. Although, didn’t you guys have a cupcake party with the others today? —Oop. Speak of the devils. Hi Rarity, Fluttershy!”
“Greetings, ladies! A pleasure to see you all! A simply marvelous day we’re having, don’t you agree? And we even have a special visitor joining us I see!”
“Hello Twilight! Pinkie! And Sunset! My goodness, I didn’t realize Sunset Shimmer was going to be joining us, otherwise I would have brought more lemonade... And vanilla cupcakes. Oh dear. I don’t know if I’ve brought enough. M-Maybe I’ll just go back home and make some more—”
“Ah, ah! Don’t you dare move, darling. I just spent the better part of an hour straining out pulp, seeds, and bunny hair from the beverages, I shan’t allow either of us to be quagmired in such a dreadful task again.”
“Oh, but Rarity, Angel only wanted to help—”
“My dear, sweet, gullible Fluttershy. What he wanted was an extra helping of salad sundae, and his efforts at buttering you up were dreadfully sloppy at that. You mustn’t let your furry charges manipulate you so!”
“I… suppose we have spent enough time preparing the lemonade—”
“Ha! And then some.”
“—I only hope we have enough for everypony to enjoy.”
“Don’t worry about me, guys. I didn’t mean to intrude on your cupcake party. Twilight, maybe we can do a raincheck?”
“No, no, don’t trouble yourself, Sunset Shimmer. It is a delight to see you! How have you been? I’d love it if you could spare a word of those curious human fashions over some of my glazed strawberry cupcakes.”
“Oh my, yes! Please don’t leave on our account! We’d love it if you stayed. Though… I’m a little curious about what has you visiting?”
“I was stopping by for more enchanted paper and some other errands, that’s all. I like to stop and chat with Twilight whenever she’s free, and we just sorta gotta sucked into this deep conversation.”
“Yeah, yeah! Ooh, can I tell them? So Sunset and Twi were talking about quantum mechanics, and cats being dead and not dead—don’t worry, they aren’t zombies—and how me doing my thang means I got a kooky noodle wavelength, and—!”
“Pinkie, Sunset and I didn’t say you were capable of superposition, we just wanted to study—”
“Wait, wait! There are cats dying? How? WHERE?? Oh dear, oh my, ohhh—”
“...!”
“...!”
“...!”
“...!”
“Sweet Celestia, she fainted!”
“Blast! And here I am without my fainting couch.”
“Oopsy doopsy. I’m guessing the cat thing was not a Fluttershy-safe topic. My bad.”
“Twilight, quick, do you have smelling salts? Wait, is that even a thing in Equestria?? We have them in our first aid kits back at Canterlot High!”
“Um, ah—y-yes! Somewhere! SPIKE?”
“I’ll look over here! Hey, can you guys make sure she’s lying down and breathing okay? Aw shoot, it figures this would happen when I visit. What am I, a jinx—?”
“Okey dokey lokey! Pinkie is on it! Or on Fluttershy, I guess… Hey Rarity, a little help?”
“Naturally! Hmph. Pinkie, I am well accustomed to the rather uncouth and outlandish discussions that may occur within realms of academe, but would someone please explain to me how in the bloody blazes you three carried on about such ghoulish things as dead cats?”
“Oh sure! You see, there was this human scientist named Schleswig who was like, ‘I wanna explain how stuff can be and not be at the same time!’ So then he came up with this mondo weird thought experiment about a cat in a box and a deadly device, and basically, we’re all just a blur of probability that—oh. Oooohhhhh…”
“Pinkie? Pinkie Pie? Oh, bother. Well… At least I can see the cogs are turning.”
Author's Note
I tried to get the science right. I really did. 
For anyone wondering, "Schleswig" was suggested to me by bkc56 as an MLP-themed name for Erwin Rudolf Josef Alexander Schrödinger, the physicist known for the Schrödinger's Cat thought experiment. Schleswig Coldbloods are a breed of draft horse originating from the Jutland Peninsula, historically part of Denmark and Germany. While Schrödinger was Austrian, I chose 'Schleswig' because it has a Germanic sound and begins with S.
Okay, now that my over-explanation is out of the way, thanks for reading! I appreciate all thoughts and feedback. And hey, pet a cat today! Please don't stick it in a box! Give it a cupcake! (Wait, no don't. That's bad for them.) 
