PLAN TO PROVE THE IMPOSSIBILITY OF TIME TRAVEL
(BACKWARDS ALONG A FIXED TIMELINE)

1. General Remark. Time Travel is Not Scientifically Possible. (As this could lead to a violation of causality).
2. Observation. In apparent contradiction of (1), I was visited by Myself From The Future during the affair with Cerberus, Madame Pinkie Pie and the Disaster That Never Was, and subsequently travelled back in time in an attempt to tell myself there was nothing to worry about.
3. Further Observation. Had I succeeded in (2), this would have been impossible, as if I had persuaded myself not to worry, then I would not have subsequently travelled back in time to warn myself. But then, if I hadn’t travelled back in time, I could not have persuaded myself not to worry. (So what would have happened?)
4. This is Not Scientifically Possible (see 1.) (What is going on here?)
5. Postulate. Was the Me who visited me not from my future, but from some other future in some Parallel Universe? (see Many Worlds Interpretation of Quantum Mechanics, Shelf Ref: 530.12). So my apparent travelling back in time created two parallel timelines, one in which I successfully changed the future, and one in which nothing changed. This would appear to explain the apparent logical impossibility. (Eureka!)
6. However. I did not succeed in (2), but ended up creating the very thing I was trying to stop. Hence (2) did not actually prove anything at all.
7. Therefore. To conclusively prove this, I need to demonstrate that time travel creates a logical impossibility by going back in time in order to stop myself going back in time. (Experiment time—Yay!)
8. Obstacle. I can't do this as the spell only works once for any pony. I need the help of a friend who can work magic and pays attention to details. (Rarity!)

9. Plan of Action. We wait two minutes. If nothing happens, go to step 10. If we are visited by a Future Rarity, go to step 11.
10. Rarity travels two minutes back in time to greet herself. This is impossible as she didn't see herself two minutes ago.
11. Rarity does not travel back in time to greet herself. This is impossible as we just saw her.
12. Either way we have created a logical impossibility and thus proven (1).

13. Note. To ensure scientific integrity, it is vital to follow the plan as stated and not be distracted by anything that may happen, such as Rainbow Dash crashing through the ceiling (note: conduct experiment during her nap time to minimise risk of this happening). Likewise if Pinkie Pie, Applejack, Fluttershy, or any of our friends stop by, or if Ponyville is attacked by a monster, we have to ask them (politely) to hang on for two minutes until we finish the experiment.

With a proud clear voice, Twilight read out the plan which she had spent the previous night perfecting, now covered with numerous coffee stains and highlighting, with extra notes and references tidily written in parentheses. On reaching the final item, she smiled and looked up at Rarity.

“Is everything clear?”

“Not exactly,” said Rarity. “How in Equestria is my travelling in time supposed to prove that time travel is impossible?”

Twilight bashed a forehoof against her forehead. “I explained this Rarity. Travel back along a given time line can’t be possible, because it could violate causality—do you know about the Grandmother Paradox?”

“No.”

“Suppose you travelled back in time to a point before you, and your parents, were born, and you killed your maternal grandmother…”

“Why would I ever want to do that? My grandmother is a delightful lady. Even if she does insist on wearing those dreadful frocks with such old fashioned sleeves.”

“But hypothetically—suppose you did—by accident—”

“I would have trouble—she’s a tough old mare.”

“Yes, but, theoretically you could do it—when she was young. But then she would never give birth to your mother, so you would never be born—so you could never go back in time to kill her—it’s a paradox—see!”

“So how does time travel work?”

“It can’t—it’s impossible.”

Rarity was not convinced by this logic.

“But I thought you said you were visited by yourself from the future?”

“Yes, but she—or I—can’t have come from our future, she must have come from some other future in a parallel universe. According to my theory, whenever you apparently travel backwards in time, you create two parallel universes, so you can alter the future in one, but not in the universe you came from. It all makes sense.”

Twilight clapped her forehooves together with satisfaction before continuing the lecture. “In some theories, a parallel universe is created whenever some decision is made—one thing happens in our world, but the alternative possibility is played out in an alternative reality. There could be a vast number of such universes—and some could be very different to our own. There may be a universe in which Nightmare Moon triumphed and banished Celestia to the moon for a thousand years. Or where Rainbow Dash never did her sonic rainboom. Or where Pinkie Pie is a paranoid schizophrenic!”

“What a frightful thought.”

“But this has never been proven. I’ve read through all the historical accounts of time travel I could find. They’re mostly unreliable fables, or they tell something like my experience, where the traveller inadvertently causes whatever-it-was they were trying to prevent. We can be the first to demonstrate a logical impossibility under scientifically controlled conditions and prove the theory!”

Rarity frowned. “Is this entirely sensible Twilight? Mightn’t a logical impossibility destroy the world or something?”

Twilight shook her head. “Don’t be silly. That sort of thing only happens in bad science fiction novels. If a failure of logic could destroy the world, Equestria would have stopped existing long ago. Pinkie Pie is proof of that.”

“But if your theory is wrong? What would happen?”

“Well—the only other conceivable scenario is that time travel is possible, but somehow the universe prevents such a paradox ever happening. But that makes no sense! We can surely produce a logical inconsistency if we try, and what can the universe do to stop us? If we can’t do this, it would imply that our fate is predetermined, that we have no free will, and we are doomed to follow a plan drawn up by some quasi-divine script writer! All your spectacular fashion creations would not really be your own individual work as all our actions would be predetermined.”

“Well that can’t possibly be true, now can it? So how can I assist you disproving such an idea?”

“As I said, the plan is to wait two minutes. If nothing happens, then you use the spell—I’ll teach it to you—to go back in time. But as we didn’t see that happen, you would have to be going to some other parallel timeline. On the other hoof, if we do receive a visit from your future self, then you don’t go back in time, so she must have come from some other universe. Either way we have shown that travelling backwards along a single timeline is impossible!” She clapped her hooves together again in satisfaction. “But we have to stick to the plan. If—hypothetically—the universe was somehow able to stop us creating an inconsistency, then presumably we should expect a visit from Rainbow Dash or something similar, which would distract us and prevent us from causing a paradox. So we must not let that happen.”

“So either way, I will meet myself?”

“Yes.”

“Oh my! What a thought! Just a minute.” Rarity rushed over to a mirror and started running a comb through her mane.

“Rarity—you don’t need to style your hair—you’re only going to be visiting yourself. It won't be much different to looking in a mirror, except you will be able to talk to yourself.”

“But I always try to look my best when I receive important visitors. And I would certainly put myself in that category. I don't want to disappoint myself.”

After ten minutes work on her vanity, Rarity was satisfied.

“Okay,” said Twilight, levitating a timer in front of her. “So, two minutes, starting from now.”

For fifty seconds nothing happened. Then a mysterious wind started blowing around the room, shaking the clothes hung up on rails. A moment later, a second Rarity appeared in the front on them in a bright flash of white light.

“Rarity, you've got to listen to me!” said Rarity.

“Well I never,” replied Rarity. “It's me. Oh my. Oh I really like how you did your eye shadow—I mean, how I did it—or should that be, how I will do? It really accentuates the beauty of your irises—or is it my irises?”

“Rarity please!” said Rarity-from-the-future. “I don’t have much time!”

“And where did you get that beret?”

“It's from the Cheval Chapeaux boutique on Manefair in Canterlot, but that's not important!”

“Hang on,” said Twilight. “Why are you wearing a hat? You're from two minutes into the future. You haven't had time to change your attire.”

“Actually, I'm from next August.”

“What? But that messes up the experiment!”

“Well I had to come back to this particular minute, otherwise I would waste my one shot at time travel on your experiment. Now I've only got a moment, so please listen to me Rarity! Full length pleated skirts, crew neck jackets and triple puffed sleeves are the thing this season.”

“Puff sleeves!?”

“Hoity-Toity has done his entire line this way. Photo Finish can't get enough of it. It's all Canterlot is talking about. Rarity—you have to follow this—it's the biggest new look in a generation—if you start now you can ride the wave—you must—”

With a flash of light, Rarity-with-a-beret was sucked back into the future.

“Urgh!” Twilight stomped a hoof and gnashed her teeth. Grey smoke drifted out of her ears. “You messed up the experiment!”

“But I didn’t do anything,” said Rarity.

“I mean—you will do,” said Twilight. “Don't you see—even if you don't go back in time now, it doesn't prove anything as the Future You came from next August! Hang on—” The smoke stopped and she started smiling. “We can still do it. We can create a logical inconsistency if you now go back to just-before-she-arrived! We didn't see you then!”

“I thought you said we had to follow the plan and not be distracted by whatever may happen?”

“Yes... But... Aargh—why do experiments never go as you planned!”

“Twilight...” Rarity placed a hoof on her friend's shoulder. “I think what we need now, is a nice cup of tea, and then a trip to the spa to relax. And anyway, I would say that proved your theory very well.”

“How?”

“She was obviously a visitor from some strange parallel universe where they have a very odd taste in fashion. You heard what she said—triple-puffed sleeves? Really? But I did like that beret. I shall have to visit that boutique.”

Author's Notes:

Thanks for reading. Thanks to everyone who gave helpful comments on the writeoff version.