Harry Potter and the Methods of Rationality

"Excuse me," Harry managed to say, "but this sounds really really really REALLY DANGEROUS!" Harry's voice didn't quite rise to a shriek, he couldn't possibly scream loud enough to do this situation justice so there was no point in trying.

Professor McGonagall looked upon him with tolerant affection. "I'm glad you're taking this seriously, Mr. Potter, but Time-Turners aren't that dangerous. We wouldn't give them to children if they were."

"Really," Harry said. "Ahahahaha. Of course you wouldn't give time machines to children if they were dangerous, what was I thinking? So just to be clear, sneezing on this device will not send me into the Middle Ages where I will run over Gutenberg with a horse cart and prevent the Enlightenment? Because, you know, I hate it when that happens to me."

McGonagall's lips were twitching in that way she had when she was trying not to smile. She offered Harry the pamphlet she was holding, but Harry was carefully holding out the necklace with both hands and staring at the hourglass to make sure it wasn't about to turn. "Don't worry," McGonagall said after a momentary pause, when it became clear that Harry wasn't going to move, "that can't possibly happen, Mr. Potter. The Time-Turner cannot be used to move more than six hours backwards. It can't be used more than six times in any day."

"Oh, good, very good, that. And if someone bumps into me the Time-Turner will not break and will not trap the whole castle of Hogwarts in an endlessly repeating loop of Thursdays."

"Well, they can be fragile..." said McGonagall. "And I do think I've heard about strange things happening if they're broken. But nothing like that!"

"Perhaps," Harry said when he could speak again, "you ought to provide your time machines with some sort of protective shell, rather than leaving the glass exposed, so as to prevent that from happening."

McGonagall looked quite struck. "That's an excellent idea, Mr. Potter. I shall inform the Ministry of it."

That's it, it's official now, they've ratified it in Parliament, everyone in the wizarding world is completely stupid.

"And while I hate to get all PHILOSOPHICAL," Harry desperately tried to lower his voice to something under a shriek, "has anyone thought about the IMPLICATIONS of going back six hours and doing something that changes time which would pretty much DELETE ALL THE PEOPLE AFFECTED and REPLACE THEM WITH DIFFERENT VERSIONS -"

"Oh, you can't change time!" Professor McGonagall interrupted. "Good heavens, Mr. Potter, do you think these would be allowed students if that was possible? What if someone tried to change their test scores?"

Harry took a moment to process this. His hands relaxed, just a little, from their white grip on the hourglass chain. Like he wasn't holding a time machine, just a live nuclear warhead.

"So..." Harry said slowly. "People just find that the universe... happens to be self-consistent, somehow, even though it has time-travel in it. If I and my future self interact then I'll see the same thing as both of me, even though, on my own first run through, my future self is already acting in full knowledge of things that, from my own perspective, haven't happened yet..." Harry's voice trailed off into the inadequacy of English.

"Correct, I think," said Professor McGonagall. "Although wizards are advised to avoid being seen by their past selves. If you're attending two classes at the same time and you need to cross paths with yourself, for example, the first version of you should step aside and close his eyes at a known time - you have a watch already, good - so that the future you can pass. It's all there in the pamphlet."

"Ahahahaa. And what happens when someone ignores that advice?"

Professor McGonagall pursed her lips. "I understand that it can be quite disconcerting."

"And it doesn't, say, create a paradox that destroys the universe."

She smiled tolerantly. "Mr. Potter, I think I'd remember hearing if that had ever happened."

"THAT IS NOT REASSURING! HAVEN'T YOU PEOPLE EVER HEARD OF THE ANTHROPIC PRINCIPLE? AND WHAT IDIOT EVER BUILT ONE OF THESE THINGS FOR THE FIRST TIME?"

Professor McGonagall actually laughed. It was a pleasant, glad sound that seemed surprisingly out of place on that stern face. "You're having another 'you turned into a cat' moment, aren't you, Mr. Potter. You probably don't want to hear this, but it's quite endearingly cute."