Harry Potter and the Methods of Rationality

pattern, hear the rhythm of the world."

Thursday.

If you wanted to be specific, 7:24am on Thursday morning.

Harry was sitting on his bed, a textbook lying limp in his motionless hands.

Harry had just had an idea for a truly brilliant experimental test.

It would mean waiting an extra hour for breakfast, but that was why he had cereal bars. No, this idea absolutely positively had to be tested right away, immediately, now.

Harry set the textbook aside, leapt out of bed, raced around his bed, yanked out the cavern level of his trunk, ran down the stairs, and started moving boxes of books around. (He really needed to unpack and get bookcases at some point but he was in the middle of his textbook reading contest with Hermione and falling behind so he hadn't had time.)

Harry found the book he wanted and raced back upstairs.

The other boys were getting ready to go down to breakfast in the Great Hall and start the day.

"Excuse me can you do something for me?" said Harry. He was flipping through the book's index as he spoke, found the page with the first ten thousand primes, flipped to that page, and thrust the book at Anthony Goldstein. "Pick two three-digit numbers from this list. Don't tell me what they are. Just multiply them together and tell me the product. Oh, and can you do the calculation twice to double-check? Please make really sure you've got the right answer, I'm not sure what's going to happen to me or the universe if you make a multiplication error."

It said a lot about what life in that dorm had been like over the past few days that Anthony didn't even bother saying anything like "Why'd you suddenly flip out?" or "That seems really weird, what are your reasons for asking?" or "What do you mean, you're not sure what's going to happen to the universe?"

Anthony wordlessly accepted the book and took out a parchment and quill. Harry spun around and shut his eyes, making sure not to see anything, dancing back and forth and bouncing up and down with impatience. He got a pad of paper and a mechanical pencil and got ready to write.

"Okay," Anthony said, "One hundred and eighty-one thousand, four hundred and twenty-nine."

Harry wrote down 181,429. He repeated what he'd just written down, and Anthony confirmed it.

Then Harry raced back down into the cavern level of his trunk, glanced at his watch (the watch said 4:28 which meant 7:28) and then shut his eyes.

Around thirty seconds later, Harry heard the sound of steps, followed by the sound of the cavern level of the trunk sliding shut. (Harry wasn't worried about suffocating. An automatic Air-Freshening Charm was part of what you got if you were willing to buy a really good trunk. Wasn't magic wonderful, it didn't have to worry about electric bills.)

And when Harry opened his eyes, he saw just what he'd been hoping to see, a folded piece of paper left on the floor, the gift of his future self.

Call that piece of paper "Paper-2".

Harry tore a piece of paper off his pad.

Call that "Paper-1". It was, of course, the same piece of paper. You could even see, if you looked closely, that the ragged edges matched.

Harry reviewed in his mind the algorithm that he would follow.

If Harry opened up Paper-2 and it was blank, then he would write "101 x 101" down on Paper-1, fold it up, study for an hour, go back in time, drop off Paper-1 (which would thereby become Paper-2), and head on up out of the cavern level to join his dorm mates for breakfast.

If Harry opened up Paper-2 and it had two numbers written on it, Harry would multiply those numbers together.

If their product equaled 181,429, Harry would write down those two numbers on Paper-1 and send Paper-1 back in time.

Otherwise Harry would add 2 to the number on the right and write down the new pair of numbers on Paper-1. Unless that made the number on the right greater than 997, in which case Harry would add 2 to the number on the left and write down 101 on the right.

And if Paper-2 said 997 x 997, Harry would leave Paper-1 blank.

Which meant that the only possible stable time loop was the one in which Paper-2 contained the two prime factors of 181,429.

If this worked, Harry could use it to recover any sort of answer that was easy to check but hard to find. He wouldn't have just shown that P=NP once you had a Time-Turner, this trick was more general than that. Harry could use it to find the combinations on combination locks, or passwords of every sort. Maybe even find the entrance to Slytherin's Chamber of Secrets, if Harry could figure out some systematic way of describing all the locations in Hogwarts. It would be an awesome cheat even by Harry's standards of cheating.

Harry took Paper-2 in his trembling hand, and unfolded it.

Paper-2 said in slightly shaky handwriting:

DO NOT MESS WITH TIME

Harry wrote down "DO NOT MESS WITH TIME" on Paper-1 in slightly shaky handwriting, folded it neatly, and resolved not to do any more truly brilliant experiments on Time until he was at least fifteen years old.

To the best of Harry's knowledge, that had been the scariest experimental result in the entire history of science.

It had been somewhat difficult for Harry to focus on reading his textbook for the next hour.

That was how Harry's Thursday started.

Thursday.