Harry Potter and the Methods of Rationality

by Eliezer Yudkowsky

  Quirrell had slumped over and was trudging back to his chair.

  No. Not acceptable. They really needed to talk. Stuff the zombie act, Professor Quirrell would probably wake up if Harry poked him a couple of times. Harry started forward -

  WRONG

  DON'T

  BAD IDEA

  Harry swayed and stopped in his tracks, feeling dizzy.

  And then a flock of Ravenclaws descended on him and the discussions began.

  Chapter 17: Locating the Hypothesis

  You have always been J. K. Rowling.

  Historical note: In the Roman calendar, the "Ides" of a month referred to the 15th day of March, May, July, and October, and to the 13th day of all other months.

  "You start to see the 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.

  If you wanted to be specific, 3:32pm on Thursday afternoon.

  Harry and all the other boys in the first year were outside on a grassy field with Madam Hooch, standing next to the Hogwarts supply of broomsticks. The girls would be learning to fly separately. Apparently, for some reason, girls didn't want to learn how to fly on broomsticks in the presence of boys.

  Harry had been a little wobbly all day long. He just couldn't seem to stop wondering how that particular stable time loop had been selected out of what was, in retrospect, a rather large space of possibilities.

  Also: seriously, broomsticks? He was going to fly on, basically, a line segment? Wasn't that pretty much the single most unstable shape you could possibly find, short of attempting to hold on to a point marble? Who'd selected that design for a flying device, out of all the possibilities? Harry had been hoping that it was just a figure of speech, but no, they were standing in front of what looked for all the world like ordinary wooden kitchen broomsticks. Had someone just gotten stuck on the idea of broomsticks and failed to consider anything else? It had to be. There was no way that the optimal designs for cleaning kitchens and flying would happen to coincide if you worked them out from scratch.

  It was a clear day with a bright blue sky and a brilliant sun that was just begging to get in your eyes and make it impossible to see, if you were trying to fly around the sky. The ground was nice and dry, smelling positively baked, and somehow felt very, very hard under Harry's shoes.

  Harry kept reminding himself that the lowest common denominator of eleven-year-olds was expected to learn this and it couldn't be that hard.

  "Stick out your right hand over the broom, or left hand if you're left-handed," called Madam Hooch. "And say, UP!"

  "UP!" everyone shouted.

  The broomstick leapt eagerly into Harry's hand.

  Which put him at the head of the class, for once. Apparently saying "UP!" was a lot more difficult than it looked, and most of the broomsticks were rolling around
on the ground or trying to inch away from their would-be riders.

  (Of course Harry would have bet money that Hermione had done at least as well when it came her own turn to try, earlier in the day. There couldn't possibly be anything he could master on the first try which would baffle Hermione, and if there was and it turned out to be broomstick riding instead of anything intellectual, Harry would just die.)

  It took a while for everyone to get a broomstick in front of them. Madam Hooch showed them how to mount and then walked around the field, correcting grips and stances. Apparently even among the few children who'd been allowed to fly at home, they hadn't been taught to do it correctly.

  Madam Hooch surveyed the field of boys, and nodded. "Now, when I blow my whistle, you kick off from the ground, hard."

  Harry swallowed hard, trying to quell the queasy feeling in his stomach.

  "Keep your brooms steady, rise a few feet, and then come straight back down by leaning forwards slightly. On my whistle - three - two -"

  One of the brooms shot skyward, accompanied by a young boy's screams - of horror, not delight. The boy was spinning at an awful rate as he ascended, they only got glimpses of his white face -

  As though in slow motion, Harry was leaping back off his own broomstick and scrabbling for his wand, though he didn't really know what he planned to do with it, he'd had exactly two sessions of Charms and the last one had been the Hover Charm but Harry had only been able to cast the spell successfully one time out of three and he certainly couldn't levitate whole people -

  If there is any hidden power in me, let it reveal itself NOW!

  "Come back, boy!" shouted Madam Hooch (which had to be the most unhelpful instruction imaginable for dealing with an out-of-control broomstick, from a flying instructor, and a fully automatic section of Harry's brain added Madam Hooch to his tally of fools).

  And the boy was thrown off the broomstick.

  He seemed to move very slowly through the air, at first.

  "Wingardium Leviosa!" screamed Harry.

  The spell failed. He could feel it fail.

  There was a THUD and a distant cracking sound, and the boy lay facedown on the grass in a heap.

  Harry sheathed his wand and raced forwards at full speed. He arrived at the boy's side at the same time as Madam Hooch, and Harry reached into his pouch and tried to recall oh god what was the name never mind he'd just try "Healer's Pack!" and it popped up into his hand and -

  "Broken wrist," Madam Hooch said. "Calm down, boy, he just has a broken wrist!"

  There was a sort of mental lurch as Harry's mind snapped out of Panic Mode.

  The Emergency Healing Pack Plus lay open in front of him, and there was a syringe of liquid fire in Harry's hand, which would have kept the boy's brain oxygenated if he'd managed to snap his neck.

  "Ah..." Harry said in a rather wavering sort of voice. His heart was pounding so loudly that he almost couldn't hear himself panting for breath. "Broken bone... right... Setting String?"

  "That's for emergencies only," snapped Madam Hooch. "Put it away, he's fine." She leaned over the boy, offering him a hand. "Come on, boy, it's all right, up you get!"

  "You're not seriously going to make him ride the broomstick again?" Harry said in horror.

  Madam Hooch sent Harry a glare. "Of course not!" She pulled the boy to his feet using his good arm - Harry saw with a shock that it was Neville Longbottom again, what was with him? - and she turned to all the watching children. "None of you is to move while I take this boy to the hospital wing! You leave those brooms where they are or you'll be out of Hogwarts before you can say 'Quidditch.' Come on, dear."

  And Madam Hooch walked off with Neville, who was clutching his wrist and trying to control his sniffles.

  When they were out of earshot, one of the Slytherins started giggling.

  That set off the others.

  Harry turned and looked at them. It seemed like a good time to memorise some faces.

  And Harry saw that Draco was strolling towards him, accompanied by Mr. Crabbe and Mr. Goyle. Mr. Crabbe wasn't smiling. Mr. Goyle decidedly was. Draco himself was wearing a very controlled face that twitched occasionally, from which Harry inferred that Draco thought it was hilarious but saw no political advantage to be gained by laughing about it now instead of in the Slytherin dungeons afterwards.

  "Well, Potter," Draco said in a low voice that didn't carry, still with that very controlled face that was twitching occasionally, "Just wanted to say, when you take advantage of emergencies to demonstrate leadership, you want to look like you're in total control of the situation, rather than, say, going into a complete panic." Mr. Goyle giggled, and Draco shot him a quelling look. "But you probably scored a few points anyway. You need any help stowing that healer's kit?"

  Harry turned to look at the Healing Pack, which got his own face turned away from Draco. "I think I'm fine," Harry said. He put the syringe back in its place, redid the latches, and stood up.

  Ernie Macmillan arrived just as Harry was feeding the pack back into his mokeskin pouch.

  "Thank you, Harry Potter, on behalf of Hufflepuff," Ernie Macmillan said formally. "It was a good try and a good thought."

  "A good thought indeed," drawled Draco. "Why didn't anyone in Hufflepuff have their wands out? Maybe if you'd all helped instead of just Potter, you could've caught him. I thought Hufflepuffs were supposed to stick together?"

  Ernie looked like he was torn between getting angry and wanting to die of shame. "We didn't think of it in time -"

  "Ah," said Draco, "didn't think of it, I guess that's why it's better to have one Ravenclaw as a friend than all of Hufflepuff."

  Oh, hell, how was Harry supposed to juggle this one... "You're not helping," Harry said in a mild tone. Hoping Draco would interpret that as you're interfering with my plans, please shut up.

  "Hey, what's this?" said Mr. Goyle. He stooped to the grass and picked up something around the size of a large marble, a glass ball that seemed to be filled with a swirling white mist.

  Ernie blinked. "Neville's Remembrall!"

  "What's a Remembrall?" asked Harry.

  "It turns red if you've forgotten something," Ernie said. "It doesn't tell you what you forgot, though. Give it here, please, and I'll hand it back to Neville later." Ernie held out his hand.

  A sudden grin flashed across Mr. Goyle's face and he spun around and raced away.

  Ernie stood still for a moment in surprise, and then shouted "Hey!" and ran after Mr. Goyle.

  And Mr. Goyle grabbed a broomstick, hopped on with one smooth motion and took to the air.

  Harry's jaw dropped. Hadn't Madam Hooch said that would get him expelled?

  "That idiot!" Draco hissed. He opened his mouth to shout -

  "Hey!" shouted Ernie. "That's Neville's! Give it back!"

  The Slytherins started cheering and hooting.

  Draco's mouth snapped shut. Harry caught the sudden look of indecision on his face.

  "Draco," Harry said in a low tone, "if you don't order that idiot back on the ground, the teacher's going to get back and -"

  "Come and get it, Hufflepuffle!" shouted Mr. Goyle, and a great cheer went up from the Slytherins.

  "I can't!" whispered Draco. "Everyone in Slytherin would think I'm weak!"

  "And if Mr. Goyle gets expelled," hissed Harry, "your father is going to think you're a moron!"

  Draco's face twisted in agony.

  At that moment -

  "Hey, Slytherslime," shouted Ernie, "didn't anyone ever tell you that Hufflepuffs stick together? Wands out, Hufflepuff!"

  And there were suddenly a whole lot of wands pointed in Mr. Goyle's direction.

  Three seconds later -

  "Wands out, Slytherin!" said around five different Slytherins.

  And there were a whole lot of wands pointed in Hufflepuff's direction.

  Two seconds later -

  "Wands out, Gryffindor!"

  "Do something, Potter!" whispered Draco. "I can't be the o
ne to stop this it has to be you! I'll owe you a favour just think of something aren't you supposed to be brilliant?"

  In around five and a half seconds, realised Harry, someone was going to cast the Sumerian Simple Strike Hex and by the time it was over and the teachers were done expelling people the only boys left in his year would be Ravenclaws.

  "Wands out, Ravenclaw!" shouted Michael Corner who was apparently feeling left out of the disaster.

  "GREGORY GOYLE!" screamed Harry. "I challenge you to a contest for possession of Neville's Remembrall!"

  There was a sudden pause.

  "Oh, really?" said Draco in the loudest drawl Harry had ever heard. "That sounds interesting. What sort of contest, Potter?"

  Er...

  "Contest" had been as far as Harry's inspiration had gotten. What sort of contest, he couldn't say "chess" because Draco wouldn't be able to accept without it looking strange, he couldn't say "arm-wrestling" because Mr. Goyle would crush him -

  "How about this?" Harry said loudly. "Gregory Goyle and I stand apart from each other, and no one else is allowed to come near either of us. We don't use our wands and neither does anyone else. I don't move from where I'm standing, and neither does he. And if I can get my hands on Neville's Remembrall, then Gregory Goyle relinquishes all claim to that Remembrall he's holding and gives it to me."

  There was another pause as people's looks of relief transmuted to confusion.

  "Hah, Potter!" said Draco loudly. "I'd like to see you do that! Mr. Goyle accepts!"

  "It's on!" said Harry.

  "Potter, what?" whispered Draco, which he somehow did without moving his lips.

  Harry didn't know how to answer without moving his.

  People were putting their wands away, and Mr. Goyle swooped gracefully to the ground, looking rather confused. Some Hufflepuffs started over towards Mr. Goyle, but Harry shot them a desperately pleading look and they backed off.

  Harry walked toward Mr. Goyle and stopped when he was a few paces away, far enough apart that they couldn't reach each other.

  Slowly, deliberately, Harry sheathed his wand.