Eliezer Yudkowsky - Harry Potter and the Methods of Rationality

" Fred? George? "

"We can't figure out how to find her!" one of the Weasley twins blurted, hands twisting in distress. "We snuck out because we thought we could find Miss Granger - there has to be a quick way to find anyone inside the Hogwarts castle, we're both sure of it - but we can't figure out what it is!"

Harry stared at both of them, from where he was hanging upside down from the broomstick where his desperate maneuver had brought him, and entirely by reflex his mouth said, "Well, why were you so sure you could find her?"

"We don't know!" cried the other Weasley twin.

"Have you been able to find people inside Hogwarts before?"

" Yes! We -" and the Weasley twin who was speaking stopped abruptly, both redheads staring off into the distance with a blank expression.

There was a thundering crash, as of two huge doors being shoved open by someone very, very strong.

Harry spun around in the air to present the two open stirrup-positions on the broomstick to the Weasley twins, he didn't say anything, there was no reason for them to give away their positions if they didn't have to. Time seemed to move too slowly as the Weasley twins scrambled into the stirrups, Harry's heart beating hard despite his mental calculation that Mr. Hagrid, running, shouldn't reach even the foot of the stairway in time. Then the three of them were accelerating hard and away toward the nearest corridor, the stone floor beneath them blurring and the walls seeming to make an audible whooshing sound (though that was just the wind in their ears) as they went past; Harry remembered that he was riding a longer three-person broomstick barely in time to slow down for the next turn.

And now all the broomstick seats were occupied, but if they actually found Hermione then - Harry could put on the Cloak of Invisibility, that should hide him from the troll, and that would free up a seat for Hermione -

Harry ducked hard before a sudden archway took his head off.

"We found Jesse!" the Weasley twin seated behind Harry blurted. "I know we did! That time we needed to tell him that Filch was hunting for him!"

"How?" Harry said, most of his brain engaged in not dying in a horrible air accident. He should have slowed down for safety, but there was a tension rising in him, a sourceless dread. He couldn't slow down, something terrible would happen if he slowed down...

"We -" said the Weasley twin seated lower down. "We can't remember!"

Another sharp turn taken at, Harry estimated, roughly 0.3% of the speed of light, and they were going through a twisty curving corridor that Harry always took to get from the Great Hall to the library only it wasn't the shortest way if you were on a broomstick, he should've taken the long straight West Corridor instead -

The part of his brain that wasn't steering caught up with reality.

"Someone's been tampering with your minds!" Harry yelled, as he weaved through the curving corridor so fast that the tail-end Weasley sometimes lightly smacked into the wall as the length of the broomstick conflicted with Harry's maladapted air skills.

" What? " cried Fred or George.

"Whoever got to Hermione messed with your minds too!" It could be an Obliviation, it could be a False Memory that hadn't been planted right, but right now Harry couldn't think -

The broomstick turned and shot upward beside a spiral staircase, all three of them flattened themselves against the broomstick so they could make in through the gap in the ceiling that opened onto the third floor, and then they were in front of the library, the broomstick slowing to a halt with a shriek despite the lack of anything it could be friction-braking against. Harry shot the Weasley twins a quick glance to stay put , as he clambered off the broomstick to shove open the doors of the library, controlling his breathing as he shoved his head inside.

Hermione Granger wasn't there.

Madam Pince, who was eating a sandwich at her desk, looked up with a sudden glare. "Library's closed!"

"Have you seen Hermione Granger?" Harry said.

"I said the library's closed, boy! Lunch hours!"

"This is extremely important. Have you seen Hermione Granger or do you have any idea where she might be?"

"No, now be off!"

"Do you have any fast way of contacting Professor McGonagall in an emergency?"

"Eh?" said the librarian, startled. She rose up from behind her desk. "What is -"

"Yes or no. Please answer immediately."

"Ah - there's the Floo -"

"She's not in her office," Harry said. "Do you have any other way of reaching her. Yes or no."

"Young man, I insist that you -"

Harry's brain flagged this as I'm talking to NPCs again and he spun on his heel and dashed back for the broomstick.

"Stop!" cried Madam Pince, bursting too late from the doors as Harry and the Weasley twins shot off again, out of the librarian's sight. The pressure in Harry's mind still rising, like a physical hand squeezing his chest, he had to find Hermione and he had no other notion of where she could be, unless it was the witches' dorms in the Ravenclaw tower and that he couldn't enter. Searching all of Hogwarts bordered on a mathematical impossibility, there probably was no continuous flight path that entered all the rooms at least once - why hadn't he thought to demand for Hermione and Neville and him to be given a set of those neat little mirrors the Aurors used to communicate -

The realization that he was being stupid hit Harry like a blow to the stomach. He didn't need mirrors to send a message, he hadn't needed mirrors since January. Harry slowed the broomstick to a halt in midair of a hallway, his wand already coming into his hand, the driving will to protect Hermione Granger rising to the front of his mind like a sun of silver fire and flowing down his arm as he cried

" EXPECTO PATRONUM! "

and the blazing white humanoid burst into existence like a nova, the Weasley twins' voices crying aloud in shock.

"Tell Hermione Granger - that there's a troll loose in Hogwarts - it could be hunting for her - she needs to get into direct sunlight, now!"

The silver figure turned as though it was departing, and then vanished.

"Merlin's underpants," breathed Fred or George.

The silver outline blasted back into the world, and said in the strange outside version of Harry's own voice, "Hermione Granger says," the blazing figure's voice became higher-pitched, "AHHHHHHHHH! "

Time seemed to fracture, like everything was moving very quickly and slowly at the same time. A desperate impulse to accelerate the broomstick, fly at its maximum speed, only Harry didn't know where -

"If you know where she is," Harry shouted to the blazing humanoid figure, staring into it as though it were a sun, "then take me to her!"

The silver blaze moved and Harry accelerated after it, the Weasley twins giving out high-pitched shrieks behind him as he fired through the air like a cannonball, moving faster than sanity, he didn't focus on the walls whizzing past him or how fast he was moving, just followed the silver light through corridors and flying up staircases and blitzing through doors that Fred or George cried desperate incantations to open and it was all still taking too much time, somewhere deep inside Harry felt like he was sinking through molasses as windows and portraits shot past.

The broomstick screamed through a final turn that whacked one of the Weasley twins against the wall not quite as hard as a Bludger would hit, and then they followed the brilliant Patronus through an open space in the ceilings, blasting up and upwards, rising past one floor and then another in less than a breath.