Recommended: Harry Potter and the Methods of Rationality

This is a followup to the post about LessWrong.

Harry Potter and the Methods of Rationality is a novel-length bit of Harry Potter fanfic by Eliezer Yudkowsky (founding author of LessWrong). In it, he tries to apply the ideas about rationality developed in the original posts to the Harry Potter universe.

I’ve never seen any of the HP movies or read any of the books (and I probably won’t) but even so I’m enjoying this a lot. The HP in this universe definitely has elements of the annoying know-it-all but I believe that’s part fo the effect. Essentially this HP is a boy genius who knows more about science and rationality at age 11 than the vast majority of people in the world, so when he goes to Hogwarts things are a bit different. Namely, he tries to systematically study the world as it actually is (now that he knows that magic exists) and exploit it for the good of humanity.

Hermione points out Harry’s mistakes in the science of magic:

“Okay! So you gave me this whole long lecture about how hard it was to do basic science and how we might need to stay on the problem for thirty-five years, and then you went and expected us to make the greatest discovery in the history of magic in the first hour we were working together. You didn’t just hope, you really expected it. You’re silly.”
“Thank you. Now -”
“I’ve read all the books you gave me and I still don’t know what to call that. Overconfidence? Planning fallacy? Super duper Lake Wobegon effect? They’ll have to name it after you. Harry Bias.”
“All right!”
“But it is cute. It’s such a boy thing to do.”
“Drop dead.”

Harry actually influences Draco to test the blood-purist hypothesis:

“Good,” Harry said, “remember, it might not be happening, and then you won’t have to believe it, either. First we just want to know what’s actually going on, which world we actually live in.” Harry turned back to his work, scribbled some more, and then turned the parchment so Draco could see it. Draco leaned over the desk and Harry brought the green light closer.
Observation: Wizardry isn’t as powerful now as it was when Hogwarts was founded.
Hypotheses:
1. Magic itself is fading.
2. Wizards are interbreeding with Muggles and Squibs.
3. Knowledge to cast powerful spells is being lost.
4. Wizards are eating the wrong foods as children, or something else besides blood is making them grow up weaker.
5. Muggle technology is interfering with magic. (Since 800 years ago?)
6. Stronger wizards are having fewer children. (Draco = only child? Check if 3 powerful wizards, Quirrell / Dumbledore / Dark Lord, had any children.)
Tests:
“All right,” Harry said. His breathing sounded a little calmer. “Now when you’re dealing with a confusing problem and you have no idea what’s going on, the smart thing to do is figure out some really simple tests, things you can look at right away. We need fast tests that distinguish between these hypotheses. Observations that would come out a different way for at least one of them compared to all the other ones.”
Draco stared at the list in shock. He was suddenly realizing that he knew an awful lot of purebloods who were only children. Himself, Vincent, Gregory, practically everyone. The two most powerful wizards everyone talked about were Dumbledore and the Dark Lord and neither had any children just like Harry had suspected…

Harry finds out about the financial system in the wizarding world:

So not only is the wizarding economy almost completely decoupled from the Muggle economy, no one here has ever heard of arbitrage. The larger Muggle economy had a fluctuating trading range of gold to silver, so every time the Muggle gold-to-silver ratio got more than 5% away from the weight of seventeen Sickles to one Galleon, either gold or silver should have drained from the wizarding economy until it became impossible to maintain the exchange rate. Bring in a ton of silver, change to Sickles (and pay 5%), change the Sickles for Galleons, take the gold to the Muggle world, exchange it for more silver than you started with, and repeat.[...][O]ne competent hedge fundie could probably own the whole wizarding world within a week. Harry filed away this notion in case he ever ran out of money, or had a week free.