Chapter 1: A Day of Very Low Probability

Dis­claimer: J. K. Rowl­ing owns Harry Pot­ter, and no one owns the meth­ods of ra­tio­nal­ity.

This fic is widely con­sid­ered to have re­ally hit its stride start­ing at around Chap­ter 5. If you still don’t like it af­ter Chap­ter 10, give up.

This is not a strict sin­gle-point-of-de­par­ture fic—there ex­ists a pri­mary point of de­par­ture, at some point in the past, but also other al­ter­a­tions. The best term I’ve heard for this fic is “par­allel uni­verse”.

The text con­tains many clues: ob­vi­ous clues, not-so-ob­vi­ous clues, truly ob­scure hints which I was shocked to see some read­ers suc­cess­fully de­code, and mas­sive ev­i­dence left out in plain sight. This is a ra­tio­nal­ist story; its mys­ter­ies are solv­able, and meant to be solved.

The pac­ing of the story is that of se­rial fic­tion, i.e., that of a TV show run­ning for a pre­de­ter­mined num­ber of sea­sons, whose epi­sodes are in­di­vi­d­u­ally plot­ted but with an over­all arc build­ing to a fi­nal con­clu­sion.

All sci­ence men­tioned is real sci­ence. But please keep in mind that, be­yond the realm of sci­ence, the views of the char­ac­ters may not be those of the au­thor. Not ev­ery­thing the pro­tag­o­nist does is a les­son in wis­dom, and ad­vice offered by darker char­ac­ters may be un­trust­wor­thy or dan­ger­ously dou­ble-edged.


Be­neath the moon­light glints a tiny frag­ment of silver, a frac­tion of a line...

(black robes, fal­ling)

...blood spills out in litres, and some­one screams a word.


Every inch of wall space is cov­ered by a book­case. Each book­case has six shelves, go­ing al­most to the ceiling. Some book­shelves are stacked to the brim with hard­back books: sci­ence, maths, his­tory, and ev­ery­thing else. Other shelves have two lay­ers of pa­per­back sci­ence fic­tion, with the back layer of books propped up on old tis­sue boxes or lengths of wood, so that you can see the back layer of books above the books in front. And it still isn’t enough. Books are overflow­ing onto the ta­bles and the so­fas and mak­ing lit­tle heaps un­der the win­dows.

This is the liv­ing-room of the house oc­cu­pied by the em­i­nent Pro­fes­sor Michael Ver­res-Evans, and his wife, Mrs. Pe­tu­nia Evans-Ver­res, and their adopted son, Harry James Pot­ter-Evans-Ver­res.

There is a let­ter ly­ing on the liv­ing-room table, and an un­stamped en­velope of yel­low­ish parch­ment, ad­dressed to Mr. H. Pot­ter in emer­ald-green ink.

The Pro­fes­sor and his wife are speak­ing sharply at each other, but they are not shout­ing. The Pro­fes­sor con­sid­ers shout­ing to be un­civil­ised.

“You’re jok­ing,” Michael said to Pe­tu­nia. His tone in­di­cated that he was very much afraid that she was se­ri­ous.

“My sister was a witch,” Pe­tu­nia re­peated. She looked fright­ened, but stood her ground. “Her hus­band was a wiz­ard.”

“This is ab­surd!” Michael said sharply. “They were at our wed­ding—they vis­ited for Christ­mas—”

“I told them you weren’t to know,” Pe­tu­nia whispered. “But it’s true. I’ve seen things—”

The Pro­fes­sor rol­led his eyes. “Dear, I un­der­stand that you’re not fa­mil­iar with the scep­ti­cal liter­a­ture. You may not re­al­ise how easy it is for a trained ma­gi­cian to fake the seem­ingly im­pos­si­ble. Re­mem­ber how I taught Harry to bend spoons? If it seemed like they could always guess what you were think­ing, that’s called cold read­ing—”

“It wasn’t bend­ing spoons—”

“What was it, then?”

Pe­tu­nia bit her lip. “I can’t just tell you. You’ll think I’m—” She swal­lowed. “Listen. Michael. I wasn’t—always like this—” She ges­tured at her­self, as though to in­di­cate her lithe form. “Lily did this. Be­cause I—be­cause I begged her. For years, I begged her. Lily had always been pret­tier than me, and I’d… been mean to her, be­cause of that, and then she got magic, can you imag­ine how I felt? And I begged her to use some of that magic on me so that I could be pretty too, even if I couldn’t have her magic, at least I could be pretty.”

Tears were gath­er­ing in Pe­tu­nia’s eyes.

“And Lily would tell me no, and make up the most ridicu­lous ex­cuses, like the world would end if she were nice to her sister, or a cen­taur told her not to—the most ridicu­lous things, and I hated her for it. And when I had just grad­u­ated from uni­ver­sity, I was go­ing out with this boy, Ver­non Dursley, he was fat and he was the only boy who would talk to me. And he said he wanted chil­dren, and that his first son would be named Dudley. And I thought to my­self, what kind of par­ent names their child Dudley Dursley? It was like I saw my whole fu­ture life stretch­ing out in front of me, and I couldn’t stand it. And I wrote to my sister and told her that if she didn’t help me I’d rather just—”

Pe­tu­nia stopped.

“Any­way,” Pe­tu­nia said, her voice small, “she gave in. She told me it was dan­ger­ous, and I said I didn’t care any more, and I drank this po­tion and I was sick for weeks, but when I got bet­ter my skin cleared up and I fi­nally filled out and… I was beau­tiful, peo­ple were nice to me,” her voice broke, “and af­ter that I couldn’t hate my sister any more, es­pe­cially when I learned what her magic brought her in the end—”

“Dar­ling,” Michael said gen­tly, “you got sick, you gained some weight while rest­ing in bed, and your skin cleared up on its own. Or be­ing sick made you change your diet—”

“She was a witch,” Pe­tu­nia re­peated. “I saw it.”

“Pe­tu­nia,” Michael said. The an­noy­ance was creep­ing into his voice. “You know that can’t be true. Do I re­ally have to ex­plain why?”

Pe­tu­nia wrung her hands. She seemed to be on the verge of tears. “My love, I know I can’t win ar­gu­ments with you, but please, you have to trust me on this—”

Dad! Mum!

The two of them stopped and looked at Harry as though they’d for­got­ten there was a third per­son in the room.

Harry took a deep breath. “Mum, your par­ents didn’t have magic, did they?”

“No,” Pe­tu­nia said, look­ing puz­zled.

“Then no one in your fam­ily knew about magic when Lily got her let­ter. How did they get con­vinced?”

“Ah...” Pe­tu­nia said. “They didn’t just send a let­ter. They sent a pro­fes­sor from Hog­warts. He—” Pe­tu­nia’s eyes flicked to Michael. “He showed us some magic.”

“Then you don’t have to fight over this,” Harry said firmly. Hop­ing against hope that this time, just this once, they would listen to him. “If it’s true, we can just get a Hog­warts pro­fes­sor here and see the magic for our­selves, and Dad will ad­mit that it’s true. And if not, then Mum will ad­mit that it’s false. That’s what the ex­per­i­men­tal method is for, so that we don’t have to re­solve things just by ar­gu­ing.”

The Pro­fes­sor turned and looked down at him, dis­mis­sive as usual. “Oh, come now, Harry. Really, magic? I thought you’d know bet­ter than to take this se­ri­ously, son, even if you’re only ten. Magic is just about the most un­scien­tific thing there is!”

Harry’s mouth twisted bit­terly. He was treated well, prob­a­bly bet­ter than most ge­netic fathers treated their own chil­dren. Harry had been sent to the best pri­mary schools—and when that didn’t work out, he was pro­vided with tu­tors from the end­less pool of starv­ing stu­dents. Always Harry had been en­couraged to study what­ever caught his at­ten­tion, bought all the books that caught his fancy, spon­sored in what­ever maths or sci­ence com­pe­ti­tions he en­tered. He was given any­thing rea­son­able that he wanted, ex­cept, maybe, the slight­est shred of re­spect. A Doc­tor teach­ing bio­chem­istry at Oxford could hardly be ex­pected to listen to the ad­vice of a lit­tle boy. You would listen to Show In­ter­est, of course; that’s what a Good Par­ent would do, and so, if you con­ceived of your­self as a Good Par­ent, you would do it. But take a ten-year-old se­ri­ously? Hardly.

Some­times Harry wanted to scream at his father.

“Mum,” Harry said. “If you want to win this ar­gu­ment with Dad, look in chap­ter two of the first book of the Feyn­man Lec­tures on Physics. There’s a quote there about how philoso­phers say a great deal about what sci­ence ab­solutely re­quires, and it is all wrong, be­cause the only rule in sci­ence is that the fi­nal ar­biter is ob­ser­va­tion—that you just have to look at the world and re­port what you see. Um… off the top of my head I can’t think of where to find some­thing about how it’s an ideal of sci­ence to set­tle things by ex­per­i­ment in­stead of ar­gu­ments—”

His mother looked down at him and smiled. “Thank you, Harry. But—” her head rose back up to stare at her hus­band. “I don’t want to win an ar­gu­ment with your father. I want my hus­band to, to listen to his wife who loves him, and trust her just this once—”

Harry closed his eyes briefly. Hope­less. Both of his par­ents were just hope­less.

Now his par­ents were get­ting into one of those ar­gu­ments again, one where his mother tried to make his father feel guilty, and his father tried to make his mother feel stupid.

“I’m go­ing to go to my room,” Harry an­nounced. His voice trem­bled a lit­tle. “Please try not to fight too much about this, Mum, Dad, we’ll know soon enough how it comes out, right?”

“Of course, Harry,” said his father, and his mother gave him a re­as­sur­ing kiss, and then they went on fight­ing while Harry climbed the stairs to his bed­room.

He shut the door be­hind him and tried to think.

The funny thing was, he should have agreed with Dad. No one had ever seen any ev­i­dence of magic, and ac­cord­ing to Mum, there was a whole mag­i­cal world out there. How could any­one keep some­thing like that a se­cret? More magic? That seemed like a rather sus­pi­cious sort of ex­cuse.

It should have been a clean case for Mum jok­ing, ly­ing or be­ing in­sane, in as­cend­ing or­der of awful­ness. If Mum had sent the let­ter her­self, that would ex­plain how it ar­rived at the let­ter­box with­out a stamp. A lit­tle in­san­ity was far, far less im­prob­a­ble than the uni­verse re­ally work­ing like that.

Ex­cept that some part of Harry was ut­terly con­vinced that magic was real, and had been since the in­stant he saw the pu­ta­tive let­ter from the Hog­warts School of Witchcraft and Wizardry.

Harry rubbed his fore­head, gri­mac­ing. Don’t be­lieve ev­ery­thing you think, one of his books had said.

But this bizarre cer­tainty… Harry was find­ing him­self just ex­pect­ing that, yes, a Hog­warts pro­fes­sor would show up and wave a wand and magic would come out. The strange cer­tainty was mak­ing no effort to guard it­self against falsifi­ca­tion—wasn’t mak­ing ex­cuses in ad­vance for why there wouldn’t be a pro­fes­sor, or the pro­fes­sor would only be able to bend spoons.

Where do you come from, strange lit­tle pre­dic­tion? Harry di­rected the thought at his brain. Why do I be­lieve what I be­lieve?

Usu­ally Harry was pretty good at an­swer­ing that ques­tion, but in this par­tic­u­lar case, he had no clue what his brain was think­ing.

Harry men­tally shrugged. A flat metal plate on a door af­fords push­ing, and a han­dle on a door af­fords pul­ling, and the thing to do with a testable hy­poth­e­sis is to go and test it.

He took a piece of lined pa­per from his desk, and started writ­ing.

Dear Deputy Headmistress

Harry paused, re­flect­ing; then dis­carded the pa­per for an­other, tap­ping an­other mil­lime­tre of graphite from his me­chan­i­cal pen­cil. This called for care­ful cal­lig­ra­phy.

Dear Deputy Head­mistress Min­erva McGon­a­gall,

Or Whom­so­ever It May Con­cern:

I re­cently re­ceived your let­ter of ac­cep­tance to Hog­warts, ad­dressed to Mr. H. Pot­ter. You may not be aware that my ge­netic par­ents, James Pot­ter and Lily Pot­ter (formerly Lily Evans) are dead. I was adopted by Lily’s sister, Pe­tu­nia Evans-Ver­res, and her hus­band, Michael Ver­res-Evans.

I am ex­tremely in­ter­ested in at­tend­ing Hog­warts, con­di­tional on such a place ac­tu­ally ex­ist­ing. Only my mother Pe­tu­nia says she knows about magic, and she can’t use it her­self. My father is highly scep­ti­cal. I my­self am un­cer­tain. I also don’t know where to ob­tain any of the books or equip­ment listed in your ac­cep­tance let­ter.

Mother men­tioned that you sent a Hog­warts rep­re­sen­ta­tive to Lily Pot­ter (then Lily Evans) in or­der to demon­strate to her fam­ily that magic was real, and, I pre­sume, help Lily ob­tain her school ma­te­ri­als. If you could do this for my own fam­ily it would be ex­tremely helpful.

Sincerely,

Harry James Pot­ter-Evans-Ver­res.

Harry added their cur­rent ad­dress, then folded up the let­ter and put it in an en­velope, which he ad­dressed to Hog­warts. Fur­ther con­sid­er­a­tion led him to ob­tain a can­dle and drip wax onto the flap of the en­velope, into which, us­ing a penknife’s tip, he im­pressed the ini­tials H.J.P.E.V. If he was go­ing to de­scend into this mad­ness, he was go­ing to do it with style.

Then he opened his door and went back down­stairs. His father was sit­ting in the liv­ing-room and read­ing a book of higher maths to show how smart he was; and his mother was in the kitchen prepar­ing one of his father’s favourite meals to show how lov­ing she was. It didn’t look like they were talk­ing to one an­other at all. As scary as ar­gu­ments could be, not ar­gu­ing was some­how much worse.

“Mum,” Harry said into the un­nerv­ing silence, “I’m go­ing to test the hy­poth­e­sis. Ac­cord­ing to your the­ory, how do I send an owl to Hog­warts?”

His mother turned from the kitchen sink to stare at him, look­ing shocked. “I—I don’t know, I think you just have to own a magic owl.”

That should’ve sounded highly sus­pi­cious, oh, so there’s no way to test your the­ory then, but the pe­cu­liar cer­tainty in Harry seemed will­ing to stick its neck out even fur­ther.

“Well, the let­ter got here some­how,” Harry said, “so I’ll just wave it around out­side and call ‘let­ter for Hog­warts!’ and see if an owl picks it up. Dad, do you want to come and watch?”

His father shook his head minutely and kept on read­ing. Of course, Harry thought to him­self. Magic was a dis­grace­ful thing that only stupid peo­ple be­lieved in; if his father went so far as to test the hy­poth­e­sis, or even watch it be­ing tested, that would feel like as­so­ci­at­ing him­self with that...

Only as Harry stumped out the back door, into the back gar­den, did it oc­cur to him that if an owl did come down and snatch the let­ter, he was go­ing to have some trou­ble tel­ling Dad about it.

But—well—that can’t re­ally hap­pen, can it? No mat­ter what my brain seems to be­lieve. If an owl re­ally comes down and grabs this en­velope, I’m go­ing to have wor­ries a lot more im­por­tant than what Dad thinks.

Harry took a deep breath, and raised the en­velope into the air.

He swal­lowed.

Cal­ling out Let­ter for Hog­warts! while hold­ing an en­velope high in the air in the mid­dle of your own back gar­den was… ac­tu­ally pretty em­bar­rass­ing, now that he thought about it.

No. I’m bet­ter than Dad. I will use the sci­en­tific method even if it makes me feel stupid.

“Let­ter—” Harry said, but it ac­tu­ally came out as more of a whispered croak.

Harry steeled his will, and shouted into the empty sky, “Let­ter for Hog­warts! Can I get an owl?

“Harry?” asked a be­mused woman’s voice, one of the neigh­bours.

Harry pul­led down his hand like it was on fire and hid the en­velope be­hind his back like it was drug money. His whole face was hot with shame.

An old woman’s face peered out from above the neigh­bour­ing fence, griz­zled grey hair es­cap­ing from her hair­net. Mrs. Figg, the oc­ca­sional babysit­ter. “What are you do­ing, Harry?”

“Noth­ing,” Harry said in a stran­gled voice. “Just—test­ing a re­ally silly the­ory—”

“Did you get your ac­cep­tance let­ter from Hog­warts?”

Harry froze in place.

“Yes,” Harry’s lips said a lit­tle while later. “I got a let­ter from Hog­warts. They say they want my owl by the 31st of July, but—”

“But you don’t have an owl. Poor dear! I can’t imag­ine what some­one must have been think­ing, send­ing you just the stan­dard let­ter.”

A wrin­kled arm stretched out over the fence, and opened an ex­pec­tant hand. Hardly even think­ing at this point, Harry gave over his en­velope.

“Just leave it to me, dear,” said Mrs. Figg, “and in a jiffy or two I’ll have some­one over.”

And her face dis­ap­peared from over the fence.

There was a long silence in the gar­den.

Then a boy’s voice said, calmly and quietly, “What.”