Chapter five:

There is a range of possible observations, each with a different probability. According to the many-worlds interpretation, each of these possible observations corresponds to a different universe. Suppose adieis thrown that contains six sides and that the numeric result of the throw corresponds to a quantum mechanicsobservable. All six possible ways the die can fall correspond to six different universes.

The many-worlds interpretation is very vague about the ways to determine when splitting happens, and nowadays usually the criterion is that the two branches have decohered. However, present day understanding of decoherence does not allow a completely precise, self-contained way to say when the two branches have decohered/"do not interact", and hence many-worlds interpretation remains arbitrary. This objection is saying that it is not clear what is precisely meant by branching, and point to the lack of self-contained criteria specifying branching.

I stared at the pages, willing my brain to understand what I was reading.

Also, it is a common misconception to think that branches are completely separate. In Everett's formulation, they may in principlequantum interfere(i.e., "merge" instead of "splitting") with each other in the future,although this requires all "memory" of the earlier branching event to be lost, so no observer ever sees two branches of reality.

So, multiple universes can exist. And the reason is…oh, bollocks.

The splitting of worlds forward in time, but not backwards in time (i.e., merging worlds), is time asymmetric and incompatible with the time symmetric nature ofSchrödinger's equation, orCPT invariancein general.

What on earth did this mean?

"You up yet, Harry?"

I slid the book under my covers quickly and pulled back the curtains around my bed, pretending to yawn. "Yeah, just woke up."

Ron's hair was sticking up in the back as he began digging for a clean uniform. "I hope Flitwick doesn't go overboard on the homework again today. You finished it, yet?"

"No, I was gonna work on it at breakfast," I said, standing up and stretching.

There ended up being such a demand to host quidditch try-outs that I booked the pitch that weekend. I talked Ron into trying out, and mentioned to Ginny that she should, too. I also made sure to approach my new brother Ben—according to the other Harry's journals, he often played with his friends on school holidays.

"Nah, that's all right," he said, shrugging.

"Really?"

"I never had much interest in joining the team. Besides, that way no one would think you had to put me on because I was your brother."

"I'd put you on because you were good," I said sternly. "What anyone else thinks is rubbish."

He shrugged again. "Thanks though. Hey, Mum wants to you write and tell her how your classes are."

"Er, yeah," I said. I had to keep reminding myself I had family that wanted to hear about my life. It was weird. I all but forgot about Ben on the first day of classes, and it wasn't until he joined us at breakfast the following day that I remembered this Harry had a younger brother.

Just as I remembered from my world, the professors weren't shy about assigning ridiculous amounts of homework.

"You are in a N.E.W.T. level class, and your workload will reflect as such," said Professor McGonagall during a particularly grueling Transfiguration lesson. "Many of you intend to advance on to higher education, and will be expected to have top scores."

Luckily I had taken all of these classes before. I was barely scraping by in Potions, but I was top of the class in all my other subjects.

I didn't have Defense Against the Dark Arts until Thursday, but I had secretly been looking forward to it all week. It was easily my best subject, and I was sure I was way ahead of the rest of the class. It would be nice to have a subject I could slack off in while I worked out theories for multiple universes and time travel.

Ron, Dean, and Seamus were all in good spirits as well at the start of class, but I'm pretty sure it was for a different reason. We shared the lecture with the Hufflepuff class, and I could see Hemione sitting alongside Hannah Abbot on the other side of the room.

Dowson entered the classroom then with nothing more than a clipboard in hand. Now that I got a good look at her, I saw she was rather short and broad-shouldered. But she definitely had a beautiful face, and out of the corner of my eye I could see Seamus straightening up in his seat instantly.

"I'm starting a dueling club for all students fourth-year and up," she said by way of greeting. "If anyone is interested," she said, waving the clipboard. "Sign your name here. If you join, you'll receive a point of extra credit for each session you attend."

The class all exchanged glances before suddenly getting to its feet and rushing the sign-up sheet. I would have joined the club regardless, but I couldn't turn down the prospect of easy extra-credit in a N.E.W.T. level class. Neither, apparently, could anyone else in the room. Once Justin Finch-Fletchley had signed his name and returned to his seat, Dowson turned to us again.

"As you all know, I'm Professor Dowson. I graduated Hogwarts five years ago, and joined the Auror Academy right afterwards. I decided being an Auror wasn't the career for me, and Professor Dumbledore offered me this position.

"While I know it's difficult to go through so many professors on a single, complicated subject, you will be expected to keep up. I teach beyond the N.E.W.T. level—there is no sense to mastering the bare minimum. Most of you will be attending higher education after Hogwarts, and it is my hope that you will be over-prepared."

Constant vigilance, I thought. I wonder if she had the opportunity to meet Mad-Eye Moody in her Auror training.

"We will be covering Dark Creatures until Halloween—it is my understanding you've had a brief course covering the major topics, so we'll be catching up. Halloween through Christmas break we'll cover identifying Dark Magic. The rest of the year will be balanced between defensive spells and counter-curses. Are there any questions?"

She spoke so quickly that I almost didn't keep up.

"Excellent," she said crisply, pulling a chalkboard toward her. She flipped it over and revealed a large picture of a dementor.

"You've covered the basics of dementors in previous classes," she said, brushing chalk dust off her hands. "Putting your knowledge and skill together is very different in the presence of a dementor. I obviously cannot bring the real thing to this class, so we'll be practicing on a substitute next week."

I relaxed a little at this while the class stiffened and shot each other wide-eyed looks. I had battled a hundred dementors at one time—even if it was a sort of fluke.

"Who here has seen a dementor in real life?"

I almost raised my hand; I quickly played it off by scratching the back of my neck.

"And hopefully none of you ever will," she said, looking over the motionless class. "As you know, the only known defense against a dementor is the Patronus Charm. And when I say 'defense' I mean that literally; there is no known spell or enchantment to destroy a dementor. The Patronus Charm is meant to buy you time until you can escape from the situation.

"Within Great Britain, the Ministry has offered a deal with the dementors in our borders. In exchange for employment at Azkaban prison, dementors will not prey on innocent witches or wizards. This is not a guarantee that you may never encounter one, as different countries have different laws and regulations. It is also ignorant to presume that an entity of witches and wizards can have total control over a Dark Creature."

She sat on the edge of her desk and pointed her wand at the chalkboard.

"There are a few defenses you can utilize if cornered by a dementor," she continued. "Though they vary greatly from one individual to the next. The first is Occlumency," she listed, the chalk writing the word quickly on the board. "Who can tell me what that is?"

There was a brief pause. I expected Hermione to raise her hand, but it seems this was one of the rare subjects she was unfamiliar with. I raised mine slowly, feeling all the eyes of the room land on me.

"Yes?" she said, nodding in my direction.

"The act of blocking magical penetration into your mind," I said, remembering my hazardous training with Snape. "Its opposite practice is Legilimency."

"Very good," she said. "Dementors feed on any happy memory you have—if you can block access to these memories, focusing only on neutral experiences, you may be able to ward off a dementor's effects longer. Occlumency is a very difficult practice to learn, and most are not well-trained enough for Occlumency to be effective.

"A second defense is Transfiguration," she continued. "Either of yourself into a creature or object with less complex emotions than a human, or of objects into complex beings. This is to be considered a last defense, as you are merely confusing the dementor until help arrives. This is also very difficult unless you are proficient in Advanced Transfiguration."

She conjured up a pot of tea and we all watched in silence as she poured herself a cup. "The last and most effective defense is of course the Patronus Charm," she said, taking a sip. "Some of you may have had a little practice with it last year." She set down her tea cup and stood up. "What most of you will achieve is a sort of white cloud—an iridescent, shapeless Patronus." With a quick flick of her wand, a shimmering silver cloud shot out like a wisp of smoke. "A true Patronus takes a distinct shape, often an animal significant to the witch or wizard. A corporeal Patronus is difficult to cast with minimal distractions, and near impossible when overwhelmed by dementors." Dowson raised her wand again, and a huge hawk shot out, soaring around the classroom twice before disappearing.

"Now, who remembers what's necessary to summon a Patronus?"

Hermione's hand shot into the air.

"Yes—Granger, isn't it?"

"Yes. In order to conjure a Patronus, the witch or wizard must think of a powerful memory—their happiest memory."

"Correct. This may sound simple enough, but it is very, very difficult. It requires you to search through your memories to bring the happiest ones forward—this takes a great deal of mental control. You will often find in this class that defense against Dark Magic—and even Dark Magic itself—is based very little in spells and wand-waving. Having the power to control your mind is pivotal in casting and fighting Dark Magic. Now," she said, clapping her hands together. "I want you all to have a think about your happiest memory—think about the sounds around you. Laughter, music, talking, nature—whatever it was. And what did it smell like? Was the sun hot on your skin, was there rain? Close your eyes and think."

I looked around at the other students before doing as instructed. So much surfaced to my mind, but it was all complicated. All my good memories were marred by something awful—I couldn't think about Sirius, Ginny, or my friends without Voldemort creeping in. I dug back further, to a time when Voldemort was little more than a name to me. The night in an old shack out to sea when Hagrid first told me about Hogwarts.

"Now, who wants to try first?" Dowson asked. I opened my eyes. The class were all looking at each other to see who would volunteer first. "Come on, you're all going to have to do it," she prodded.

I knew I could do it, but I was sure this world's Harry had never so much as attempted the Patronus Charm, and it would definitely raise suspicion if I managed to summon a full, corporeal stag in the classroom. I supposed I could try to be bad on purpose…

"Mr. Potter, you look like you're thinking about something quite hard—why don't you give it a go?"

Oh crap.

I got to my feet and cleared my throat. "Just…up here?" I asked, looking around the room. Everyone was watching me.

"You can't hurt anybody with this."

"Right." I cleared my throat again. I tried to think of a halfway-happy memory. Er, there was the time Gryffindor won the Quidditch Cup last year—"

I had never tried to deliberately screw up my spellwork before. I raised my hand, thinking hard of the after-party in the Common Room, and hoped for the best. "Expecto Patronum!"

A huge white cloud burst from my wand, but it had no shape to it. Thank Merlin. I had been hoping to achieve the pathetic mist I struggled with in third year so it looked more believable, but I supposed this would work.

Dowson looked pleasantly surprised. "That's the first time I've seen someone cast more than mist on their first go-around," she said, nodding.

Oh crap.

"Try it again," she encouraged, folding her arms.

I bit my lip, feeling all eyes in the room upon me. On one hand it'd be kind of cool to just go for it and impress everone, but that would have to raise suspicions. I searched my mind for another halfway happy memory, and settled on the Christmas I got a Firebolt.

Once again, shapeless silver clouds.

"You look distracted when you're casting," said Dowson. "Close your eyes."

Constant vigilance creeped into the back of my head, but I did as I was told.

"Tell me when you have your memory."

"I have it," I said automatically. I would go with making the Gryffindor Quidditch team.

"Okay. Now I want you to think about when this occurred—what season was it? Can you remember what the weather was like? Out loud, it'll help your focus."

"It was early fall."

"How was the weather?"

I thought. "Er, sunny. Not too warm or cold yet."

"Were there other people around?"

"Yeah."

"Think about their faces. What did they look like, what were they doing?"

The memory of Malfoy knicking Neville's Remembrall crept into my mind; how I had managed to snatch it from a hundred-foot freefall my first time on a broom.

"Think on why this made you happy—were you surrounded by family or friends, was there a thrill of excitement? Focus on that feeling."

It had been the first time I was effortlessly good at anything. The Dursleys used to keep me so down-trodden, and I wasn't used to being considered talented or useful.

Or accepted.

"Do it!"

I jumped and blurted the spell before I could clear my mind. A huge silver cloud shot forth, dancing around the room. It wasn't much more than mist and smoke, but I know Dowson didn't miss the slight dog-like shape it took before disappearing.

Dowson looked ecstatic. "Very good! That was very good! You see, by perfecting your ability to focus, you add so much more power to the same exact spellwork. You can sit, Potter. Who wants to try next?"

My brain was fuzzy. I slumped into my chair without really being aware of the awed looks or playful punches Ron, Dean, and Seamus gave me.

All I could think about was how my Patronus had always been a stag.

But this was a dog.

A giant, bear-like dog that was just like one I knew from a previous life.

It was hard to focus the rest of the day. Word quickly spread about my almost-patronus, and I had several faces come up to me and want to talk about it. Transfiguration was a blur, but double Herbology kept everyone busy and distracted, giving me an opportunity to be left alone with my thoughts.

Was I replacing my dad with Sirius?

The try-outs were ridiculously messy. The whole affair lasted the better part three hours, and the fact that almost every position had to be replaced didn't help. Katie and I were the only original teammates left, and by the time the third wave of prospects—a group of giggling girls including Romilda Vane and that Clare Philips—came through, Katie looked like she had given up on life.

Groups showed up without brooms. There were students from other Houses, and several first-year students who had clearly never flown before. There were a couple of crashes and a lot of broken teeth. After her try-out, Ginny was gracious enough to lead a group of loudly-complaining students to the hospital wing.

Ginny showed up and was excellent, just as I figured. I hadn't seen much of her that first week of term, but watching her fly around the Quidditch pitch brought forth memories of our impromptu kiss. I had to keep reminding myself that this was a different Ginny.

Two hours in, I was ready to rip out my hair. I was sure I had my Chasers and one Beater picked out, but that still left two open seats. I ran my hands through my hair for the millionth time that day, and called out the next to last group. Cormac McLaggen had been busy discussing his Quidditch expertise with the other students and gave me an affable smile. I was too irritated to return it.

"Right, I'm going to divide you into Chasers and Beaters," I said, eyeballing the group real quick to make sure they all had brooms.

"What about Keeper?" Cormac asked.

I tried not to be annoyed. I really didn't have a reason to be annoyed with Cormac McLaggen, except for the fact that he was competing for what was supposed to be Ron's spot on the team.

Ron, who still hadn't shown up.

"Okay, fine, you can play Keeper," I said distractedly. "Let's go."

I was disappointed to see that Cormac played just as well as he did in my world. I sat on my broom, glowering at the scene in front of me: he blocked every shot with precision and shot the giggling girls down below a toothy grin. Good, I thought snarkily. Maybe Romilda and Clare will go after him and leave me alone.

Finally, when I was running out of excuses to prolong the end of try-outs, Ron finally showed up with a Nimbus in hand, a broom that looked suspiciously like the one the Whomping Willow destroyed in my third year.

"Finally!" I said to Ron, meeting him on the edge of the Quidditch Pitch. "Where've you been?"

"Don't distract me—let's get this over with."

I raised my eyebrows but didn't argue. "Er, okay."

I set up one last group—putting Ginny on as Chaser again—and Katie and I mounted our brooms.

Ron fumbled the first ball that came his way, but he did manage to block it from going through any of the hoops. He looked nervous, but quickly got over it once he made a close save five minutes in. The group of spectators down below cheered, and this seemed to give Ron the boost of confidence he needed.

"All right, that's it!" I shouted, blowing my whistle. "Results will be posted to the Common Room in a few days! Do not come find me and ask about it before that, or I'll ban you from the team!"

"Think I did all right?" Ron asked as Katie and I began to lock up the quidditch balls. Most of the crowd had dispersed at this point, eager to spend the remainder of their Saturday lounging around.

"You were excellent!" I said genuinely. "I'll see you later, Katie—" I added while she went to go put the equipment away.

"Yeah, I had no idea I wouldn't be rubbish at Keeper," said Ron, still a little breathless. "But Cormac wasn't bad, either."

I glanced up to make sure no one could overhear me. "Cormac's an idiot—spent half the time watching the girls down below. I should have let a bludger out—teach him to pay attention during a game."

Ron grinned at that. "Well, I'm starving. Let's go get some food, yeah?"

I learned from last year, and was careful not to accept any gifts or favors until after the new team list was posted. I avoided the common room as much as possible that weekend, preferring to hide out with Ron in the library or by the lake. Our homework load was ridiculous, and the assignments from Snape easily matched three of my other classes put together. It was lucky that I had studied most of this before; Ron and I didn't have Hermione to help us get through our work this time around.

Hermione and I were paired together two nights a week for patrol—this was mostly spent walking around the corridors at night, telling off students who were breaking curfew. Hermione wasn't much of a talker in the beginning, but I convinced her that having a conversation wouldn't detract from our work performance. She grudgingly obliged, and I had to wonder how on earth this version of her was so rigid. I would bet my Firebolt that she hadn't broken a single rule in this world.

"I've seen you in the library a lot," she said one night. Usually I had to start the conversations. "What are you studying?"

"Huh? Oh, I was just trying to get through McGonagall's homework earlier—between her and Snape, I think I'm going to explode."

"No, I mean on your own. I saw you with some books that we're not using for class. Are you studying something on your own?"

I had wanted Hermione's help with the subject, but this wasn't quite how I pictured bringing it all up.

"Er, yeah. Sirius—er, my godfather—gave me this book over the summer and so I've been doing some research on my own about it. And I'm pretty much still stuck at page one. It doesn't really make any sense."

Hermione gave me a sort of bemused, patient look. "Can I ask what subject it is?"

"Huh? Oh, right—er, alternate universes? You know, like multiple timelines."

"That does sound complicated."

"The idea of it is straightforward. Say you have a die you cast six times," I said, recalling the information from one of the books I had checked out. "Each side corresponds to a potential future—each one is acted out in a separate line."

"That would be interesting," said Hermione, sounding genuine. "It's like with Time-Turners—I've always wondered what happens to the world that you leave when you travel back—does it continue on and then get replaced by the events from traveling back? And if those events are replaced, can you argue that the first one never existed?"

I remembered the night Ron, Hermione and I saved Sirius and Buckbeak with Hermione's Time Turner.

"I dunno. There are mostly just theories," I said. "And they're all equally confusing. But they all agree that once the timeline separates—once two observable universes are distinct from each other—they cannot touch again—meaning you can't hop from one to the other."

"I wonder when they would split," said Hermione, sounding the most relaxed I had yet seen her. "Each day you wake up—every decision you make, every person you meet, is an opportunity for a different path. It sounds like if anything, the universe is branched into millions and millions of parts, rather than just two straight lines diverging at some point and running parallel."

"Huh. I guess I didn't think of it that way," I said. We were making rounds on the seventh floor, and had rounded another empty corner. "So every situation could lead to a new timeline? It doesn't have to be significant?"

Hermione shrugged. "Well, if our basis is that each separate branch occurs for every probability, I guess so. You'd make a thousand new potential futures in a single day," she said, raising her eyebrows at the thought. "And if you travel back in time, you can adjust any of them—and then create new ones."

"What about moving between alternate timelines?" I asked. "Not just time-traveling, but actually moving from one universe to another. Think that's possible?"

Hermione thought for a long moment. "I don't know," she finally said. "It might be do-able in theory—but how you would actually achieve it is hard to imagine. How did you get on this subject again? It seems awfully dense for a self-study."

"Er, my godfather," I lied. "He travels a lot for work, so he's always bringing home these crazy books and stuff. This one just sounded really interesting."

"Well, I do know that the Ministry studies things like this," Hermione added. "You know, if you're serious about it, and want to pursue it to that degree."

"What, like a job?"

She shrugged. "It's not too far from now that we'll be graduating," she said.

"Yeah, I s'pose so."

We had circled the floor again, and were outside the Fat Lady's portrait. I checked my watch. "Five past ten. Guess our time's up."

Hermione gave the password and we entered the empty common room together.

"Er, well, see you in the morning," I said awkwardly, heading up the boys' staircase.

She nodded. "Right. Good night."

The others were all asleep. I yanked my bed's hangings shut and pulled out several books from underneath my pillow.

"Lumos," I whispered, searching for some parchment and a quill. I used one of the thick textbooks as a writing surface, and began detailing everything I had learned into a letter to Sirius, including Hermione's insight. By the time I was done, my letter was almost three pages long, back-to-front.

I hastily wrote a second one to my parents, this one much shorter and lighter in tone. I told them about the quidditch try-outs, about how classes were going. It was very superficial compared to the novel I wrote Sirius, but there was nothing else I could say. I couldn't tell them how I wasn't really their Harry, and that Merlin knows where the real one went. No doubt Sirius was used to stranger things, and even he could barely handle the idea.

My mind drifted back to my changed patronus in class the other day.

The dog.

Not a stag.

I pulled out my want, trying to focus very clearly on my happiest memory. I recalled how unexplainably enormous Hagrid had seemed, and remembered the squished birthday cake he had made for me. I remembered my complete shock and delight at being told I was a wizard who would soon attend Hogwarts.

A bright white light shot out of my wand, straight through my four poster's hangings. I yanked them back to get a good look at the animal circling around the room, and saw the unmistakable shape of antlers.

"What the hell?"

"'s going on?"

My patronus dissipated quickly as Ron yanked his bed hangings to the side. "The bloody hell's that?" he muttered sleepily.

"Er, nothing. Go back to sleep."

Ron flopped back on his bed, and began snoring almost instantly.

I pulled my hangings shut again. I felt relieved at seeing the stag again, but I couldn't help wonder why it had changed at all in the Defense classroom. I was torn—on one hand I didn't want the stag to be replaced, but I also felt guilty for not wanting Sirius in that role.

In a lot of ways, Sirius was like a father.

And now in this world I had both. It was way more complicated than I had expected it to be.