Harry Potter and the Methods of Rationality (21 page)

“So…” Harry said slowly. “People just find that the universe… happens to be self-consistent, somehow, even though it has time-travel in it. If I and my future self interact then I’ll see the same thing as both of me, even though, on my own first run through, my future self is already acting in full knowledge of things that, from my own perspective, haven’t happened yet…” Harry’s voice trailed off into the inadequacy of English.

“Correct, I think,” said Professor McGonagall. “Although wizards
are
advised to avoid being seen by their past selves. If you’re attending two classes at the same time and you need to cross paths with yourself, for example, the first version of you should step aside and close his eyes at a known time - you have a watch already, good - so that the future you can pass. It’s all there in the pamphlet.”

“Ahahahaa. And what happens when someone
ignores
that advice?”

Professor McGonagall pursed her lips. “I understand that it can be quite disconcerting.”

“And it doesn’t, say, create a paradox that destroys the universe.”

She smiled tolerantly. “Mr. Potter, I think I’d remember hearing if
that
had ever happened.”

“
THAT IS NOT REASSURING! HAVEN’T YOU PEOPLE EVER HEARD OF THE ANTHROPIC PRINCIPLE? AND WHAT IDIOT EVER BUILT ONE OF THESE THINGS FOR THE FIRST TIME?
”

Professor McGonagall actually laughed. It was a pleasant, glad sound that seemed surprisingly out of place on that stern face. “You’re having another ‘you turned into a cat’ moment, aren’t you, Mr. Potter. You probably don’t want to hear this, but it’s quite endearingly cute.”

“Turning into a cat doesn’t even
BEGIN
to compare to this. You know right up until this moment I had this awful suppressed thought somewhere in the back of my mind that the only remaining answer was that my whole universe was a computer simulation like in the book
Simulacron 3
but now
even that is ruled out
because this little toy
ISN’T TURING COMPUTABLE!
A Turing machine could simulate going back into a defined moment of the past and computing a different future from there, an oracle machine could rely on the halting behavior of lower-order machines, but what you’re saying is that reality somehow self-consistently computes in one sweep using information that hasn’t… happened… yet…”

Realisation struck Harry a pile-driver blow.

It all made sense now. It all
finally
made sense.

“
SO THAT’S HOW THE COMED-TEA WORKS!
Of
course!
The spell doesn’t
force
funny events to happen, it just makes you
feel an impulse to drink
right before funny things are going to happen anyway! I’m such a fool, I should have realised when I felt the impulse to drink the Comed-Tea before Dumbledore’s second speech,
didn’t
drink it, and then choked on my own saliva instead - drinking the Comed-Tea doesn’t cause the comedy, the comedy causes you to drink the Comed-Tea! I saw the two events were correlated and assumed the Comed-Tea had to be the cause and the comedy had to be the effect because I thought temporal order restrained causation and causal graphs had to be acyclic BUT IT ALL MAKES SENSE ONCE YOU DRAW THE CAUSAL ARROWS GOING
BACKWARDS IN TIME!
”

Realisation struck Harry the
second
pile-driver.

This one he managed to keep quiet, making only a small strangling sound like a dying kitten as he realised who’d put the note on his bed this morning.

Professor McGonagall’s eyes were alight. “After you graduate, or possibly even before, you really
must
teach some of these Muggle theories at Hogwarts, Mr. Potter. They sound quite fascinating, even if they’re all wrong.”

“Glehhahhh…”

Professor McGonagall offered him a few more pleasantries, demanded a few more promises to which Harry nodded, said something about not talking to snakes where anyone could hear him, reminded him to read the pamphlet, and then somehow Harry found himself standing outside her office with the door closed firmly behind him.

“Gaahhhrrrraa…” Harry said.

Why yes his mind
was
blown.

Not least by the fact that, if not for the Prank, he might well have never obtained a Time-Turner in the first place.

Or would Professor McGonagall have given it to him anyway, only later in the day, whenever he got around to asking about his sleep disorder or telling her about the Sorting Hat’s message? And would he, at that time, have wanted to pull a prank on himself which would have led to him getting the Time-Turner
earlier?
So that the only
self-consistent
possibility was the one in which the Prank started before he even woke up in the morning…?

Harry found himself considering, for the first time in his life, that the answer to his question might be literally
inconceivable.
That since his own brain contained neurons that only ran forwards in time, there was
nothing
his brain could do, no operation it could perform, which was conjugate to the operation of a Time Turner.

Up until this point Harry had lived by the admonition of E. T. Jaynes that if you were ignorant about a phenomenon, that was a fact about your own state of mind, not a fact about the phenomenon itself; that your uncertainty was a fact about you, not a fact about whatever you were uncertain about; that ignorance existed in the mind, not in reality; that a blank map did not correspond to a blank territory. There were mysterious questions, but a mysterious answer was a contradiction in terms. A phenomenon could be mysterious
to
some particular person, but there could be no phenomena mysterious of themselves. To worship a sacred mystery was just to worship your own ignorance.

So Harry had looked upon magic and refused to be intimidated. People had no sense of history, they learned about chemistry and biology and astronomy and thought that these matters had always been the proper meat of science, that they had
never been
mysterious. The stars had once been mysteries. Lord Kelvin had once called the nature of life and biology - the response of muscles to human will and the generation of trees from seeds - a mystery “infinitely beyond” the reach of science. (Not just a little beyond, mind you, but
infinitely
beyond. Lord Kelvin certainly had felt a huge emotional charge from
not knowing something
.) Every mystery ever solved had been a puzzle from the dawn of the human species right up until someone solved it.

Now, for the first time, he was up against the prospect of a mystery that was threatening to be
permanent
. If Time didn’t work by acyclic causal networks then Harry didn’t understand what was meant by cause and effect; and if Harry didn’t understand causes and effects then he didn’t understand what sort of stuff reality might be made of instead; and it was entirely possible that his human mind never
could
understand, because his brain was made of
old-fashioned linear-time neurons,
and this had turned out to be an impoverished subset of reality.

On the plus side, the Comed-Tea, which had once seemed all-powerful and all-unbelievable, had turned out to have a much simpler explanation. Which he’d missed
merely
because the truth was completely outside his hypothesis space or anything that his brain had evolved to comprehend. But now he actually
had
figured it, probably. Which was sort of encouraging. Sort of.

Harry glanced down at his watch. It was nearly 11AM, he’d gotten to sleep last night at 1AM, so in the natural state of affairs he’d go to sleep tonight at 3AM. So to go to sleep at 10PM and wake up at 7AM, he should go back five hours total. Which meant that if he wanted to get back to his dorm at around 6AM, before anyone was awake, he’d better hurry up and…

Even in
retrospect
Harry didn’t understand how he’d pulled off
half
the stuff involved in the Prank. Where had the
pie
come from?

Harry was starting to seriously fear time travel.

On the other hand, he had to admit that it
had
been an irreplaceable opportunity. A prank you could only pull on yourself once in a lifetime, within six hours of when you first found out about Time-Turners.

In fact that was even
more
puzzling, when Harry thought about it. Time had presented him with the finished Prank as a
fait accompli,
and yet it was, quite clearly, his own handiwork. Concept and execution and writing style. Every last part, even the ones he still didn’t understand.

Well, time was a-wasting and there were at most thirty hours in a day. Harry did know
some
of what he had to do, and he might figure out the rest, like the pie, while he was working. There was no point putting it off. He couldn’t exactly accomplish anything stuck here in the
future.

Five hours earlier, Harry was sneaking into his dorm with his robes pulled up over his head as a thin sort of disguise, just in case someone was already up and about and saw him at the same time as Harry lying in his bed. He didn’t want to have to explain to anyone about his little medical problem with Spontaneous Duplication.

Fortunately it seemed that everyone was still asleep.

And there also seemed to be a box, wrapped in red and green paper with a bright golden ribbon, lying next to his bed. The perfect, stereotypical image of a Christmas present, although it wasn’t Christmas.

Harry crept in as softly as he could manage, just in case someone had their Quieter turned down low.

There was an envelope attached to the box, closed by plain clear wax without a seal impressed.

Harry carefully pried the envelope open, and took out the letter inside.

The letter said:

This is the Cloak of Invisibility of Ignotus Peverell, passed down through his descendants the Potters. Unlike lesser cloaks and spells it has the power to keep you
hidden
, not merely invisible. Your father lent it to me to study shortly before he died, and I confess that I have received much good use of it over the years.

In the future I shall have to get along with Disillusionment, I fear. It is time the Cloak was returned to you, its heir. I had thought to make this a Christmas present, but it wished to come back to your hand before then. It seems to expect you to have need of it. Use it well.

No doubt you are already thinking of all manner of wonderful pranks, as your father committed in his day. If his full misdeeds were known, every woman in Gryffindor would gather to desecrate his grave. I shall not try to stop history from repeating, but be MOST careful not to reveal yourself. If Dumbledore saw a chance to possess one of the Deathly Hallows, he would never let it escape his grasp until the day he died.

A Very Merry Christmas to you.

The note was unsigned.

“Hold on,” Harry said, pulling up short as the other boys were about to leave the Ravenclaw dorm. “Sorry, there’s something else I’ve got to do with my trunk. I’ll be along to breakfast in a couple of minutes.”

Terry Boot scowled at Harry. “You’d better not be planning to go through any of our things.”

Harry held up one hand. “I swear that I intend to do nothing of the sort to any of your things, that I only intend to access objects that I myself own, that I have no pranking or otherwise questionable intentions towards any of you, and that I do not anticipate those intentions changing before I get to breakfast in the Great Hall.”

Terry frowned. “Wait, is that -”

“Don’t worry,” said Penelope Clearwater, who was there to guide them. “There were no loopholes. Well-worded, Potter, you should be a lawyer.”

Harry Potter blinked at that. Ah, yes, Ravenclaw
prefect.
“Thank you,” he said. “I think.”

“When you try to find the Great Hall, you will get lost.” Penelope stated this in the tones of a flat, unarguable fact. “As soon as you do, ask a portrait how to get to the first floor. Ask another portrait the
instant
you suspect you might be lost again.
Especially
if it seems like you’re going up higher and higher. If you are higher than the whole castle ought to be,
stop
and wait for search parties. Otherwise we shall see you again four months later and you will be five months older and dressed in a loincloth and covered in snow and
that’s if you stay inside the castle.
”

“Understood,” said Harry, swallowing hard. “Um, shouldn’t you tell students all that sort of stuff right away?”

Penelope sighed. “What,
all
of it? That would take weeks. You’ll pick it up as you go along.” She turned to go, followed by the other students. “If I don’t see you at breakfast in thirty minutes, Potter, I’ll start the search.”

Once everyone was gone, Harry attached the note to his bed - he’d already written it and all the other notes, working in his cavern level before everyone else woke up. Then he carefully reached inside the Quietus field and pulled the Cloak of Invisibility off Harry-1‘s still-sleeping form.

And just for the sake of mischief, Harry put the Cloak into Harry-1‘s pouch, knowing it would thereby already be in his own.

“I can see that the message is passed on to Cornelion Flubberwalt,” said the painting of a man with aristocratic airs and, in fact, a perfectly normal nose. “But might I ask where it came from
originally?
”

Harry shrugged with artful helplessness. “I was told that it was spoken by a hollow voice that belled forth from a gap within the air itself, a gap that opened upon a fiery abyss.”

“Hey!” Hermione said in tones of indignation from her place on the other side of the breakfast table. “That’s
everyone’s
dessert! You can’t just take one whole pie and put it in your pouch!”

“I’m not taking one pie, I’m taking two. Sorry everyone, gotta run now!” Harry ignored the cries of outrage and left the Great Hall. He needed to arrive at Herbology class a little early.

Professor Sprout eyed him sharply. “And how do
you
know what the Slytherins are planning?”