The Senior Common Room was unusually crowded.
Professor Quillibrace had placed a copy of The House That Had Two Histories on the table.
Mr Blottisham was standing beside it.
Miss Elowen Stray was examining the sugar bowl.
“I've solved it,” said Blottisham.
Quillibrace looked up.
“You have?”
“Obviously.”
“Would you care to explain?”
“It is about quantum mechanics.”
Quillibrace nodded.
“Yes.”
“Specifically, quantum uncertainty.”
“Yes.”
“And therefore anything can happen.”
“No.”
Blottisham frowned.
“That is what quantum uncertainty means.”
“It does not.”
“Then what does it mean?”
“That we cannot assign simultaneous arbitrary precision to certain pairs of physical quantities.”
Blottisham looked disappointed.
“That is considerably less useful.”
Miss Stray put down the sugar bowl.
“Perhaps that's the point.”
Blottisham picked up the book.
“The house has two histories.”
“Apparently,” said Quillibrace.
“Therefore it exists in two states at once.”
“Not necessarily.”
“But the story says so.”
“The story says the house appears to have two histories.”
“That is the same thing.”
“No.”
Blottisham sighed.
“Why do you insist on making everything difficult?”
“Because you insist on making everything simple.”
Miss Stray smiled.
“I think the important thing is what happens when Anna asks which history is the real one.”
“What happens?”
“The house gives her an answer.”
“Which one?”
“Whichever history is selected by what she does.”
Blottisham brightened.
“So consciousness collapses the wave function.”
Quillibrace closed his eyes.
“No.”
“Observation does.”
“No.”
“Looking does.”
“No.”
“Measurement does.”
“Yes.”
“Aha!”
“Do not celebrate yet.”
Quillibrace turned to the passage about the coin.
“The coin is a useful example.”
“It is a coin,” said Blottisham.
“Indeed.”
“It has heads or tails.”
“Yes.”
“Therefore it has a definite state.”
“Classically, yes.”
“And the story says it has both.”
“The story says Anna cannot infer which state it occupies merely by refusing to look.”
Blottisham looked puzzled.
“But surely it must be one or the other.”
Quillibrace paused.
“In quantum mechanics, the distinction between having a definite value and being in a state that can be represented as a superposition of possible measurement outcomes is crucial.”
Blottisham considered this.
“So the coin is both.”
“No.”
“Neither?”
“No.”
“Somewhere in between?”
“No.”
“Then what?”
Quillibrace sighed.
“It is in a quantum state.”
Blottisham nodded solemnly.
“Both.”
“No.”
“Not both.”
“No.”
“Neither.”
“No.”
Blottisham looked triumphant.
“Exactly.”
Miss Stray turned a page.
“I think Anna's notebook is more interesting.”
“The notebook?” said Blottisham.
“She first writes, ‘The red ball was on the stairs.’ Then she changes it to, ‘The red ball may have been on the stairs.’ Finally she writes, ‘When I looked, the red ball was on the stairs.’”
“Yes,” said Quillibrace.
“Why is the last sentence better?”
“Because it reports an event of observation without pretending that the observation simply revealed a pre-existing classical fact.”
Blottisham looked doubtful.
“But the ball was there.”
“When she looked.”
“So it was there.”
“When she looked.”
“You're being pedantic.”
“I am being precise.”
Miss Stray said, “The distinction is subtle. The story isn't saying that nothing existed before Anna looked. It's questioning whether the properties she wants to assign to it can always be treated as though they had definite classical values independently of the measurement context.”
Blottisham stared at her.
“That is exactly what I meant.”
“No,” said Quillibrace.
There was a pause.
Then Blottisham pointed to the two front doors.
“This proves my point.”
“What point?”
“That observation changes reality.”
“No.”
“But the doors change when she looks.”
“Yes.”
“Therefore—”
“Therefore the story has constructed a fictional house in which the outcome depends on the interaction.”
“Which is what quantum mechanics says.”
“No.”
Blottisham frowned.
“Why not?”
“Because quantum mechanics does not say that human observation magically changes whatever we look at.”
“Then what does measurement do?”
“Measurement is a physical interaction between a system and something capable of recording a result.”
“So looking isn't necessary?”
“Not in the everyday sense.”
Blottisham considered this.
“That's disappointing.”
Miss Stray said, “Perhaps the story deliberately uses looking because it gives us a familiar metaphor for interaction.”
“Exactly,” said Quillibrace.
“But the deeper point is not that the mind creates the world.”
“No.”
“It is that obtaining a definite result requires an interaction that changes the situation.”
“Yes.”
Blottisham looked pleased.
“So the observer is part of the experiment.”
“Yes.”
“I knew that.”
“You said consciousness collapses reality.”
“I was simplifying.”
“You were wrong.”
Miss Stray returned to the photograph.
“This is the part I find most interesting.”
“The photograph?” said Blottisham.
“The woman says the photograph changes when Anna looks at it.”
“That's impossible.”
“Yes.”
“Unless it's quantum.”
“No.”
“Unless it's magic.”
“No.”
“Unless—”
“It's a story.”
Blottisham sat down.
“Fine.”
Miss Stray continued.
“The photograph gives us a visual version of something quantum mechanics makes conceptually difficult: the temptation to imagine that there must be one complete history sitting behind every observation.”
Quillibrace nodded.
“We are accustomed to thinking that the world possesses a single definite history, whether or not anyone knows it.”
“And quantum theory complicates that picture.”
“It certainly complicates our classical intuitions.”
Blottisham raised a finger.
“So the world doesn't have a history.”
“No.”
“It has several.”
“No.”
“One?”
“Not necessarily in the classical sense you mean.”
Blottisham lowered his finger.
“This is getting tiresome.”
Quillibrace looked at the chapter concerning the two doors.
“Now we come to superposition.”
Blottisham sat up.
“At last.”
“The two doors represent alternatives.”
“Yes.”
“Anna opens one.”
“Yes.”
“Then the story presents a definite outcome.”
“Yes.”
“So what was happening before she opened it?”
Blottisham answered immediately.
“Both possibilities existed.”
Quillibrace nodded.
“That is one way of describing a superposition.”
“Therefore I was right.”
“No.”
“Again?”
“The two possibilities are not simply two ordinary classical states waiting in storage.”
“Why not?”
“Because a superposition contains phase relations that have no classical analogue.”
Blottisham stared at him.
“Phase relations.”
“Yes.”
“Are they important?”
“Very.”
“Do they matter to the story?”
Quillibrace looked at the book.
“Not explicitly.”
Miss Stray said, “Perhaps they are there anyway.”
Quillibrace smiled.
“Yes.”
Blottisham stood again.
“I have another point.”
Quillibrace sighed.
“Proceed.”
“The house has two histories because the alternatives interfere.”
Miss Stray looked interested.
“Go on.”
“If there are two histories, they must interfere.”
“That is not quite how it works.”
“But interference occurs when alternatives are available.”
“Quantum amplitudes interfere.”
“Yes.”
“Not histories in the ordinary narrative sense.”
“Close enough.”
“No.”
Blottisham waved his hand.
“You see the problem.”
“What problem?”
“Scientists insist on describing things so precisely that nobody can understand them.”
Quillibrace replied:
“Scientists insist on describing things precisely because otherwise nobody knows what they are talking about.”
Miss Stray laughed.
She looked at the passage where Anna opens both doors.
“I think the story becomes stranger there.”
“The part where the house gets very large?” asked Blottisham.
“Yes.”
“She sees all the possibilities.”
“Does she?”
Blottisham looked at her.
“That's what it says.”
“It says she sees them.”
“Exactly.”
“But seeing all the possibilities isn't the same as observing a quantum superposition directly.”
Quillibrace nodded.
“Correct.”
“Then what is she seeing?”
“Perhaps the structure of the alternatives.”
Blottisham frowned.
“That's rather abstract.”
“Perhaps.”
Miss Stray looked at the book.
“She realises that the house doesn't have two histories. It has no single history until something selects one.”
Quillibrace was quiet for a moment.
“That's a useful fictional representation of the measurement problem.”
“Useful?”
“Yes.”
“Not literally true?”
“No.”
“But it helps us think about the question.”
“Exactly.”
Blottisham nodded.
“I see.”
Everyone looked at him.
“I don't.”
Miss Stray smiled.
“What about the man?”
“The dead man?” asked Blottisham.
“Yes.”
“Definitely a quantum effect.”
Quillibrace shook his head.
“Why?”
“Because he's dead and alive.”
“No.”
“He is.”
“In the story.”
“That's enough.”
“It isn't.”
Miss Stray said, “He is also an interesting way of making two histories meet.”
“Yes,” said Quillibrace.
“One history contains his death. Another contains his continued presence.”
“And Anna encounters both.”
“So perhaps the man is not a quantum cat.”
“No.”
“He is a narrative device for making alternative histories visible.”
“Precisely.”
Blottisham looked disappointed.
“You've removed all the fun.”
There was a pause.
Then Miss Stray said:
“I wonder whether entanglement is hidden in the story.”
Quillibrace looked at her.
“Where?”
“The woman and the man.”
Blottisham brightened immediately.
“I knew it.”
“You know nothing.”
“Thank you.”
Miss Stray continued.
“They are separated. One is dead in one history and alive in another. Yet the state of one seems to constrain how the other can appear.”
Quillibrace nodded slowly.
“That is suggestive.”
“Not literal entanglement?”
“No.”
“Why not?”
“Because entanglement is a specific quantum relation between systems, not merely the fact that two things have correlated histories.”
Blottisham said:
“So they're married.”
Quillibrace looked at him.
“What?”
“Correlated histories.”
Miss Stray laughed.
Quillibrace sighed.
“An excellent example of why physicists avoid metaphor.”
Blottisham picked up the book again.
“I think the final scene is the key.”
“The second Anna,” said Miss Stray.
“Yes.”
“She is standing behind the window.”
“And the first Anna is outside.”
“Yes.”
“So which is the real one?”
Miss Stray smiled.
“Exactly.”
Blottisham looked triumphant.
“There is no real one.”
Quillibrace said:
“Why not?”
“Because they're both Anna.”
“That's not an answer.”
“They're both observing each other.”
“Still not an answer.”
Blottisham looked irritated.
“Then which one is real?”
Quillibrace looked at Miss Stray.
She looked at him.
Neither answered.
Blottisham waited.
“That's the point, isn't it?”
Quillibrace nodded.
“Yes.”
Miss Stray said, “The story has made the observer part of the problem.”
“And then made the observer observable.”
“Yes.”
“So there is no privileged viewpoint.”
“Perhaps.”
Blottisham looked at the photograph again.
“That's rather uncomfortable.”
“Yes,” said Quillibrace.
The room was quiet for a moment.
Then Blottisham said:
“So quantum mechanics tells us that reality is uncertain.”
Quillibrace shook his head.
“No.”
“That reality is subjective.”
“No.”
“That consciousness creates reality.”
“No.”
“That everything is connected.”
“No.”
“That there are infinite universes.”
“No.”
“That nothing exists until we look at it.”
“No.”
Blottisham paused.
“What does it tell us, then?”
Quillibrace looked at the book.
“That our classical picture of what it means for a physical system to have definite properties is inadequate.”
Blottisham thought about this.
“That is considerably less memorable.”
“It has the advantage of being closer to the truth.”
Miss Stray looked out of the window.
“Perhaps that's what the house was doing.”
“What?”
“Making ordinary assumptions stop working.”
Quillibrace nodded.
“The house never tells Anna that reality is impossible.”
“It simply stops behaving as though reality had to be classical.”
Blottisham smiled.
“So the house is a physicist.”
“No.”
“A very confusing physicist.”
“No.”
“A physicist who lives in a house with two doors.”
Quillibrace looked at him.
“Now you're getting somewhere.”
Outside, the afternoon light had begun to fade.
Miss Stray closed the book.
“I wonder whether the strangest thing is actually the simplest.”
“What?” asked Blottisham.
“We assume that the world has already decided what it is.”
Quillibrace nodded.
“And quantum mechanics asks us to be more careful about what we mean by decided.”
Blottisham looked thoughtful.
“So the world doesn't answer questions until we ask them?”
“No.”
“Sometimes.”
“No.”
“Only certain questions?”
Quillibrace smiled.
“Now you're asking better questions.”
Blottisham looked pleased.
“I knew I would eventually.”
Miss Stray stood.
“And perhaps that is why the story ends with the coin.”
“The coin?”
“Yes.”
“What happens to it?”
“We never find out.”
Blottisham frowned.
“But surely it landed.”
Quillibrace looked at him.
“Did you see it?”
“No.”
“Then perhaps,” said Miss Stray, “we should leave it there.”
They all looked at the closed book.
For a moment, nobody moved.
Then something rolled across the table.
A small coin.
It spun once.
Twice.
Three times.
Blottisham reached for it.
Quillibrace said:
“Don't.”
Blottisham stopped.
The coin continued spinning.
Miss Stray smiled.
“Perhaps we should find out.”
Quillibrace looked at her.
“No.”
The coin slowed.
It wobbled.
Then stopped.
All three stared at it.
Blottisham leaned forward.
“Which side is up?”
Quillibrace did not answer.
Miss Stray did not answer.
Very slowly, Blottisham reached out.
And turned the coin over.
Nobody saw which side had been showing before he did.
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