The Last Unquestion

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"How long am I supposed to stay inside that thing?" Noah asked once he’d surveyed the lab.
"That thing" was a silver box about three feet on a side with a thick bundle of wires and cables coming out the top and a hole in the bottom large enough to stick one's head in. Thick metal rods attached it to the wall, and an adjustable chair was bolted to the floor underneath.
"Maybe twelve minutes, fifteen tops," Liz said.
"And you're sure it's safe?" Noah asked.
"There is a small chance it'll give you superpowers," she said, and shrugged, then grinned. "I'm kidding, of course it's safe."
"OK," he said.
"If anything," she added, "being inside it is actually safer than being anywhere else."
He frowned. "Why's that?"
"I'll tell you after we're done."
So Noah sat down in the chair, and Liz put a sort of metal mesh net over his head, with some little silver disks embedded in it, and then she pumped a pedal that raised up the chair, and his head went into the hole in the box.
Twelve-to-fifteen minutes later, the chair lowered. When Noah's eyes regained focus, Liz was looking at him with an expression he didn't recognize.
"How'd did it go?" he asked.
"Fine," Liz said, and walked across the lab to her desk.
Noah stood up and followed her.
"Is that it?"
"Almost." She handed him a printed page. "Just answer these questions, and we'll be done."
"Did you eat breakfast this morning?"
"Do you prefer pie to cake?"
"Have you ever traveled outside of the country?"
And so on, fifteen questions in all.
When he handed her his answers, she said "Thanks" without looking up.
"So what's the deal?" he asked.
"What do you mean?"
"Before, you said you'd explain it after. Now it's after."
She looked at him. "In the two years we've been housemates, have you ever heard me say the words 'interaction-free measurement?' "
"Sure, but what does that mean?"
"It's a way of performing an experiment where you can learn something about an object without interacting with it."
He squinted. "I don't think that's possible."
"That's what makes it so cool. The key is to create what's called a coherent superposition, a situation where your measurement device is split into two paths--two realities, kind of--only one of which interacts with the object. If you set things up just right, the possibility of having interacted with the object affects the measurement device even when, in retrospect, it turns out that it took the non-interacting path. So you can get an answer without ever having asked the question."
Noah glanced warily back at the silver box. "Is that what you did to me?"
"Yup. The technique is well-known in quantum physics, but we’re the first neuroscientists to use it. The apparatus probed your brain for answers to these questions," she said, holding up the paper, "and now I have your real answers to compare to."
"Sounds like mind-reading."
"Maybe the most useless kind of mind-reading imaginable. The experiment probes the outcome of asking you the questions, so it can only get answers you'd have freely given if asked."
"OK. So how did it turn out?"
"Take a look."
She handed him back his questionnaire along with another printed page, which had the same questions, but instead of his handwritten answers, each had a printed answer and a probability. All the probabilities were between 77% and 96%, and all of the printed answers matched his own.
"This is amazing!"
"I guess," she said, her voice flat. "There was some luck involved--based on the probabilities, I'd have expected two wrong answers."
"Even thirteen out of fifteen would have been really impressive. Liz, I had no idea your research was so interesting!"
She didn't say anything, but the corners of her mouth tightened.
"Hey Liz, I wanted to ask you--I was wondering--I was thinking of going to see the Discoveralls, you know, that cover band, they're playing downtown tomorrow night--would you want to come?"
Her eyes narrowed. "Like a date?"
"I guess."
"Are you joking?"
He blushed, shook his head. "No--but hey, if you don't want to, that's fine."
She chuckled and rolled her eyes. "Unbelievable!"
"What? What is it?"
"When you were in the apparatus, after I was done with the experimental questions, I had it probe you for the answer to 'Do you want to go on a date with Liz Kline?' The answer came back as 'no,' with 94% probability."
His face contorted with disbelief. "There’s no--really?"
She raised her eyebrows. "Yes."
He was shaking his head. "I don’t--wait! The way the measurement works, in the hypothetical world where I'm asked the question, do I know that it's you that's asking?"
"No, it's just supposed to be a neutral--oh!"
"Yeah. If I was just filling out some questionnaire and saw that question, I'd probably be too disturbed to answer at all."
She nodded. "Well, there goes my million-dollar idea for the technology."
"What do you mean?"
"I was trying to think of a situation where people would be willing to go to all this trouble to know the answer to some innocuous question without asking it. Finding out if someone would want to go on a date with you without risk of rejection seemed like the perfect application."
"OK, well, sorry it didn't work."
"I'm not," she said, smiling. "Interaction-free measurement is interesting, but interaction is more fun. I'd love to go see the Discoveralls with you, Noah."
"Great. Hey Liz, did you probe me for any other answers while I was in there?"
She held his gaze for a second or two, then shrugged her shoulders and said, "I guess you'll never know."

About the Author: 
Igor Teper lives with his family in the San Francisco Bay Area and teaches old atoms new tricks at temperatures near absolute zero. His fiction has appeared in _Asimov's Science Fiction_ and _Nature_, and his science essays have appeared in _American Scholar_ and _Nautilus_. For more about him, see igorteper.com.