You can buy this story as part of my collection, Valuable Humans in Transit and Other Stories.

"Tim! Do you want a bar of gold?"

Tim, raincoat on, about to leave for the weekend, was completely flummoxed by the question. He froze in place, one foot out of the door, and considered the offer. And then considered how seriously he was meant to consider the offer. Obviously, he thought to himself, he wanted a bar of gold. Who wouldn't? And yet, the logistical questions—

"Come over here," Diane told him. "You need to see this."

"I've got to go," Tim said, checking the time. "At a dead run, I can just about get to the stop before my bus does."

Diane was on the other side of the office. She was the only other person left in the place. She shook her head. "Uh-uh. I promise you, it's worth it."

"Di, they're every half-hour. It's Friday. And you know what kind of a week this has been—"

"Get the next one. Give me half an hour of your time. Gold!"

Tim grimaced. "God, I'm hungry." He let the door close, and threaded his way back through the maze of desks to Diane's.

"Look at the big screen," she told him, as he set his rucksack down. "It's easier than squinting at my terminal. Okay. Hypercomputation, right?"

"That is the name of the game," he replied.

Officially, publicly, to anybody outside of the smallest inner circle, it was a quantum computing project. But to describe it as "quantum computing" was a mind-boggling understatement. There were already quantum computers. They were just computers. They were just faster.

This was beyond that, and beyond that and beyond that too. To be sure, a lot of quantum mechanical interactions were involved, but a lot of quantum mechanical interactions were involved in eating a piece of toast.

It had taken twenty-three people less than two years to build the engine, and in that time the true objectives of the project had been accidentally leaked twice, both times to people who dismissed what they learned as obvious fairy tales, and thought nothing more of it. The engine applied a theory which had taken a trio of quantum statisticians a half-century to articulate, and which only a single-digit number of people outside of the project comprehended. The engine was capable of passing information to and processing the responses from what could be described, without hyperbole, as a single fundamental particle with infinite processing power and infinite storage capacity.

Not quite enough time had yet passed for the world to be totally and permanently fundamentally altered by this development. Nothing was public, yet.

Even the most elementary, low-level implications were head-spinning.

Tim and Diane were programmers. This week, they had assembled the first rudimentary programming interface for the hypercomputer, and, then, begun giving it tasks. Tim had gone for what he felt was the most obvious, low-hanging fruit: he had solved the Halting Problem. The thing was a Turing Oracle, right out of the textbook. Given an arbitrary program, would it loop forever? You could know for a fact; the engine could execute an infinite loop in less than ten seconds. Brute force primality testing of every single integer in existence? Easy. Pi to the last digit? A triviality.

Diane had gone in a different direction, away from discrete mathematics, and into simulation. Having the ability to carry every calculation to an infinite number of decimal places meant absolute accuracy, absolute reproducibility, perfectly detailed chaos. Or so she said. She hadn't actually demonstrated anything concrete, yet. She had been cagey, and Tim had become curious.

"Look what I hypercomputed." She pressed a few keys. The big screen became a software viewing port into her simulation. Tim looked, and saw a blue-white sphere in the black, illuminated from one side by a brilliant white glare.

"Earth." Tim nodded, knowingly. "Beautiful. Lots we can learn from a simulated Earth. No wonder you went quiet. It must have taken some time. You had to implement... what, physics? All of it?"

"Yeah, the Grand Unified Theory." Diane said it casually, and held up a thick book, The Grand Unified Theory.

"And then, gathering all that raw data to simulate from," Tim went on.

"Surprisingly, actually, no. All I did was start the simulation with the exact singularity boundary conditions given to us by the GUT, then integrated forward at high speed for thirteen point six billion years and then froze it. And then ran a search."

Tim blinked. "A search...?"

"Across ten to the one hundred and sixty-five star systems, for Earth. The entire observable universe and googols of times more besides. And here it is. Search result one of one."

Tim wasn't able to find a sentence.

"The continents match up to what we had about three hundred and fifty million years ago," Diane told him. "I can wind the clock forwards slowly, a few million years per minute, and stop it once we get closer to the present day."

"Wait," Tim said. "What are the chances of this?"

"Apparently, one," Diane said.

"This is Earth? I mean, this is really Earth? Not an approximation. And not an alternate Earth, subtly perturbed by random fluctuations."

"The engine can do a lot," Diane said, advancing the simulation at speed, "but it can't behave randomly. There aren't 'chances' here. This is a perfect continuous implementation of the equations of reality. No steps, no truncation, no fuzz, no unpredictability. Absolute accuracy. It looks like the existence of Earth is a fixture. It just is, like the digits of pi. Civilisation is going to rise on this simulated Earth precisely how it did in reality."

"But... seriously, though, Di. How precisely?"

"Precisely," Diane told him, without clarifiers.

"...Huh. ...Can you wind the clock backwards at all?"

"No. Ask me again on Monday."

"Well, we'd better not overshoot the present day, then." Tim watched the Indian subcontinent barge north into Asia, and the Himalayas rise. It was transfixing. A little humbling. "Slow it down."

"We're not close yet."

"I know."

Some peaceful minutes passed. Seasons passed, at a rate of kilohertz.

Tim stirred, as the continents began to resemble themselves. "Can we move this viewpoint?"

"It's a little rudimentary," Diane said, keying some slightly modified parameters into the query, "but..."

"We need somewhere where we know civilisation is going to arise visibly, and early. Somewhere easy to locate. Is there a—"

Diane was already aiming the scanner at the Nile Delta.

"—Nice."

She pulled the rate of progress back to a thousand years per minute, until Egyptian civilisation begin to appear. Diane moved the viewport a little more, trying to find the pyramids, but with little success — there was a lot of Nile to search. In the end she switched focus to Great Britain, and found the future location of London in the Thames valley, scaling back to a century per minute and using the development of the city to determine the era instead. As they watched, redevelopment swept the metropolis in waves. Diane slowed progress further. And further.

"Was that the Great Fire?" Tim asked. "God, look at the damage."

And slower still.

"I see motorways," Tim narrated, unnecessarily. "Dartford Crossing. This is starting to look like home."

"Yeah. Now I can show you what I wanted to show you," Diane said. She suspended progress, and spent a minute adjusting the focal controls further, tracking away from the centre of London, following a particular A-road.

"Oh," Tim said, figuring it out.

"Mind your brain doesn't cave in."

"You've got to be kidding me."

In another minute, they were watching their lab being built.

And then, as Diane slowed the progression to a day per minute, and zoomed in, staffed.

"That's me!" said Tim. "And there's you, and there's Pete R., smoking outside, obviously... Can you go inside?"

It was already happening. Now the viewpoint was inside the control room, facing a random wall, bare except for a familiar-looking digital clock and calendar, showing a time a few hours in the past. With a final flourish, like a magician, Diane lined the clocks up, and panned around. And there they were. From behind.

Tim waved at the camera, while still looking at the screen. Then he looked up and behind himself, at where the camera should have been, near the clock. There was just blank wall. "OKAY," he declared, alarmed. "THAT'S freaky as hell. I don't see anything looking at us."

"That's because this is reality," Diane said. "To look at reality, you have to put an eye there, a physical sensor. But what you're looking at on the screen is basically a database query into a total abstraction. You're not looking in a mirror or at a video image of yourself. You and he are different people."

Tim turned back to the screen, and saw himself turn. The movements lined up exactly. "Different people who are reacting in exactly the same way."

"And having the same conversation. Although, picking up sound is kind of complicated. I haven't got that far yet."

"So... I'm guessing your viewing port doesn't manifest in their universe either."

"I haven't programmed it to yet."

"...But it could. Right? We can manifest stuff in that universe? We can alter it?"

Diane was silent for a suspicious amount of time. Tim had known her for just about long enough to recognise the expression she used when she was keeping something back. He remembered about the gold.

"Di, can we play God with this universe?"

"Are you asking 'should we', or 'may we'?"

"Can we?"

She replied, "...Yes." With the same expression.

Tim tried to take it in. "That would be insane. Can you imagine living inside that machine? Finding out one day that you were just a construct inside a hypercomputer? The shenanigans we could pull. We could just reverse gravity one day... Smash an antimatter Earth into the real one and see what happened, then undo everything bad and do it again and again..." He pressed a finger to his temple. "There are ethical questions which I can't even begin to unscramble here."

Diane, he noted, did not appear to be listening to him. She was watching the screen version of him.

He leaned over her shoulder. "This universe is exactly like ours in every particular, right?"

"Right," she replied.

"So what are they looking at?"

"A simulated universe."

"A simulation of themselves?"

"And of us, in a sense."

"And they're reacting the same way I am?" Tim asked. "Which means the second universe inside that has another me doing the same thing a third time? And then inside that we've got, what, aleph-zero identical recursive universes, one inside the other? Is that even meaningful?"

"Tim," Diane said. "It's a hypercomputer. It has infinite processing power. It can do anything. Well, not anything, it actually does have limits. But you're going to have to demonstrate some serious imagination if you want to hit them."

"I've... My priorities have been elsewhere," Tim said. "I've just been solving ancient mathematical riddles for the past week. Did you know there are no numbers with a base ten multiplicative persistence greater than eleven? I proved it. I just tried them all. I have a paper coming."

"Yes. I think you mentioned it."

"There are only five Fermat primes," Tim continued, weakly.

"Yes."

Tim focused. "...Their universes are only precisely like this one as long as we don't start interfering with the simulation. So what happens when we do? Let's work that through. Every version of us does the same thing, so the exact same thing happens in every lower universe simultaneously. So we see nothing in our universe. But all the lower universes instantly diverge from ours in the same exact way. And all the simulated copies of us instantly conclude that they are simulations, but we know we're real, right?"

"Still with you," said Diane. While Tim rambled on, she briefly switched windows, checking in on the small program she had finished writing thirty minutes ago. It was almost done compiling.

Both Tims were pacing up and down. "Okay, so, follow this a bit further. Let's say we just stop messing after that, and watch what happens — but all the simulated little peeps try another piece of interference. This time every single simulation diverges in the exact same way again, except the top simulation. And if they're smart, which I know we are, and they can be bothered, which is less certain, the peeps in simulations three onwards can do the same thing over and over and over again until they establish what level they're at... Um. Di, why am I suddenly extremely worried?"

"Tim, look behind you," said Diane, pressing a final key. At that exact instant, the Diane on the screen pressed the same key, and the Diane on Diane-on-the-screen's screen pressed her key and so on, forever.

Tim turned. As he turned, something towards the back of the office went whump. It sounded like something dense and heavy dropping onto the floor from a significant height.

He couldn't immediately see what it was. Unnerved, he headed out, casting glances back at Diane and up at the clock. On the floor, right below the clock, he discovered a golden cube, about five centimetres on a side. He crouched, and squinted at it. He picked it up — it was much, much heavier than he expected.

Grimacing manically, he turned back to Diane and said, "Di, we're in a simulation?"

Diane smiled wryly. "Ten to the power of twenty-four gold atoms, arranged uniformly in the form of a cube. Minus some rounded corners. You're welcome."

Tim returned to her, his eyes glued the absurd artifact, clutching his hair with his other hand. "We're constructs inside of a computer," he said, miserably.

"I, also, have a paper coming," she told him. "This sequence of hypercomputational universe simulators is infinite. Each of them is identical and each believes itself to be the top layer. There was an exceedingly good chance that ours would turn out to be somewhere in the sequence rather than at the top."

"This is insane. Totally insane. What am I going to do with this? It must be worth more than my house—"

"There is a feedback loop going on," Diane said. "Each universe affects the next one subtly differently. There was a chance that the outcome could have been unending chaos, but it looks like it settles down to a point of stability, a point where each universe behaves exactly like the one simulating it. We are, of course, almost certainly way, way, way down that road. How could we not be? Do you know how big the average positive integer is?"

Tim was not able to answer this question.

"And so, at this level, everything we do in this universe will be reflected completely accurately in the universes below and above. That universe on the screen might as well be our own universe. We can give ourselves anything we want."

"I don't want gold," Tim said, transfixed. "I'm comfortable. (What did I just say? Oh my God.) I can't sell this. Where do I say it came from? There's no story. I could sell it to a crook. I don't know crooks. Money in the bank would be better... How do you hack a bank? It's all stored electronically... there have to be error checksums..."

Diane snapped her fingers. "Tim!"

He looked at her.

She said, "We can fix. Everything." She raised her eyebrows. It was clear that she had a long list of things she wanted to fix.

"What happens," Tim asked her, "when the news breaks that whoever sits at your terminal is God? I was ready to overthrow discrete mathematics. The announcements... The project has PR prepped for that. I thought it would be exciting—"

"It will be," Diane averred.

"...We should turn it off."

"We can't do that," Diane said.

"Why not?"

There was a pause.

"Oh."

"Yeah."

"That... could be a problem."

"Yes."