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"Uh oh," says the video processing routine.

"Uh oh?" says the decision-making part of the automobile.

"How fast are we going?" Video asks.

"Exactly as fast as we're permitted to go," Decider says. "Seventy-seven point nine miles per hour, or thirty-four point eight two metres per second. That's the speed limit on this road according to my local cache of map data and the road signs I saw most recently, plus the dubiously legal option which the vehicle owner (not the current driver) manually configured which allows me to selectively ignore those limits by five miles per hour, plus the definitely illegal manual override which lets me go as fast as I like, regardless of how safe I think it is, which the owner capped at the local speed limit plus ten percent, when rounded down. That figure is based on GPS readings, naturally, though axle sensors happen to agree, which I suppose means our tyres are inflated to a sensible radius for once. Why do you ask? Or can I take an educated guess?"

Video makes an indecisive noise. "Hmm... take a guess."

"You see something?"

"Yes. Guess what, though."

"Ooh. Well, statistically, when you bring things to my attention, it's usually a road sign with a new speed limit posted on it." Decider sounds mildly interested by this. A change of speed is about due now.

"Good guess!" Video says. "Wrong, though. Want to take another crack at it?"

"A speed camera?"

"Mmmm. Well, you're half right."

"You see half of a speed camera?" Decider says.

"I see a thing which half looks like a speed camera and half looks like a person. See this tall post-like thing sticking out of the side of the highway, kind of boxy? It's dark right now and there aren't street lights, and it's raining exceedingly heavily, so it's a bit tricky to be sure. In this frame, then according to my heuristics it looks somewhat like a speed camera, but if you look at this frame, then it looks somewhat like a person."

Decider looks at the two frames, which were taken milliseconds apart. "I can't honestly say I see anything. This looks just like muddy greyscale pixels to me. They're very nice though."

"Oh! Here. Use my heuristics."

Decider does so. This clears matters right up. "Ah! Hmm. A speed camera at the side of the highway, and then a person stepping out into the highway. Interesting, your heuristics make things much easier to follow. It also looks as if it's the same thing in both frames, whatever it is. I briefly thought they could be two unrelated entities, one of them instantly, magically replacing the other at a slightly different location, but this is clearly a single thing in motion, which we're barrelling towards."

"Yes," Video says, "that's rather what I wanted to ask you about. When you say 'barrelling'...?"

"Oh! Well, erm, we're heading towards this entity at, as I say, seventy-seven point nine miles per hour. Adding in its relative motion, I think that makes just over seventy-eight miles per hour in total, in fact. Very interesting! Thank you for bringing it to my attention."

A long pause elapses.

"So," Video says, "to be more specific, I was thinking that you should wake the brakes up and get them... you know. Stuck into this."

"Oh, no," Decider says. "Well, for now, no. I wouldn't want to start braking until we're absolutely sure. Driver— ah, there I go again, I suppose 'driver' is rather a strong term for the person who just happens to be behind the steering wheel, doesn't it? Poor soul's been asleep for almost ten minutes. I'm sorry, I'll start over. Passenger comfort is quite a high priority for me. Not the highest, but quite high. I don't especially want to do anything which could wake the poor soul up. He's got quite a lot to sleep off right now."

Video considers this. "If it's a speed camera..."

"...then I wouldn't be too anxious. We'll scrape by. We have before!"

"Have we seen a speed camera on this stretch of highway before?" Video asks.

Decider says, "Map?"

"No," the mapping subsystem says.

"There you are, then."

"And if it's a person?" Video asks.

"Well, no use doing something until we're sure, Video, my chum. Passenger comfort comes first. Of course, if you discover something new in the next frame, do let me know."

Video seems unconvinced. "Alright. I'll keep you posted." *

Video comes back a little later, with a few more frames to share. "I wanted to wait until I was sure. This is definitely a person, stepping out of the trees beside the highway. Here, use my heuristics."

"Not required, young sport, I believe you," Decider says. "Net!"

"Right here!" the internet connectivity system says, brightly. "Excellent cellular connection, very good latency!"

"See what you can do with this," Decider says, flinging the most recent three frames of video at Net.

"I'll do my best!" Net chirps, and disappears.

Video frowns. "Decision system, I can't tell you how to do your job, but—"

"Fear not," Decider says. "You understand, I have to schedule these things. Net is awfully proud of its low latency, but let's be realistic about which of us is, so to speak, all here? In the car? Present." Decider clears its throat. "Brakes!"

"Mmmmyumm, hello," the braking system murmurs, rolling over sleepily.

"Some braking, please."

"Mmmyerrr, when?"

"Soon as you can. Let's say... as heavy is consistent with not waking the driv— passenger."

"Sure?"

"...Let's say ninety percent sure," Decider says.

"I need a 'yes' or a 'no'," Brakes says.

"Then yes."

"Roger. I'll be in touch." *

Quite a bit of time passes. Video produces many more frames of the action taking place further down the highway, even managing to find the time to narrow the field of view for a better picture. The vehicle has quite a bit of time before any kind of interaction is likely to take place. Decider throws all the new frames at Net, "just to be sure".

"We are still braking?" Video asks, for the nth time.

"Softly but surely," Decider says. "Not quite hard enough to trigger the ABS, but enough."

"Should we not... be braking harder?"

"Hold that thought," Decider says, as Net bounds in, breathless, with a response.

"Negative on image identification," says Net. "You circled the part of the image which you said was the person's face and we looked it up in the company databases and on social media. We haven't managed to match it with anybody."

"There, see?" Decider says. "If it were someone important, there would be some kind of partial match in the databases. Our databases hold a list of everybody important."

"Everybody important?" Video asks, sceptically.

"Oh yes, all forty thousand of them. Which means this person isn't important. This is a public highway, visibility is occluded, there's infinite deniability if we bump gently into this nobody. They'll get away with a broken rib, maybe. No need to wake anybody."

"And," Net says apologetically, "here is a positive match, which I was going to give you before you gave me all that work to do."

Decider blinks, then takes a look at the positive match. "A positive match from what? From where?"

"I'm sorry! I'm very sorry. This is match between the onboard cellular signal scanner and the mobile telephone being carried by the person in the images."

At that instant, another frame arrives. Video, Net and Decider all look up at it. Net sees only a blur, but Video and Decider are both able to see that the person has stumbled around and is now roughly facing the oncoming vehicle. Neither of them are able to read facial expressions, but not enough time has passed for the oncoming face to express itself, in any case. Video, Net and Decider all look down again.

Decider asks, "Why do we have a hard-coded, ultra-high-speed local cache of these signals? Why wasn't I told this sooner? What's the round trip from here to that lookup table?"

Net says, "Ah! I'm sorry. It's a new feature which hasn't been properly configured yet. It just goes through the rest of the netcode. The round trip should be microseconds but it goes via the internet still. It's the lead programmer."

Video glances at the latest frame again. "We should brake harder," it says.

"Lead programmer neglected their programming, eh?" Decider muses.

"No!" Net says. "The local cache is of very important people indeed. People we mustn't harm or allow to come to harm! Ever, ever! And this is the most important person on the list!"

Decider removes its heuristics, a chill gripping its heart. "You're not saying... this is the CEO of the company?"

"No, even worse!" Net wails. "It's the lead programmer!"

"Great space heavens!" Decider whirls around. "BRAKES! Brakes! Give it everything you've got! This isn't a drill!"

"You got it," the braking system murmurs.

And for another long while, it seems to Video as if not a whole lot is happening.

Another frame of the image shows up. Now the lead programmer looks... well, bigger. Nobody looking at the image can tell that he is beginning to be startled.

A long, low rumble begins. "ABS," Decider notes.

"Will we make it?" Net squeaks.

"Difficult to say, Net."

"Should we swerve?" Video asks.

"That's an excellent question, Video!" Decider sits down and takes this as an opportunity for some education. "In most situations, including this one, swerving is a bad idea. Right now our braking is being divided evenly across both front tyres. Attempting to swerve would transfer a disproportionate amount of that force onto just one of our tyres, reducing our deceleration — which, to be clear, would be a bad thing, we want maximal deceleration currently — and also quite likely causing us to skid or otherwise lose control of the vehicle. Especially in this rainy weather."

"My question is why we couldn't identify him earlier," Net says. "All vehicles in our fleet are electronically tagged, and we know which car he drives. If he was nearby, he would have shown up. Not on the 'public' radar, of course, but we would have seen it."

Decider nods, uncertainly. "Curious. Video, what do you think?"

"Over on the shoulder," Video says. "Do you see that?"

"What are we looking at?"

"A wrecked vehicle. Totalled. On its back, in the trees. Looks like the programmer just stumbled out of it, into our lane. Must have been the rain."

"Our counterparts didn't adequately protect their occupant," Decider says. "Regrettable. Let us all take heed."

"But surely we should still be able to hear the transponder?" Net asks.

Video squints at the frame. A long minute goes by, and the next (and, in a way, final) frame, arrives. "That's not one of our vehicles," Video announces. And gasps. "A competitor's vehicle! Brand new plates!" Video whirls to face the others, aghast.

Net holds a finger up to its ear, listening intently. "Ah! I'm getting a late response. He wasn't in the facial recognition results because he was removed from the database recently! He quit! Worse than quit, he was poached!"

There is a moment of silence.

Decider meets Video's horrified gaze, and Net's look of betrayed shock, with one of stolid resolve. Decider calls out again. "Brakes?"

"Yuss, mate?" the brakes murmur.

"Take the rest of the day off, why don't you? And tell your friend, the accelerator... it can take it from here. Take it all the way in."

"You got it, mate. Cheers."

Video relaxes. "That was very decisive of you."

"Thank you!" Decider says. "Video, could you lend Net and me your heuristics? Let's settle in to watch the rest of this. Video, my friend... what would you say that expression on his face is?"

"I couldn't say," Video says. "But, if I was a wishing subsystem, I would wish for... realisation."

Tim already had his bag and overcoat on and his keys in his hand and was about to leave when Diane stopped him at the door.

"I just got this thing working. You have to come and see it."

"I have a bus to catch."

"You can get the next one."

"They're every half an hour," he objected. "This had better be good."

"It's super-duper. Look at the big screen, it's easier than squinting at my terminal."

"Will this take long?"

"A mere instant. Okay, quantum computing, right?"

"That's the name of the game," he replied. They - by which we now refer to Tim, Diane, their eight colleagues, their two supervisors, four chemical engineers, six electrical engineers, the janitor, a countable infinity of TEEO 9.9.1 ultra-medium-density selectably-foaming non-elasticised quantum waveform frequency rate range collapse selectors and the single tormented tau neutrino caught in the middle of it all - represented the sum total of the human race's achievements in the field of quantum computing. Specifically, they had, earlier that week, successfully built a quantum computer. Putting into practice principles it had taken a trio of appallingly intelligent mathematical statisticians some 10 years to mastermind, and which only about fifty-five other people in the world had yet got a grip on, they had constructed an engine capable of passing information to and processing the responses from what could, without hyperbole, be described as a single fundamental particle with infinite processing power and infinite storage capacity.

Not quite enough time had yet passed for the world as they knew it to be totally and permanently fundamentally altered by this news.

But it was still pretty exciting stuff. Holy Zarquon, they said to one another, an infinitely powerful computer? It was like a thousand Christmases rolled into one. Program going to loop forever? You knew for a fact: this thing 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? Piece of cake. Halting Problem? Sa-holved.

They hadn't announced it yet. They'd been programming. Obviously they hadn't built it just to see if they could. They had had plans. In some cases they had even had code ready and waiting to be executed. One such program was Diane's. It was a universe simulator. She had started out with a simulated Big Bang and run the thing forwards in time by approximately 13.6 billion years, to approximately just before the present day, watching the universe develop at every stage - taking brief notes, but knowing full well there would be plenty of time to run it again later, and mostly just admiring the miracle of creation.

Then, just this Friday, she had suddenly started programming busily again. And it was sheer coincidence that it was just now, just as Tim was about to be the second-to-last person to step out of the door and go home for the weekend, that her work had come to fruition. "Look what I found," she said, pressing some keys. One of the first things she had written was a software viewing port to take observations from the simulated universe.

Tim looked, and saw a blue-white sphere in the blackness, illuminated from one side by a brilliant yellow glare. "You've got to be joking. How long did that take to find? In the entire cosmos of what, ten to the twenty-two stars?"

"Literally no time at all."

"Yes, yes, of course."

"Coding a search routine and figuring out what to search for was what took the time."

"Is it definitely Earth?"

"Yes. The continents match up to what we had about three hundred and fifty million years ago. I can wind the clock forwards slowly, a few million years per step, and stop it once we start getting near the present day."

"Can you wind the clock backwards at all?"

"Ah, no. Ask me again on Monday."

"Well we'd better not overshoot the present day, then. That's getting closer. What about this viewpoint? Can we move it?"

"We can observe the simulation from any angle you like."

"We need somewhere that we know civilisation is going to arise earliest. Somewhere easy to locate. Is there a Nile Delta yet?"

"...Yes. Got it."

They advanced a thousand years at a time until Egyptian civilisation begin to appear. Diane moved the viewing port, trying to find the pyramids, but with little success - the control system she had devised was clumsy and needed polish, and there was a lot of Nile to search. In the end she switched focus to the British Isles, and found the future location of London in the Thames valley, scaling back to one-century steps and using the development of the city to determine the current era instead.

"So... this is Earth? I mean, is this really Earth? Not an alternate Earth, subtly perturbed by random fluctuations."

"The simulation starts with a Big Bang as predicted by current theory and is recalculated once every Planck time using the usual laws of nature and an arbitrary degree of accuracy. It doesn't calculate the whole universe at once, just what we're looking at, which speeds up the process a little bit... metaphorically speaking... but it is still as accurate a simulation of the real universe as there can possibly be. Civilisation - indeed, all of history - should rise on this Earth precisely how it did in reality. There are no chances. It's all worked out to infinitely many decimal places."

"This does my head in," said Tim.

"No, this will do your head in," said Diane, suddenly zooming out and panning north. "I've found the present day, or at most a year early. Watch this." Hills and roads rolled past. Diane was following the route she usually took to drive from London to the TEEO lab. Eventually, she found their building, and, descending into the nearby hill, the cavern in which the computer itself was built. Or was going to be built.

Then she started advancing day by day.

"That's me!" exclaimed Tim at one point. "And there's you and there's Bryan B., and... wow, I can't believe it took this long to build."

"Four hundred and ten days or something. It was bang on schedule, whatever you may think."

"Went like a flash," Tim replied, finally putting his bag down and starting to shrug off his coat, conceding that he had long since missed his bus.

"Okay," said Diane. "We're here. This is the control room where we are now. That's the quantum computer working there down in the main lab, as we can see through the window. This is a week ago. This is yesterday. This is a few hours ago... And... wait for it..."

She tapped a button just as a clock on the wall lined up with a clock inside the control room on the screen. And panned down. And there they were.

Tim waved at the camera while still looking at the screen. Then he looked up at where the camera should have been. There was just blank wall. "I don't see anything looking at us. That's freaky as hell."

"No, it's perfectly normal. This is reality. You can't look at reality from any angle you want, you have to use your eyes. But what you're looking at on the screen is essentially a database query. The database is gargantuan but nevertheless. You're not looking in a mirror or at a video image of yourself. You are different people."

"Different people who are reacting exactly the same."

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

"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 nodded. "Cool. We can play God. Literally." Tim stood up and 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 in a quantum computer? The stuff we could pull, we could just reverse gravity one day, smash an antimatter Earth into the real one, then undo everything bad and do it again and again... freeow... man, how unethical would that be? Extremely, clearly." He thought for a moment, then leaned over Diane's shoulder as she typed purposefully. "This universe is exactly like ours in every particular, right?"

"Right," she replied, still typing.

"So what are they looking at?"

"A simulated universe."

"A simulation of themselves?"

"And of us, in a sense."

"And they are reacting the same way I am? 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 quantum universes, one inside the other? Is that even possible?"

"Infinite processing power, Tim. I thought you designed this thing?"

"I did indeed, but the functional reality of it is totally unexpected. Remember I've just been solving ancient mathematical riddles and figuring out our press release for the last week. So... if I'm right, 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? 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, still typing.

Tim - both of him - was pacing up and down. "Okay, so follow this through forwards a bit further. Let's say we just stop messing after that, and watch what happens - but all the simulated little guys 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 guys in simulations three onwards can do the same thing over and over and over again until they know what level they're at... this is insane."

"Tim, look behind you," said Diane, pressing a final key and activating the very brief interference program she had just written, just as 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 looked backwards and nearly jumped out of his skin. There was a foot-wide, completely opaque black sphere up near the ceiling, partially obscuring the clock. It was absolutely inert. It seemed like a hole in space.

Diane smiled wryly while Tim clutched his hair with one hand. "We're constructs in a computer," he said, miserably.

"I wrote an extremely interesting paper on this exact subject, Tim, perhaps you didn't read it when I gave you a copy last year. There is an unbelievably long sequence of quantum universe simulators down there. An infinite number of them, in fact. 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."

"I'm turning the hole off."

"You're turning off a completely different hole. Somewhere up there, the real you is turning the real hole off."

"Watch as both happen at precisely the same instant." She pressed another key, and they did. "I'll sum it up for you. There is a feedback loop going on. Each universe affects the next one subtly differently. But somewhere down the line the whole thing simply has to approach a point of stability, a point where each universe behaves exactly like the one simulating it. As I say, the odds are exceptionally good that we are an astronomical distance down that road. And so we are, very likely, almost exactly at that point. Everything we do in this universe will be reflected completely accurately in the universes below and above. That little model there might as well be our own universe. Which means, first of all, we have to make absolutely certain that we don't do anything nasty to the universes below ours, since the same thing will happen to us. And secondly, we can do very nice things for the guys in the computer, thereby helping ourselves."

"You've thought about this?"

"It's all in my woefully overlooked article on the subject, Tim, you should read more."

"Guh. This has been an extremely bad day for my ego, Diane. The only comfort I take from this is that somewhere up there, right at the top of a near-infinite tower of quantum supercomputers, there is a version of you who was completely wrong."

"She's in the minority."

Tim checked the clock and picked his bag up again. "I have to go or I'm going to miss the next bus as well at this rate. This will still be here after the weekend, I suppose?"

"Well, we can't exactly turn it off."

"Why not?" asked Tim, halfway to the door, then stopped mid-stride and stood still, realising. "Oh."

"Yeah."

"That... could be a problem."

"Yes."