It might take only one. 

A single robot, a few billionths of a meter wide, dropped somewhere in the woods or on a front lawn. In a matter of days or weeks, the entire North American continent would be dust. No trees, no insects, no animals—not even a single bacterium. All of this organic matter will have been converted into copies of that first, mischievous little nanobot.

This is the so-called “gray goo” scenario, which K. Eric Drexler famously discussed in his 1986 book Engines of Creation. More than thirty years later, the danger looms closer. Whereas current 3D printers add one layer of plastic at a time, a nanofactory would move one atom or molecule at a time to build products from the very bottom up. The result would be “atomically precise manufacturing”—APM, for short. 

For example, two computers made this way would not only appear identical, but their atoms would also be in exactly the same position. 

Once this technology arrives—perhaps by 2040—a misanthropic terrorist could design nanobots for the explicit purpose of disassembling organic matter and creating clones of themselves. Thus, the first nanobot would replicate itself, resulting in two nanobots. These two nanobots would then replicate themselves, resulting in four nanobots. As the nanobot population grows, they could be blown around by the wind, like loblolly pollen, until everything that can be converted into more nanobots has been.

While buildings, roads, and other signs of civilization would survive, cities like Durham, Raleigh, and Chapel Hill would be ghost towns. The streets would be silent. The parks would be gone. The lights that illuminate the roads at night would eventually flicker out. And, if the army of self-replicating nano-scale robots were to spread across the continents, the human species could kick the bucket, like the dodo and dinosaurs before us. 

How fast might this happen? Quite quickly, since the replication process would be exponential, and we are notoriously bad at grasping exponential functions. We let them sneak up on us. 

So let me give you an example.

Picture a lovely little pond—say, the fishpond in Duke Gardens—that contains one water lily. But this isn’t an ordinary water lily. It’s a super-fast-growing plant that doubles in size every minute and will cover the entire pond in just an hour. How long would it take for the water lily to cover one-quarter of the pond? 

The easiest way to answer is to work backward: If the entire pond is covered after sixty minutes, then, given the water lily’s rate of growth, it will be half-covered after fifty-nine minutes. Since a quarter is half of a half, it will be one-quarter covered after fifty-eight minutes.

And that’s the answer: Just two minutes before the entire pond is covered, a whopping three-fourths of the pond will be completely untouched by the super-fast-growing plant. This is the power of exponentials!

Right now, no one knows exactly how dangerous self-replicating nanobots might be. But there are people out there who would destroy the world—even the entire biosphere—if they could. I know, because I’ve studied them. 

To be sure, APM could introduce many profound benefits to humanity: Imagine a nanofactory “printing out” virtually anything you could possibly want in your own home for virtually zero cost. Say goodbye to Amazon! 

But it could also empower bad actors with a death wish for humanity to push a metaphorical “doomsday button” that ends the human story once and for all.

Phil Torres, a North Carolina ex-pat, is an existential-risk scholar who can be found online at Comment on this story at Click here to read the rest of our 2040 predictions.

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