[music playing] Space is big, maybe even too big to be easily colonized by real, living aliens.

But what about almost-living machines?

I mean spacecraft capable of replicating themselves and exponentially spreading across the galaxy.

I'm talking about Von Neumann probes.

Our galaxy is depressingly natural looking.

Stars for the most part act very star-like, their brightnesses and colors slavishly following the equations of stellar physics.

Space stuff gravitates around in slow, stately arcs that would make Newton proud.

Even the unusual denizens of the galaxy like pulsars and black holes just do what they do.

And Nature rules the sky.

No one seems to be messing around with stuff up there.

As far as we've seen, humanity is the only species ever to build anything bigger than a beaver dam in the entire galaxy.

And yet the Milky Way has been around for long enough that any previous civilizations with any inclination to expansion or exploration should have been able to cross, even colonize the entire galaxy.

So we get back to the famous Fermi Paradox.

Some suggest that the resolution to this paradox is that advanced civilizations never make it to an interstellar state, perhaps self-destructing before building the great generation ships needed to seed new star systems.

Others suggest that interstellar travel is just too hard, and that any sufficiently advanced civilization will find better things to do with its eternity, like turn inwards into complex virtual worlds.

Today I want to argue that even if these points are true, there are reasons to expect a galaxy full of the evidence of past technological life.

Why?

Because of Von Neumann probes.

Von what the?

Self-replicating robotic spacecraft-- that's right, completely unmanned or unkerbled vessels capable of traveling between star systems, and capable of extracting resources at their destinations to build copies of themselves to continue exploration of the galaxy.

Well, that sure sounds science fiction-y.

However it's an idea that a number of people who should know better have taken and are taking quite seriously-- not necessarily as something we should do, but as something that someone, somewhere, surely would have done.

We touched on the idea of self-replicating machines recently when we talked about the Dyson Swarm.

It's time to generalize.

Because these may be the future of space development.

Imagine-- a single machine that can build a huge variety of other machines, including itself.

Not so hard to conceive these days.

Very soon, we'll have 3-D printers that will be able to print most of their own parts.

But even in the late '40s, John Von Neumann, Hungarian mathematician, physicist, inventor, and general co-founder of the modern technological world, laid down a theoretical framework for a self-replicating automaton.

He called it a "universal assembler."

These days, we call it a Von Neumann Machine.

A number of luminaries have proposed uses for such a device.

Edward Moore conceived of desert or ocean-dwelling self-replicators whose only purpose was to build copies of themselves, which humans would then harvest for parts.

Freeman Dyson imagined several types of Von Neumann machine, including a Martian terraformer and the Astro-chicken.

This is a tiny spacecraft that will be powered by a solar sail-fed ion drive that could harvest planetary resources to build more of itself-- a self-replicating spacecraft, a Von Neumann probe-- albeit one only capable of exploring this Solar System.

But with modern advances in zero-g 3-D printing, material science, nanofabrication, and automation software that verges on AI, we can now realistically project much further into the future.

We can imagine a Von Neumann Machine much smaller than the infrastructure it is able to build.

This suddenly makes it reasonable to use such machines for some pretty cosmic scale endeavors.

Let me outline how such a device could populate the galaxy with robotic probes.

Other applications, like asteroid mining, Dyson swarm construction, and terraforming-- really any large-scale automatable space operation-- might be best handled with self-replicating machines.

This outline is inspired by Robert Freitas' vision for a self-replicating version of the Daedalus spacecraft.

Although with modern advances, it's now possible to be even more ambitious.

So a spacecraft is launched from the home Solar System with an engine capable of taking it to 10% or 20% lightspeed.

Fusion engines might be a good candidate, because the fuel is abundant everywhere.

The vessel contains a universal assembler and minimal mining and/or processing machinery.

After several decades, it decelerates into a neighboring star system, and parks in orbit, or lands on a nice, big asteroid or gas giant moon.

It deploys initial solar panels and mining bots, and uses these resources to build a factory.

That factory includes larger solar power plant, a strip-mining operation, and perhaps more assemblers.

It builds fuel collectors-- maybe orbiters to harvest deuterium or tritium from gas giant atmospheres.

And it launches probes to actually explore the planetary system, and stream the data back home, or terraform the system, or build a Dyson swarm, or annihilate all life-- whatever these aliens are into.

At some point, the assembler starts building new Von Neumann probes which, one by one, launch to new, more distant star systems.

This whole process takes a while.

Assume an average 10% light speed, 10 light year jumps for each probe, and up to 500 years for production of the first daughter probe at each jump.

It might take several million years to cross the galaxy this way.

But the exponential nature of the process means that the entire galaxy would be covered in these things in that amount of time.

This brings us back to the Fermi paradox.

I think it's fair to say that A, Von Neumann probes are possible to build.

In fact, I think we could build one in a few 100 years at most.

And B, once a successful probe is built, the galaxy will be swarming with them in 10 million years, max.

Given B, we should see replication factories in our own solar system.

So we have to conclude that either no civilizations ever choose to build these things, or there were very few technological civilizations 10 million years ago.

Both options are difficult to buy.

Let's talk about numbers before we get into the intricacies of alien psychology.

There are, at a minimum, tens of billions of terrestrial planets with liquid water in our galaxy.

The Kepler Space Telescope showed us this.

The random events that led to technological life dominating the Earth could have happened at least tens, but perhaps hundreds of millions of years earlier on our own planet.

So if complex life is even remotely common-- say it evolves in one in 1,000 habitable planets-- and another one in 1,000 of these evolve technological species, that still means tens of thousands of planets in our galaxy get tech at some point.

Even if Earth is in the earliest 10% of these, that's thousands of civilizations before us, many of which may have had thousands of generations to do lots of crazy stuff.

OK, so alien psychology-- the exclusivity argument, that civilizations will never do certain things, is fatally flawed.

We know very well from our own recent history that it only takes an individual, sometimes with questionable motives, to drive some pretty crazy and large-scale programs.

So thousands of generations of thousands of civilizations, which means potentially quadrillions of individuals, and not one of them builds a single self-replicating spacecraft?

Something we could do pretty soon?

No.

The only reasonable conclusion is that those numbers are wrong.

They are wildly wrong.

There have been so few civilizations capable of doing this that it hasn't been done-- yet.

This doesn't mean that there are no advanced civilizations out there.

It just means that they are probably few and far between-- few enough that the numbers game doesn't guarantee that one will commit this single, relatively easy act-- building one Von Neumann probe.

So how are we here if technological life is so rare?

Well, sort of luck, but not really.

Perhaps a variation of the Anthropic Principle needs to be invoked.

In any universe that produces intelligence, someone, somewhere, at some point has to ask, why are we alone?

Perhaps that's us, preparing to explore the young and still untamed reaches of this space-time.

[music playing]