Could we move beyond our small blue planet

to establish colonies in the multitude of star systems out there?

This question's a pretty daunting one.

There are around 300 billion stars in the galaxy,

which is about 160,000 light-years across.

So far we've sent a single spacecraft outside our solar system,

trudging along at 0.006% of the speed of light.

At that rate, it would take over 2.5 billion years

just to get from one end of the galaxy to the other.

And then there's the question of human survival.

The gulf between stars is simply enormous.

We couldn't live sustainably on most planets,

and we require a lot of resources to stay alive.

And yet, decades ago, scholars found that it's theoretically possible

to not just spread human civilization across the galaxy,

but to do so quite quickly,

without breaking any known laws of physics.

Their idea is based on the work of a mathematician named John von Neumann,

who designed on paper machines that could self-replicate

and create new generations of themselves.

These would later come to be known as von Neumann machines.

In the context of space exploration,

von Neumann machines could be built on Earth

and launched into space.

There, the self-sufficient machines would land on distant planets.

They would then mine the available resources and harvest energy,

build replicas of themselves,

launch those to the nearest planets,

and continue the cycle.

The result is the creation of millions of probes

spreading outwards into the universe like a drop of ink in a fishbowl.

Scholars crunched the numbers and found that a single von Neumann machine

traveling at 5% of the speed of light

should be able to replicate throughout our galaxy in 4 million years or less.

That may sound like a long time,

but when you consider that our universe is 14 billion years old,

on a cosmic scale, it's incredibly fast -

the equivalent of about 2.5 hours in an entire year.

Creating von Neumann machines would require a few technologies

we don't have yet,

including advanced artificial intelligence,

miniaturization,

and better propulsion systems.

If we wanted to use them to spread actual humans throughout the galaxy,

we would need yet another technological leap -

the ability to artificially grow biological organisms and bodies

using raw elements and genetic information.

Regardless, if in the last billion years

an alien civilization created such a machine

and set it multiplying its way toward us,

our galaxy would be swarming with them by now.

So then where are all these machines?

Some astronomers, like Carl Sagan,

say that intelligent aliens wouldn't build self-replicating machines at all.

They might hurtle out of control,

scavaging planets to their cores in order to keep replicating.

Others take the machines' absence as proof

that intelligent alien civilizations don't exist,

or that they go extinct before they can develop the necessary technologies.

But all this hasn't stopped people from imagining

what it would be like if they were out there.

Science fiction author David Brin

writes about an universe in which many different von Neumann machines exist

and proliferate simultaneously.

Some are designed to greet young civilizations,

others to locate and destroy them before they become a threat.

In fact, in Brin's story "Lungfish,"

some von Neumann machines are keeping a close watch over the Earth right now,

waiting for us to reach a certain level of sophistication

before they make their move.

For now, all we have is curiosity and theory.

But the next time you look at the night sky,

consider that billions of self-replicating machines

could be advancing between stars in our galaxy right now.

If they exist, one of them will eventually land on Earth,

or maybe, just maybe, they're already here.