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Could human civilization eventually spread
across the whole Milky Way galaxy?

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Could we move beyond 
our small blue planet

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

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This question's a pretty daunting one.

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There are around 300 billion stars
in the galaxy,

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which is about 160,000 light-years across.

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

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trudging along at 0.006% of 
the speed of light.

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At that rate, it would take over 
2.5 billion years

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

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And then there's the question 
of human survival.

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The gulf between stars is simply enormous.

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We couldn't live sustainably 
on most planets,

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and we require a lot of resources
to stay alive.

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

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to not just spread human civilization
across the galaxy,

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but to do so quite quickly,

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without breaking any known 
laws of physics.

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

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who designed on paper machines 
that could self-replicate

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and create new generations of themselves.

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

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In the context of space exploration,

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von Neumann machines could be
built on Earth

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and launched into space.

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

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They would then mine the available
resources and harvest energy,

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build replicas of themselves,

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launch those to the nearest planets,

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and continue the cycle.

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The result is the creation 
of millions of probes

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

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

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traveling at 5% of the speed of light

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

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That may sound like a long time,

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

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on a cosmic scale, it's incredibly fast -

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the equivalent of about 2.5 hours
in an entire year.

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Creating von Neumann machines
would require a few technologies

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we don't have yet,

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including advanced 
artificial intelligence,

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miniaturization,

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and better propulsion systems.

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

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we would need 
yet another technological leap -

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the ability to artificially 
grow biological organisms and bodies

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using raw elements 
and genetic information.

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Regardless, if in the last billion years

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an alien civilization 
created such a machine

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and set it multiplying its way toward us,

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our galaxy would be 
swarming with them by now.

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So then where are all these machines?

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Some astronomers, like Carl Sagan,

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

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They might hurtle out of control,

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scavaging planets to their cores
in order to keep replicating.

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Others take the machines absence as proof

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that intelligent alien civilizations 
don't exist,

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

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But all this hasn't stopped people from
imagining what it would be like

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if they were out there.

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Science fiction author David Brin

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

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and proliferate simultaneously.

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Some are designed 
to greet young civilizations,

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

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

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

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waiting for us to reach a certain level
of sophistication

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before they make their move.

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For now, all we have is curiosity 
and theory.

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But the next time 
you look at the night sky,

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consider that billions of
self-replicating machines

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could be advancing between stars
in our galaxy right now.

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

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