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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.