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This episode is supported by the Great Courses Plus.
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.
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
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?
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
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.
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In a recent episode, we asked whether it's even possible,
or a good idea, to build a Dyson swarm to capture
all of our sun's energy.
You guys had a lot of good questions.
Paul Baba asks whether such a Dyson swarm would
cause the Earth to freeze.
Well, a full Dyson swarm interior to Earth's orbit
would, indeed, block sunlight and freeze our planet.
There's no way to make a consistent hole in the swarm
if you have full spherical coverage.
However, it is conceivable that you
could cause the solar collectors to fill up
as they transited the sun, leaving a gap for sunlight.
A couple of you pointed out that the Dyson swarm proposal
is both ridiculous and nowhere near close
to being logistically possible yet.
And for that reason, we shouldn't even talk about it.
Look-- if we only ever talked about things
that are immediately doable and not slightly ridiculous,
then we wouldn't have Segways, or sporks,
or Joss Whedon's "Firefly."
Cobra60six would maybe like us to talk
about more doable energy solutions like thorium power
Yeah, good point.
Dyson swarms will not solve the current energy crisis.
But there are several solutions, and safer nuclear power-- maybe
thorium-- could be an answer.
But even solar power stations at Earth's surface
will soon be a viable solution for most of our current energy
The issue is, of course, political.
Mr. Mercury is a little uncomfortable
with building a power source that has the potential
to destroy our solar system.
Nice try, Mercury.
I'm afraid you have outlived your usefulness.
Shawn Tripp imagines how cool would
be if we started exploring the Kuiper Belt,
only to find out that it is the remnants of an ancient Dyson
Hey, Joss-- hey, yeah, I got this idea for you.
Yeah, I came up with it myself.