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[narrator] Fifty years ago,
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astronomers picked up
this radio signal coming from deep space.
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The signal repeated so regularly,
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it kept time better than an atomic clock.
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[imitates signal] What could that be?
That couldn't be natural.
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[narrator] They thought it might be
an alien transmission,
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so they nicknamed the signal LGM-1,
for Little Green Men.
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It turned out to be a pulsar,
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radio waves from a neutron star
collapsing 5.5 million years ago.
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A lot of us put aliens
in the same category as ghosts
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or the Loch Ness Monster,
a subject for science fiction.
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Or left to the cranks, kooks
and conspiracy theorists.
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[man] Immediately, I was just lifted
from the ground,
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to about the height
that they were off the ground.
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That's when I first saw this thing coming
straight down, just like an elevator.
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This is a center for the distribution
of information
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coming through me telepathically
from the space people.
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[narrator] But time and again,
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serious scientists
have thought they've found evidence
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of extraterrestrial life.
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[man] Are these really canals on Mars?
Are the polar caps frozen water?
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[narrator] As recently as 2016,
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astronomers proposed that
never-before-seen dimming patterns
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from a star could be evidence
of gigantic structures built
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by an advanced civilization
to harness the star's energy.
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It turned out to be dust.
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Scientists feel confident
that there is biology beyond Earth.
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Not because we've found it,
we haven't found it.
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The reason that we think
that they're out there is simply, if not,
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then Earth is some sort of miracle.
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[narrator] For most scientists
who study the universe,
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searching for aliens isn't crazy.
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What's crazy is that
we haven't found them.
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In a universe so vast, where is everybody?
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[Stephen Hawking] It is important to us
to know if we are alone in the dark.
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Unidentified objects that
sound warning klaxons around the world.
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[man] Oh, my gosh, look at that thing.
It's resting!
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[Ronald Reagan]
How quickly our differences would vanish
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if we were facing an alien threat
from outside this world.
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Out there is a million other
civilizations.
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They all look fabulously ugly.
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And they're all a lot smarter than us.
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[theme music playing]
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Trash cans been vanishing
from city sidewalks in alarming numbers.
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[narrator] Stolen trash cans are
a time-honored public nuisance.
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You may have seen local news reports
about it.
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The New Yorker magazine even published
a cartoon about it back in 1950,
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blaming mischievous aliens.
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That silly joke inspired one
of the most profound insights
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in modern scientific history.
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Because the physicist Enrico Fermi
saw that cartoon,
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and the story goes, blurted out,
"Where is everybody?"
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The fact that we haven't found
any evidence of aliens
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became known as the Fermi Paradox.
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There are about ten
to the power of 22 total stars.
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That's about 10,000 stars
for every grain of sand on Earth.
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A conservative scientific estimate says 5%
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of those stars are similar
to our sun,
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which means 500 billion billion suns
in the universe.
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Many scientists are more confident
than ever that aliens exist
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because of some game changing discoveries
in the last few decades.
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Nobody could say for sure
if there were any planets
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outside of our solar system,
until the 1990s.
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Now, scientists think one in five
sun-like stars is a planet...
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similar to our own.
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When I get asked what are the chances
there's life out there,
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I always answer 100%.
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Just because there's
so many stars and planets...
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we think pretty much every star
has planets.
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[narrator] We've also discovered
life on Earth
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in environments where nobody
expected to find it.
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We see life all the way deep
in the sub-surface of the planet,
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miles down, in like, gold mines.
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We see life near volcanic calderas.
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We see life on nuclear reactors.
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We see life in the most extremes.
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That actually gives us lot of hope
for the search for life elsewhere,
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'cause we can't necessarily expect
that all planets
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will have just the same conditions
as Earth.
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[narrator] Estimates of how many
Earth-like planets will develop life vary.
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So, let's say even with this new
scientific confidence,
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it's just one out of every thousand.
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That means every tenth grain of sand
on Earth
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represents a planet with life on it.
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And if just one out of every thousand
of those planets develop intelligent life,
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that's a quadrillion intelligent alien
civilizations in the universe.
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10,000 just in our galaxy.
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Extraterrestrial life also has time
on its side.
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Earth is only about a third
as old as the universe.
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And so there's been plenty of time
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for life to evolve
to advanced civilizations and for these civilizations to spread
across the galaxy.
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[narrator] With all that time and space,
the math seems pretty clear.
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We should've found aliens by now,
or they should have found us.
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There's one popular explanation for
why we haven't found evidence of aliens.
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We have found it.
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Governments have just covered it up.
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I believe that the flying saucers seen
by veteran airline and Air Force pilots
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are objects from another planet.
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Our critics continually charge
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that the United States Air Force
is withholding information
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from the general public on this subject.
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This is absolutely untrue.
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[narrator] Every so often,
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something comes out
that gives this theory new life,
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like the revelation in 2017 that
the US government had spent millions
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on a secret program
to investigate UFO sightings.
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[narrator] But as almost any scientist
will tell you,
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looking into UFOs isn't the same
as searching for extraterrestrial life.
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The word UFO is
"unidentified flying object."
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It's unidentified.
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So, by definition, we have to
leave it open.
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It doesn't mean it's been identified
as an alien spacecraft.
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[narrator] Scientists have their own
favorite theories about
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why we haven't found aliens.
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It could be that they came here,
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didn't like what they found and moved on.
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Imagine for a moment, you get
an infestation of ants in your house.
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It happens.
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Now let's say you wanna
have a conversation with those ants.
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Say, "Excuse me, can you please leave?"
How would you even do that?
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I like that theory, that we're
just so dumb right now.
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We're not even at the level where
if they wanted to talk to us,
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these so-called intelligent creatures out
there could even communicate with us.
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It could be that they've got better things
to do
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than just waft around the galaxy.
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They've seen our planet,
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they just don't wanna interfere with us
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until we get to this point
of technological or societal advancement
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where we're ready to be interacted with.
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Maybe the galaxy is colonized,
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maybe it's heavily colonized,
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but just not where we are.
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In other words,
the fact that we seem to be alone
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may be only that we're in a backwater.
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[narrator] But it's important to remember
that when Fermi calculated the odds
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that alien life is out there,
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it was just an educated guess.
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With so many stars and planets,
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he bet at least some of them
will develop life,
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which would evolve and spread out.
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The trouble with this bet is
there's a lot we don't know about life.
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The great filter theory helps us think
about what we don't know.
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Imagine the evolution of life
as a series of hurdles.
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First, molecules
start replicating themselves,
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which evolves into single-cell life,
then multi-cell life
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and then animals with large brains
that can use tools,
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and then smarter animals
that create even better tools. That's us.
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And finally, animals that can figure out
how to colonize the galaxy.
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Given the size and age of the universe,
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it seems like a lot of alien species
should have beat us to that last stage,
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unless one of those stages
is much harder than we think.
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[Paul Davies]
The view seems to be that
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given the right conditions,
life will obligingly pop up,
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but the truth is, nobody has a clue.
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We have no idea how non-life
turns into life.
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We know how life structures itself,
but our gaps are in the major transitions.
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[narrator] We know what the major hurdles
are in the evolution of life.
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Just not how hard they are to get past.
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Another example of life would help us
understand life better.
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But so far, no aliens have contacted us.
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So, it's up to us to find them.
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[man] Listen to the sound of the sun
and the stars.
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[Jill Tarter] I've spent my career
at the SETI Institute, S-E-T-I.
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Search for Extraterrestrial Intelligence,
that's the acronym.
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But in fact, we don't know how
to detect intelligence directly.
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[narrator] When scientists look
for intelligent aliens,
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they look for what are called
technosignatures...
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evidence of alien technology.
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We want to find
extraterrestrial intelligence,
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by finding something that's engineered,
something that's artificial,
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something that nature can't produce.
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If I find technology...
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I'm going to presume,
at least at some time...
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the existence
of an intelligent technologist.
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[narrator] From up close,
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Earth has technosignatures
in the form of city lights.
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From further out, aliens might notice
the satellites
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and space stations orbiting our planet.
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From even further,
they might pick up radio signals
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or stumble across the Voyager probes
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that are hurtling
across interstellar space.
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Sometimes the things we think
are technosignatures
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turn out to be natural phenomena,
like that pulsar.
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But since radio signals are still
our most promising leads,
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scientists do a lot of listening
to the sky.
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[Tarter] If you've seen the movie Contact,
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there's Jodie Foster on the hood
of the car with the earphones on.
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It's a bit ridiculous. Because in fact,
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the computer back
in the observatory control room
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is doing the signal processing.
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They're analyzing the equivalent of
the Encyclopedia Britannica every second.
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[narrator] Jill Tarter would know.
Jodie Foster's character was based on her.
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Holy shit!
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She gets to say, "Holy shit!," you know?
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We hope to some day have that moment.
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[narrator] But that's just one way
to search for extraterrestrial life.
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There are also biosignatures.
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Biosignatures are indications
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that life existed or once did exist
in any given environment.
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[narrator] If aliens came to Earth looking
for life after we were long gone,
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they would find biosignatures
in the form of fossils
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and chemical evidence of life processes.
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My favorite line to say with kids
is that all life poops.
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So we know that all life takes in energy
and releases waste products.
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[narrator]
If aliens were observing us from afar,
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they would see biosignatures
in the form of water
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and the gases in our atmosphere.
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Oxygen is so reactive
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that it can only be in our atmosphere
if it's being continuously produced.
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Without life, Earth's atmosphere
would have no oxygen,
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so we're trying to look for gases
that don't belong,
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that might be attributed to life,
and we call them biosignature gases.
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[narrator] Searching for biosignatures
on other planets is really hard.
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We can't even see planets
outside our solar system. Stars are so much brighter than planets.
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It's like trying to see a firefly
in a spotlight.
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Today, we have a planet finding technique
called the transit technique.
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When a planet goes in front of its star,
the starlight drops by a tiny amount.
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[narrator]
These drops in light give scientists clues
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about whether a planet might have life
on it.
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Like the distance from its star.
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We call the "Goldilocks zone"
the distance from the star,
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where the planet, as heated by the star,
is not too hot,
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not too cold, but just right for life.
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[narrator] Researchers have been able to
surmise some amazing things about planets
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just from these light patterns.
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Scientists think they found a super Earth
with really intense gravity.
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A planetary system with seven planets
all crammed into the Goldilocks zone.
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And even a planet that could have
red vegetation,
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from the different wavelengths
of light it receives.
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We now know of over 3,500 planets
outside of our solar system.
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Most of them were discovered
in just the last five years.
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And tools are only getting better.
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The next generation of space telescopes
will be able to see more distant galaxies.
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A newly launched satellite will survey
the entire sky for possible planets,
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rather than just small sections.
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And astronomers are developing
new technologies
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that would let them see
distant planets directly.
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The line between what is considered
completely crazy
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and what is mainstream
is constantly shifting.
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[narrator] For all the exciting new ways
to search for life in deep space,
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scientists are also searching
a lot closer to home.
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-[man] We have landed.
-[man 2] Roger.
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[narrator] In the 1970s,
we sent two landers to Mars
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to test the soil for evidence of life.
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The first and only time we've ever tried.
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One of the experiments came back negative.
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But another came back positive
for evidence of a process
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that we only associate with living things.
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When some of the experiments
came back positive,
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and the others came back negative,
it was controversial,
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because it was ambiguous.
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So it was hard to say,
did we actually really find life?
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[narrator] The contradiction could mean
an unknown chemical reaction occurred
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that only looked like a living
thing consuming energy.
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But since the '70s, we've learned
that life in extreme environments
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uses energy differently and leaves
different markers on its environment.
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The experiments that were designed
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were designed based on life as we knew it
back then,
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which was a very limited view
of life just here on Earth.
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We need to go back to Mars
and do the experiment again.
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[narrator]
The Mars 2020 mission is our next shot.
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Unlike the Viking experiments,
it won't test for currently living things,
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but it will look for signs that life once
did exist in certain Martian environments.
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A mission is also in the works
to look for biosignatures
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in the frozen oceans
of Jupiter's moon Europa.
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In the search for intelligent life,
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scientists are also trying to expand
their thinking and their search.
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Now, the only example we have
of intelligent life is indeed us.
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You know... [laughs]
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in Star Trek, I guess it was the doctor
on board, Bones,
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who'd occasionally say,
"It's life, Jim, but not as we know it."
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[narrator]
Actually, this is a common misquote.
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The line is a lyric in the song
"Star Trekkin" by The Firm
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and was never said in the show.
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It's life, Jim, but not as we know it. [narrator]
Spock, however, said something similar
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in Season One, episode 29.
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It's not life as we know or understand it.
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It is obviously alive. It exists.
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[narrator] Advances in our own technology
give us new ideas
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about what intelligent aliens
might be like.
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One thing that we're doing in this century
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and certainly in the first half
of this century, it seems,
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is to develop artificial intelligence
that does more
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than just play a good game of chess.
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Humans are the best known reference
for intelligence.
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What a great standard
to try to live up to.
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We think of the aliens as being
like a soft and squishy biology,
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whereas in fact the majority
of the intelligence in the universe
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could very well be synthetic intelligence.
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One good thing in terms of helping us
to think about what we don't know
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is to read science fiction.
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Actually, Arrival was one
of my favorite movies.
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Just because of the concept
that the aliens could be so different
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from intelligent humanoids.
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I really think that's
how it's gonna end up being.
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[narrator] Science fiction has shaped
our space programs
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from the very beginning.
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[Kennda Lynch] The Martian Chronicles,
War of the Worlds, many senior scientists
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were inspired by those early,
early novels,
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and they've actually created
the science reality of Mars exploration
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that we have today.
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[Sara Seager]
It's a multi-generational search.
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We're just starting now.
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We're just kind of planting the seeds
for a really long endeavor.
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[man] Velocity build up
in feet per second.
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[man 2] Okay.
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[Tarter] Consider the volume
of all the Earth's oceans.
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All right, and let's say,
that's the volume of search space,
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where we might find a signal.
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Well, in 50 years,
how much of that ocean have we searched?
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It's a pretty disappointing
one glass of water.
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[narrator] Whether we find
extraterrestrial life
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or learn that we are alone,
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it will tell us a lot
about our civilization
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and what our future might be.
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Think back to that great filter theory.
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It could be even life rarely gets started
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or that the universe is teeming with life,
but none of it...
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as smart as us.
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That is good news
for the future of our civilization.
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It means that we are maybe
the only planet in the galaxy
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that got as far as intelligent life,
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and there's no reason
we can't be set fair
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for thousands or millions of years
in the future.
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[narrator] Or maybe the hardest stage
is ahead of us.
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And some unknown challenge
awaits humanity.
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If life on Earth is typical
and we are typical,
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but the typical thing is
you don't survive very long then,
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that doesn't say much about our future.
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The importance for the search
for life elsewhere in the universe
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is kind of the search
in understanding ourselves.
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It's important to understanding
how did we as a planet come here.
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And how rare are we
or how rare are we not?
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And for humans, it's an understanding of,
you know,
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what's the next big step for us.