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00:00In our galaxy, there may be ships of other worlds that dare to venture into the cosmic deep.
00:12Perhaps they travel from star to star, searching for worlds where life has taken hold,
00:19to get a closer look at the emergent properties of life that even they can't predict.
00:26This particular vessel is on just such a survey mission.
00:33Who, scanning this world, would have pronounced it a fit nursery for life?
00:40What wild optimist could have foreseen that peonies and eagles would one day spring from this hellhole?
00:48In its infancy, four billion years ago, Earth held little apparent promise.
00:59Now, Venus, that's more like it.
01:05Back then, it may have had oceans and land, and possibly life.
01:11This long-ago epoch was Venus' moment to flourish, its time in the habitable zone.
01:24For any world, that's a period when its relationship to its star means that it's not too hot and not too cold.
01:31It's a time in a world's existence, when it can foster and sustain life.
01:39But the grace of the habitable zone is a fleeting thing, and for no world lasts forever.
01:47We reside in our star's habitable zone, but it is moving outwards at the rate of about three feet per year.
01:59Earth has already passed through 70% of its most hospitable time, but no need to worry.
02:06That still leaves us hundreds of millions of years to plan and execute our exit strategy.
02:13Where will we go when the sun's grace leaves us behind for other worlds, and Earth is no longer a garden for life?
02:23Will our species have set sail for distant islands in the vast ocean of the Milky Way?
02:43O worms'e has been raised by the sun.
02:57Bayou
02:58Bayou
02:59Bayou
03:00Bayou
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03:01Bayou
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03:03Bayou
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03:07Bayou
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03:10Bayou
03:11Bayou
03:44There is no refuge from change in the cosmos, no safe place to hide for more than a few hundred million years. Someday, all of this will surrender to the churning cycles of birth, destruction, and rebirth mandated by the laws of nature.
04:02The universe evolves beautiful things, then smashes them to bits before making new ones out of the shattered pieces. Any species that wants to survive long term on any possible world will have to learn how to engineer interplanetary and ultimately interstellar mass transit.
04:22How do we know this? The little we've learned about the universe allows us glimpses of the future.
04:33I'm not talking about the near term where climate change caused by human activity poses a danger to our civilization.
04:40If we want to endure for thousands, millions, even billions of years, we'll have to stop dumping all that carbon dioxide into the atmosphere right now.
04:51But I'm going to give us the benefit of the doubt and take the long view.
05:00The sun is aging just like the rest of us.
05:04Some day, it will exhaust the hydrogen fuel at its core.
05:16Five or six billion years from now, the zone of hydrogen fusion will slowly migrate outward.
05:23An expanding shell of thermonuclear reactions until the temperatures are less than about 10 million degrees.
05:34It will go from being a yellow dwarf to a red giant.
05:44Its gravitational hold on Venus and Earth will diminish, allowing them to migrate to a safer distance.
05:52For a little while, this red giant sun, ruddy and bloated, will envelop and devour the planet Mercury.
06:01The grace of the habitable zone will be moving outward, farther and faster.
06:12By now, the intense light and heat of the sun's expansion will reach all the way out to the Jupiter system.
06:19Its clouds of ammonia and water will escape and be lost to space as vapor.
06:28And for the first time, the more dowdy, hidden layers down beneath Jupiter's gaudy upper atmosphere will be exposed.
06:37Could we make a home on one of Jupiter's frozen moons?
06:48The thick layers of ice encasing Europa and Callisto will defrost, exposing the liquid oceans beneath to harsh sunlight thousands of times stronger than before.
07:00This will liberate large amounts of water vapor, starting a runaway greenhouse effect.
07:11Ganymede's once thin atmosphere will become steamy and dense.
07:16If life was swimming in those oceans all along, here's a new chance to flourish and evolve.
07:29Then Ganymede will belong to those beings.
07:32It's just as well, because we'll want our next home to be at a safer distance from the sun.
07:37Solar evolution is inevitable, but we have a billion years to go house hunting.
07:47Plenty of time to seek those worlds in the cosmos that could become our new homes.
07:52Saturn, what has the sun done to you?
07:58She's stolen your glory, your rings.
08:01Your rings.
08:05And Titan, she's robbed you of your atmosphere.
08:15Hey, we're running out of possible worlds here.
08:18Just in time, Neptune's moon, Triton. A world named for the sun of the Roman god Neptune.
08:32First known to the Greeks as Poseidon, the god of the sea.
08:48No one's named this ocean yet. It only began to exist when the heat of the red giant sun melted the ammonia and water ices of this once frigid moon.
09:03Our distant descendants will live to different rhythms that we do.
09:08A day on Triton will be 144 hours long, and the winters will be brutal.
09:13They'll be nearly 50 years long.
09:17But still, the Triton of a few billion years from now could be a great home for us.
09:23It's got everything. An atmosphere, water oceans, the chemical building blocks that would make life possible.
09:30Okay, it's chilly, but not much worse than upstate New York in January.
09:35And that means great skiing all year round.
09:38But one day, the sun will exhaust itself completely, and the fleeting grace of the habitable zone will end here, too.
09:53When the sun's torrid red giant phase is over.
09:58It will strip itself naked, revealing the small white dwarf beneath.
10:04A star without even enough energy left to warm her surviving children.
10:09The moons of the outer solar system.
10:11So, if we're looking for a long lease on a new home, say, more than just a couple hundred million years, we'd have to travel even farther out.
10:30We'll have to leave our solar system and brave the vast, bottomless ocean of interstellar space.
10:38I know what you're thinking.
10:40Are we to venture to the distant stars?
10:43We once made a few baby steps to the moon before we lost our will and scurried back to the safety of our mother.
10:49What makes us think we could survive a voyage between the stars, the nearest of which is a hundred million times farther than our moon?
11:00Wouldn't our tiny ships be swallowed up by the great unknown?
11:04I think we can do it.
11:06Why?
11:08Because we've done it before.
11:10We dream of sailing among the island worlds of the Milky Way.
11:30Catching photons with our sails.
11:33Daring to go tetherless beyond the point of no return.
11:37We've passed this way before.
11:43Once, there was a people who chose the unknown.
11:47They risked everything to go forth on uncharted seas.
11:51And their courage was rewarded.
11:54They found paradise.
11:57I want to tell you their story.
12:07We call these people the Lapita.
12:10But that was never their name.
12:12It was just the result of a misunderstanding made decades ago when we first began to discover the broken shards of their pottery.
12:21To me, they're not the Lapita.
12:24They are the Voyagers.
12:25A name far more worthy of them.
12:31About 10,000 years ago, when the population of settlements in southern China began to swell,
12:37there were those who chose to pioneer the frontier farther south to what is now Taiwan.
12:43And our Voyagers settled there, happily for thousands of years, until the place began to get too crowded again.
12:55Just as we, of this planet, came of age in a kind of cosmic quarantine, cut off from any hope of knowing about, much less reaching the other worlds of the cosmos,
13:08our distant ancestors were prisoners of the land.
13:12If you wanted to travel a great distance, you had to walk there.
13:17And if you walked as far as you could, you would be hemmed in at the water's edge.
13:22This was before the age of the great seafaring civilizations.
13:26The Phoenicians of the Middle East and the Minoans of Crete.
13:30And for most of their history, they hugged the shore.
13:34Their fishing and trading expeditions were careful to keep land in sight.
13:43For our ancestors, this was the edge of the cosmic ocean.
13:52We do not know what first inspired the Voyagers to attempt the seemingly impossible.
13:59Could they no longer trust the Earth?
14:05They were living on a tectonic plate, where earthquakes and volcanic eruptions were common.
14:10Or did hostile neighbors there make life intolerable for them?
14:16Did a change in climate threaten their livelihood?
14:21Were there new population pressures?
14:25Did they begin to exhaust the resources of their island by over-hunting and over-fishing?
14:31Or was it simply something innately human in them that made them want to know what was out there?
14:41To reach for the mysterious distance, no matter how dangerous that might be.
14:46Whatever their motives, over time, they conquered their fear.
14:51And made preparations to venture when no one had gone before.
14:57Qu tots.
15:08Quovers.
15:18Quatreon.
15:20Quatreon.
15:22Quatreon.
15:24Come here, buddy.
15:54Come here, buddy.
16:24Come here, buddy.
16:54Come here, buddy.
17:24Come here.
17:28Come here.
17:37Oh, my God.
18:07The Voyagers use the careful observations of their ancestors over generations to develop navigational techniques that are still viable today.
18:34The seasonal migratory flight patterns of the birds was their GPS.
18:40They also brought high-flying frigate birds with them, releasing them at the proper time to find the shortest distance to the nearest land.
18:48They could read water, feeling the ocean currents in their fingertips and the messages written in the clouds.
18:54These Voyagers were scientists, and all of nature was their laboratory.
19:00Oh, no!
19:11Oh, no!
19:13Oh, no!
19:14Oh, no!
19:16Oh, no!
19:17Oh, no!
19:18Oh, no!
19:19Oh, no!
19:20Oh, no!
19:21Oh, no!
19:21Oh, no!
19:22Oh, no!
19:23The Philippine Islands was where they settled first.
19:50After lingering there for a thousand years, they were ready to set sail again.
19:57New generations of voyagers, Polynesians, mounted successful missions of exploration to Indonesia,
20:04the Melanesian Islands, Venuatu, Fiji, Samoa, and on to the Marquesas,
20:14and then to the most isolated island group on Earth, the Hawaiian Islands.
20:21Tahiti, Tonga, New Zealand, Pitcairn, and Easter Island.
20:29Their empire of water covered nearly 20 million square miles of sea,
20:34and they accomplished this without a single nail or metal tool of any kind.
20:40As contact between the islands became less frequent,
20:43the language the Polynesians brought with them evolved into different tongues in isolation.
20:48Many words changed, but one word remained the same in all the languages of the wide Pacific.
20:56Layar, the word for sail.
21:01Where are we bound for next?
21:29To that place where you can read the Book of Worlds.
21:57The ship of the imagination is on a mission.
22:00Not to any particular world, but to an empty space in the interstellar ocean.
22:07Why there? Come with me.
22:10We're headed to a place 50 billion miles from our sun.
22:15What I'm about to show you is a gift of thousands of generations of searchers.
22:20We've been studying light for millennia and gravity for centuries.
22:25Among Einstein's many insights was an understanding of how one could affect the other.
22:31The way gravity bends light makes it possible to turn any star, including our own, into a kind of lens for a cosmic telescope, one 50 billion miles long.
22:44Our most powerful space-based telescopes of the present day can only see the worlds of other suns as mere dots.
22:51A cosmic telescope could give us detailed images of the mountains, oceans, glaciers, and who knows, maybe even the cities of these worlds.
23:06This is the cosmic telescope's detector array.
23:13Collecting the light bouncing off a distant world.
23:17It then sends a signal back to Earth, becoming, in effect, the telescope's eyepiece.
23:27See that brightest star in the sky?
23:29That's our sun.
23:31And the lens of this telescope.
23:34So how can a star, which you can't see through, be turned into a lens?
23:40When all the rays of light from a distant planet pass very close to the sun, the sun's gravity bends those rays ever so slightly.
23:50Where they converge in space is called the focal point.
23:54Because that's where the object you're looking at comes into focus.
24:01So what can you see through a 50 billion mile long telescope?
24:06Virtually anything you want.
24:09Galileo's best telescope could magnify an image 30 times, making Jupiter, say, appear 30 times closer.
24:17Our cosmic telescope can make things appear 100 billion times closer.
24:23And we can aim it in almost any direction.
24:26Our detector array moves 360 degrees around the sun.
24:32There's only one part of the cosmos that's off limits to us.
24:35And that's the heart of our own Milky Way galaxy, which is just too bright.
24:40Its radiance is blinding.
24:41But with a telescope like this one, so much else that has been foreclosed to us would be made visible.
24:47A possible world, perhaps.
24:59The mixture of gases in its atmosphere can tell us if life is there.
25:05Molecules have specific color signatures.
25:07If we look at this atmosphere through a spectroscope, an instrument that breaks down light into its constituent colors, we'll be able to identify the molecules that make up the atmosphere.
25:19The presence of oxygen and methane are telltale signs of life.
25:26This world is alive.
25:29And our cosmic telescope could give us a complete picture of its entire surface.
25:36It's not just an optical telescope, one that can only see visible light.
25:42It's also a radio telescope.
25:45Just as it can magnify light from distant worlds 100 billion times, it can do the same for radio waves.
25:52There's something astronomers call a water hole.
25:56It's named after that place where the lions and the water buffalo gather to drink and bathe.
26:03It's a region of the radio spectrum where interference is at a minimum.
26:07And we can eavesdrop on even the faintest transmissions between far-flung civilizations.
26:13We would need to use all of our computing power to decrypt the signals hidden in the noise.
26:22And this vast telescope is also a means for seeing back in time.
26:43You can't look across space without seeing an object in the past.
26:49That's because the speed of light is finite.
26:53In the morning, look up at the sun and see it as it was 8 minutes and 20 seconds ago.
27:00You'll never see it any other way.
27:03That's because it takes that long for light from the sun to travel the 93 million miles to Earth.
27:10And when we look at any world through this telescope, we're seeing it in the past.
27:20Now imagine the cosmic telescope of another civilization.
27:24Say, one that's 5,000 light years from Earth.
27:28The astronomers of that world could witness the building of the pyramids in Egypt or follow the Polynesian voyages as they bravely made their way across the Pacific.
27:39But perhaps the most important use of the cosmic telescope would be our search for new Earths.
27:46What I can't understand is why we haven't built one.
27:50We already know how to do it.
27:52We have the technology right now.
27:55When would you like the future to begin?
28:02Okay, we've got the biggest dreams of putting our eyes on other worlds, traveling to them, making them our home.
28:30But how do we get there?
28:32The stars are so far apart.
28:35We would need sailing ships that could sustain human crews over the longest haul of all time.
28:41The nearest star is four light years away.
28:45That's 24 trillion miles to Proxima Centuri.
28:50Just to give you some idea of how far away that point of light really is, if NASA's Voyager 1 spacecraft, which moves at a pretty good clip, 38,000 miles an hour, was headed for Proxima Centuri, it would take 70,000 years to get there.
29:12And that's only the nearest star out of the hundreds of billions in our galaxy alone.
29:19So if we want to endure as a species beyond the projected shelf life of our own planet, we'd better act like the Polynesians.
29:38We need to take what we know of nature and build sailing ships that can ride the light as they once rode the wind.
29:51These sails are enormous, miles high, but they're very thin, a thousand times thinner than a garbage bag.
30:04When a photon of light strikes those magnificent sails, it gives them a little push.
30:23This means that in the vacuum of space, even the tiniest push from a photon will propel them ever faster until they're moving at a significant fraction of the speed of light.
30:46When you get too far from your star and the light dwindles, lasers can do the trick.
31:16If we were to light sail our way to Proxima Centuri, it wouldn't take 70,000 years, but only 20 years.
31:34Proxima b lies in the habitable zone of its star, but we don't yet know if it could support life.
31:41Does it have the kind of protective magnetic field that has sheltered the evolution of life on the surface of our world?
31:51Another consequence of Proxima b's close location to its star is that the planet is probably tidally locked.
31:59One side perpetually facing the star, the other doomed to endless night.
32:11one side perpetually is now spoils!
32:14One side世界 is still a massive hit.
32:16One side perpetually causes the same.
32:25These little stars may be lukewarm, but they have a long future ahead of them.
32:32Trillions of years!
32:34Think of the continuity and growth potential of a civilization with a
32:40future measured in trillions of years. It's always magic hour on the strip of
32:47land that lies between day and night on this world. If Proxima B is habitable,
32:53its life would be confined to this twilight zone. It could be a home for the
32:59indigenous life here or a possible campsite for our descendants. The gravity
33:07on Proxima B is about 10% greater than ours on Earth. No real problem for us,
33:13just a little like exercising with weights on. Remote scanning from orbit
33:22found no apparent signs of life, making Proxima B a way station on a much
33:28grander interstellar diaspora of Earthlings. But for those longer trips we're gonna
33:37need a faster boat. Let's say we found a system located a hundred light years from
33:43home, one with several potentially habitable worlds. For light sailors that
33:49would be a 500 year long trip. Is it possible to build a ship that could break
33:55the cosmic speed limit? A mathematical physicist, Miguel Alcubierre of Mexico,
34:01inspired by the original Star Trek television series, conceived the
34:06calculations for a ship that could theoretically travel faster than the
34:10speed of light. If successful, it could cut the travel time between our Sun and
34:16this distant star system down to a single year or even less. But wait a minute,
34:22isn't it a cardinal rule of science that thou shalt not travel faster than light? It
34:28is. But here's the thing about the Alcubierre Drive. It doesn't move. The cosmos does. The
34:37ship itself would be enclosed in its own space-time bubble, where it needn't
34:41violate any laws of physics. Harold White of the United States ironed out some of the
34:47kinks, such as prohibitively enormous energy requirements to fly it. But it
34:53remains far beyond our immediate grasp. The Alcubierre Drive ship is a
35:01gravitational wave-making machine that compresses the ocean of space-time in
35:05front of it and expands that ocean in its wake. Jet skis for joyriding through the
35:13galaxy and beyond. Who knows, maybe the entire Laniakea supercluster could one day
35:20become our pond. That's a hundred thousand galaxies. Laniakea, the phrase in Hawaiian
35:28for immeasurable heaven. An advanced version of our Alcubierre Drive could do
35:36six hundred trillion miles in the blink of an eye.
35:43Before you know it, you're in the planetary system of a distant star. Let's
35:49call it the Hoku system. A red dwarf star surrounded by a retinue of rocky and ice
35:55giant planets. Somewhere among them is a world that we have come to call home. Our
36:02cosmic telescope sifted through all the stars within a radius of a hundred light
36:07years and pointed the way to this one. All seven of these planets huddle closer to
36:13their star than Mercury does to our Sun.
36:19Haomiya is just on the outskirts of Hoku's habitable zone. Those warm green
36:26colors seem inviting, but we're not seeing the tops of forests. That green comes from
36:32methane and ammonia. Even at a distance of only 27 million miles, the star Hoku is
36:38too weak to keep this planet warm.
36:42We are now in the sweet spot of Hoku's habitable zone. And this is the planet Tangaroa, where
36:54the latest chapter in the saga of our species is playing out.
37:04It took a few hundred years for humans to terraform this lifeless world. But now, even the air tastes
37:12as sweet as home.
37:22But this was only Indonesia, one of the earlier stops on our nomadic odyssey throughout the Milky
37:28way. There were still so many islands that lay ahead.
37:41And in this dream future of ours, with our faster-than-light craft, there would come a
37:46time when we could place our cosmic telescope far enough away from our home planet to
37:52see firsthand those of our nameless ancestors, who first set sail on unknown seas.
38:22Carl Sagan made this drawing when he was a child, imagining the unfolding of the next golden
38:40age of exploration.
38:52As a scientist, he played a central role in it. And now, he speaks to us, out of the past,
39:01about our dreams of the future.
39:07Are we to venture out into space, move worlds, re-engineer planets, spread to neighboring star
39:15systems? We who cannot even put our own planetary home in order, riven with rivalries and hatreds,
39:23despoiling our environment, murdering one another through irritation and inattention, as well
39:29as on deadly purpose. And moreover, a species that, until only recently, was convinced that the
39:36whole universe was made for its sole benefit. I do not imagine that it is precisely we, with
39:47our present customs and social conventions, who will be out there. If we continue to accumulate
39:54only power and not wisdom, we will surely destroy ourselves. Our very existence in that distant time
40:04requires that we will have changed our institutions and ourselves.
40:12How can I dare to guess about humans in the far future? It is, I think, only a matter of natural selection.
40:20If we become even slightly more violent, short-sighted, ignorant and selfish than we are now, almost
40:30certainly, we will have no future. If you're young, it's just possible that we will be taking
40:40our first steps on near-Earth asteroids and Mars during your lifetime. By the time we're ready to settle even the nearest other
40:50planetary systems, we will have changed. The simple passage of so many generations will have changed us.
40:58The different circumstances we will be living under will have changed us. We're an adaptable species.
41:10It will not be we who reach Alpha Centauri and the other nearby stars. It will be a species very like us,
41:22but with more of our strengths and fewer of our weaknesses.
41:28A species returned to circumstances more like those for which it was originally evolved. More confident,
41:40far-seeing, capable and prudent. The sorts of beings we would want to represent us in a universe that,
41:50for all we know, is filled with species much older, much more powerful and very different.
42:06The vast distances that separate the stars are providential. Beings and worlds are quarantined
42:16one from another. The quarantine is lifted only for those with sufficient self-knowledge
42:24and judgment to have safely traveled from star to star.
42:36Our remote descendants, safely arrayed on many worlds through the solar system and beyond,
42:48will be unified by their common heritage, by their regard for their home planet,
42:56and by the knowledge that whatever other life may be, the only humans in all the universe
43:04come from Earth.
43:14Thank you, Mother.
43:34Thank you, Mother.