#CarlSagan covers a wide range of scientific subjects, including the origin of life and a #perspective of our place in the universe...
A 13-part #documentary #series that covers a wide range of #scientific #subjects, including the #origin of #life and a #perspective of our place in the universe narrated by famous American #Scientist – #Carl #Sagan.
A 13-part #documentary #series that covers a wide range of #scientific #subjects, including the #origin of #life and a #perspective of our place in the universe narrated by famous American #Scientist – #Carl #Sagan.
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LearningTranscript
00:00:00Transcribed by ESO, translated by —
00:01:00We are drifting in the great ocean of space and time.
00:01:06In that ocean, the events that shape the future are working themselves out.
00:01:13Each creature and every world, to the remotest star, owe their existence to the great, coursing, implacable forces of nature, but also to minor happenstance.
00:01:25We are carried with our planet around the sun.
00:01:32The earth has made more than 4 billion circuits of our star since its origin.
00:01:36The sun itself travels about the core of the Milky Way galaxy.
00:01:43Our galaxy is moving among the other galaxies.
00:01:46We have always been space travelers.
00:01:48These fine sand grains are all more or less uniform in size.
00:01:57They've been produced from bigger rocks through ages of jostling and rubbing, abrasion and erosion, driven in part by the distant moon and sun.
00:02:08So the roots of the present lie buried in the past.
00:02:13We are also travelers in time.
00:02:15But trapped on earth, we've had little to say about where we're going in time and space, or how fast.
00:02:27But now, we're thinking about true journeys in time, and real voyages to the distant stars.
00:02:34A handful of sand contains about 10,000 grains, more than the total number of stars we can see with the naked eye on a clear night.
00:02:47But the number of stars we can see is only the tiniest fraction of the number of stars there are.
00:02:52What we see at night is the nearest smattering of the nearest stars, with a few more distant bright stars thrown in for good measure.
00:03:03Meanwhile, the cosmos is rich beyond measure.
00:03:07The total number of stars in the universe is larger than all the grains of sand on all the beaches of the planet Earth.
00:03:14Long ago, before we had figured out that the stars are distant suns, they seemed to us to make pictures in the sky.
00:03:27Just follow the dots.
00:03:31The constellation called the Big Dipper today in North America has had many other incarnations.
00:03:36Every culture, ancient and modern, has placed its totems and concerns among the stars, from a Chinese bureaucrat to a German wagon.
00:03:51But very ancient cultures would have seen different constellations, because the stars move with respect to one another.
00:03:58We can give a computer the present three-dimensional positions and motions of the nearby stars, and then run the patterns back into time.
00:04:09Every constellation is a single frame in a cosmic movie.
00:04:13But because our lives are so short, because the star patterns change so slowly, we tend not to notice it's a movie.
00:04:21A million years ago, there was no Big Dipper.
00:04:24Our ancestors, looking up and wondering about the stars, saw some other pattern in the northern skies.
00:04:33We can also run a constellation, Leo the Lion, say, forward in time, and see what the patterns in the stars will be in the future.
00:04:44A million years from now, Leo might be renamed the constellation of the Radio Telescope.
00:04:49Although I suspect that radio telescopes then will be as obsolete as stone spears are now.
00:04:55Or, here's the constellation of Cetus, the whale.
00:05:08A million years ago, it may have been called something else.
00:05:12Perhaps, the spear.
00:05:14Now, let's run fast forward.
00:05:29Millions of years from now, some other, very different image, will be featured in this cosmic movie.
00:05:35In Orion, the hunter, things are changing, not only because the stars are moving, but also because the stars are evolving.
00:05:52Many of Orion's stars are hot, young, and short-lived.
00:05:55They're born, live, and die within a span of only a few million years.
00:05:59If we run Orion forward in time, we see the births and explosive deaths of dozens of stars flashing on and winking off like fireflies in the night.
00:06:11If we wait long enough, we see the constellations change.
00:06:18But if we go far enough, we also see the star patterns alter.
00:06:22The two-dimensional constellations are only the appearance of stars strewn through three dimensions.
00:06:28Some are dim and near, others are bright but farther away.
00:06:33Could a space traveler actually see the patterns of the constellations change?
00:06:38For that, you must travel roughly as far as the constellation is from us.
00:06:45Here, we're traveling hundreds of light years, circling all the way around the stars of the Big Dipper.
00:06:55The inhabitants of planets around other stars will see very different constellations than we do,
00:07:01because their vantage points are different.
00:07:08Here we are in the constellation Andromeda, or at least a model of it, next to the constellation Perseus.
00:07:21Andromeda, in the Greek myth, was the maiden who was saved by Perseus from a sea monster.
00:07:28This star just above me is Beta Andromeda, the second brightest star in the constellation, 75 light years from the Earth.
00:07:40The light by which we see, this star, has spent 75 years traversing into stellar space on its journey to the Earth.
00:07:49In the unlikely event that Beta Andromeda blew itself up a week ago Tuesday,
00:07:56we will not know of it for another 75 years, as this interesting information traveling at the speed of light crosses the enormous interstellar distances.
00:08:07When the light we see from the star set out on its long interstellar voyage,
00:08:13the young Albert Einstein, working as a Swiss patent clerk,
00:08:19had just published his epical special theory of relativity here on Earth.
00:08:25We see that space and time are intertwined.
00:08:31We cannot look out into space without looking back into time.
00:08:38The speed of light is very fast, but space is very empty, and the stars are very far apart.
00:08:48In fact, the distances that we've been talking about up to now are very small by the usual astronomical standards.
00:08:55In fact, the distance from the Earth to the center of the Milky Way galaxy is 30,000 light years.
00:09:04From our galaxy to the nearest spiral galaxy like our own, called M31,
00:09:12and which is also within, that means behind, the constellation Andromeda,
00:09:17is 2 million light years.
00:09:21When the light we see today from M31 left on its journey for Earth,
00:09:29there were no human beings on the Earth,
00:09:31although our ancestors were nicely evolving and very rapidly to our present form.
00:09:38There are much greater distances in astronomy.
00:09:41The distance from the Earth to the most distant quasars is 8 or 10 billion light years.
00:09:48We see them as they were before the Earth itself accumulated, before the Milky Way galaxy was formed.
00:09:57The fastest space vehicles ever launched by the human species are the Voyager spacecraft.
00:10:03They are traveling so fast that it's only 10,000 times slower than the speed of light.
00:10:10The Voyager spacecraft will take 40,000 years to go the distance to the nearest stars,
00:10:15and they're not even headed towards the nearest stars.
00:10:18But is there a method by which we could travel in a conveniently short time to the stars?
00:10:25Can we travel close to the speed of light?
00:10:28And what's magic about the speed of light?
00:10:31Can't we travel faster than that?
00:10:35It turns out that there is something very strange about the speed of light,
00:10:41something that provides the key to our understanding of time and space.
00:10:49The story of its discovery takes us to Tuscany in northern Italy.
00:10:56There's something almost timeless about this place.
00:10:59A century ago, it probably looked very much the same.
00:11:02If you had traveled these roads in the summer of 1895,
00:11:20you might have come upon a 16-year-old German high school dropout.
00:11:24His teacher had told him that he would never amount to anything,
00:11:27that his attitude destroyed classroom discipline,
00:11:30that he'd be better off out of school.
00:11:33So he left and came here,
00:11:35where he enjoyed wandering these roads
00:11:37and giving his mind free reign to explore.
00:11:42One day, he began to think about light,
00:11:45about how fast it travels.
00:11:47In our everyday life, we always measure the speed of a moving object
00:11:50relative to something else.
00:11:52I'm moving at about 10 kilometers an hour relative to the ground.
00:11:57But the ground isn't at rest.
00:11:58The Earth is turning at more than 1,600 kilometers an hour.
00:12:03The Earth itself is in orbit around the Sun.
00:12:05The Sun is moving among the drifting stars, and so on.
00:12:09It was hard for the young man to imagine some absolute standard
00:12:13to measure all these relative motions against.
00:12:15He knew that sound waves are a vibration of the air,
00:12:29and their speed is measured relative to the air itself.
00:12:33But sunlight travels across the vacuum of empty space.
00:12:37Do light waves move relative to something else?
00:12:39And if so, he wondered, relative to what?
00:12:42That teenage dropout's name was Albert Einstein,
00:12:53and his ruminations changed the world.
00:12:55He had been fascinated by Bernstein's 1869
00:13:05People's Book of Natural Science.
00:13:10Here, on its very first page,
00:13:14it describes the astonishing speed of electricity
00:13:17through wires and light through space.
00:13:20Einstein wondered, perhaps for the first time
00:13:24here in northern Italy,
00:13:25what the world would look like
00:13:27if you could travel on a wave of light.
00:13:30To travel at the speed of light.
00:13:33What an engaging and magical thought
00:13:35for a teenage boy on the road
00:13:37where the countryside is dappled
00:13:39and rippling in sunlight.
00:13:41You couldn't tell you were on a light wave
00:13:55if you were traveling with it.
00:13:57If you started on a wave crest,
00:14:00you would stay on the crest
00:14:01and lose all notion of it being a wave.
00:14:04Something funny happens at the speed of light.
00:14:09So I don't think about that.
00:14:38The more Einstein thought about such questions, the more troubling they became.
00:14:44Paradoxes seemed to pop up all over if you could travel at the speed of light.
00:14:50Certain ideas had been accepted as true without sufficiently careful thought.
00:14:58One of those ideas had to do with the light from a moving object.
00:15:03The images by which we see the world are made of light, and are carried at the speed of
00:15:08light, 300,000 kilometers a second.
00:15:13You might think that the image of me should be moving out ahead of me, at the speed of
00:15:17light plus the speed of the bicycle.
00:15:20If I'm moving towards you faster than a horse and a cart, then my image should be approaching
00:15:25you exactly that much faster.
00:15:27My image ought to arrive earlier.
00:15:32But in reality, you don't see any time delay.
00:15:35In a near collision, for example, you always see everything happen at once.
00:15:39Horse, cart, swerve, bicycle, all simultaneous.
00:15:43But how would it look if it were proper to add the velocities?
00:15:48Since I'm heading towards you, you'd add my speed to the speed of light.
00:15:51So my image ought to arrive before the image of the horse and cart.
00:15:57I'd be cycling towards you quite normally.
00:15:59To me, a collision would suddenly seem imminent.
00:16:03But you'd see me swerve for no apparent reason and have a collision with nothing.
00:16:10Now, the horse and cart aren't headed towards you.
00:16:12Their image would arrive only at the speed of light.
00:16:17Could it seem to me that I just missed colliding while, to you, it wasn't even close?
00:16:23In precise laboratory experiments, scientists have never observed any such thing.
00:16:29If the world is to be understood, if we are to avoid logical paradoxes when traveling at high speeds, then there are certain rules which must be obeyed.
00:16:40Einstein called these rules the special theory of relativity.
00:16:45Light from a moving object travels at the same speed, no matter whether the object is at rest or in motion.
00:16:51Thou shalt not add my speed to the speed of light.
00:16:56Also, no material object can travel at or beyond the speed of light.
00:17:01There's nothing in physics that prevents you from traveling as close to the speed of light as you like.
00:17:0599.9% of the speed of light is just fine.
00:17:09But no matter how hard you try, you can never gain that last decimal point.
00:17:15For the world to be logically consistent, there must be a cosmic speed limit.
00:17:21The crack of a whip is due to its tip moving faster than the speed of sound.
00:17:29It makes a shockwave a small sonic boom in the Italian countryside.
00:17:34The thunderclap has a similar origin.
00:17:37So does the sound of a supersonic airplane.
00:17:40So why is the speed of light a barrier any more than the speed of sound?
00:17:47The answer is not just that light travels about a million times faster than sound.
00:17:52It's not merely an engineering problem like the supersonic airplane.
00:17:56Instead, the light barrier is a fundamental law of nature, as basic as gravity.
00:18:03Einstein found his absolute framework for the world,
00:18:06this sturdy pillar among all the relative motions of the cosmos.
00:18:11Light travels just as fast no matter how its source is moving.
00:18:15The speed of light is constant relative to everything else.
00:18:19Nothing can ever catch up with light.
00:18:22Einstein's prohibition against traveling faster than light seems to clash with our common-sense notions.
00:18:32But why should we expect our common-sense notions to have any reliability in a matter of this sort?
00:18:38Why should our experience at 10 kilometers an hour constrain the laws of nature at 300,000 kilometers a second?
00:18:48Relativity sets limits on what humans ultimately can do.
00:18:56The universe is not required to be in perfect harmony with human ambition.
00:19:03Imagine a place where the speed of light isn't its true value of 300,000 kilometers a second, but something a lot less.
00:19:16Let's say, 40 kilometers an hour, and strictly enforced.
00:19:23Just as in the real world, we can never reach the speed of light.
00:19:27The commandment here is, still, thou shalt not travel faster than light.
00:19:32But we can do thought experiments on what happens near the speed of light, here, 40 kilometers an hour.
00:19:39The speed of a motor scooter.
00:19:45You can't break the laws of nature.
00:19:47There are no penalties for doing so.
00:19:50The real world, and this one, are merely so arranged that transgressions can't happen.
00:19:56The job of physics is to find out what those laws are.
00:20:03Before Einstein, physicists thought that there were privileged frames of reference, some special places and times against which everything else had to be measured.
00:20:14Einstein encountered a similar notion in human affairs.
00:20:17The idea that the customs of a particular nation, his native Germany or Italy or anywhere, are the standard against which all other societies must be measured.
00:20:27But Einstein rejected the strident nationalism of his time.
00:20:32He believed every culture had its own validity.
00:20:36And also in physics.
00:20:37He understood that there are no privileged frames of reference.
00:20:40Every observer, in any place, time or motion, must deduce the same laws of nature.
00:20:50A speed is simply how much space you cover in a given time.
00:20:54As any kid on a motor scooter knows.
00:20:56Since near the velocity of light, we cannot simply add speeds.
00:21:04The familiar notions of absolute space and absolute time, independent of your relative motion, must give way.
00:21:11That's why, as Einstein showed, funny things have to happen close to the speed of light.
00:21:18There are conventional perspectives of space and time strangely change.
00:21:25Your nose is just a little closer to me than your ears.
00:21:32Light reflected off your nose reaches me just an instant in time before your ears.
00:21:37But suppose I had a magic camera so that I could see your nose and your ears at precisely the same instant.
00:21:49With such a camera, you could take some pretty interesting pictures.
00:21:53Paolo says goodbye to his little brother, Vincenzo.
00:22:01And rides off.
00:22:03He's now going more than half the speed of light.
00:22:05He's almost catching up with his own light waves.
00:22:08This compresses the light waves in front of him and his image becomes blue.
00:22:12The shorter wavelength is what makes blue light waves blue.
00:22:17Also, Paolo becomes skinny in the direction of motion.
00:22:21This isn't just some optical illusion.
00:22:23It really happens when you travel near the speed of light.
00:22:27As he roars away, he leaves his own light waves stretched out behind him.
00:22:31Long light waves are red.
00:22:33We say that his receding image is red shifted.
00:22:40Now, Paolo leaves for a short tour of the countryside.
00:22:45He experiences something even stranger.
00:22:51Everything he can see is squeezed into a moving window just ahead of him.
00:22:55Blue shifted at the center, red shifted at the edges.
00:22:59To a passerby, Paolo appears blue shifted when approaching, red shifted when receding.
00:23:04But to him, the entire world is both coming and going at nearly the speed of light.
00:23:09Roadside houses and trees that he's already gone past still appear to him at the edge of his forward field of view, but distorted and red shifted.
00:23:21When he slows down, everything again looks normal.
00:23:26Only very close to the speed of light does the visible world get squeezed into a kind of tunnel.
00:23:33You would really see these distortions if you could travel near the speed of light.
00:23:37Someday, perhaps, interstellar navigators will take their bearings on stars behind them,
00:23:43whose images have all crowded together on the forward view screen.
00:23:49The most bizarre aspect of traveling near the speed of light is that time slows down.
00:23:55All clocks, mechanical and biological, tick more slowly near the speed of light.
00:24:01But stationary clocks tick at their usual rate.
00:24:05If we travel close to light speed, we age more slowly than those we left behind.
00:24:16Paolo's watch and his internal sense of time show that he's been gone from his friends for only a few minutes.
00:24:22But from their point of view, he has been away for many decades.
00:24:27His friends have grown up, moved on, and died.
00:24:32And his younger brother has been patiently waiting for him all this time.
00:24:38The two brothers experience the paradox of time dilation.
00:24:44They've encountered Einstein's special relativity.
00:24:49Any chance.
00:24:51This was just a thought experiment.
00:25:06But atomic particles traveling near the speed of light do decay more slowly than stationary particles.
00:25:12As strange and counterintuitive as it seems, time dilation is a law of nature.
00:25:19Traveling close to the speed of light is a kind of elixir of life.
00:25:27Because time slows down close to the speed of light,
00:25:31special relativity provides us with a means of going to the stars.
00:25:39This region of northern Italy is not only the cauldron of some of the thinking of the young Albert Einstein.
00:25:46It is also the home of another great genius who lived 400 years earlier.
00:25:52Leonardo da Vinci.
00:25:56Leonardo delighted in climbing these hills and viewing the ground from a great height as if he were soaring like a bird.
00:26:07He drew the first aerial views of landscapes, villages, fortifications.
00:26:13I've been talking about Einstein in and around this town of Vinci in which Leonardo grew up.
00:26:20Einstein greatly respected Leonardo.
00:26:23And their spirits in some sense inhabit this countryside still.
00:26:29Still.
00:26:30Still.
00:26:31Still.
00:26:32Still.
00:26:34Still.
00:26:35Still.
00:26:36Still.
00:26:37Still.
00:26:38Still.
00:26:39Still.
00:26:40diversify Mother.
00:26:41ciamozzle-assism.
00:26:43Amone the Leonardo's many accomplishments in painting sculpture or architecture, natural history or anatomy geology,
00:26:47Among Leonardo's many accomplishments in painting, sculpture, architecture, natural history, anatomy, geology, civil and military engineering, he had a great passion.
00:27:08He wished to construct a machine which would fly. He made sketches of such machines, built miniature models, constructed great full-scale prototypes.
00:27:23And not a one of them ever worked. Mainly because there were no machines of adequate capacity available in his time. The technology was just not ready.
00:27:37The designs, however, were brilliant. For example, this bird-like machine here in the Leonardo Museum in the town of Vinci.
00:27:49Leonardo's great designs encouraged engineers in later epochs, although Leonardo himself was very depressed at these failures.
00:28:00But it's not his fault he was trapped in the 15th century.
00:28:05A somewhat similar case occurred in 1939, when a group of engineers calling themselves the British Interplanetary Society decided to design a ship which would carry people to the moon.
00:28:19Now, it was by no means the same design as the Apollo ship which actually took people to the moon some years later.
00:28:26But that design suggested that a mission to the moon might one day be a practical engineering possibility.
00:28:34Today, we have preliminary designs of ships which will take people to the stars.
00:28:45They are constructed in Earth orbit and from there they venture on their great interstellar journeys.
00:28:55One of them is called Project Orion.
00:29:01It utilizes nuclear weapons, hydrogen bombs, against an inertial plate.
00:29:07Each explosion providing a kind of putt-putt, a vast nuclear motorboat in space.
00:29:15Orion seems entirely practical and was under serious development in the United States until the signing of the international treaty forbidding nuclear weapons explosions in space.
00:29:28Personally, the Orion starship is the best use of nuclear weapons I can think of, provided the ships don't depart from very near the Earth.
00:29:49The אםists call, Thenilus, Dedalus is a recent design of the British Interplanetary Society.
00:29:54It assumes the existence of a nuclear fusion reactor.
00:29:58Something much safer as well as more efficient than the existing nuclear fission power plants.
00:30:07We do not yet have fusion reactors.
00:30:10One day, quite soon, we may.
00:30:13Orion and Daedalus might go 10% of the speed of light.
00:30:25So a trip to Alpha Centauri four and a half light years away would take 45 years, less than a human lifetime.
00:30:33Such ships could not travel close enough to the speed of light for the time-slowing effects of special relativity to become important.
00:30:43It does not seem likely that such ships would be built before the middle of the 21st century, although we could build an Orion starship now.
00:30:53For voyages beyond the nearest stars, something must be added.
00:30:57Perhaps they could be used as multi-generation ships, so those arriving would be the remote descendants of those who had originally set out centuries before.
00:31:07Or perhaps some safe means of human hibernation might be found, so that the space travelers might be frozen and then thawed out when they arrive at the destination centuries later.
00:31:21But fast interstellar spaceflight approaching the speed of light is much more difficult.
00:31:27That's an objective not for a hundred years, but for a thousand, or for ten thousand.
00:31:34But it also is possible.
00:31:36A kind of interstellar ramjet has been proposed, which scoops up the hydrogen atoms which float between the stars, accelerates them into an engine, and spits them out the back.
00:31:51But in deep space, there is one atom for every ten cubic centimeters of space.
00:31:58For the ramjet to work, it has to have a frontal scoop hundreds of kilometers across.
00:32:06When the ship reaches relativistic velocities, the hydrogen atoms will be moving, with respect to the interstellar spaceship, at close to the speed of light.
00:32:16If precautions aren't taken, the passengers will be fried by these induced cosmic rays.
00:32:23There's a proposed solution.
00:32:25A laser is used to strip electrons off the atoms and make them electrically charged while they're still some distance away.
00:32:31And an extremely strong magnetic field is used to deflect the charged atoms into the scoop and away from the rest of the spacecraft.
00:32:40This is engineering on a scale so far unprecedented on the Earth.
00:32:46We are talking of engines the size of small worlds.
00:32:50Suppose that the spacecraft is designed to accelerate at 1G, so we'd be comfortable aboard it.
00:33:07We'd be going closer and closer to the speed of light until the midpoint of the journey.
00:33:13Then the spacecraft is turned around and we decelerate at 1G to the destination.
00:33:18For most of the trip, the velocity would be very close to the speed of light and time would slow down enormously.
00:33:27By how much?
00:33:30Barnard's star could be reached by such a ship in 8 years ship time.
00:33:36The center of the Milky Way galaxy in 21 years.
00:33:41The Andromeda galaxy in 28 years.
00:33:44Of course, the people left behind on the Earth would see things somewhat differently.
00:33:50Instead of 21 years to the center of the galaxy, they would measure it as 30,000 years.
00:33:56When we got back, very few of our friends would be around to greet us.
00:34:00In principle, such a journey, mounting the decimal points closer and closer to the speed of light,
00:34:08would even permit us to circumnavigate the known universe in 56 years ship time.
00:34:15We would return tens of billions of years in the far future with the Earth a charred cinder and the sun dead.
00:34:29Relativistic space flight makes the universe accessible to advanced civilizations,
00:34:35but only to those who go on the journey, not to those who stay home.
00:34:39These designs are probably further from the actual interstellar spacecraft of the future
00:34:48than Leonardo's models are from the supersonic transports of the present.
00:34:56But if we do not destroy ourselves, I believe that we will one day venture to the stars.
00:35:05When our solar system is all explored, the planets of other stars will beckon.
00:35:11This is the newaya sea fagera, which travel the state of discoverable states that may or may impact the sun have significantly more.
00:35:24The new media is pumped to the earth more is beyond the builtAs of view type to the current title.
00:35:30You can't go to the stratified infrastructure, which will pop up to find the air canopy.
00:35:34Space travel and time travel are connected.
00:35:50To travel fast into space is to travel fast into the future.
00:35:58We travel into the future, although slowly, all the time.
00:36:03But what about the past?
00:36:04Could we journey into yesterday?
00:36:07Many physicists think that this is fundamentally impossible,
00:36:10that there is no way we could build a device which would carry us backwards into time.
00:36:16Some say that even if we were to build such a device,
00:36:20it wouldn't do us much good, that we couldn't significantly affect the past.
00:36:23For example, suppose you traveled into the past
00:36:28and somehow or other prevented your own parents from meeting.
00:36:33Why, then you would probably never have been born,
00:36:36which is something of a contradiction, isn't it, since you're clearly there.
00:36:41Other people think that the two alternative histories have equal validity,
00:36:46that they're parallel threads, schemes of time,
00:36:50that they could exist side by side.
00:36:56The history in which you were never born and the history that you know all about.
00:37:02Perhaps time itself has many potential dimensions,
00:37:06despite the fact that we are condemned to experience only one of those dimensions.
00:37:11Now, suppose you could go back into the past and really change it
00:37:17by, oh, let's say something like persuading Queen Isabella not to bankroll Christopher Columbus.
00:37:24Then, you would have set into motion a different sequence of historical events,
00:37:30which those people you left behind you in our time would never get to know about.
00:37:34If that kind of time travel were possible,
00:37:38then every imaginable sequence of alternative history might in some sense really exist.
00:37:47Would it be possible for a time traveler to change the course of history in a major way?
00:37:53Well, let's think about that.
00:37:55History consists, for the most part, of a complex multitude of deeply interwoven threads,
00:38:06biological, economic, and social forces that are not so easily unraveled.
00:38:12The ancient Greeks imagined the course of human events to be a kind of tapestry,
00:38:17created by three goddesses, the fates.
00:38:20Random minor events generally have no long-range consequences,
00:38:26but some, which occur at critical junctures, may alter the weave of history.
00:38:33There may be cases where profound changes can be made by relatively trivial adjustments.
00:38:39The further in the past such an event is, the more powerful its influence.
00:38:44What if our time traveler had persuaded Queen Isabella that Columbus's geography was wrong?
00:38:50Almost certainly, some other European would have sailed west to the New World soon after.
00:38:54There were many inducements, the lure of the spice trade, improvements in navigation,
00:39:00competition among rival European powers.
00:39:02The discovery of America, around 1500, was inevitable.
00:39:06Of course, then, there wouldn't be any postage stamps showing Columbus,
00:39:10and the Republic of Colombia would have some other name.
00:39:13But the big picture would have turned out more or less the same.
00:39:20In order to affect the future profoundly, a time traveler would have to pick and choose.
00:39:28He'd probably have to intervene in a number of events which are very carefully selected
00:39:34so he could change the weave of history.
00:39:39If you had H.G. Wells' time machine, maybe you could understand how history really works.
00:39:56If an apparently pivotal person had never lived,
00:39:59Paul the Apostle or Peter the Great or Pythagoras,
00:40:03how different would the world really be?
00:40:05What if the scientific tradition of the ancient Ionian Greeks had prospered and flourished?
00:40:15It would have required many social factors of the time to have been different,
00:40:19including the common feeling that slavery was right and natural.
00:40:25But what if that light that had dawned on the eastern Mediterranean some 2500 years ago
00:40:32had not flickered out?
00:40:35What if the scientific method and experiment had been vigorously pursued
00:40:402,000 years before the Industrial Revolution, our Industrial Revolution?
00:40:44What if the power of this new mode of thought, the scientific method, had been generally appreciated?
00:40:52I think we might have saved 10 or 20 centuries.
00:40:56Perhaps the contributions that Leonardo made would have been made 1,000 years earlier,
00:41:02and the contributions of Einstein 500 years ago.
00:41:06Not that it would have been those people who would have made those contributions,
00:41:10because they live only in our timeline.
00:41:14If the Ionians had won, we might by now, I think, be going to the stars.
00:41:22We might at this moment have the first survey ships returning with astonishing results
00:41:29from Alpha Centauri and Barnard Star, Sirius and Tau Cetai.
00:41:37There would now be great fleets of interstellar transports being constructed in Earth orbit.
00:41:45Small unmanned survey ships, liners for immigrants, perhaps.
00:41:52Great trading ships to ply the spaces between the stars.
00:41:56On all these ships, there would be symbols and inscriptions on the sides.
00:42:04The inscriptions, if we look closely, would be written in Greek.
00:42:09The symbol, perhaps, would be the dodecahedron.
00:42:15And the inscription on the sides of the ships to the stars,
00:42:19something like Starship Theodorus of the planet Earth.
00:42:26If you were a really ambitious time traveler,
00:42:33you might not dally with human history or even pause to examine the evolution of life on Earth.
00:42:41Instead, you would journey back to witness the origin of our solar system
00:42:45from the gas and dust between the stars.
00:42:49Five billion years ago, an interstellar cloud was collapsing to form our solar system.
00:42:56Most clumps of matter gravitated towards the center and were destined to form the sun.
00:43:02Smaller peripheral clumps would become the planets.
00:43:07Long ago, there was a kind of natural selection among the worlds.
00:43:11Those on highly elliptical orbits tended to collide and be destroyed,
00:43:15but planets in circular orbits tended to survive.
00:43:19But if events had been only a little different,
00:43:22the Earth would never have formed,
00:43:24and some other planet at some other distance from the sun would be around.
00:43:28We owe the existence of our world to random collisions in a long-vanished cloud.
00:43:33Soon, the central mass became very hot.
00:43:41Thermonuclear reactions were initiated,
00:43:43and the sun turned on, flooding the solar system with light.
00:43:46But the growing smaller lumps would never achieve such high temperatures
00:43:54and would never generate thermonuclear reactions.
00:43:57They would become the Earth and the other planets,
00:44:01heated not from within, but mainly by the distant sun.
00:44:05The accretion continued until almost all the gas and dust and small worldlets
00:44:15were swept up by the surviving planets.
00:44:21Our time traveler would witness the collisions that made the worlds.
00:44:26Except for the comets and asteroids,
00:44:35the chaos of the early solar system was reduced to a remarkable simplicity.
00:44:41Nine or so principal planets in almost circular orbits and a few dozen moons.
00:44:51Now, let's take a different look.
00:44:53If we view the solar system edge-on and move the sun off-screen to the left,
00:45:01we see that the small terrestrial planets,
00:45:04the ones about as massive as the Earth,
00:45:06tend to be close to the sun.
00:45:08The big Jupiter-like planets tend to be much further from the sun.
00:45:12But is that the way it has to be?
00:45:16Computer studies suggest that there may be many similar systems about other stars,
00:45:20with the terrestrials in close and the Jovian planets further away.
00:45:29But some systems might have Jovians and terrestrials mixed together.
00:45:33There may be great worlds like Jupiter looming in other skies.
00:45:39Rarely, the Jovian planets may form close to the star,
00:45:43the terrestrials trailing away towards interstellar space.
00:45:46Our familiar arrangement of planets is only one, perhaps typical, case
00:45:53in the vast, expanse of systems.
00:45:58Often, one fledgling planet accumulates so much gas and dust
00:46:03that thermonuclear reactions do occur.
00:46:05It becomes a second sun.
00:46:07A binary star system is formed.
00:46:14For most of these worlds, the vistas will be dazzling.
00:46:19Not a one of them will be identical to the Earth.
00:46:22A few will be hospitable.
00:46:24Many will appear hostile.
00:46:27Where there are two suns in the sky,
00:46:30every object will cast two shadows.
00:46:32What wonders are waiting for us
00:46:40on the planets of the nearby stars?
00:46:43Are there radically different kinds of worlds?
00:46:46Unimaginably exotic forms of life?
00:46:52Perhaps in another century or two,
00:46:55when our solar system is all explored,
00:46:57we will also have put our own planet in order.
00:47:00Then, we will set sail for the stars
00:47:04and the beckoning worlds around them.
00:47:10In that day,
00:47:12our machines and our descendants,
00:47:14approaching the speed of light,
00:47:16will skim the light years,
00:47:18leaping ahead through time,
00:47:20seeking new worlds.
00:47:23Einstein has shown us
00:47:24that it's possible.
00:47:25We will journey simultaneously
00:47:29to distant planets
00:47:31and to the far future.
00:47:34Some worlds, like this one,
00:47:36will look out onto a vast, gaseous nebula,
00:47:39the remains of a star
00:47:41that once was and is no longer.
00:47:44In all those skies,
00:47:49rich and distant
00:47:50and exotic constellations,
00:47:52there may be a faint yellow star,
00:47:56perhaps barely visible to the naked eye,
00:47:59perhaps seen only through the telescope.
00:48:02The home star
00:48:04of a fleet of interstellar transports
00:48:07exploring this tiny region
00:48:09of the great Milky Way galaxy.
00:48:12The themes of space and time
00:48:15are intertwined.
00:48:17Worlds and stars,
00:48:19like people,
00:48:20are born,
00:48:22live,
00:48:23and die.
00:48:24The lifetime of a human being
00:48:26is measured in decades,
00:48:27but the lifetime of the sun
00:48:29is a hundred million times longer.
00:48:32Matter is much older than life.
00:48:38Billions of years
00:48:38before the sun and earth
00:48:39even formed,
00:48:41atoms were being synthesized
00:48:43in the insides of hot stars
00:48:44and then returned to space
00:48:46when the stars blew themselves up.
00:48:49Newly formed planets
00:48:50were made of this stellar debris.
00:48:53The earth and every living thing
00:48:54are made of star stuff.
00:48:56But how slowly
00:49:03in our human perspective
00:49:05life evolved
00:49:05from the molecules
00:49:07of the early oceans
00:49:08to the first bacteria.
00:49:14The reason evolution
00:49:15is not immediately obvious
00:49:16to everybody
00:49:17is because it moves so slowly
00:49:19and takes so long.
00:49:21How can creatures
00:49:22who live for only 70 years
00:49:24detect events
00:49:25that take 70 million years
00:49:27to unfold
00:49:27or 4 billion?
00:49:34By the time
00:49:35one-celled animals
00:49:36had evolved,
00:49:37the history of life
00:49:38on earth
00:49:39was half over.
00:49:45Not very far along
00:49:46to us,
00:49:47you might think,
00:49:48but by now
00:49:49almost all the basic
00:49:50chemistry of life
00:49:51had been established.
00:49:54Forget our human
00:49:55time perspective
00:49:56from the point of view
00:49:57of a star.
00:49:59Evolution was weaving
00:50:00intricate new patterns
00:50:01from the star stuff
00:50:03on the planet earth
00:50:04and very rapidly.
00:50:09Most evolutionary lines
00:50:10became extinct.
00:50:12Many lines
00:50:12became stagnant.
00:50:14If things had gone
00:50:15a little differently,
00:50:16a small change
00:50:17of climate, say,
00:50:18or a new mutation
00:50:20or the accidental death
00:50:21of a different
00:50:21humble organism,
00:50:23the entire future
00:50:24history of life
00:50:25might have been
00:50:26very different.
00:50:30Perhaps the line
00:50:31to an intelligent
00:50:32technological species
00:50:33would have passed
00:50:34through worms.
00:50:38Perhaps the present
00:50:40masters of the planet
00:50:41would have had ancestors
00:50:42who were
00:50:43tunicates.
00:50:44We might not
00:50:48have evolved.
00:50:49Someone else,
00:50:50someone very different,
00:50:52would be here now
00:50:53in our stead,
00:50:54maybe pondering
00:50:55their origins.
00:50:59But that's
00:51:00not what happened.
00:51:02There was a particular
00:51:02sequence of environmental
00:51:04accidents and random
00:51:05mutations in the
00:51:06hereditary material.
00:51:08One particular timeline
00:51:10for life on earth
00:51:12in this universe.
00:51:17As a result,
00:51:19the dominant organisms
00:51:20on the planet today
00:51:21come from fish.
00:51:25Along the way,
00:51:27many more species
00:51:27became extinct
00:51:28and now exist.
00:51:30If history
00:51:30had a slightly
00:51:31different weave,
00:51:33some of those
00:51:34extinct organisms
00:51:34might have survived
00:51:36and prospered.
00:51:38But occasionally,
00:51:40a creature thought
00:51:40to have become extinct
00:51:42hundreds of millions
00:51:43of years ago
00:51:43turns out to be
00:51:45alive and well.
00:51:47The coelacanth,
00:51:48for example.
00:51:51For three and a half
00:51:53billion years,
00:51:54life had lived
00:51:54exclusively in the water.
00:51:57But now,
00:51:57in a great
00:51:58breathtaking adventure,
00:51:59it took to the land.
00:52:01But if things
00:52:02had gone a little
00:52:02differently,
00:52:03the dominant species
00:52:04might still be in the ocean
00:52:05or they might have
00:52:06developed spaceships
00:52:07to carry them off
00:52:08the planet altogether.
00:52:09from our ancestors,
00:52:17the reptiles,
00:52:18there developed
00:52:19many successful lines,
00:52:21including the dinosaurs.
00:52:23Some were fast,
00:52:25dexterous,
00:52:26and intelligent.
00:52:27A visitor from
00:52:28another world or time
00:52:29might have thought
00:52:29them the wave
00:52:30of the future.
00:52:32But after nearly
00:52:33200 million years,
00:52:34they were suddenly
00:52:35all wiped out.
00:52:36perhaps it was
00:52:37a great meteorite
00:52:38colliding with the earth,
00:52:40spewing debris
00:52:41into the air,
00:52:42blotting out the sun
00:52:43and killing the plants
00:52:44that the dinosaurs ate.
00:52:45I wonder
00:52:46when they first sensed
00:52:47that something was wrong.
00:52:52The successors
00:52:53of the dinosaurs
00:52:53came from the same
00:52:54reptilian stock,
00:52:55but they were able
00:52:57to survive the catastrophe
00:52:58that destroyed
00:52:59their cousins.
00:52:59again,
00:53:04there were many branches
00:53:04which became extinct.
00:53:06And again,
00:53:07had events been
00:53:07only a little different,
00:53:08those branches
00:53:09might have led
00:53:10to the dominant form
00:53:11today.
00:53:15For 40 million years,
00:53:16a visitor would not
00:53:17have been much impressed
00:53:18by these timid
00:53:19little creatures,
00:53:20but they led
00:53:21to all the familiar
00:53:22mammals of today.
00:53:23And that includes
00:53:27the primates.
00:53:30About 20 million years ago,
00:53:32a space-time traveler
00:53:33might have recognized
00:53:35these guys
00:53:35as promising,
00:53:37bright,
00:53:37quick,
00:53:38agile,
00:53:38sociable,
00:53:39curious.
00:53:41Their ancestors
00:53:41were once atoms
00:53:42made in stars.
00:53:44Them,
00:53:44simple molecules,
00:53:46single cells,
00:53:47polyps stuck
00:53:48to the ocean floor,
00:53:49fish,
00:53:50amphibians,
00:53:51reptiles,
00:53:51shrews.
00:53:52But then,
00:53:54they came down
00:53:55from the trees
00:53:56and stood upright.
00:53:58They grew
00:53:58an enormous brain.
00:54:00They developed
00:54:01culture,
00:54:02invented tools,
00:54:04domesticated fire.
00:54:09They discovered
00:54:10language and writing.
00:54:12They developed
00:54:13agriculture.
00:54:14They built cities
00:54:15and forged metal.
00:54:19And ultimately,
00:54:20they set out
00:54:22for the stars
00:54:23from which
00:54:24they had come
00:54:25five billion years
00:54:27earlier.
00:54:30We are star stuff
00:54:32which has taken
00:54:33its destiny
00:54:34into its own hands.
00:54:38The loom of time
00:54:40and space
00:54:40works the most
00:54:41astonishing transformations
00:54:43of matter.
00:54:44Our own planet
00:54:46is only a tiny part
00:54:48of the vast
00:54:49cosmic tapestry,
00:54:51a starry fabric
00:54:52of worlds
00:54:54yet untold.
00:55:03Those worlds
00:55:04in space
00:55:05are as countless
00:55:07as all the grains
00:55:08of sand
00:55:08on all the beaches
00:55:09of the Earth.
00:55:10each of those worlds
00:55:12is as real
00:55:13as ours.
00:55:14In every one
00:55:15of them,
00:55:16there's a succession
00:55:16of incidents,
00:55:18events,
00:55:19occurrences
00:55:19which influence
00:55:20its future.
00:55:22Countless worlds,
00:55:24numberless moments,
00:55:26an immensity
00:55:27of space
00:55:28and time.
00:55:29And our small planet
00:55:31at this moment,
00:55:33here we face
00:55:34a critical branch point
00:55:36in history.
00:55:37what we do
00:55:38with our world
00:55:38right now
00:55:40will propagate
00:55:41down through
00:55:42the centuries
00:55:42and powerfully
00:55:43affect the destiny
00:55:44of our descendants.
00:55:46It is well
00:55:47within our power
00:55:48to destroy
00:55:49our civilization
00:55:50and perhaps
00:55:51our species
00:55:52as well.
00:55:53If we capitulate
00:55:54to superstition
00:55:55or greed
00:55:56or stupidity,
00:55:58we can plunge
00:55:59our world
00:56:00into a darkness
00:56:00deeper than the time
00:56:02between the collapse
00:56:03of classical civilization
00:56:04and the Italian
00:56:05Renaissance.
00:56:05but we are also
00:56:08capable of using
00:56:09our compassion
00:56:10and our intelligence,
00:56:12our technology
00:56:13and our wealth
00:56:14to make an abundant
00:56:16and meaningful life
00:56:18for every inhabitant
00:56:19of this planet,
00:56:20to enhance enormously
00:56:22our understanding
00:56:23of the universe
00:56:24and to carry us
00:56:26to the stars.
00:56:28in our motorbike sequence,
00:56:46we showed how the landscape
00:56:47might look
00:56:48if we were
00:56:48barreling through it
00:56:50at close to the speed
00:56:50of light.
00:56:51Since then,
00:56:53inspired by this sequence,
00:56:55Ping Kang Sung
00:56:56at Carnegie Mellon University
00:56:58produced an exact
00:57:00computer animation.
00:57:01This is what you'd see
00:57:02if you were travelling
00:57:03at ordinary speeds
00:57:04through this red
00:57:05and white lattice.
00:57:07But this is how
00:57:08it would appear
00:57:08if you were travelling
00:57:10the same route
00:57:10at close to the speed
00:57:12of light.
00:57:12We're probably
00:57:14many centuries away
00:57:16from travelling
00:57:16close to the speed
00:57:17of light
00:57:17and experiencing
00:57:19time dilation.
00:57:21But even then,
00:57:22it might not be
00:57:23fast enough
00:57:24if we wanted to travel
00:57:25to some distant place
00:57:26in the galaxy, say,
00:57:28and then come back
00:57:28to Earth
00:57:29in our own epoch.
00:57:31Some years after
00:57:32completing Cosmos,
00:57:33I found myself
00:57:35taking time out
00:57:36from my scientific work
00:57:37to write a novel.
00:57:39A novel about travel
00:57:41to the centre.
00:57:42of the Milky Way galaxy.
00:57:44I was willing
00:57:45to imagine beings
00:57:46and civilisations
00:57:47far more advanced
00:57:49than we,
00:57:50but I wasn't willing
00:57:51to ignore
00:57:52the laws of physics.
00:57:54Was there,
00:57:55even in principle,
00:57:56a way to get
00:57:57very quickly
00:57:58to 30,000 light years
00:58:00from Earth?
00:58:02So I put this question
00:58:03to my friend
00:58:04Kip Thorne
00:58:05of the California
00:58:06Institute of Technology.
00:58:07He's a leading expert
00:58:08on the nature
00:58:09of space and time.
00:58:11Kip thought about it
00:58:11for a while
00:58:12and then answered
00:58:14with about 50 lines
00:58:15of equations,
00:58:16which showed
00:58:17that a really advanced
00:58:19civilisation
00:58:19might establish
00:58:20and hold open
00:58:21wormholes,
00:58:24which we might think
00:58:26of as tubes
00:58:27through the fourth dimension,
00:58:29which connect the Earth
00:58:31with another place
00:58:31in the universe
00:58:32without having to
00:58:33traverse
00:58:35the intervening distance,
00:58:36something like crawling
00:58:37through a wormhole
00:58:39in an apple.
00:58:40I was very happy
00:58:41with this result
00:58:42and I used it
00:58:43as a key plot device
00:58:44in Contact.
00:58:46But such wormholes
00:58:47through space
00:58:48would also be
00:58:50time machines,
00:58:50it seemed to me.
00:58:52And I used that notion
00:58:53in my novel Contact
00:58:54as well.
00:58:55Kip Thorne and his colleagues
00:58:56later proved,
00:58:58or so it seemed,
00:58:59that time travel
00:59:00of this sort
00:59:01was possible.
00:59:02Here,
00:59:03look at this.
00:59:05The key question
00:59:07being explored now
00:59:08is whether such time travel
00:59:10can be done
00:59:11consistently
00:59:12with causes
00:59:13preceding effects,
00:59:14say,
00:59:15rather than following them.
00:59:17Does nature contrive it
00:59:18so that even
00:59:19with a time machine
00:59:20you can intervene
00:59:21to prevent
00:59:22your own conception,
00:59:23for example?
00:59:25Even if time travel
00:59:26of this sort
00:59:27is really possible,
00:59:28it's far
00:59:29in our technological future.
00:59:32But maybe,
00:59:33other beings,
00:59:34much more advanced
00:59:35than we,
00:59:36are voyaging
00:59:36to the far future
00:59:37and the remote past,
00:59:39not a measly 40 years ago
00:59:41on Earth,
00:59:42but to witness
00:59:43the death of the sun,
00:59:44say,
00:59:45or the origin
00:59:45of the cosmos.
00:59:46The earth.
00:59:47The earth.
00:59:48The earth.
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