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00:05:06A million years ago, it may have been called something else.
00:05:12Perhaps, the spear.
00:05:19Now, let's run fast forward.
00:05:29Millions of years from now, some other, very different image, will be featured in this cosmic movie.
00:05:36In 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.
00:06:32Could a space traveler actually see the patterns of the constellations change?
00:06:39For that, you must travel roughly as far as the constellation is from us.
00:06:44Here, we're traveling hundreds of light years, circling all the way around the stars of the Big Dipper.
00:06:51The inhabitants of planets around other stars will see very different constellations than we do, because their vantage points are different.
00:07:03Here 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.
00:07:50In the unlikely event that Beta Andromeda blew itself up a week ago Tuesday, we will not know of it.
00:07:58For 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 this star set out on its long interstellar voyage,
00:08:13the young Albert Einstein, working as a Swiss patent clerk, had just published his epical special theory of relativity here on Earth.
00:08:24We see that space and time are intertwined.
00:08:31We cannot look out into space without looking back into time.
00:08:37The 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, and which is also within, that means behind, the constellation Andromeda,
00:09:16is 2 million light years.
00:09:20When the light we see today from M31 left on its journey for Earth, there were no human beings on the Earth,
00:09:31although our ancestors were nicely evolving and very rapidly to our present form.
00:09:36There are much greater distances in astronomy.
00:09:40The distance from the Earth to the most distant quasars is 8 or 10 billion light years.
00:09:47We see them as they were before the Earth itself accumulated, before the Milky Way galaxy was formed.
00:09:55The fastest space vehicles ever launched by the human species are the Voyager spacecraft.
00:10:01They are traveling so fast that it's only 10,000 times slower than the speed of light.
00:10:09The Voyager spacecraft will take 40,000 years to go the distance to the nearest stars,
00:10:14and 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?
00:10:24Can we travel close to the speed of light?
00:10:27And what's magic about the speed of light?
00:10:30Can't we travel faster than that?
00:10:34It turns out that there is something very strange about the speed of light.
00:10:40Something that provides the key to our understanding of time and space.
00:10:46The story of its discovery takes us to Tuscany in Northern Italy.
00:10:53There's something almost timeless about this place.
00:10:58A century ago, it probably looked very much the same.
00:11:02If you had traveled these roads in the summer of 1895, you might have come upon a 16-year-old German high school dropout.
00:11:23His teacher had told him that he would never amount to anything, that his attitude destroyed classroom discipline, that he'd be better off out of school.
00:11:32So he left and came here, where he enjoyed wandering these roads and giving his mind free reign to explore.
00:11:40One day, he began to think about light, about how fast it travels.
00:11:46In our everyday life, we always measure the speed of a moving object relative to something else.
00:11:51I'm moving at about 10 kilometers an hour relative to the ground.
00:11:55But the ground isn't at rest.
00:11:57The Earth is turning at more than 1,600 kilometers an hour.
00:12:01The Earth itself is in orbit around the Sun.
00:12:04The Sun is moving among the drifting stars, and so on.
00:12:08It was hard for the young man to imagine some absolute standard to measure all these relative motions against.
00:12:14He knew that sound waves are a vibration of the air, and their speed is measured relative to the air itself.
00:12:31But sunlight travels across the vacuum of empty space.
00:12:35Do light waves move relative to something else?
00:12:38And if so, he wondered, relative to what?
00:12:41That teenage dropout's name was Albert Einstein, and his ruminations changed the world.
00:13:00He had been fascinated by Bernstein's 1869 People's Book of Natural Science.
00:13:09Here, on its very first page, it describes the astonishing speed of electricity through wires and light through space.
00:13:20Einstein wondered, perhaps for the first time here in northern Italy, what the world would look like if you could travel on a wave of light.
00:13:29To travel at the speed of light.
00:13:31What an engaging and magical thought for a teenage boy on the road where the countryside is dappled and rippling in sunlight.
00:13:41You couldn't tell you were on a light wave if you were traveling with it.
00:13:56If you started on a wave crest, you would stay on the crest and lose all notion of it being a wave.
00:14:04Something funny happens at the speed of light.
00:14:09...
00:14:10...
00:14:11...
00:14:12...
00:14:15...
00:14:16...
00:14:17...
00:14:18The more Einstein thought about such questions, the more troubling they became.
00:14:45Paradoxes seemed to pop up all over.
00:14:47If you could travel at the speed of light, certain ideas had been accepted as true without sufficiently careful thought.
00:14:57One 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 light, 300,000 kilometers a second.
00:15:11You might think that the image of me should be moving out ahead of me at the speed of light plus the speed of the bicycle.
00:15:19If I'm moving towards you faster than a horse and a cart, then my image should be approaching you exactly that much faster.
00:15:27My image were to arrive earlier.
00:15:29But in reality, you don't see any time delay.
00:15:35In a near collision, for example, you always see everything happen at once.
00:15:38Horse, cart, swerve, bicycle, all simultaneous.
00:15:41But how would it look if it were proper to add the velocities?
00:15:47Since I'm heading towards you, you'd add my speed to the speed of light.
00:15:50So my image ought to arrive before the image of the horse and cart.
00:15:56I'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.
00:16:08Now, 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.
00:16:27If the world is to be understood, if we are to avoid logical paradoxes when traveling at high speeds,
00:16:37then there are certain rules which must be obeyed.
00:16:40Einstein called these rules the special theory of relativity.
00:16:44Light 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.
00:16:55Also, 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.
00:17:14For the world to be logically consistent, there must be a cosmic speed limit.
00:17:19The crack of a whip is due to its tip moving faster than the speed of sound.
00:17:28It makes a shockwave, a small sonic boom in the Italian countryside.
00:17:34A thunderclap has a similar origin.
00:17:37So does the sound of a supersonic airplane.
00:17:39So 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.
00:18:01Einstein found his absolute framework for the world, this sturdy pillar, among all the relative motions of the cosmos.
00:18:10Light 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.
00:18:21Einstein'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?
00:18:47Relativity sets limits on what humans ultimately can do.
00:18:56The universe is not required to be in perfect harmony with human ambition.
00:19:07Imagine a place where the speed of light isn't its true value of 300,000 kilometers a second, but something a lot less.
00:19:16It's, let'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.
00:19:33But we can do thought experiments on what happens near the speed of light.
00:19:37Here, 40 kilometers an hour, the speed of a motor scooter.
00:19:41You can't break the laws of nature.
00:19:47There are no penalties for doing so.
00:19:49The 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.
00:19:59Before Einstein, physicists thought that there were privileged frames of reference, some special places and times against which everything else had to be measured.
00:20:13Einstein encountered a similar notion in human affairs, the 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.
00:20:35And 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.
00:20:45A speed is simply how much space you cover in a given time.
00:20:54As any kid on a motor scooter knows, since near the velocity of light, we cannot simply add speeds.
00:21:05The 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.
00:21:19There are conventional perspectives of space and time strangely change.
00:21:25light 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.
00:21:45With such a camera, you could take some pretty interesting pictures.
00:21:54Paolo says goodbye to his little brother Vincenzo.
00:21:58Oh, Vincenzo.
00:21:59And rides off.
00:22:03He's now going more than half the speed of light.
00:22:06He'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.
00:22:13The shorter wavelength is what makes blue light waves blue.
00:22:18Also, Paolo becomes skinny in the direction of motion.
00:22:22This isn't just some optical illusion.
00:22:24It really happens when you travel near the speed of light.
00:22:28As he roars away, he leaves his own light waves stretched out behind him.
00:22:33Long light waves are red.
00:22:34We say that his receding image is red-shifted.
00:22:38Now, Paolo leaves for a short tour of the countryside.
00:22:44He experiences something even stranger.
00:22:50Everything he can see is squeezed into a moving window just ahead of him.
00:22:56Blue-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.
00:23:10Roadside houses and trees that he's already gone past still appear to him at the edge of his forward field of view,
00:23:17but distorted and red-shifted.
00:23:19When 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.
00:23:32You 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.
00:23:46The most bizarre aspect of traveling near the speed of light is that time slows down.
00:23:56All 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.
00:24:10Paolo's watch and his internal sense of time show that he's been gone from his friends for only a few minutes.
00:24:23But from their point of view, he has been away for many decades.
00:24:28His friends have grown up, moved on, and died.
00:24:32And his younger brother has been patiently waiting for him all this time.
00:24:37The two brothers experience the paradox of time dilation.
00:24:45They've encountered Einstein's special relativity.
00:24:49No chance.
00:25:04This was just a thought experiment.
00:25:07But atomic particles traveling near the speed of light do decay more slowly than stationary particles.
00:25:14As strange and counterintuitive as it seems, time dilation is a law of nature.
00:25:23Traveling close to the speed of light is a kind of elixir of life.
00:25:28Because time slows down close to the speed of light, special relativity provides us with a means of going to the stars.
00:25:38This 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, Leonardo da Vinci.
00:25:55Leonardo 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.
00:26:12I've been talking about Einstein in and around this town of Vinci in which Leonardo grew up.
00:26:20Einstein greatly respected Leonardo and their spirits in some sense inhabit this countryside still.
00:26:30He drew the first place to see the world in the world in the world in the world.
00:26:54Among Leonardo's many accomplishments in painting, sculpture, architecture, natural history, anatomy, geology, civil and military engineering, he had a great passion.
00:27:09He wished to construct a machine which would fly.
00:27:14He made sketches of such machines, built miniature models, constructed great full-scale prototypes.
00:27:22And not a one of them ever worked, mainly because there were no machines of adequate capacity available in his time.
00:27:34The technology was just not ready.
00:27:38The designs, however, were brilliant.
00:27:42For example, this bird-like machine here in the Leonardo Museum in the town of Vinci.
00:27:48Leonardo'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.
00:28:27But that design suggested that a mission to the moon might one day be a practical engineering possibility.
00:28:34Today, today, we have preliminary designs of ships which will take people to the stars.
00:28:44They are constructed in Earth orbit, and from there, they venture on their great interstellar journeys.
00:28:54One of them is called Project Orion.
00:28:59It utilizes nuclear weapons, hydrogen bombs, against an inertial plate.
00:29:08Each explosion providing a kind of putt-putt, a vast nuclear motorboat in space.
00:29:16Orion seems entirely practical, and was under serious development in the United States
00:29:22until the signing of the International Treaty, forbidding nuclear weapons explosions in space.
00:29:30Personally, the Orion starship is the best use of nuclear weapons I can think of,
00:29:35provided the ships don't depart from very near the Earth.
00:29:38Project Daedalus 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.
00:30:03We do not yet have fusion reactors.
00:30:10One day, quite soon, we may.
00:30:19Orion and Daedalus might go 10% the speed of light.
00:30:24So, a trip to Alpha Centauri, four and a half light years away, would take 45 years, less than a human lifetime.
00:30:34Such ships could not travel close enough to the speed of light
00:30:37for 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,
00:30:48although 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,
00:31:01so 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,
00:31:12so that the space travelers might be frozen and then bought out
00:31:16when they arrive at the destination centuries later.
00:31:21But fast interstellar space flying, approaching the speed of light, is much more difficult.
00:31:28That'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,
00:31:42which scoops up the hydrogen atoms which float between the stars,
00:31:47accelerates 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,
00:32:11with 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.
00:32:22There's a proposed solution.
00:32:23A laser is used to strip electrons off the atoms and make them electrically charged while they're still some distance away.
00:32:32And 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.
00:32:45We 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.
00:33:12Then the spacecraft is turned around and we'd decelerate at 1G to the destination.
00:33:19For 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?
00:33:28Barnard'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.
00:33:45Of 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.
00:33:55When we got back, very few of our friends would be around to greet us.
00:34:01In 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 spaceflight 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.
00:35:10Or do not use our lenses, which are capsizing doubtful, unless the atmosphere is far into the dark of the stars and planes of the stars.
00:35:26Or do not support the stars.
00:35:31Or soon as we find ancient powers that stay home as our heads of the stars will,
00:35:37space travel and time travel are connected to travel fast into space is to travel fast into
00:35:54the future we travel into the future although slowly all the time but what about the past
00:36:04could we journey into yesterday many 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 some say
00:36:16that even if we were to build such a device it wouldn't do us much good that we couldn't
00:36:21significantly affect the past for example suppose you traveled into the past and somehow or other
00:36:29prevented your own parents from meeting why then you would probably never have been born
00:36:36which is something of a contradiction isn't it since you're clearly there other people think
00:36:42that the two alternative histories have equal validity that they're parallel threads schemes
00:36:49of time that they could exist side by side the history in which you were never born and the
00:37:00history that you know all about perhaps time itself has many potential dimensions despite the fact that
00:37:07we are condemned to experience only one of those dimensions now suppose you could go back into the
00:37:15past and really change it by oh let's say something like persuading queen isabella not to bankroll
00:37:22christopher columbus then you would have set into motion a different sequence of historical events
00:37:29which those people you left behind you in our time would never get to know about if that kind of time
00:37:37travel were possible then every imaginable sequence of alternative history might in some sense really
00:37:45exist would it be possible for a time traveler to change the course of history in a major way
00:37:52well let's think about that history consists for the most part of a complex multitude of deeply
00:38:04interwoven threads biological economic and social forces that are not so easily unraveled
00:38:10the ancient greeks imagined the course of human events to be a kind of tapestry created by three
00:38:18goddesses the fates
00:38:20random minor events generally have no long-range consequences but some which occur at critical
00:38:29junctures may alter the weave of history there may be cases where profound changes can be made by
00:38:36relatively trivial adjustments the further in the past such an event is the more powerful its influence
00:38:43what if our time traveler had persuaded queen isabella that columbus's geography was wrong
00:38:49almost 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 navigation competition among
00:39:00rival european powers the discovery of america around 1500 was inevitable of course then there wouldn't
00:39:07be any postage stamps showing columbus and the republic of columbia would have some other name but the big
00:39:14picture would have turned out more or less the same
00:39:16in order to affect the future profoundly a time traveler would have to pick and choose he'd probably have
00:39:28to intervene in a number of events which are very carefully selected so he could change
00:39:36the weave of history it's a lovely fantasy to explore those other worlds that never were
00:39:46if you had hg wells's time machine maybe you could understand how history really works
00:39:55if an apparently pivotal person had never lived paul the apostle or peter the great or pythagoras how
00:40:03different would the world really be what if the scientific tradition of the ancient ionian greeks had
00:40:11prospered and flourished it 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
00:40:24but what if that light that had dawned on the eastern mediterranean
00:40:30some 2500 years ago had not flickered out
00:40:34what if the scientific method and experiment had been vigorously pursued
00:40:402 000 years before the industrial revolution
00:40:43our industrial revolution what if the power of this new mode of thought the scientific method had been
00:40:50generally appreciated i think we might have saved 10 or 20 centuries
00:40:55perhaps the contributions that leonardo made would have been made
00:40:59a thousand years earlier and the contributions of einstein 500 years ago
00:41:05not that it would have been those people who would have made those contributions
00:41:09because they live only in our timeline
00:41:13if the ionians had won we might by now i think be going to the stars
00:41:21we might at this moment have the first survey ships returning with astonishing results
00:41:29from alpha centauri and barnard star sirius and ta sitai
00:41:36there would now be great fleets of interstellar transports being constructed in earth orbit
00:41:44small unmanned survey ships
00:41:47liners for immigrants perhaps
00:41:51great trading ships to ply the spaces between the stars
00:41:56on all these ships there would be symbols and inscriptions on the sides
00:42:03the inscriptions if we look closely would be written in greek
00:42:08the symbol perhaps would be the dodecahedron
00:42:14and the inscription on the sides of the ships to the stars
00:42:18something like starship theodorus of the planet earth
00:42:25if you were a really ambitious time traveler
00:42:32you might not dally with human history or even pause to examine the evolution of life on earth
00:42:40instead 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
00:42:55most clumps of matter gravitated towards the center and were destined to form the sun
00:43:01smaller peripheral clumps would become the planets
00:43:05long ago there was a kind of natural selection among the worlds those are highly elliptical orbits tended to collide and be destroyed but planets in circular orbits tended to survive
00:43:17but if events had been only a little different the earth would never have formed and some other planet that some other distance from the sun would be around
00:43:24we owe the existence of our world to random collisions in a long vanished cloud
00:43:36soon the central mass became very hot thermonuclear reactions were initiated and the sun turned on flooding the solar system with light
00:43:46but the growing smaller lumps would never achieve such high temperatures and would never generate thermonuclear reactions they would become the earth and the other planets heated not from within but mainly by the distant sun
00:44:04the accretion continued until almost all the gas and dust and small worldlets were swept up by the surviving planets
00:44:18our time traveler would witness the collisions that made the worlds
00:44:26except for the comets and asteroids the chaos of the early solar system was reduced to a remarkable simplicity
00:44:40nine or so principal planets in almost circular orbits and a few dozen moons
00:44:46now 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
00:45:00we see that the small terrestrial planets the ones about as massive as the earth tend to be close to the sun
00:45:06the big jupiter-like planets tend to be much further from the sun
00:45:10but is that the way it has to be computer studies suggest that there may be many similar systems about other stars with the terrestrials in close and the jovian planets further away
00:45:28but some systems might have jovians and terrestrials mixed together there may be great worlds like jupiter looming in other skies
00:45:36rarely the jovian planets may form close to the star
00:45:42the terrestrials trailing away towards interstellar space
00:45:46our familiar arrangement of planets is only one perhaps typical case in the vast expanse of systems
00:45:56often one fledgling planet accumulates so much gas and dust that thermonuclear reactions do occur
00:46:04it becomes a second sun a binary star system has formed
00:46:14for most of these worlds the vistas will be dazzling
00:46:17not a one of them will be identical to the earth
00:46:20a few will be hospitable many will appear hostile
00:46:24where there are two suns in the sky
00:46:28every object will cast two shadows
00:46:36what wonders are waiting for us on the planets of the nearby stars
00:46:41are there radically different kinds of worlds
00:46:44unimaginably exotic forms of life
00:46:46perhaps in another century or two when our solar system is all explored
00:46:56we will also have put our own planet in order
00:47:00then we will set sail for the stars and the beckoning worlds around them
00:47:06in that day our machines and our descendants approaching the speed of light will skim the light years
00:47:18leaping ahead through time seeking new worlds
00:47:22Einstein has shown us that it's possible
00:47:26we will journey simultaneously
00:47:28to distant planets
00:47:30and to the far future
00:47:32some worlds like this one
00:47:34will look out onto a
00:47:36vast gaseous nebula
00:47:38the remains of a star
00:47:40that once was
00:47:42and is no longer
00:47:44in all those skies rich and distant and exotic constellations
00:47:52there may be a faint yellow star
00:47:56perhaps barely visible to the naked eye
00:47:58perhaps seen only through the telescope
00:48:02the home star
00:48:04of a fleet of interstellar transports
00:48:06exploring this tiny region
00:48:08of the great Milky Way galaxy
00:48:12the themes of space and time are intertwined
00:48:16worlds and stars
00:48:18like people
00:48:20are born
00:48:21live
00:48:22and die
00:48:24the lifetime of human being is measured in decades
00:48:26but the lifetime of the sun
00:48:28is a hundred million times longer
00:48:34matter is much older than life
00:48:36billions of years before the sun and earth even formed
00:48:40atoms were being synthesized
00:48:42in the insides of hot stars
00:48:44and then returned to space
00:48:46when the stars blew themselves up
00:48:48newly formed planets were made
00:48:50of this stellar debris
00:48:52the earth and every living thing
00:48:54are made of star stuff
00:49:01but how slowly in our human perspective
00:49:04life evolved from the molecules of the early oceans
00:49:07to the first bacteria
00:49:13the reason evolution is not immediately obvious to everybody
00:49:15is because it moves so slowly
00:49:17and takes so long
00:49:19how can creatures who live for only 70 years
00:49:22detect events that take 70 million years to unfold
00:49:25or 4 billion
00:49:33by the time one-celled animals had evolved
00:49:35the history of life on earth
00:49:37was half over
00:49:39not very far along to us you might think
00:49:47but by now almost all the basic chemistry of life had been established
00:49:51forget our human time perspective
00:49:55from the point of view of a star
00:49:57evolution was weaving intricate new patterns
00:50:01from the star stuff on the planet earth
00:50:03and very rapidly
00:50:09most evolutionary lines became extinct
00:50:11many lines became stagnant
00:50:13if things had gone a little differently
00:50:15a small change of climate say
00:50:17or a new mutation
00:50:19or the accidental death of a different
00:50:21humble organism
00:50:23the entire future history of life
00:50:25might have been very different
00:50:29perhaps the line to an intelligent technological species
00:50:32would have passed through worms
00:50:38perhaps the present masters of the planet
00:50:40would have had ancestors who were
00:50:42tunicates
00:50:43we might not have evolved
00:50:48someone else
00:50:49someone
00:50:50very different
00:50:51would be here now in our stead
00:50:53maybe pondering
00:50:54their origins
00:50:58but that's
00:50:59not what happened
00:51:00there was a particular sequence
00:51:02of environmental accidents
00:51:04and random mutations
00:51:05in the hereditary material
00:51:07one
00:51:08particular timeline
00:51:09for life on earth
00:51:11in this universe
00:51:12as a result
00:51:17the dominant organisms
00:51:19on the planet today
00:51:20come from
00:51:21fish
00:51:23along the way
00:51:25many more species
00:51:26became extinct
00:51:27than now exist
00:51:28if history
00:51:29had a slightly different
00:51:30weave
00:51:31some of those extinct organisms
00:51:33might have survived
00:51:34and prospered
00:51:35but occasionally
00:51:37a creature thought to have
00:51:39become extinct
00:51:40hundreds of millions of years
00:51:41hundreds of millions of years ago
00:51:43turns out to be
00:51:44alive
00:51:45and well
00:51:46the coelacanth
00:51:47for example
00:51:51for three and a half billion years
00:51:53life had lived
00:51:54exclusively
00:51:55in the water
00:51:56but now in a great
00:51:57breathtaking adventure
00:51:58it took to the land
00:52:00but if things had gone
00:52:01a little differently
00:52:02the dominant species
00:52:03might still be
00:52:04in the ocean
00:52:05or they might have
00:52:06developed spaceships
00:52:07to carry them off the planet
00:52:08all together
00:52:15from our ancestors
00:52:16the reptiles
00:52:17there developed many successful lines
00:52:19including
00:52:20the dinosaurs
00:52:22some were fast
00:52:24dexterous
00:52:25and intelligent
00:52:26a visitor from another world
00:52:27or time
00:52:28might have thought them
00:52:29the wave of the future
00:52:31but after nearly
00:52:32200 million years
00:52:33they were suddenly all
00:52:34wiped out
00:52:35perhaps it was a great meteorite
00:52:37colliding with the earth
00:52:38spewing debris into the air
00:52:40blotting out the sun
00:52:42and killing the plants
00:52:43that the dinosaurs ate
00:52:44I wonder
00:52:45when they first sensed
00:52:46that something was wrong
00:52:48the successors of the dinosaurs
00:52:52came from the same reptilian stock
00:52:54but
00:52:55they were able to survive
00:52:56the catastrophe
00:52:57that destroyed
00:52:58their cousins
00:53:02again
00:53:03there were many branches
00:53:04which became extinct
00:53:05and again
00:53:06had events been only a little different
00:53:07those branches
00:53:08might have led
00:53:09to the dominant form today
00:53:14for 40 million years
00:53:15a visitor would not have been
00:53:16much impressed
00:53:17by these
00:53:18timid little creatures
00:53:19but they led
00:53:20to all the familiar
00:53:21mammals of today
00:53:22and that includes
00:53:27the primates
00:53:29about 20 million years ago
00:53:31a space-time traveler
00:53:33might have recognized
00:53:34these guys as promising
00:53:36bright, quick, agile, sociable, curious
00:53:39their ancestors were once
00:53:41atoms made in stars
00:53:43then simple molecules
00:53:45single cells
00:53:46polyps stuck to the ocean floor
00:53:48fish, amphibians, reptiles, shrews
00:53:51but then
00:53:53they came down
00:53:54from the trees
00:53:55and stood upright
00:53:56they grew
00:53:57an enormous brain
00:53:59they developed
00:54:00culture
00:54:01invented tools
00:54:02domesticated fire
00:54:08they discovered language
00:54:09and writing
00:54:10they developed agriculture
00:54:12they built cities
00:54:14and forged metal
00:54:16and ultimately
00:54:19they set out for the stars
00:54:23from which they had come
00:54:24five billion years earlier
00:54:26we are star stuff
00:54:31which has taken its destiny
00:54:33into its own hands
00:54:34the loom of time and space
00:54:39works the most astonishing transformations of matter
00:54:42our own planet
00:54:45our own planet
00:54:46is only a tiny part
00:54:48of the vast cosmic tapestry
00:54:50a starry fabric
00:54:52of worlds yet untold
00:55:02those worlds in space
00:55:04are as countless
00:55:06as all the grains of sand
00:55:07and all the beaches of the earth
00:55:09each of those worlds
00:55:11each of those worlds
00:55:12is as real as ours
00:55:13in every one of them
00:55:14there is a succession of
00:55:16incidents
00:55:17events
00:55:18occurrences
00:55:19which influence its future
00:55:20countless worlds
00:55:22numberless moments
00:55:24an immensity of space and time
00:55:27and our small planet
00:55:30at this moment
00:55:31here we face
00:55:32a critical branch point
00:55:34in history
00:55:35what we do with our world
00:55:38right now
00:55:39will propagate down
00:55:40through the centuries
00:55:41and powerfully affect
00:55:42the destiny
00:55:43of our descendants
00:55:44it is well within our power
00:55:47to destroy our civilization
00:55:49and perhaps
00:55:50our species as well
00:55:52if we capitulate
00:55:53to superstition
00:55:54or greed
00:55:55or stupidity
00:55:56we can plunge our world
00:55:58into a darkness
00:55:59deeper than the time
00:56:00between
00:56:01the collapse of classical civilization
00:56:03and the Italian Renaissance
00:56:05but
00:56:06we are also capable
00:56:08of using our compassion
00:56:10and our intelligence
00:56:11our technology
00:56:12and our wealth
00:56:13to make
00:56:14an abundant
00:56:15and meaningful life
00:56:17for every inhabitant
00:56:18of this planet
00:56:19to enhance enormously
00:56:21our understanding
00:56:22of the universe
00:56:23and to carry us
00:56:25to the stars
00:56:27in our motorbike sequence
00:56:45we showed how the landscape
00:56:46might look
00:56:47if we were
00:56:48barreling through it
00:56:49at close to the speed of light
00:56:50since then
00:56:52inspired by this sequence
00:56:53Ping Kang Siung
00:56:55at Carnegie Mellon University
00:56:57produced an exact
00:56:58computer animation
00:57:00this is what you'd see
00:57:01if you were travelling
00:57:02at ordinary speeds
00:57:03through this red and white lattice
00:57:05but this is how it would appear
00:57:07if you were travelling
00:57:08the same route
00:57:09at close to the speed of light
00:57:11we're probably many centuries away
00:57:14from travelling close to the speed of light
00:57:16and experiencing time dilation
00:57:19but even then
00:57:21it might not be fast enough
00:57:23if we wanted to travel
00:57:24to some distant place
00:57:25in the galaxy
00:57:26say
00:57:27and then come back to Earth
00:57:28in our own epoch
00:57:29some years after completing Cosmos
00:57:32I found myself taking time out
00:57:35from my scientific work
00:57:36to write a novel
00:57:37a novel about travel
00:57:40to the centre
00:57:41of the Milky Way galaxy
00:57:43I was willing to imagine beings
00:57:45and civilizations
00:57:46far more advanced
00:57:48than we
00:57:49but I wasn't willing
00:57:50to ignore the laws of physics
00:57:52was there even in principle
00:57:55a way to get very quickly
00:57:57to 30,000 light years from Earth
00:58:00so I put this question
00:58:02to my friend Kip Thorne
00:58:03of the California Institute of Technology
00:58:06he's a leading expert
00:58:07on the nature of space and time
00:58:09Kip thought about it for a while
00:58:11and then answered
00:58:13with about 50 lines of equations
00:58:15which showed
00:58:16which showed
00:58:17that a really advanced civilization
00:58:19might establish
00:58:20and hold open
00:58:21wormholes
00:58:23which
00:58:24we might think of as tubes
00:58:27through the fourth dimension
00:58:29which connect the Earth
00:58:30with another place in the universe
00:58:32without having to
00:58:33traverse
00:58:34the intervening distance
00:58:35something like crawling
00:58:37through a wormhole
00:58:38in an apple
00:58:39I was very happy with this result
00:58:41and I used it as a key plot device
00:58:43in Contact
00:58:45but such wormholes through space
00:58:48would also be time machines
00:58:49it seemed to me
00:58:51and I used that notion
00:58:52in my novel Contact as well
00:58:54Kip Thorne and his colleagues
00:58:56later proved
00:58:57or so it seemed
00:58:58that time travel of this sort
00:59:00was possible
00:59:01here
00:59:02look at this
00:59:03the key question being explored now
00:59:07is whether such time travel
00:59:09can be done consistently
00:59:11with causes preceding effects
00:59:13say
00:59:14rather than following them
00:59:15does nature contrive it
00:59:17so that even with a time machine
00:59:19you can intervene
00:59:20to prevent your own conception
00:59:22for example
00:59:23even if time travel of this sort
00:59:25is really possible
00:59:26it's far in our technological future
00:59:29but maybe
00:59:32other beings
00:59:33much more advanced than we
00:59:35are voyaging
00:59:36to the far future
00:59:37in the remote past
00:59:38not a measly 40 years ago
00:59:40on earth
00:59:41but to witness
00:59:42the death of the sun say
00:59:44or the origin of the cosmos
00:59:59here
01:00:00below
01:00:01the moon
01:00:03or the moon
01:00:04and there
01:00:05are we going to be
01:00:06that says
01:00:07all the sun say
01:00:08this is a dream
01:00:09that is not going to be
01:00:10for us
01:00:11this is it
01:00:12or these are the two
01:00:13people who have
01:00:15to look at
01:00:15the moon
01:00:16that was going on
01:00:16to the moon
01:00:18and there
01:00:19this is a dream
01:00:19there
01:00:20that is not going to be
01:00:21for us
01:00:22that is how we're going to
01:00:23do it
01:00:24the future
01:00:25is actually
01:00:26to have a dream
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