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Cosmos: A Personal Voyage - Episode 8 (Carl Sagan)

  • 0:53 - 0:59
    SAGAN: We are drifting in
    a great ocean of space and time.
  • 0:59 - 1:02
    In that ocean, the events
    that shape the future...
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    ...are working themselves out.
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    Each creature and every world,
    to the remotest star...
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    ...owe their existence to...
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    ...the great, coursing,
    implacable forces of nature...
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    ...but also, to minor happenstance.
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    We are carried with our planet
    around the sun.
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    The Earth has made more than
    4 billion circuits of our star...
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    ...since its origin.
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    The sun itself travels about
    the core of the Milky Way galaxy.
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    Our galaxy is moving
    among the other galaxies.
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    We have always been space travelers.
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    These fine sand grains are all,
    more or less, uniform in size.
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    They're produced from bigger rocks
    through ages of...
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    ...jostling and rubbing,
    abrasion and erosion.
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    Driven in part by
    the distant moon and sun.
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    So the roots of the present
    lie buried in the past.
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    We are also travelers in time.
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    But trapped on Earth...
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    ...we've had little to say about
    where we go in time and space...
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    ...or how fast.
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    But now we're thinking
    about true journeys in time...
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    ...and real voyages
    to the distant stars.
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    A handful of sand contains
    about 10,000 grains...
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    ...more than all the stars
    we can see...
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    ...with the naked eye
    on a clear night.
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    But the number of stars we can see...
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    ...is only the tiniest fraction
    of the number of stars that are.
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    What we see at night
    is the merest smattering...
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    ...of the nearest stars...
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    ...with a few more distant bright
    stars thrown in for good measure.
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    Meanwhile, the cosmos
    is rich beyond measure.
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    The number of stars
    in the universe...
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    ...is larger than all the grains
    of sand on all the beaches...
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    ...of the planet Earth.
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    Long ago, before we had figured out
    that the stars are distant suns...
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    ...they seemed to us
    to make pictures in the sky.
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    Just follow the dots.
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    The Big Dipper constellation
    today in North America...
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    ...has had many other incarnations.
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    Every culture, ancient and modern...
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    ...has placed its totems
    and concerns among the stars.
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    From a Chinese bureaucrat
    to a German wagon.
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    But very ancient cultures would have
    seen different constellations...
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    ...because the stars move
    with respect to one another.
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    We can give a computer the present
    positions and motions of stars...
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    ...and then run the patterns
    back into time.
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    Every constellation is a single frame
    in a cosmic movie...
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    ...but because our lives
    are so short...
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    ...because star patterns
    change slowly...
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    ...we tend not to notice
    it's a movie.
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    A million years ago,
    there was no Big Dipper.
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    Our ancestors, looking up
    and wondering about the stars...
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    ...saw some other pattern
    in the northern skies.
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    We can also run a constellation,
    Leo the Lion, say, forward in time...
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    ...and see what the patterns
    in the stars will be in the future.
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    A million years from now,
    Leo might be renamed...
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    ...the constellation
    of the Radio Telescope.
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    Although I suspect radio telescopes
    then will be as obsolete...
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    ...as stone spears are now.
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    Or, here's the constellation
    of Cetus the Whale.
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    A million years ago, it may have
    been called something else.
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    Perhaps the Spear.
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    Now, let's run fast-forward
    through a billion nights.
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    Millions of years from now...
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    ...some other very different image
    will be featured in this cosmic movie.
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    In Orion the Hunter,
    things are changing...
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    ...not only because
    the stars are moving...
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    ...but also because
    the stars are evolving.
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    Many of Orion's stars are
    hot, young and short-lived.
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    They're born, live and die within
    a span of only a few million years.
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    If we run Orion forward in time...
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    ...we see the births
    and explosive deaths...
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    ...of dozens of stars...
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    ...flashing on and winking off
    like fireflies in the night.
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    If we wait long enough,
    we see the constellations change.
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    But if we go far enough,
    we also see the star patterns alter.
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    Two-dimensional constellations...
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    ...are only the appearance of stars
    strewn through three dimensions.
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    Some are dim and near,
    others are bright but farther away.
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    Could a space traveler
    actually see...
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    ...the patterns of
    the constellations change?
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    For that, you must travel roughly as
    far as the constellation is from us.
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    Here, we're traveling
    hundreds of light-years...
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    ...circling all the way around
    the stars of the Big Dipper.
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    Inhabitants of planets
    around other stars...
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    ...will see different constellations
    than us...
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    ...because their vantage points
    are different.
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    Here we are
    in the constellation Andromeda...
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    ...or at least a model of it
    next to the constellation Perseus.
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    Andromeda, in the Greek myth...
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    ...was the maiden
    who was saved by Perseus...
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    ...from a sea monster.
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    This star just above me
    is Beta Andromedae...
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    ...the second brightest star
    in the constellation...
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    ...75 light-years from the Earth.
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    The light by which we see this star...
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    ...has spent 75 years
    traversing interstellar space...
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    ...on its journey to the Earth.
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    In the unlikely event
    that Beta Andromedae...
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    ...blew itself up
    a week ago Tuesday...
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    ...we will not know of it
    for another 75 years...
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    ...as this interesting information,
    traveling at the speed of light...
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    ...crosses the enormous
    interstellar distances.
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    When the light we see
    from this star set out...
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    ...on its long
    interstellar voyage...
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    ...the young Albert Einstein...
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    ...working as a Swiss patent clerk...
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    ...had just published his epochal
    special theory of relativity...
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    ...here on Earth.
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    We see...
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    ...that space and time
    are intertwined.
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    We cannot look out into space...
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    ...without looking back into time.
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    The speed of light is very fast...
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    ...but space is very empty...
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    ...and the stars are very far apart.
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    The distances that we've been
    talking about up to now...
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    ...are very small
    by the usual astronomical standards.
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    In fact, the distance
    from the Earth...
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    ...to the center
    of the Milky Way galaxy...
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    ...is 30,000 light-years.
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    From our galaxy to the nearest
    spiral galaxy like our own...
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    ...called M31 ...
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    ...and which is also within,
    that means behind...
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    ...the constellation Andromeda...
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    ...is 2 million light-years.
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    When the light we see today
    from M31 ...
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    ...left on its journey for Earth...
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    ...there were no human beings...
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    ...although our ancestors
    were nicely evolving...
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    ...and very rapidly,
    to our present form.
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    There are much greater distances
    in astronomy.
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    The distance from the Earth
    to the most distant quasars...
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    ...is 8 or 10 billion light-years.
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    We see them as they were before
    the Earth itself accumulated...
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    ...before the Milky Way galaxy
    was formed.
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    The fastest space vehicles ever
    launched by the human species...
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    ...are the Voyager spacecraft.
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    They are traveling so fast...
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    ...that it's only
    10,000 times slower...
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    ...than the speed of light.
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    The Voyager spacecraft
    will take 40,000 years...
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    ...to go the distance
    to the nearest stars...
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    ...and they're not even headed
    towards the nearest stars.
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    But is there a method
    by which we could travel...
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    ...in a conveniently short time
    to the stars?
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    Can we travel close
    to the speed of light?
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    And what's magic
    about the speed of light?
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    Can't we travel faster than that?
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    It turns out that
    there is something very strange...
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    ...about the speed of light.
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    Something that provides the key...
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    ...to our understanding
    of time and space.
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    The story of its discovery...
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    ...takes us to Tuscany
    in northern Italy.
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    There's something timeless
    about this place.
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    A century ago, it probably
    looked very much the same.
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    If you had traveled these roads
    in the summer of 1895...
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    ...you might have come upon a
    16-year-old German high-school dropout.
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    His teacher told him that
    he'd never amount to anything...
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    ...that his attitude destroyed
    classroom discipline...
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    ...that he should drop out.
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    So he left and came here...
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    ...where he enjoyed
    wandering these roads...
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    ...and giving his mind
    free rein to explore.
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    One day, he began
    to think about light...
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    ...about how fast it travels.
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    We always measure
    the speed of a moving object...
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    ...relative to something else.
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    I'm moving at about 10 kilometers
    an hour relative to the ground.
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    But the ground isn't at rest.
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    The Earth is turning at more
    than 1600 kilometers an hour.
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    The Earth itself is
    in orbit around the sun.
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    The sun is moving among
    the drifting stars, and so on.
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    It was hard for the young man
    to imagine some absolute standard...
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    ...to measure all these
    relative motions against.
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    He knew that sound waves are
    a vibration of the air...
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    ...and their speed is measured
    relative to the air itself.
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    But sunlight travels across
    the vacuum of empty space.
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    "Do light waves move
    relative to something else?
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    And if so," he wondered,
    "relative to what?"
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    That teenage dropout's name...
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    ...was Albert Einstein.
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    And his ruminations changed the world.
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    He had been fascinated
    by Bernstein's 1869...
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    ...People's Book of Natural Science.
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    Here, on its very first page...
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    ...it describes the astonishing speed
    of electricity through wires...
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    ...and light through space.
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    Einstein wondered, perhaps for
    the first time, in northern Italy...
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    ...what the world would look like if
    you could travel on a wave of light.
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    To travel at the speed of light.
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    What an engaging and magical thought
    for a teenage boy on the road...
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    ...where the countryside is dappled
    and rippling in sunlight.
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    You couldn't tell you were on a light
    wave if you were traveling with it.
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    If you started on a wave crest...
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    ...you would stay on the crest and
    lose all notion of it being a wave.
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    Something funny happens
    at the speed of light.
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    The more Einstein thought about it,
    the more troubling it became.
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    Paradoxes seemed to pop up all over...
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    ...if you could travel
    at the speed of light.
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    Certain ideas had been
    accepted as true...
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    ...without sufficiently
    careful thought.
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    One of those ideas had to do
    with the light from a moving object.
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    The images by which we see the world
    are made of light...
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    ...and are carried
    at the speed of light...
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    ...300,000 kilometers a second.
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    You might think that the image of me
    should be moving out ahead of me...
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    ...at the speed of light
    plus the speed of the bicycle.
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    If I'm moving towards you
    faster than a horse-and-cart...
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    ...then my image should be
    approaching you that much faster.
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    My image ought to arrive earlier.
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    But in reality
    you don't see any time delay.
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    In a near collision, for example,
    you see everything happen at once.
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    Horse, cart, swerve, bicycle.
    All simultaneous.
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    But how would it look if
    it were proper to add the velocities?
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    Since I'm heading toward you, you'd
    add my speed to the speed of light.
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    So my image ought to arrive before
    the image of the horse-and-cart.
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    I'd be cycling towards you
    quite normally.
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    To me, a collision
    would seem imminent.
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    But you'd see me swerve
    for no apparent reason...
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    ...and have a collision with nothing.
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    Now, the horse-and-cart
    aren't headed towards you.
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    Their image would arrive only
    at the speed of light.
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    Could it seem to me that
    I just missed colliding...
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    ...while to you it wasn't even close?
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    In precise laboratory experiments...
  • 16:18 - 16:22
    ...scientists have never observed
    any such thing.
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    If the world is to be understood...
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    ...if we are to avoid logical paradoxes
    when traveling at high speeds...
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    ...then there are rules
    which must be obeyed.
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    Einstein called these rules
    the special theory of relativity.
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    Light from a moving object
    travels at the same speed...
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    ...no matter whether the object
    is at rest or in motion.
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    "Thou shalt not add my speed
    to the speed of light."
  • 16:49 - 16:54
    Also, no material object can travel
    at or beyond the speed of light.
  • 16:54 - 16:58
    Nothing in physics prevents you from
    traveling close to the speed of light.
  • 16:58 - 17:02
    99.9 percent the speed of light
    is just fine.
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    But no matter how hard you try...
  • 17:05 - 17:08
    ...you can never gain
    that last decimal point.
  • 17:08 - 17:10
    For the world
    to be logically consistent...
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    ...there must be a cosmic speed limit.
  • 17:14 - 17:17
    The crack of a whip is,
    due to its tip...
  • 17:17 - 17:22
    ...moving faster
    than the speed of sound.
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    It makes a shock wave...
  • 17:23 - 17:27
    ...a small sonic boom
    in the Italian countryside.
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    A thunderclap has a similar origin.
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    So does the sound of
    a supersonic airplane.
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    So why is the speed of light a barrier
    any more than the speed of sound?
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    The answer is not just that...
  • 17:42 - 17:45
    ...light travels a million times
    faster than sound.
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    It's not merely an engineering problem
    like the supersonic airplane.
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    Instead, the light barrier is
    a fundamental law of nature...
  • 17:53 - 17:55
    ...as basic as gravity.
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    Einstein found his absolute framework
    for the world:
  • 17:59 - 18:04
    This sturdy pillar among all
    the relative motions of the cosmos.
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    Light travels just as fast,
    no matter how its source is moving.
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    The speed of light is constant,
    relative to everything else.
  • 18:12 - 18:18
    Nothing can ever catch up with light.
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    Einstein's prohibition against
    traveling faster than light...
  • 18:22 - 18:25
    ...seems to clash with
    our common sense notions.
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    But why should we expect
    our common sense notions...
  • 18:28 - 18:31
    ...to have any reliability
    in a matter of this sort?
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    Why should our experience
    at 10 kilometers an hour...
  • 18:35 - 18:38
    ...constrain the laws of nature...
  • 18:38 - 18:43
    ...at 300,000 kilometers a second?
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    Relativity sets limits...
  • 18:46 - 18:50
    ...on what humans ultimately can do.
  • 18:50 - 18:52
    The universe is not required...
  • 18:52 - 19:00
    ...to be in perfect harmony
    with human ambition.
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    Imagine a place
    where the speed of light...
  • 19:03 - 19:07
    ...isn't its true value
    of 300,000 kilometers a second...
  • 19:07 - 19:10
    ...but something a lot less.
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    Let's say, 40 kilometers an hour...
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    ...and strictly enforced.
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    Just as in the real world we can
    never reach the speed of light...
  • 19:20 - 19:22
    ...the commandment here is still...
  • 19:22 - 19:26
    ..."Thou shalt not travel
    faster than light."
  • 19:26 - 19:31
    We can do thought experiments on
    what happens near the speed of light...
  • 19:31 - 19:38
    ...here 40 kilometers per hour,
    the speed of a motor scooter.
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    You can't break the laws of nature.
    There are no penalties for doing so.
  • 19:43 - 19:45
    The real world and this one...
  • 19:45 - 19:49
    ...are merely so arranged
    that transgressions can't happen.
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    The job of physics is to find out
    what those laws are.
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    Before Einstein,
    physicists thought that...
  • 19:59 - 20:01
    ...there were privileged frames
    of reference...
  • 20:01 - 20:04
    ...some special places and times...
  • 20:04 - 20:07
    ...against which everything else
    had to be measured.
  • 20:07 - 20:10
    Einstein encountered
    a similar notion in human affairs.
  • 20:10 - 20:13
    The idea that the customs
    of a particular nation...
  • 20:13 - 20:17
    ...his native Germany
    or Italy or anywhere...
  • 20:17 - 20:22
    ...are the standard which all
    other societies must be measured.
  • 20:22 - 20:25
    But Einstein rejected the strident
    nationalism of his time.
  • 20:25 - 20:29
    He believed every culture
    had its own validity.
  • 20:29 - 20:31
    Also in physics,
    he understood that...
  • 20:31 - 20:33
    ...there are no privileged
    frames of reference.
  • 20:33 - 20:37
    Every observer,
    in any place, time or motion...
  • 20:37 - 20:40
    ...must deduce
    the same laws of nature.
  • 20:40 - 20:43
    (SPEAKING IN ITALIAN)
  • 20:43 - 20:47
    A speed is simply how much space
    you cover in a given time...
  • 20:47 - 20:53
    ...as any kid on
    a motor scooter knows.
  • 20:53 - 20:55
    Since near the velocity of light...
  • 20:55 - 20:58
    ...we cannot simply add speeds...
  • 20:58 - 21:02
    ...the familiar notions of
    absolute space and absolute time...
  • 21:02 - 21:05
    ...independent of your
    relative motion, must give way.
  • 21:05 - 21:08
    That's why, as Einstein showed...
  • 21:08 - 21:12
    ...funny things have to happen
    close to the speed of light.
  • 21:12 - 21:16
    There, our conventional perspectives
    of space and time...
  • 21:16 - 21:21
    ...strangely change.
  • 21:21 - 21:25
    Your nose is just a little closer
    to me than your ears.
  • 21:25 - 21:28
    Light reflected off your nose
    reaches me...
  • 21:28 - 21:29
    ...an instant in time
    before your ears.
  • 21:29 - 21:33
    But suppose I had a magic camera...
  • 21:33 - 21:36
    ...so that I could see
    your nose and your ears...
  • 21:36 - 21:39
    ...at precisely the same instant?
  • 21:39 - 21:40
    (SCOOTER STARTS UP)
  • 21:40 - 21:42
    (SCOOTER HONKS)
  • 21:42 - 21:48
    With such a camera you could take
    some pretty interesting pictures.
  • 21:48 - 21:52
    Paolo says goodbye to
    his little brother, Vincenzo...
  • 21:52 - 21:55
    -Ciao, Vincenzo.
    -Ciao, Paolo.
  • 21:55 - 21:56
    ...and rides off.
  • 21:56 - 21:59
    He's now going more than
    half the speed of light.
  • 21:59 - 22:02
    He is almost catching up
    with his own light waves.
  • 22:02 - 22:05
    This compresses the light waves
    in front of him...
  • 22:05 - 22:06
    ...and his image becomes blue.
  • 22:06 - 22:11
    The shorter wavelength is
    what makes blue light waves blue.
  • 22:11 - 22:15
    Also Paolo becomes skinny
    in the direction of motion.
  • 22:15 - 22:17
    This isn't just some optical illusion.
  • 22:17 - 22:21
    It really happens when you travel
    near the speed of light.
  • 22:21 - 22:26
    As he roars away, he leaves his own
    light waves stretched out behind him.
  • 22:26 - 22:27
    Long light waves are red.
  • 22:27 - 22:32
    We say that his receding image
    is red-shifted.
  • 22:32 - 22:38
    Now Paolo leaves for
    a short tour of the countryside.
  • 22:38 - 22:43
    He experiences something
    even stranger.
  • 22:43 - 22:46
    Everything he can see is squeezed...
  • 22:46 - 22:49
    ...into a moving window
    just ahead of him...
  • 22:49 - 22:52
    ...blue-shifted at the center,
    red-shifted at the edges.
  • 22:52 - 22:56
    To a passerby, Paolo appears
    blue-shifted when approaching...
  • 22:56 - 22:58
    ...red-shifted when receding.
  • 22:58 - 23:01
    But to him, the entire world
    is both coming and going...
  • 23:01 - 23:03
    ...at nearly the speed of light.
  • 23:03 - 23:06
    Roadside houses and trees
    that has already gone past...
  • 23:06 - 23:10
    ...still appear to him at the edge
    of his forward field of view...
  • 23:10 - 23:14
    ...but distorted and red-shifted.
  • 23:14 - 23:20
    When he slows down,
    everything again looks normal.
  • 23:20 - 23:22
    Only very close
    to the speed of light...
  • 23:22 - 23:27
    ...does the visible world
    get squeezed into a kind of tunnel.
  • 23:27 - 23:30
    You'd see these distortions if you
    traveled near the speed of light.
  • 23:30 - 23:33
    Someday, perhaps,
    interstellar navigators...
  • 23:33 - 23:36
    ...will take their bearings
    on stars behind them...
  • 23:36 - 23:43
    ...whose images have all crowded
    together on the forward view screen.
  • 23:43 - 23:46
    The most bizarre aspect of traveling
    near the speed of light...
  • 23:46 - 23:50
    ...is that time slows down.
  • 23:50 - 23:52
    All clocks,
    mechanical and biological...
  • 23:52 - 23:55
    ...tick more slowly
    near the speed of light.
  • 23:55 - 23:59
    But stationary clocks tick
    at their usual rate.
  • 23:59 - 24:01
    If we travel close to light speed...
  • 24:01 - 24:09
    ...we age more slowly
    than those we left behind.
  • 24:09 - 24:13
    Paolo's watch and his internal
    sense of time show...
  • 24:13 - 24:16
    ...that he has been gone from
    his friends for only a few minutes.
  • 24:16 - 24:21
    But from their point of view,
    he has been away for many decades.
  • 24:21 - 24:26
    His friends have grown up,
    moved on and died.
  • 24:26 - 24:28
    And his younger brother has been...
  • 24:28 - 24:33
    ...patiently waiting
    for him all this time.
  • 24:33 - 24:38
    The two brothers experience
    the paradox of time dilation.
  • 24:38 - 24:43
    They've encountered
    Einstein's special relativity.
  • 24:43 - 24:58
    Vincenzo.
  • 24:58 - 25:00
    This was just a thought experiment.
  • 25:00 - 25:03
    But atomic particles traveling
    near the speed of light...
  • 25:03 - 25:07
    ...do decay more slowly
    than stationary particles.
  • 25:07 - 25:11
    As strange and counterintuitive
    as it seems...
  • 25:11 - 25:16
    ...time dilation is a law of nature.
  • 25:16 - 25:19
    Traveling close
    to the speed of light...
  • 25:19 - 25:23
    ...is a kind of elixir of life.
  • 25:23 - 25:26
    Because time slows down
    close to the speed of light...
  • 25:26 - 25:29
    ...special relativity provides us...
  • 25:29 - 25:33
    ...with a means of going to the stars.
  • 25:33 - 25:36
    This region of northern Italy
    is not only the caldron...
  • 25:36 - 25:40
    ...of some of the thinking
    of the young Albert Einstein...
  • 25:40 - 25:44
    ...it is also the home
    of another great genius...
  • 25:44 - 25:46
    ...who lived 400 years earlier.
  • 25:46 - 25:50
    Leonardo da Vinci.
  • 25:50 - 25:54
    Leonardo delighted
    in climbing these hills...
  • 25:54 - 25:58
    ...and viewing the ground
    from a great height...
  • 25:58 - 26:00
    ...as if he were soaring like a bird.
  • 26:00 - 26:03
    He drew the first aerial views...
  • 26:03 - 26:07
    ...of landscapes, villages,
    fortifications.
  • 26:07 - 26:11
    I've been talking about Einstein
    in and around this town of Vinci...
  • 26:11 - 26:14
    ...in which Leonardo grew up.
  • 26:14 - 26:17
    Einstein greatly respected Leonardo...
  • 26:17 - 26:20
    ...and their spirits, in some sense...
  • 26:20 - 26:48
    ...inhabit this countryside still.
  • 26:48 - 26:51
    Among Leonardo's
    many accomplishments...
  • 26:51 - 26:55
    ...in painting, sculpture,
    architecture, natural history...
  • 26:55 - 27:00
    ...anatomy, geology,
    civil and military engineering...
  • 27:00 - 27:02
    ...he had a great passion.
  • 27:02 - 27:06
    He wished to construct a machine...
  • 27:06 - 27:08
    ...which would fly.
  • 27:08 - 27:12
    He made sketches of such machines,
    built miniature models...
  • 27:12 - 27:18
    ...constructed great,
    full-scale prototypes.
  • 27:18 - 27:23
    And not a one of them ever worked.
  • 27:23 - 27:27
    There were no machines of adequate
    capacity available in his time.
  • 27:27 - 27:32
    The technology was just not ready.
  • 27:32 - 27:35
    The designs, however, were brilliant.
  • 27:35 - 27:38
    For example, this bird-like machine...
  • 27:38 - 27:44
    ...here in the Leonardo Museum
    in the town of Vinci.
  • 27:44 - 27:49
    Leonardo's great designs encouraged
    engineers in later epochs...
  • 27:49 - 27:53
    ...although Leonardo himself
    was very depressed at these failures.
  • 27:53 - 27:55
    But it's not his fault...
  • 27:55 - 27:59
    ...he was trapped in the 15th century.
  • 27:59 - 28:03
    A somewhat similar case
    occurred in 1939...
  • 28:03 - 28:08
    ...when a group of engineers called
    the British Interplanetary Society...
  • 28:08 - 28:10
    ...decided to design a ship...
  • 28:10 - 28:13
    ...which would carry people
    to the moon.
  • 28:13 - 28:16
    Now, it was by no means
    the same design...
  • 28:16 - 28:21
    ...as the Apollo ship which actually
    took people to the moon years later.
  • 28:21 - 28:23
    But that design suggested that...
  • 28:23 - 28:25
    ...a mission to the moon
    might one day...
  • 28:25 - 28:28
    ...be a practical
    engineering possibility.
  • 28:28 - 28:30
    Today...
  • 28:30 - 28:35
    ...we have preliminary
    designs of ships...
  • 28:35 - 28:38
    ...which will take people
    to the stars.
  • 28:38 - 28:43
    They are constructed in Earth orbit
    and from there...
  • 28:43 - 28:48
    ...they venture on their great
    interstellar journeys.
  • 28:48 - 28:50
    One of them...
  • 28:50 - 28:54
    ...is called Project Orion.
  • 28:54 - 28:56
    It utilizes nuclear weapons...
  • 28:56 - 29:00
    ...hydrogen bombs
    against an inertial plate.
  • 29:00 - 29:05
    Each explosion providing
    a kind of "putt-putt"...
  • 29:05 - 29:09
    ...a vast nuclear motorboat in space.
  • 29:09 - 29:13
    Orion seems entirely practical...
  • 29:13 - 29:15
    ...and was under development
    in the U.S...
  • 29:15 - 29:19
    ...until the signing
    of the international treaty...
  • 29:19 - 29:23
    ...forbidding nuclear weapons
    explosions in space.
  • 29:23 - 29:28
    I think, the Orion starship
    is the best use of nuclear weapons...
  • 29:28 - 29:41
    ...provided the ships don't depart
    from very near the Earth.
  • 29:41 - 29:44
    Project Daedalus is
    a recent design...
  • 29:44 - 29:47
    ...of the British
    Interplanetary Society.
  • 29:47 - 29:51
    It assumes the existence
    of a nuclear fusion reactor...
  • 29:51 - 29:53
    ...something much safer
    and more efficient...
  • 29:53 - 30:01
    ...than the existing nuclear
    fission power plants.
  • 30:01 - 30:03
    We do not yet have fusion reactors.
  • 30:03 - 30:12
    One day, quite soon, we may.
  • 30:12 - 30:16
    Orion and Daedalus might go...
  • 30:16 - 30:19
    ...10 percent the speed of light.
  • 30:19 - 30:23
    So a trip to Alpha Centauri,
    4 1/2 light-years away...
  • 30:23 - 30:27
    ...would take 45 years,
    less than a human lifetime.
  • 30:27 - 30:31
    Such ships could not travel
    close enough to the speed of light...
  • 30:31 - 30:34
    ...for the time-slowing effects
    of special relativity...
  • 30:34 - 30:36
    ...to become important.
  • 30:36 - 30:39
    It does not seem likely
    that such ships...
  • 30:39 - 30:42
    ...would be built before
    the middle of the 21 st century...
  • 30:42 - 30:46
    ...although we could build
    an Orion starship now.
  • 30:46 - 30:51
    For voyages beyond the nearest stars,
    something must be added.
  • 30:51 - 30:54
    Perhaps they could be used
    as multigeneration ships...
  • 30:54 - 30:57
    ...so those arriving would be
    the remote descendants...
  • 30:57 - 31:02
    ...of those who had originally
    set out centuries before.
  • 31:02 - 31:06
    Or perhaps some safe means
    of human hibernation might be found...
  • 31:06 - 31:10
    ...so that space travelers might be
    frozen and then thawed out...
  • 31:10 - 31:14
    ...when they arrive at
    the destination centuries later.
  • 31:14 - 31:19
    But fast interstellar space flight
    approaching the speed of light...
  • 31:19 - 31:21
    ...is much more difficult.
  • 31:21 - 31:24
    That's an objective
    not for a hundred years...
  • 31:24 - 31:27
    ...but for a thousand
    or for 10 thousand...
  • 31:27 - 31:32
    ...but it also is possible.
  • 31:32 - 31:35
    A kind of interstellar ramjet
    has been proposed...
  • 31:35 - 31:38
    ...which scoops up
    the hydrogen atoms...
  • 31:38 - 31:40
    ...which float between the stars...
  • 31:40 - 31:45
    ...accelerates them into an engine
    and spits them out the back.
  • 31:45 - 31:48
    But in deep space,
    there is one atom...
  • 31:48 - 31:52
    ...for every 10 cubic centimeters
    of space.
  • 31:52 - 31:54
    For the ramjet to work...
  • 31:54 - 31:57
    ...it has to have a frontal scoop...
  • 31:57 - 32:00
    ...hundreds of kilometers across.
  • 32:00 - 32:04
    Reaching relativistic velocities,
    the hydrogen atoms will be moving...
  • 32:04 - 32:07
    ...with respect
    to the interstellar spaceship...
  • 32:07 - 32:09
    ...at close to the speed of light.
  • 32:09 - 32:11
    If precautions aren't taken...
  • 32:11 - 32:16
    ...the passengers will be fried
    by these induced cosmic rays.
  • 32:16 - 32:18
    There's a proposed solution:
  • 32:18 - 32:21
    A laser is used to strip
    electrons off the atoms...
  • 32:21 - 32:25
    ...and electrically charge them
    while they're some distance away.
  • 32:25 - 32:28
    And an extremely strong
    magnetic field...
  • 32:28 - 32:32
    ...is used to deflect
    the charged atoms into the scoop...
  • 32:32 - 32:33
    ...and away from the spacecraft.
  • 32:33 - 32:35
    This is engineering...
  • 32:35 - 32:39
    ...on a scale so far
    unprecedented on the Earth.
  • 32:39 - 32:53
    We are talking of engines
    the size of small worlds.
  • 32:53 - 32:58
    Suppose that the spacecraft is
    designed to accelerate at 1 g...
  • 32:58 - 33:01
    ...so we'd be comfortable aboard it.
  • 33:01 - 33:03
    We'd go closer and closer
    to the speed of light...
  • 33:03 - 33:06
    ...until the midpoint of the journey.
  • 33:06 - 33:08
    Then the spacecraft is
    turned around...
  • 33:08 - 33:13
    ...and we decelerate at 1 g
    to the destination.
  • 33:13 - 33:17
    For most of the trip, the velocity
    would be close to the speed of light...
  • 33:17 - 33:20
    ...and time would
    slow down enormously.
  • 33:20 - 33:23
    By how much?
  • 33:23 - 33:27
    Barnard's Star could be reached
    by such a ship...
  • 33:27 - 33:30
    ...in eight years, ship time.
  • 33:30 - 33:34
    The center of the Milky Way galaxy
    in 21 years.
  • 33:34 - 33:38
    The Andromeda galaxy in 28 years.
  • 33:38 - 33:41
    Of course, the people
    left behind on the Earth...
  • 33:41 - 33:43
    ...would see things
    somewhat differently.
  • 33:43 - 33:45
    Instead of 21 years to the galaxy...
  • 33:45 - 33:49
    ...they would measure it
    as 30,000 years.
  • 33:49 - 33:50
    When we got back...
  • 33:50 - 33:55
    ...very few of our friends
    would be around to greet us.
  • 33:55 - 33:57
    In principle, such a journey...
  • 33:57 - 34:01
    ...mounting the decimal points closer
    and closer to the speed of light...
  • 34:01 - 34:05
    ...would even permit us to
    circumnavigate the known universe...
  • 34:05 - 34:09
    ...in 56 years, ship time.
  • 34:09 - 34:14
    We would return tens
    of billions of years...
  • 34:14 - 34:16
    ...in the far future...
  • 34:16 - 34:19
    ...with the Earth a charred cinder...
  • 34:19 - 34:22
    ...and the sun dead.
  • 34:22 - 34:26
    Relativistic space flight makes
    the universe accessible...
  • 34:26 - 34:29
    ...to advanced civilizations...
  • 34:29 - 34:31
    ...but only to those
    who go on the journey...
  • 34:31 - 34:34
    ...not to those who stay home.
  • 34:34 - 34:39
    These designs are probably further...
  • 34:39 - 34:44
    ...from the actual interstellar
    spacecraft of the future...
  • 34:44 - 34:47
    ...than Leonardo's models are...
  • 34:47 - 34:51
    ...from the supersonic transports
    of the present.
  • 34:51 - 34:53
    But if we do not destroy ourselves...
  • 34:53 - 34:58
    ...I believe that we will,
    one day, venture to the stars.
  • 34:58 - 35:01
    When our solar system
    is all explored...
  • 35:01 - 35:37
    ...the planets of other stars
    will beckon.
  • 35:37 - 35:43
    Space travel and time travel
    are connected.
  • 35:43 - 35:45
    To travel fast into space...
  • 35:45 - 35:51
    ...is to travel fast into the future.
  • 35:51 - 35:56
    We travel into the future,
    although slowly, all the time.
  • 35:56 - 36:00
    But what about the past?
    Could we journey into yesterday?
  • 36:00 - 36:04
    Many physicists think this is
    fundamentally impossible...
  • 36:04 - 36:06
    ...that we could
    not build a device...
  • 36:06 - 36:09
    ...which would carry us
    backwards into time.
  • 36:09 - 36:13
    Some say that even if we were
    to build such a device...
  • 36:13 - 36:14
    ...it wouldn't do much good.
  • 36:14 - 36:17
    We couldn't significantly
    affect the past.
  • 36:17 - 36:20
    For example, suppose you
    traveled into the past...
  • 36:20 - 36:23
    ...and somehow or other prevented...
  • 36:23 - 36:26
    ...your own parents from meeting.
  • 36:26 - 36:30
    Why, then you would probably
    never have been born...
  • 36:30 - 36:32
    ...which is something
    of a contradiction, isn't it...
  • 36:32 - 36:35
    ...since you are clearly there.
  • 36:35 - 36:36
    Other people think that...
  • 36:36 - 36:40
    ...the two alternative histories
    have equal validity...
  • 36:40 - 36:43
    ...that they're parallel threads,
    skeins of time...
  • 36:43 - 36:50
    ...that they could exist side by side.
  • 36:50 - 36:52
    The history in which
    you were never born...
  • 36:52 - 36:56
    ...and the history that
    you know all about.
  • 36:56 - 36:59
    Perhaps time itself has
    many potential dimensions...
  • 36:59 - 37:03
    ...despite the fact that
    we are condemned to experience...
  • 37:03 - 37:05
    ...only one of those dimensions.
  • 37:05 - 37:09
    Now, suppose you could go back
    into the past...
  • 37:09 - 37:13
    ...and really change it by,
    let's say something like...
  • 37:13 - 37:17
    ...persuading Queen Isabella not
    to bankroll Christopher Columbus.
  • 37:17 - 37:20
    Then you would have set into motion...
  • 37:20 - 37:23
    ...a different sequence
    of historical events...
  • 37:23 - 37:26
    ...which those people
    you left behind you in our time...
  • 37:26 - 37:29
    ...would never get to know about.
  • 37:29 - 37:31
    If that kind of time travel
    were possible...
  • 37:31 - 37:34
    ...then every imaginable sequence...
  • 37:34 - 37:37
    ...of alternative history...
  • 37:37 - 37:40
    ...might in some sense really exist.
  • 37:40 - 37:43
    Would it be possible
    for a time traveler...
  • 37:43 - 37:46
    ...to change the course of history
    in a major way?
  • 37:46 - 37:52
    Well, let's think about that.
  • 37:52 - 37:54
    History consists for the most part...
  • 37:54 - 37:59
    ...of a complex multitude
    of deeply interwoven threads...
  • 37:59 - 38:01
    ...biological, economic
    and social forces...
  • 38:01 - 38:05
    ...that are not so easily unraveled.
  • 38:05 - 38:10
    The ancient Greeks imagined the course
    of human events to be a tapestry...
  • 38:10 - 38:16
    ...created by three goddesses:
    the Fates.
  • 38:16 - 38:20
    Random minor events generally
    have no long-range consequences.
  • 38:20 - 38:23
    But some which occur
    at critical junctures...
  • 38:23 - 38:26
    ...may alter the weave of history.
  • 38:26 - 38:29
    There may be cases where
    profound changes can be made...
  • 38:29 - 38:32
    ...by relatively trivial adjustments.
  • 38:32 - 38:37
    The further in the past such an event
    is, the more powerful its influence.
  • 38:37 - 38:41
    What if our time traveler had
    persuaded Queen Isabella that...
  • 38:41 - 38:43
    ...Columbus' geography was wrong?
  • 38:43 - 38:47
    Almost certainly, some other European
    would have sailed to the New World.
  • 38:47 - 38:49
    There were many inducements:
  • 38:49 - 38:53
    The lure of the spice trade,
    improvements in navigation...
  • 38:53 - 38:55
    ...competition among
    rival European powers.
  • 38:55 - 38:59
    The discovery of America
    around 1500 was inevitable.
  • 38:59 - 39:03
    Of course, there wouldn't be any
    postage stamps showing Columbus...
  • 39:03 - 39:06
    ...and the Republic of Colombia
    would have another name.
  • 39:06 - 39:14
    But the big picture would have
    turned out more or less the same.
  • 39:14 - 39:18
    In order to affect
    the future profoundly...
  • 39:18 - 39:21
    ...a time traveler
    has to pick and choose.
  • 39:21 - 39:25
    He'd probably have to intervene
    in a number of events...
  • 39:25 - 39:27
    ...which are
    very carefully selected...
  • 39:27 - 39:33
    ...so he could change
    the weave of history.
  • 39:33 - 39:35
    It's a lovely fantasy...
  • 39:35 - 39:42
    ...to explore those other worlds
    that never were.
  • 39:42 - 39:46
    If you had H.G. Wells' time machine...
  • 39:46 - 39:49
    ...maybe you could understand
    how history really works.
  • 39:49 - 39:52
    If an apparently pivotal person
    had never lived...
  • 39:52 - 39:56
    ...Paul the Apostle or Peter the Great
    or Pythagoras...
  • 39:56 - 40:00
    ...how different would
    the world really be?
  • 40:00 - 40:02
    What if the scientific tradition...
  • 40:02 - 40:05
    ...of the ancient Ionian Greeks...
  • 40:05 - 40:08
    ...had prospered and flourished?
  • 40:08 - 40:11
    It would have required
    many social factors at the time...
  • 40:11 - 40:13
    ...to have been different...
  • 40:13 - 40:16
    ...including the common feeling...
  • 40:16 - 40:18
    ...that slavery was right and natural.
  • 40:18 - 40:22
    But what if that light
    that had dawned...
  • 40:22 - 40:26
    ...on the eastern Mediterranean
    some 2500 years ago...
  • 40:26 - 40:28
    ...had not flickered out?
  • 40:28 - 40:32
    What if scientific method
    and experiment...
  • 40:32 - 40:34
    ...had been vigorously pursued...
  • 40:34 - 40:36
    ...2000 years before
    the industrial revolution...
  • 40:36 - 40:38
    ...our industrial revolution?
  • 40:38 - 40:43
    What if the power of this new mode
    of thought, the scientific method...
  • 40:43 - 40:45
    ...had been generally appreciated?
  • 40:45 - 40:49
    I think we might have saved
    10 or 20 centuries.
  • 40:49 - 40:52
    Perhaps the contributions
    that Leonardo made...
  • 40:52 - 40:55
    ...would have been made
    1000 years earlier...
  • 40:55 - 40:59
    ...and the contributions
    of Einstein 500 years ago.
  • 40:59 - 41:01
    Not that it would have
    been those people...
  • 41:01 - 41:04
    ...who would've made
    those contributions...
  • 41:04 - 41:08
    ...because they lived only
    in our timeline.
  • 41:08 - 41:11
    If the Ionians had won...
  • 41:11 - 41:15
    ...we might by now, I think,
    be going to the stars.
  • 41:15 - 41:20
    We might at this moment have
    the first survey ships...
  • 41:20 - 41:25
    ...returning with astonishing results
    from Alpha Centauri...
  • 41:25 - 41:30
    ...and Barnard's Star,
    Sirius and Tau Ceti.
  • 41:30 - 41:33
    There would now be great fleets...
  • 41:33 - 41:35
    ...of interstellar transports...
  • 41:35 - 41:38
    ...being constructed in Earth orbit...
  • 41:38 - 41:41
    ...small, unmanned survey ships...
  • 41:41 - 41:44
    ...liners for immigrants, perhaps...
  • 41:44 - 41:47
    ...great trading ships...
  • 41:47 - 41:50
    ...to ply the spaces
    between the stars.
  • 41:50 - 41:54
    On all these ships
    there would be symbols...
  • 41:54 - 41:57
    ...and inscriptions on the sides.
  • 41:57 - 41:59
    The inscriptions,
    if we looked closely...
  • 41:59 - 42:02
    ...would be written in Greek.
  • 42:02 - 42:04
    The symbol...
  • 42:04 - 42:08
    ...perhaps, would be the dodecahedron.
  • 42:08 - 42:12
    And the inscription on the sides
    of the ships to the stars...
  • 42:12 - 42:14
    ...something like:
  • 42:14 - 42:22
    "Starship Theodorus
    of the Planet Earth."
  • 42:22 - 42:28
    If you were a really
    ambitious time traveler...
  • 42:28 - 42:31
    ...you might not dally
    with human history...
  • 42:31 - 42:34
    ...or even pause to examine
    the evolution on Earth.
  • 42:34 - 42:36
    Instead, you would journey back...
  • 42:36 - 42:38
    ...to witness the origin
    of our solar system...
  • 42:38 - 42:44
    ...from the gas and dust
    between the stars.
  • 42:44 - 42:45
    Five billion years ago...
  • 42:45 - 42:49
    ...an interstellar cloud was
    collapsing to form our solar system.
  • 42:49 - 42:53
    Most clumps of matter
    gravitated towards the center...
  • 42:53 - 42:55
    ...and were destined
    to form the sun.
  • 42:55 - 43:00
    Smaller peripheral clumps
    would become the planets.
  • 43:00 - 43:04
    Long ago, there was a kind of
    natural selection among the worlds.
  • 43:04 - 43:09
    Those on highly elliptical orbits
    tended to collide and be destroyed...
  • 43:09 - 43:13
    ...but planets in circular orbits
    tended to survive.
  • 43:13 - 43:15
    But if events had been
    a little different...
  • 43:15 - 43:17
    ...the Earth would never have formed...
  • 43:17 - 43:21
    ...and another planet at another
    distance from the sun would be around.
  • 43:21 - 43:24
    We owe the existence of our world...
  • 43:24 - 43:30
    ...to random collisions
    in a long-vanished cloud.
  • 43:30 - 43:34
    Soon, the central mass
    became very hot.
  • 43:34 - 43:38
    Thermonuclear reactions were initiated
    and the sun turned on...
  • 43:38 - 43:43
    ...flooding the solar system
    with light.
  • 43:43 - 43:44
    But the growing smaller lumps...
  • 43:44 - 43:47
    ...would never achieve
    such high temperatures...
  • 43:47 - 43:50
    ...and would never generate
    thermonuclear reactions.
  • 43:50 - 43:54
    They would become
    the Earth and the other planets...
  • 43:54 - 44:03
    ...heated not from within,
    but mainly by the distant sun.
  • 44:03 - 44:05
    The accretion continued until...
  • 44:05 - 44:08
    ...almost all the gas and dust
    and small worldlets...
  • 44:08 - 44:15
    ...were swept up
    by the surviving planets.
  • 44:15 - 44:17
    Our time traveler would witness...
  • 44:17 - 44:26
    ...the collisions
    that made the worlds.
  • 44:26 - 44:28
    Except for the comets and asteroids...
  • 44:28 - 44:31
    ...the chaos of the early
    solar system was reduced...
  • 44:31 - 44:34
    ...to a remarkable simplicity:
  • 44:34 - 44:38
    Nine or so principal planets
    in almost circular orbits...
  • 44:38 - 44:44
    ...and a few dozen moons.
  • 44:44 - 44:49
    Now, let's take a different look.
  • 44:49 - 44:51
    If we view the solar system edge on...
  • 44:51 - 44:54
    ...and move the sun
    off-screen to the left...
  • 44:54 - 44:57
    ...we see that
    the small terrestrial planets...
  • 44:57 - 45:01
    ...the ones about as massive as Earth,
    tend to be close to the sun.
  • 45:01 - 45:05
    The big Jupiter-like planets tend
    to be much further from the sun.
  • 45:05 - 45:09
    But is that the way it has to be?
  • 45:09 - 45:11
    Computer studies suggest...
  • 45:11 - 45:14
    ...that there may be many
    similar systems about stars...
  • 45:14 - 45:22
    ...with the terrestrials in close
    and the Jovian planets further away.
  • 45:22 - 45:26
    But some systems might have Jovians
    and terrestrials mixed together.
  • 45:26 - 45:31
    There may be great worlds
    like Jupiter looming in other skies.
  • 45:31 - 45:36
    Rarely, the Jovian planets
    may form close to the star...
  • 45:36 - 45:42
    ...the terrestrials trailing away
    towards interstellar space.
  • 45:42 - 45:44
    Our familiar arrangement of planets...
  • 45:44 - 45:47
    ...is only one,
    perhaps typical, case...
  • 45:47 - 45:51
    ...in the vast expanse of systems.
  • 45:51 - 45:56
    Often, one fledgling planet
    accumulates so much gas and dust...
  • 45:56 - 45:58
    ...that thermonuclear reactions
    do occur.
  • 45:58 - 46:01
    It becomes a second sun.
  • 46:01 - 46:08
    A binary star system has formed.
  • 46:08 - 46:12
    From most of these worlds,
    the vistas will be dazzling.
  • 46:12 - 46:15
    Not one of them will be
    identical to the Earth.
  • 46:15 - 46:20
    A few will be hospitable.
    Many will appear hostile.
  • 46:20 - 46:23
    Where there are two suns in the sky...
  • 46:23 - 46:30
    ...every object will cast two shadows.
  • 46:30 - 46:33
    What wonders are waiting for us...
  • 46:33 - 46:36
    ...on the planets of the nearby stars?
  • 46:36 - 46:38
    Are there radically
    different kinds of worlds...
  • 46:38 - 46:45
    ...unimaginably exotic forms of life?
  • 46:45 - 46:48
    Perhaps in another century or two...
  • 46:48 - 46:51
    ...when our solar system
    is all explored...
  • 46:51 - 46:54
    ...we will also have put
    our own planet in order.
  • 46:54 - 46:58
    Then we will set sail for the stars...
  • 46:58 - 47:04
    ...and the beckoning worlds
    around them.
  • 47:04 - 47:07
    In that day, our machines
    and our descendants...
  • 47:07 - 47:11
    ...approaching the speed of light,
    will skim the light-years...
  • 47:11 - 47:16
    ...leaping ahead through time,
    seeking new worlds.
  • 47:16 - 47:21
    Einstein has shown us
    that it's possible.
  • 47:21 - 47:23
    We will journey simultaneously...
  • 47:23 - 47:27
    ...to distant planets
    and to the far future.
  • 47:27 - 47:29
    Some worlds, like this one...
  • 47:29 - 47:32
    ...will look out onto
    a vast gaseous nebula...
  • 47:32 - 47:34
    ...the remains of a star...
  • 47:34 - 47:40
    ...that once was and is no longer.
  • 47:40 - 47:43
    In all those skies,
    rich and distant...
  • 47:43 - 47:46
    ...and exotic constellations...
  • 47:46 - 47:50
    ...there may be a faint yellow star...
  • 47:50 - 47:53
    ...perhaps barely visible
    to the naked eye...
  • 47:53 - 47:56
    ...perhaps seen only
    through the telescope.
  • 47:56 - 48:00
    The home star of a fleet
    of interstellar transports...
  • 48:00 - 48:02
    ...exploring this tiny region...
  • 48:02 - 48:06
    ...of the great Milky Way galaxy.
  • 48:06 - 48:10
    The themes of space and time
    are intertwined.
  • 48:10 - 48:14
    Worlds and stars, like people...
  • 48:14 - 48:18
    ...are born, live and die.
  • 48:18 - 48:20
    The lifetime of a human being
    is measured in decades.
  • 48:20 - 48:23
    But the lifetime of the sun...
  • 48:23 - 48:28
    ...is a hundred million times longer.
  • 48:28 - 48:31
    Matter is much older than life.
  • 48:31 - 48:34
    Billions of years before
    the sun and Earth even formed...
  • 48:34 - 48:38
    ...atoms were being synthesized
    in the insides of hot stars...
  • 48:38 - 48:42
    ...and then returned to space
    when the stars blew themselves up.
  • 48:42 - 48:46
    Newly formed planets were
    made of this stellar debris.
  • 48:46 - 48:55
    The Earth and every living thing
    are made of star stuff.
  • 48:55 - 48:58
    But how slowly, in our human
    perspective, life evolved...
  • 48:58 - 49:07
    ...from the molecules of the early
    oceans to the first bacteria.
  • 49:07 - 49:10
    Evolution is not immediately
    obvious to everybody...
  • 49:10 - 49:14
    ...because it moves
    so slowly and takes so long.
  • 49:14 - 49:17
    How can creatures who
    live for only 70 years...
  • 49:17 - 49:21
    ...detect events that
    take 70 million years to unfold?
  • 49:21 - 49:28
    Or 4 billion?
  • 49:28 - 49:30
    By the time
    one-celled animals had evolved...
  • 49:30 - 49:38
    ...the history of life
    on Earth was half over.
  • 49:38 - 49:41
    Not very far along to us,
    you might think...
  • 49:41 - 49:44
    ...but by now almost all
    the basic chemistry of life...
  • 49:44 - 49:47
    ...had been established.
  • 49:47 - 49:49
    Forget our human time perspective.
  • 49:49 - 49:52
    From the point of view of a star...
  • 49:52 - 49:55
    ...evolution was weaving
    intricate new patterns...
  • 49:55 - 50:01
    ...from the star stuff on
    the planet Earth, and very rapidly.
  • 50:01 - 50:05
    Most evolutionary lines
    became extinct.
  • 50:05 - 50:07
    Many lines became stagnant.
  • 50:07 - 50:09
    If things had gone
    a bit differently...
  • 50:09 - 50:11
    ...a small change of climate,
    say, or...
  • 50:11 - 50:13
    ...a new mutation...
  • 50:13 - 50:16
    ...or the accidental death
    of a different humble organism...
  • 50:16 - 50:23
    ...the entire future history of life
    might have been very different.
  • 50:23 - 50:26
    Maybe the line to an intelligent
    technological species...
  • 50:26 - 50:32
    ...would have passed through worms.
  • 50:32 - 50:34
    Maybe the present masters
    of the planet...
  • 50:34 - 50:40
    ...would have had ancestors
    who were tunicates.
  • 50:40 - 50:42
    We might not have evolved.
  • 50:42 - 50:45
    Someone else,
    someone very different...
  • 50:45 - 50:52
    ...would be here now in our stead,
    maybe pondering their origins.
  • 50:52 - 50:55
    But that's not what happened.
  • 50:55 - 50:58
    There's a particular sequence
    of environmental accidents...
  • 50:58 - 51:01
    ...and random mutations
    in the hereditary material.
  • 51:01 - 51:05
    One particular timeline
    for life on Earth...
  • 51:05 - 51:11
    ...in this universe.
  • 51:11 - 51:14
    As a result, the dominant organisms
    on the planet today...
  • 51:14 - 51:18
    ...come from fish.
  • 51:18 - 51:23
    Along the way, many more species
    became extinct than now exist.
  • 51:23 - 51:26
    If history had
    a slightly different weave...
  • 51:26 - 51:31
    ...some of those extinct organisms
    might have survived and prospered.
  • 51:31 - 51:35
    But occasionally, a creature
    thought to have become extinct...
  • 51:35 - 51:37
    ...hundreds of millions
    of years ago...
  • 51:37 - 51:40
    ...turns out to be alive and well.
  • 51:40 - 51:45
    The coelacanth, for example.
  • 51:45 - 51:50
    For 3 1/2 billion years, life had
    lived exclusively in the water.
  • 51:50 - 51:52
    But now, in a great
    breathtaking adventure...
  • 51:52 - 51:54
    ...it took to the land.
  • 51:54 - 51:56
    But if things had gone
    a little differently...
  • 51:56 - 51:59
    ...the dominant species might
    still be in the ocean...
  • 51:59 - 52:09
    ...or developed spaceships to
    carry them off the planet altogether.
  • 52:09 - 52:11
    From our ancestors, the reptiles...
  • 52:11 - 52:14
    ...there developed
    many successful lines...
  • 52:14 - 52:17
    ...including the dinosaurs.
  • 52:17 - 52:20
    Some were fast, dexterous
    and intelligent.
  • 52:20 - 52:22
    A visitor from
    another world or time...
  • 52:22 - 52:25
    ...might have thought them
    the wave of the future.
  • 52:25 - 52:30
    But after nearly 200 million years,
    they were suddenly all wiped out.
  • 52:30 - 52:33
    Perhaps it was a great meteorite
    colliding with the Earth...
  • 52:33 - 52:35
    ...spewing debris into the air,
    blotting out the sun...
  • 52:35 - 52:38
    ...and killing the plants
    that the dinosaurs ate.
  • 52:38 - 52:45
    I wonder when they first sensed
    that something was wrong.
  • 52:45 - 52:49
    The successors of the dinosaurs
    came from the same reptilian stock...
  • 52:49 - 52:56
    ...but they survived the catastrophe
    that destroyed their cousins.
  • 52:56 - 52:59
    Again, there were many branches
    which became extinct.
  • 52:59 - 53:01
    And had events been
    a little different...
  • 53:01 - 53:08
    ...those branches might have led
    to the dominant form today.
  • 53:08 - 53:11
    For 40 million years, a visitor
    would not have been impressed...
  • 53:11 - 53:13
    ...by these timid little creatures...
  • 53:13 - 53:20
    ...but they led to all
    the familiar mammals of today.
  • 53:20 - 53:23
    And that includes the primates.
  • 53:23 - 53:27
    About 20 million years ago,
    a space time traveler...
  • 53:27 - 53:30
    ...might have recognized
    these guys as promising...
  • 53:30 - 53:34
    ...bright, quick, agile,
    sociable, curious.
  • 53:34 - 53:37
    Their ancestors were once
    atoms made in stars...
  • 53:37 - 53:40
    ...then simple molecules,
    single cells...
  • 53:40 - 53:42
    ...polyps stuck to the ocean floor...
  • 53:42 - 53:46
    ...fish, amphibians, reptiles, shrews.
  • 53:46 - 53:51
    But then they came down
    from the trees and stood upright.
  • 53:51 - 53:53
    They grew an enormous brain...
  • 53:53 - 53:57
    ...they developed culture,
    invented tools...
  • 53:57 - 54:02
    ...domesticated fire.
  • 54:02 - 54:05
    They discovered language and writing.
  • 54:05 - 54:07
    They developed agriculture.
  • 54:07 - 54:13
    They built cities and forged metal.
  • 54:13 - 54:17
    And ultimately,
    they set out for the stars...
  • 54:17 - 54:24
    ...from which they had come
    5 billion years earlier.
  • 54:24 - 54:25
    We are star stuff...
  • 54:25 - 54:32
    ...which has taken its destiny
    into its own hands.
  • 54:32 - 54:34
    The loom of time and space...
  • 54:34 - 54:39
    ...works the most astonishing
    transformations of matter.
  • 54:39 - 54:41
    Our own planet is only a tiny part...
  • 54:41 - 54:44
    ...of the vast cosmic tapestry...
  • 54:44 - 54:56
    ...a starry fabric
    of worlds yet untold.
  • 54:56 - 55:00
    Those worlds in space
    are as countless...
  • 55:00 - 55:04
    ...as all the grains of sand
    on all the beaches of the Earth.
  • 55:04 - 55:08
    Each of those worlds
    is as real as ours.
  • 55:08 - 55:10
    In every one of them,
    there's a succession of...
  • 55:10 - 55:15
    ...incidents, events, occurrences
    which influence its future.
  • 55:15 - 55:19
    Countless worlds,
    numberless moments...
  • 55:19 - 55:23
    ...an immensity of space and time.
  • 55:23 - 55:25
    And our small planet,
    at this moment...
  • 55:25 - 55:30
    ...here, we face
    a critical branchpoint in history.
  • 55:30 - 55:33
    What we do with our world right now...
  • 55:33 - 55:36
    ...will propagate down
    through the centuries...
  • 55:36 - 55:39
    ...and powerfully affect
    the destiny of our descendants.
  • 55:39 - 55:43
    It is well within our power
    to destroy our civilization...
  • 55:43 - 55:46
    ...and perhaps our species as well.
  • 55:46 - 55:49
    If we capitulate to superstition...
  • 55:49 - 55:52
    ...or greed or stupidity...
  • 55:52 - 55:56
    ...we can plunge our world into
    a darkness deeper than the time...
  • 55:56 - 56:00
    ...between the collapse of classical
    civilization and Italian Renaissance.
  • 56:00 - 56:03
    But we are also capable...
  • 56:03 - 56:05
    ...of using our compassion
    and our intelligence...
  • 56:05 - 56:08
    ...our technology and our wealth...
  • 56:08 - 56:11
    ...to make an abundant
    and meaningful life...
  • 56:11 - 56:13
    ...for every inhabitant
    of this planet...
  • 56:13 - 56:18
    ...to enhance enormously
    our understanding of the universe...
  • 56:18 - 56:37
    ...and to carry us to the stars.
  • 56:37 - 56:39
    In our motorbike sequence...
  • 56:39 - 56:42
    ...we showed how
    the landscape might look...
  • 56:42 - 56:45
    ...if we barreled through it
    at close to light speed.
  • 56:45 - 56:48
    Since then,
    inspired by this sequence...
  • 56:48 - 56:52
    ...Ping-Kang Hsiung
    at Carnegie Mellon University...
  • 56:52 - 56:54
    ...produced an exact
    computer animation.
  • 56:54 - 56:58
    This is what you'd see if you
    traveled at ordinary speeds...
  • 56:58 - 57:00
    ...through this red and white lattice.
  • 57:00 - 57:02
    But this is how it would appear...
  • 57:02 - 57:07
    ...if you were traveling
    at close to the speed of light.
  • 57:07 - 57:11
    We're probably many centuries away
    from traveling close to light speed...
  • 57:11 - 57:14
    ...and experiencing time dilation.
  • 57:14 - 57:17
    But even then,
    it might not be fast enough...
  • 57:17 - 57:21
    ...if we wanted to travel
    to some distant place in the galaxy...
  • 57:21 - 57:24
    ...and then come back to Earth
    in our own epoch.
  • 57:24 - 57:27
    Some years after completing Cosmos...
  • 57:27 - 57:32
    ...I took time out from
    my scientific work to write a novel.
  • 57:32 - 57:34
    A novel about travel...
  • 57:34 - 57:38
    ...to the center
    of the Milky Way galaxy.
  • 57:38 - 57:41
    I was willing to imagine
    beings and civilizations...
  • 57:41 - 57:43
    ...far more advanced than we...
  • 57:43 - 57:47
    ...but I wasn't willing
    to ignore the laws of physics.
  • 57:47 - 57:52
    Was there, even in principle,
    a way to get very quickly...
  • 57:52 - 57:55
    ...to 30,000 light-years from Earth?
  • 57:55 - 57:57
    So I asked my friend...
  • 57:57 - 58:00
    ...Kip Thorne of the California
    Institute of Technology.
  • 58:00 - 58:04
    He's a leading expert
    on the nature of space and time.
  • 58:04 - 58:06
    Kip thought about it for a while...
  • 58:06 - 58:10
    ...and then answered with
    about 50 lines of equations...
  • 58:10 - 58:12
    ...which showed that
    a really advanced civilization...
  • 58:12 - 58:18
    ...might establish
    and hold open wormholes...
  • 58:18 - 58:23
    ...which we might think of as tubes
    through the fourth dimension...
  • 58:23 - 58:25
    ...which connect the Earth
    with another place...
  • 58:25 - 58:29
    ...without having to traverse
    the intervening distance.
  • 58:29 - 58:34
    Something like crawling
    through a wormhole in an apple.
  • 58:34 - 58:36
    I was happy with this result...
  • 58:36 - 58:40
    ...and used it as
    a key plot device in Contact.
  • 58:40 - 58:42
    But such wormholes through space...
  • 58:42 - 58:45
    ...would also be time machines,
    it seemed to me.
  • 58:45 - 58:48
    And I used that notion
    in my novel Contact as well.
  • 58:48 - 58:52
    Kip Thorne and his colleagues
    later proved, or so it seemed...
  • 58:52 - 58:55
    ...that time travel
    of this sort was possible.
  • 58:55 - 58:59
    Here, look at this.
  • 58:59 - 59:02
    The key question being explored now...
  • 59:02 - 59:05
    ...is whether such time travel
    can be done consistently...
  • 59:05 - 59:10
    ...with causes preceding effects, say,
    rather than following them.
  • 59:10 - 59:12
    Does nature contrive it...
  • 59:12 - 59:15
    ...so that even with a time machine,
    you can't intervene...
  • 59:15 - 59:18
    ...to prevent your own conception,
    for example?
  • 59:18 - 59:22
    Even if time travel of this sort
    is really possible...
  • 59:22 - 59:25
    ...it's far in
    our technological future.
  • 59:25 - 59:29
    But maybe other beings
    much more advanced than we...
  • 59:29 - 59:32
    ...are voyaging to the far future
    and the remote past...
  • 59:32 - 59:35
    ...not a measly 40 years ago
    on Earth...
  • 59:35 - 59:38
    ...but to witness
    the death of the sun, say...
  • 59:38 -
    ...or the origin of the cosmos.
Title:
Cosmos: A Personal Voyage - Episode 8 (Carl Sagan)
Description:

Episode 8: "Travels in Space and Time"

English, Spanish, and Hebrew subtitles included.

If you like this series, buy it!
- http://www.amazon.com/Cosmos-Carl-Sagan-DVD-Set/dp/B000055ZOB
- http://www.dvdtalk.com/reviews/4774/cosmos-carl-sagan/

Playlist: http://www.youtube.com/playlist?p=PL474A7F1BA0FCEF8C

Content property of Koch Entertainment (Entertainment One) - http://bit.ly/gWINrr

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Duration:
01:01:36
Amara Bot edited English subtitles for Cosmos: A Personal Voyage - Episode 8 (Carl Sagan)
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