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¶ ¶
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NEIL DEGRASSE TYSON:
Seeing is not believing.
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Our senses can deceive us.
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Even the stars are not
what they appear to be.
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The cosmos,
as revealed by science,
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is stranger than
we ever could have imagined.
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Light and time and space
and gravity
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conspire to create realities
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which lie
beyond human experience.
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That's where we're headed.
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Come with me.
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Back in 1802,
on a night like this,
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the astronomer William Herschel
strolled the beach
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on the English coast,
with his son John.
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Herschel was
the first person ever
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to see into the deeper waters
of the cosmic ocean.
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There he glimpsed
the magic trick
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that light does with time.
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Father...
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do you believe in ghosts?
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Why, yes, my son!
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You, you do?
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I would not have thought so.
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Oh, no, not in the
human kind of ghost.
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No... not at all.
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But look up, my boy,
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and see a sky full of them.
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The stars, Father?
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I do not follow.
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Every star is a sun as big,
as bright as our own.
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Just imagine how far away from
us you'd have to move the Sun
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to make it appear as small
and faint as a star.
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The light from the stars
travels very fast...
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faster than anything...
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but not infinitely fast.
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It takes time for their
light to reach us.
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For the nearest ones,
it takes years.
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For others, centuries.
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Some stars are so far away,
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it takes eons for their
light to get to Earth.
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By the time the light
from some stars gets here,
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they are already dead.
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For those stars,
we see only their ghosts.
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We see their light,
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but their bodies perished
long, long ago.
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John, I have seen
further back in time
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than any man before me--
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millions of years
into the past.
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DEGRASSE TYSON:
William Herschel was the first
person to understand
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that a telescope
is a time machine.
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We cannot look out into space
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without seeing back in time.
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In one second, light travels
300,000 kilometers,
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or 186,000 miles.
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That's nearly the distance
from the Earth to the Moon.
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So, the Moon is about
one light-second away.
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The next time
you look at the Moon,
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you'll be seeing
one second into the past.
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¶ ¶
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¶ ¶
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¶ ¶
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¶ ¶
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¶ ¶
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¶ ¶
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DEGRASSE TYSON:
That Sun...
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it's not really there.
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It won't actually be
above the horizon
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for another two minutes.
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The sunrise is an illusion.
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Earth's atmosphere bends
the incoming rays of sunlight
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like a lens
or a glass of water.
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So we see the image of the Sun
projected above the horizon...
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before the physical Sun
is actually there.
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That Sun behind me is a mirage.
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No more real
than the shimmering image
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that hovers in the distance
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over a desert road on a hot day.
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Sunlight takes about
eight minutes to reach Earth,
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so the Sun is eight
light-minutes away.
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From Earth, we can
only ever see the Sun
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as it was eight minutes ago.
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And another thing,
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the Sun doesn't really
"rise" at all.
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The Earth turns
and we turn with it.
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It may not look like it,
but right at this moment,
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I'm moving faster
than a jet plane
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and so are you
and everyone on Earth.
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While I'm at it,
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that horizon...
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it's not really there at all.
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There's no edge.
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The horizon is just
another illusion.
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The distance between Earth
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and the outermost planet Neptune
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varies as the planets
orbit the Sun.
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On average, the light
makes that trip in four hours.
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So for us on Earth,
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the Neptune we see is always
four hours in the past--
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four light-hours away.
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But the distances
to the planets,
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even the farthest one...
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are mere baby steps
on a much grander scale
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of the stars and galaxies.
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As soon as we leave the Sun's
immediate neighborhood,
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we need to change
the unitive distance
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from light-hours to light-years.
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A light-year
is the yardstick of the cosmos.
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A single one is nearly
ten trillion kilometers,
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or about six trillion miles.
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It's a unitive distance,
just like a meter or a mile.
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It's the distance
light travels in a year.
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The nearest star to the Sun,
Proxima Centauri,
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is a little more than four
light-years away from Earth.
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How far away
is four light-years?
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NASA's Voyager spacecraft moves
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at more than
56,000 kilometers an hour.
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Even at that astonishing speed,
it would take Voyager
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more than 80,000 years
to reach the nearest star.
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And the stars of the
Pleiades cluster,
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400 light-years away.
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The Ship of the Imagination
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is equipped with a
highly unusual capability--
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one-of-a-kind, actually.
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It makes it possible for us
to see what was happening
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when the light from a distant
star or galaxy first set out
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on its long journey to Earth.
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¶ ¶
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DEGRASSE TYSON:
When that light left the
Pleiades, about 400 years ago,
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Galileo was taking his first
look through a telescope.
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A few years later, he tried to
measure the speed of light,
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but he couldn't do it.
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He had a very clever plan,
but the technology of that era
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just wasn't good enough to
measure the motion of anything
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that moves as fast as light.
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When we look at
the Crab Nebula from Earth,
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we're seeing much farther
back in time.
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The Crab Nebula
was once a giant star,
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ten times the mass of the Sun,
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until it exploded
in a supernova.
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At its heart is a pulsar,
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a collapsed star
the size of a city,
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spinning 30 times a second.
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This pulsar's
whirling magnetic field
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whips nearby electrons
into a frenzy,
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accelerating them
to almost the speed of light.
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They shine with a blue glow that
lights up the tendrils of gas
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still unraveling
from the supernova.
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The Crab Nebula
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is about 6,500
light-years from Earth.
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According to some beliefs,
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that's the age
of the whole universe.
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But if the universe were only
6,500 years old,
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how could we see the light
from anything more distant
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than the Crab Nebula?
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We couldn't.
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There wouldn't have been
enough time for the light
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to get to Earth
from anywhere farther away
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than 6,500 light-years
in any direction.
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That's just enough time
for light to travel
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through a tiny portion
of our Milky Way galaxy.
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To believe in a universe
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as young as
6,000 or 7,000 years old
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is to extinguish the light
from most of the galaxy,
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not to mention the light
from all the 100 billion
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other galaxies
in the observable universe.
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The center of our own galaxy
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is about 30,000 light-years
from Earth.
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The light we see today
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coming from the core
of the Milky Way left there...
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when our ancestors
were perfecting a way
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to vanquish death...
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...by making art with the power
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to inspire those who would come
long after they were gone.
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¶ ¶
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The light we see
coming from the Sombrero Galaxy
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is 30 million years old.
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Our ancestors
were living in trees
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when that light started out.
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They weighed about five kilos
and had long tails.
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But even 30 million
light-years away
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is still in our own
cosmic backyard.
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That galaxy is part
of the Coma Cluster,
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320 million light-years away.
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What was going on back home
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when the light you are seeing
began its trip to Earth?
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No familiar continents,
oceans or rivers.
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Our distant ancestors were just
leaving the water for the land.
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That's pretty old light,
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but not nearly
the oldest light we can see.
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The oldest light is very faint,
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a pale ghost in the night.
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See that red blob
inside the circle?
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That's one of the oldest
galaxies we've ever seen.
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You're looking at 13.4-billion-
year-old starlight
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as captured by
the Hubble space telescope.
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It's coming from the very first
generation of stars.
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What was happening on Earth
back then?
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Absolutely nothing.
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There was no Earth,
no Sun, no Milky Way.
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They would not come to be
for billions of years.
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When we try to look even farther
into the universe,
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we come to what appears
to be the end of space...
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but actually...
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it's the beginning of time.
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DEGRASSE TYSON:
Earth pulls on us.
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Our lives are a relentless
struggle with gravity.
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That little girl is trying
her best to climb out
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of a gravitational well.
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00:13:10,172 --> 00:13:14,042
From our first efforts to stand
to our final surrender,
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we are struggling to overcome
the Earth's pull.
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We are born, live and die
in a force field--
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one that is almost as old
as the universe itself.
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00:13:26,956 --> 00:13:29,557
And how old is that?
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00:13:29,592 --> 00:13:33,161
To visualize the 13.8 billion
year age of the universe,
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00:13:33,195 --> 00:13:34,729
we've compressed all
of cosmic time
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into a single
year-at-a-glance calendar.
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Midnight on December 31
is this very moment right now.
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And January 1
is the beginning of time.
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See that glowing fog out there?
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It's radiation left over
from the Big Bang,
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the explosion that made
the universe
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00:13:53,716 --> 00:13:57,018
13.8 billion years ago.
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00:13:57,052 --> 00:14:02,957
Right now, we're at the very
edge of known space and time.
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00:14:04,727 --> 00:14:06,928
So what happened before
the Big Bang?
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Nobody knows.
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00:14:08,631 --> 00:14:11,432
No evidence survives
from before that moment.
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00:14:11,467 --> 00:14:13,268
We've got
some pretty crazy ideas
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00:14:13,302 --> 00:14:14,769
about where
the universe came from,
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which we'll get to, in time.
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00:14:17,806 --> 00:14:20,708
Where are we in the universe?
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At the very center.
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00:14:23,846 --> 00:14:27,782
In the observed universe,
everyone gets to feel special.
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00:14:27,816 --> 00:14:30,652
No matter which galaxy
you happen to live in,
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00:14:30,686 --> 00:14:33,788
when you look out to the
universe, you'll find yourself
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at the center
of the cosmic horizon.
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But this is just an illusion.
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In reality, there is no center,
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and the cosmic horizon
is no more real
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than the horizon at sea.
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It's what you get when you have
a finite speed of light
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00:14:50,205 --> 00:14:53,508
in a universe that had
a beginning in time.
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00:14:56,495 --> 00:14:59,480
A few hundred million years
after the Big Bang,
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00:14:59,515 --> 00:15:02,650
vast clouds of hydrogen
and helium condensed
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into the first stars
and galaxies.
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00:15:05,955 --> 00:15:07,488
With these new sources
of light,
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00:15:07,523 --> 00:15:10,525
the long dark ages
of the universe ended.
247
00:15:10,559 --> 00:15:13,094
As space continued to expand,
248
00:15:13,128 --> 00:15:16,831
cosmic evolution unfolded
on grander scales.
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00:15:16,865 --> 00:15:19,167
As the first generation
of stars died,
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00:15:19,201 --> 00:15:22,503
they seeded space
with heavier elements,
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00:15:22,538 --> 00:15:25,173
making possible
the formation of planets,
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and ultimately, life.
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00:15:32,381 --> 00:15:35,883
Matter and energy were formed
in the Big Bang.
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00:15:35,918 --> 00:15:37,352
But that's not all.
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00:15:37,386 --> 00:15:39,837
Space and time
were created, too,
256
00:15:39,888 --> 00:15:42,190
and all the forces
that bind matter together,
257
00:15:42,224 --> 00:15:43,725
including gravity.
258
00:15:43,759 --> 00:15:46,060
Isaac Newton discovered
a mathematical law
259
00:15:46,161 --> 00:15:48,529
that describes
how gravity works.
260
00:15:48,564 --> 00:15:51,866
With that law, he could explain
the motions of the planets.
261
00:15:51,900 --> 00:15:53,851
More than 100 years later,
262
00:15:53,902 --> 00:15:58,106
William Herschel realized
gravity could do much more.
263
00:16:05,080 --> 00:16:07,382
John, can you
keep a secret?
264
00:16:07,416 --> 00:16:09,217
Yes, Father.
265
00:16:09,251 --> 00:16:13,621
I've made a discovery and have
yet to tell another soul.
266
00:16:15,090 --> 00:16:18,259
The gravity that
holds us to the Earth--
267
00:16:18,293 --> 00:16:20,228
the same gravity
that Newton showed
268
00:16:20,262 --> 00:16:22,163
keeps the planets
in their orbits--
269
00:16:22,197 --> 00:16:23,731
I've discovered
270
00:16:23,766 --> 00:16:27,769
that it also rules
the distant stars.
271
00:16:27,803 --> 00:16:32,073
Father... but how
can you know this?
272
00:16:32,091 --> 00:16:34,475
Can you find the
constellation of the Lion?
273
00:16:36,278 --> 00:16:38,613
There.
274
00:16:38,647 --> 00:16:39,947
Well done.
275
00:16:39,949 --> 00:16:42,583
Can you now find the star
276
00:16:42,618 --> 00:16:45,920
that joins the Lion's
head to his body?
277
00:16:45,954 --> 00:16:47,255
That one.
278
00:16:47,289 --> 00:16:50,625
WILLIAM: That star
is really two stars
279
00:16:50,659 --> 00:16:54,629
so close together that
they appear to be one.
280
00:16:54,663 --> 00:16:57,265
I've been watching them
through my telescope
281
00:16:57,299 --> 00:16:59,634
since long before
you were born.
282
00:17:01,637 --> 00:17:04,539
They dance around
each other very slowly.
283
00:17:04,573 --> 00:17:08,826
More slowly than any planet
moves around the Sun.
284
00:17:10,813 --> 00:17:13,448
Many of the stars
we see tonight,
285
00:17:13,482 --> 00:17:15,450
perhaps most of them,
286
00:17:15,484 --> 00:17:18,052
dance with
invisible partners.
287
00:17:18,087 --> 00:17:23,257
Gravity's empire
governs all the heavens.
288
00:17:32,835 --> 00:17:34,168
DEGRASSE TYSON:
A century earlier,
289
00:17:34,203 --> 00:17:36,070
Isaac Newton had been haunted
290
00:17:36,105 --> 00:17:39,140
by the same absence
of a mechanism for gravity.
291
00:17:39,174 --> 00:17:41,743
How could distant bodies
affect each other
292
00:17:41,777 --> 00:17:45,513
across empty space
without actually touching?
293
00:17:45,547 --> 00:17:49,817
This "action at a distance,"
as he called it, baffled him.
294
00:17:51,520 --> 00:17:54,989
In the 19th century,
Michael Faraday discovered
295
00:17:55,023 --> 00:17:58,092
that we were surrounded
by invisible fields of force
296
00:17:58,127 --> 00:18:00,828
that explained
how gravity works.
297
00:18:00,863 --> 00:18:03,431
The apple and the Earth
don't touch each other,
298
00:18:03,433 --> 00:18:06,100
but the fields between them do.
299
00:18:06,135 --> 00:18:09,670
He imagined those lines
of gravitational force
300
00:18:09,705 --> 00:18:12,707
radiating out into space
from every massive body--
301
00:18:12,741 --> 00:18:17,995
the Earth, the Moon,
the Sun, everything.
302
00:18:18,046 --> 00:18:20,882
Here was the answer
to that question
303
00:18:20,916 --> 00:18:23,518
that had stumped Newton.
304
00:18:23,552 --> 00:18:28,322
In 1865, James Clerk Maxwell
translated Faraday's idea
305
00:18:28,324 --> 00:18:30,892
about fields of electricity
and magnetism
306
00:18:30,926 --> 00:18:33,694
into mathematical laws.
307
00:18:33,729 --> 00:18:37,732
He discovered that these fields
move through space in waves.
308
00:18:37,766 --> 00:18:40,368
When he calculated
how fast they move,
309
00:18:40,402 --> 00:18:43,371
it turned out to be
the speed of light.
310
00:18:43,405 --> 00:18:45,373
We were beginning
to discover the threads
311
00:18:45,407 --> 00:18:47,208
of the cosmic tapestry,
312
00:18:47,242 --> 00:18:50,311
but we were not yet able
to discern the rich pattern
313
00:18:50,329 --> 00:18:54,382
that time, light, space
and gravity weave.
314
00:18:54,416 --> 00:18:55,917
As Albert Einstein
worked in Berlin
315
00:18:55,951 --> 00:18:57,752
on his theory of gravity,
316
00:18:57,786 --> 00:19:01,489
he kept the portraits
of these three men before him.
317
00:19:01,523 --> 00:19:04,859
He knew he was standing
on their shoulders.
318
00:19:04,893 --> 00:19:07,845
Years before, as a teenager,
he had an insight
319
00:19:07,896 --> 00:19:11,532
that was as Earth-shaking
as any idea of theirs.
320
00:19:11,567 --> 00:19:13,234
And it happened one summer
321
00:19:13,268 --> 00:19:16,103
while he was daydreaming
in Italy.
322
00:19:20,626 --> 00:19:23,427
In the summer of 1895,
323
00:19:23,462 --> 00:19:26,264
Einstein's father's business
in Germany had failed,
324
00:19:26,298 --> 00:19:28,766
and the family had moved here
to northern Italy.
325
00:19:28,800 --> 00:19:31,602
Young Einstein
loved wandering these roads
326
00:19:31,637 --> 00:19:34,438
and giving his mind
free rein to explore.
327
00:19:34,473 --> 00:19:38,142
There's something timeless
about this place.
328
00:19:39,878 --> 00:19:41,445
Nothing here has really changed
329
00:19:41,463 --> 00:19:44,482
since the time
of Einstein's early daydreams.
330
00:19:49,821 --> 00:19:52,123
One day,
he began to think about light
331
00:19:52,157 --> 00:19:54,625
and how fast it travels.
332
00:19:54,660 --> 00:19:56,861
In everyday life,
we always measure the speed
333
00:19:56,895 --> 00:19:59,564
of a moving object with respect
to something else.
334
00:20:05,938 --> 00:20:09,740
For example, I'm moving
about ten kilometers per hour
335
00:20:09,775 --> 00:20:12,176
relative to the ground.
336
00:20:12,211 --> 00:20:15,246
But as I mentioned earlier,
the ground is moving.
337
00:20:15,280 --> 00:20:19,650
Earth is turning at more than
1,600 kilometers per hour
338
00:20:19,685 --> 00:20:21,402
while it orbits the Sun
339
00:20:21,453 --> 00:20:24,255
at more than 100,000
kilometers per hour.
340
00:20:24,289 --> 00:20:26,657
And the Sun is moving
through the galaxy
341
00:20:26,692 --> 00:20:29,727
at a half a million
miles per hour.
342
00:20:29,761 --> 00:20:31,929
And the Milky Way is moving
through the universe
343
00:20:31,964 --> 00:20:35,032
at nearly one and a half
million miles an hour.
344
00:20:35,067 --> 00:20:38,202
There is no fixed place
in the cosmos.
345
00:20:38,237 --> 00:20:40,605
All of nature is in motion.
346
00:20:42,074 --> 00:20:44,175
It was hard even for
the young Einstein
347
00:20:44,209 --> 00:20:46,277
to imagine
some absolute standard
348
00:20:46,311 --> 00:20:49,247
to measure all those relative
motions against.
349
00:21:03,762 --> 00:21:05,363
This is the very book
350
00:21:05,397 --> 00:21:07,798
that inspired Einstein
as a young boy.
351
00:21:09,434 --> 00:21:12,570
Give a kid a book
and you change the world.
352
00:21:12,604 --> 00:21:14,905
In a way, even the universe.
353
00:21:14,940 --> 00:21:18,476
Look at this--
the very first page,
354
00:21:18,510 --> 00:21:20,378
it describes
the astonishing speed
355
00:21:20,412 --> 00:21:22,146
of electricity through wires
356
00:21:22,180 --> 00:21:24,749
and light through space.
357
00:21:24,783 --> 00:21:26,951
Einstein remembered
what he'd learned as a child
358
00:21:26,985 --> 00:21:28,386
from this book,
359
00:21:28,420 --> 00:21:31,822
and perhaps,
for the first time, right here,
360
00:21:31,857 --> 00:21:33,491
wondered what the world
would look like
361
00:21:33,525 --> 00:21:36,460
if you could travel
at the speed of light.
362
00:21:41,266 --> 00:21:42,733
The more Einstein thought
about it,
363
00:21:42,801 --> 00:21:44,635
the more troubled he became.
364
00:21:44,670 --> 00:21:47,972
If you imagine traveling
at the speed of light,
365
00:21:48,006 --> 00:21:51,842
paradoxes seem
to pop up everywhere.
366
00:21:51,877 --> 00:21:54,645
Einstein was shocked
to realize that so much
367
00:21:54,680 --> 00:21:56,681
of what had been
uncritically accepted as truth
368
00:21:56,715 --> 00:21:59,250
by even the greatest
authorities on the subject
369
00:21:59,284 --> 00:22:00,851
was just plain wrong.
370
00:22:02,521 --> 00:22:04,855
When traveling at high speeds,
371
00:22:04,890 --> 00:22:07,825
there are certain rules
which must be obeyed.
372
00:22:07,859 --> 00:22:10,594
Einstein called these rules
"The Principles of Relativity."
373
00:22:10,629 --> 00:22:13,330
Imagine that young woman
who just blew past us
374
00:22:13,332 --> 00:22:14,765
on the motorbike,
375
00:22:14,800 --> 00:22:16,267
imagine she was riding her bike
376
00:22:16,269 --> 00:22:19,303
through the cosmos.
377
00:22:19,338 --> 00:22:21,939
Light from
a moving object travels
378
00:22:21,973 --> 00:22:23,941
at the same speed,
no matter whether the object
379
00:22:23,975 --> 00:22:26,844
is at rest or in motion.
380
00:22:26,878 --> 00:22:29,780
Her speed is not added
to the speed of light.
381
00:22:29,815 --> 00:22:31,615
The light from her motorbike
382
00:22:31,650 --> 00:22:34,218
still travels
at the speed of light.
383
00:22:35,854 --> 00:22:37,621
Nature commands,
384
00:22:37,656 --> 00:22:41,459
"Thou shalt not add my speed
to the speed of light."
385
00:22:41,493 --> 00:22:44,028
Also, no material object
386
00:22:44,062 --> 00:22:46,630
can travel at or faster
than the speed of light.
387
00:22:46,665 --> 00:22:48,733
There's nothing in physics
that prevents you
388
00:22:48,767 --> 00:22:51,335
from traveling as close to
the speed of light as you like.
389
00:22:51,370 --> 00:22:55,005
99.9% of the speed of light
is just fine,
390
00:22:55,040 --> 00:22:57,475
but no matter how hard you try,
391
00:22:57,509 --> 00:22:59,977
you never gain
that last decimal point.
392
00:23:00,011 --> 00:23:02,813
For reality to be
logically consistent,
393
00:23:02,848 --> 00:23:05,416
there must be
a cosmic speed limit.
394
00:23:07,052 --> 00:23:09,186
(thumps, horse whinnies)
395
00:23:10,522 --> 00:23:12,656
(whip cracks)
396
00:23:12,691 --> 00:23:15,159
The crack of that whip
is due to its tip
397
00:23:15,193 --> 00:23:17,161
moving faster
than the speed of sound.
398
00:23:17,195 --> 00:23:18,996
It makes a shockwave,
399
00:23:19,030 --> 00:23:21,382
a mini sonic boom,
in the Italian countryside.
400
00:23:24,236 --> 00:23:26,404
A thunderclap works
the same way,
401
00:23:26,438 --> 00:23:29,540
and so does the sound
of a passing supersonic jet.
402
00:23:29,574 --> 00:23:31,876
So why is the speed of light
403
00:23:31,910 --> 00:23:34,044
any more a barrier
than the speed of sound?
404
00:23:34,079 --> 00:23:36,781
The answer is not just
that light travels
405
00:23:36,815 --> 00:23:39,016
about a million times
faster than sound.
406
00:23:39,050 --> 00:23:41,452
And it's not merely
an engineering problem,
407
00:23:41,486 --> 00:23:43,020
like building
the first supersonic jet.
408
00:23:43,054 --> 00:23:45,423
Instead, the light barrier
409
00:23:45,457 --> 00:23:47,358
is a fundamental law of nature,
410
00:23:47,392 --> 00:23:49,226
as basic as gravity.
411
00:23:49,261 --> 00:23:52,530
Einstein found his absolute
framework for the world,
412
00:23:52,564 --> 00:23:55,599
this sturdy pillar
among all the relative motions
413
00:23:55,634 --> 00:23:57,301
within the motions
of the cosmos.
414
00:23:57,335 --> 00:23:59,136
Light travels just as fast,
415
00:23:59,138 --> 00:24:03,274
no matter how fast or slow
its source is moving.
416
00:24:03,308 --> 00:24:07,278
Speed of light is constant,
relative to everything else.
417
00:24:07,312 --> 00:24:09,580
Nothing can
ever catch up with light.
418
00:24:11,783 --> 00:24:13,884
The thing
about the laws of nature
419
00:24:13,919 --> 00:24:15,786
is that they're unbreakable.
420
00:24:15,821 --> 00:24:18,789
The job of physicists is
to discover these commandments,
421
00:24:18,824 --> 00:24:21,759
the ones that do not vary
from culture to culture
422
00:24:21,793 --> 00:24:23,427
or time to time
423
00:24:23,462 --> 00:24:25,563
and hold true
throughout the cosmos.
424
00:24:25,597 --> 00:24:28,666
That's why, as Einstein showed,
425
00:24:28,700 --> 00:24:32,336
funny things happen
close to the speed of light.
426
00:24:36,475 --> 00:24:38,442
Traveling close
to the speed of light
427
00:24:38,460 --> 00:24:41,011
is kind of an elixir of life
428
00:24:41,013 --> 00:24:44,014
because your biological clock
slows down
429
00:24:44,016 --> 00:24:46,183
relative to those
you leave behind.
430
00:24:46,218 --> 00:24:48,853
This phenomenon
may provide us humans,
431
00:24:48,887 --> 00:24:50,921
who only live
for a century or so,
432
00:24:50,956 --> 00:24:53,357
a practical means
to travel to the stars,
433
00:24:53,391 --> 00:24:55,526
where the magic show
of spacetime
434
00:24:55,560 --> 00:24:57,962
really gets crazy.
435
00:25:06,571 --> 00:25:09,473
DEGRASSE TYSON:
The 19th-century astronomer
William Herschel
436
00:25:09,508 --> 00:25:11,775
loved to share the wonders
of the universe
437
00:25:11,810 --> 00:25:13,811
with his son John.
438
00:25:24,656 --> 00:25:28,058
I once had a friend,
very clever fellow,
439
00:25:28,093 --> 00:25:30,227
an astronomer and
a parson at Leeds,
440
00:25:30,262 --> 00:25:32,963
by the name of John Michell.
441
00:25:32,998 --> 00:25:35,566
Poor man died when
you were a babe,
442
00:25:35,600 --> 00:25:37,501
God rest his soul.
443
00:25:37,536 --> 00:25:39,570
He held that some stars
444
00:25:39,604 --> 00:25:41,572
are invisible.
445
00:25:41,606 --> 00:25:44,909
They really exist,
but we shall never see them.
446
00:25:44,943 --> 00:25:48,279
"Dark stars,"
Michell called them.
447
00:25:50,048 --> 00:25:52,149
With all due respect, Father,
448
00:25:52,183 --> 00:25:54,752
surely your friend was mistaken.
449
00:25:54,786 --> 00:25:56,420
If no one can see them,
450
00:25:56,454 --> 00:25:59,223
then how can we possibly know
they exist?
451
00:26:00,792 --> 00:26:04,128
Did you see the man
who left those footprints, John?
452
00:26:05,297 --> 00:26:06,764
Why, no, Father.
453
00:26:06,831 --> 00:26:07,765
I did not.
454
00:26:07,799 --> 00:26:10,634
But do you know that he exists?
455
00:26:27,652 --> 00:26:30,521
DEGRASSE TYSON:
John Michell is one
of the greatest scientists
456
00:26:30,555 --> 00:26:32,890
you've probably never heard of.
457
00:26:32,924 --> 00:26:35,526
He lived and worked in England
in the 18th century.
458
00:26:35,560 --> 00:26:39,163
If he ever sat for a portrait,
it no longer exists.
459
00:26:39,247 --> 00:26:41,732
He was once described
by an acquaintance
460
00:26:41,766 --> 00:26:43,400
as "a short little man,
461
00:26:43,435 --> 00:26:46,971
of black complexion, and fat."
462
00:26:47,005 --> 00:26:49,740
Michell imagined a star so big,
463
00:26:49,774 --> 00:26:53,243
so massive,
that nothing, not even light,
464
00:26:53,278 --> 00:26:55,813
could escape
its gravitational grip.
465
00:26:55,847 --> 00:26:57,748
Can you find the dark star?
466
00:26:57,782 --> 00:27:00,651
You can't see it
with your eyes, not directly,
467
00:27:00,685 --> 00:27:03,153
but it may leave
a kind of footprint
468
00:27:03,188 --> 00:27:05,155
on the cosmic shore.
469
00:27:05,190 --> 00:27:07,591
Michell realized that we might
be able to detect
470
00:27:07,626 --> 00:27:10,978
some of these dark stars because
of their extreme gravity.
471
00:27:11,029 --> 00:27:12,396
If one happened to be near
472
00:27:12,430 --> 00:27:14,665
a smaller,
luminous companion star,
473
00:27:14,699 --> 00:27:17,735
that star would appear
to travel in a tight orbit
474
00:27:17,769 --> 00:27:20,604
around nothing.
475
00:27:20,639 --> 00:27:22,106
Even though we can't see it,
476
00:27:22,140 --> 00:27:23,841
we know something
with a lot of mass
477
00:27:23,843 --> 00:27:25,509
has to be right there.
478
00:27:25,543 --> 00:27:26,994
A dark star,
479
00:27:27,045 --> 00:27:30,280
or what today we call
a black hole.
480
00:27:31,983 --> 00:27:33,684
What does a black hole look like
481
00:27:33,718 --> 00:27:35,853
and what would it
be like inside?
482
00:27:35,887 --> 00:27:38,188
We'll get there, but first,
483
00:27:38,223 --> 00:27:41,525
let's make a pit stop
in my hometown,
484
00:27:41,559 --> 00:27:43,761
New York City,
485
00:27:43,795 --> 00:27:45,129
where it's always seemed to me
486
00:27:45,163 --> 00:27:48,198
that everything
is in constant motion.
487
00:27:48,233 --> 00:27:50,934
I've lived here most
of my life.
488
00:27:50,969 --> 00:27:53,037
There's always
something new to see.
489
00:27:53,071 --> 00:27:55,873
But one thing never changes--
gravity.
490
00:27:55,907 --> 00:27:57,941
Gravity on Earth
has been the same
491
00:27:57,976 --> 00:28:00,377
for the past
four and a half billion years.
492
00:28:00,412 --> 00:28:03,547
But what if, today,
we could alter it?
493
00:28:03,581 --> 00:28:05,382
Gravity is a distortion
494
00:28:05,417 --> 00:28:07,384
in the shape of spacetime
495
00:28:07,419 --> 00:28:09,386
as Einstein showed.
496
00:28:09,421 --> 00:28:11,288
Space can expand and contract
497
00:28:11,322 --> 00:28:12,756
and warp without limit.
498
00:28:18,430 --> 00:28:20,731
If the Earth's size or density
499
00:28:20,765 --> 00:28:22,232
were even a little different,
500
00:28:22,267 --> 00:28:24,068
its gravity would be, too.
501
00:28:24,102 --> 00:28:26,136
There's an infinite range
of possibilities.
502
00:28:26,171 --> 00:28:28,005
New Yorkers feel right at home
503
00:28:28,039 --> 00:28:29,573
in the gravitational pull
of the Earth,
504
00:28:29,607 --> 00:28:32,109
called "one g."
505
00:28:36,014 --> 00:28:40,384
Suppose we turn off the gravity
on one of its streets.
506
00:28:45,790 --> 00:28:48,692
People and objects
that were already in motion
507
00:28:48,727 --> 00:28:50,861
are launched into flight.
508
00:28:59,904 --> 00:29:02,272
Now what if I turn
the gravity up
509
00:29:02,307 --> 00:29:04,842
to, say, eight or nine g's?
510
00:29:04,876 --> 00:29:06,376
Out of compassion,
511
00:29:06,411 --> 00:29:08,879
let's evacuate the area.
512
00:29:10,048 --> 00:29:11,782
This is about the same g-force
513
00:29:11,816 --> 00:29:14,551
that a fighter pilot in
a high-speed turn would feel.
514
00:29:14,586 --> 00:29:16,353
A few minutes of this
wouldn't hurt you,
515
00:29:16,387 --> 00:29:18,956
but it wouldn't be comfortable.
516
00:29:18,990 --> 00:29:21,625
At 100,000 g's,
517
00:29:21,659 --> 00:29:23,560
even fire hydrants
become crushed
518
00:29:23,595 --> 00:29:26,196
by their own enormous weight.
519
00:29:26,231 --> 00:29:27,965
But at millions of g's,
520
00:29:27,967 --> 00:29:30,701
even light bows to gravity.
521
00:29:30,735 --> 00:29:33,303
The light still moves
at its constant speed,
522
00:29:33,338 --> 00:29:35,339
but it cannot escape.
523
00:29:36,341 --> 00:29:38,575
Michell's dark star...
524
00:29:38,610 --> 00:29:40,811
our black hole.
525
00:29:40,845 --> 00:29:44,414
And the nearest one
may be closer than you think.
526
00:29:52,106 --> 00:29:54,741
Not every star can
become a black hole.
527
00:29:54,776 --> 00:29:57,077
Only about one in 1,000
is massive enough.
528
00:29:57,111 --> 00:30:00,647
The nearest one could be within
100 light-years of Earth.
529
00:30:00,682 --> 00:30:03,750
Black holes aren't the mythic,
cosmic vacuum cleaners
530
00:30:03,785 --> 00:30:05,486
of science fiction.
531
00:30:05,520 --> 00:30:08,322
They don't go around gobbling up
unsuspecting worlds.
532
00:30:08,356 --> 00:30:10,157
You've got to come to them.
533
00:30:10,191 --> 00:30:11,758
But if you do,
534
00:30:11,793 --> 00:30:14,328
it might be the last thing
you ever see.
535
00:30:14,362 --> 00:30:17,364
(rumbling)
536
00:30:17,431 --> 00:30:21,101
That was us resisting
a few million g's of gravity.
537
00:30:21,135 --> 00:30:24,538
Don't forget,
that thing swallows light.
538
00:30:24,572 --> 00:30:26,573
We'll keep our distance.
539
00:30:29,911 --> 00:30:32,713
When giant stars exhaust
their nuclear fuel,
540
00:30:32,747 --> 00:30:34,381
they can no longer
stay hot enough
541
00:30:34,415 --> 00:30:37,184
to fend off the inward pull
of their own gravity.
542
00:30:37,218 --> 00:30:40,954
The most massive stars
collapse into darkness,
543
00:30:40,989 --> 00:30:43,123
leaving only
their gravity behind.
544
00:30:43,157 --> 00:30:46,627
This black hole enshrouds
the shrunken corpse
545
00:30:46,661 --> 00:30:48,395
of a supergiant star.
546
00:30:48,429 --> 00:30:50,531
The star itself has shriveled
into something
547
00:30:50,565 --> 00:30:52,766
even smaller
than this darkness,
548
00:30:52,768 --> 00:30:55,903
only 64 kilometers wide.
549
00:30:57,755 --> 00:31:00,974
This is the first black hole
ever discovered--
550
00:31:01,009 --> 00:31:03,243
Cygnus X-1.
551
00:31:03,278 --> 00:31:05,646
How did we on Earth
ever find something
552
00:31:05,680 --> 00:31:08,715
so small and dark and far away?
553
00:31:08,750 --> 00:31:14,054
We looked at it in another
kind of light: X-rays.
554
00:31:14,088 --> 00:31:16,823
In X-ray light, we lost sight
of the blue star
555
00:31:16,858 --> 00:31:20,494
because its surface
is a tepid 30,000 degrees.
556
00:31:20,528 --> 00:31:23,096
But the disk of gas around
the black hole
557
00:31:23,131 --> 00:31:27,935
glowed brilliantly in X-rays
at 100 million degrees.
558
00:31:27,969 --> 00:31:29,770
As William Herschel discovered,
559
00:31:29,804 --> 00:31:34,508
many stars have close companions
forming a binary star system.
560
00:31:34,542 --> 00:31:36,777
But if one member of
such a pair is enormous
561
00:31:36,811 --> 00:31:38,745
and the other is compact,
562
00:31:38,780 --> 00:31:42,249
the smaller star can drain
and consume the atmosphere
563
00:31:42,283 --> 00:31:44,685
of its larger sibling.
564
00:31:44,719 --> 00:31:48,956
This neurotic relationship
can last for millions of years.
565
00:31:48,990 --> 00:31:52,960
The atmosphere of the larger
star was being siphoned onto
566
00:31:52,994 --> 00:31:55,796
a glowing hot accretion disk
that revolves around
567
00:31:55,830 --> 00:31:58,865
and spirals into a black hole.
568
00:31:58,900 --> 00:32:02,469
The overwhelming gravity was
accelerating the blue star's gas
569
00:32:02,503 --> 00:32:03,937
into a death spiral,
570
00:32:03,972 --> 00:32:06,707
crossing
the spacetime boundary,
571
00:32:06,741 --> 00:32:08,308
never to be seen again.
572
00:32:08,326 --> 00:32:11,778
The fateful boundary
that separates a black hole
573
00:32:11,780 --> 00:32:15,282
from the rest of the universe
is called an event horizon.
574
00:32:15,316 --> 00:32:18,135
From our point of view,
the substance in the disk
575
00:32:18,186 --> 00:32:20,487
slows down as it approaches
the event horizon,
576
00:32:20,521 --> 00:32:22,956
never quite reaching it.
577
00:32:22,991 --> 00:32:25,559
But if you were riding
on that spiraling gas--
578
00:32:25,593 --> 00:32:27,394
and I don't advise it--
579
00:32:27,428 --> 00:32:30,397
you would sail past the event
horizon in a matter of seconds
580
00:32:30,431 --> 00:32:35,435
into the undiscovered country
from which no traveler returns.
581
00:32:44,612 --> 00:32:47,481
We have searched the hearts
of dozens of galaxies,
582
00:32:47,515 --> 00:32:51,585
and in every case, we have found
a super-massive black hole.
583
00:32:51,686 --> 00:32:56,089
Our own galaxy is no exception.
584
00:32:56,124 --> 00:33:00,027
The stars nearest the center
of our galaxy whip around
585
00:33:00,061 --> 00:33:02,729
at more than 40 million
kilometers an hour.
586
00:33:05,299 --> 00:33:08,085
What could make them move
so fast?
587
00:33:08,136 --> 00:33:09,836
The only logical explanation
588
00:33:09,871 --> 00:33:12,205
is that something with the mass
589
00:33:12,240 --> 00:33:16,710
of four million suns
lies at the center.
590
00:33:16,744 --> 00:33:20,213
So where's the blazing light
of four million suns?
591
00:33:20,248 --> 00:33:21,682
Since we can't see it,
592
00:33:21,716 --> 00:33:24,484
it must be imprisoned
inside a black hole.
593
00:33:30,925 --> 00:33:34,127
Earth is far enough away
to be perfectly safe.
594
00:33:34,162 --> 00:33:37,330
Other worlds might not be
so lucky.
595
00:33:39,667 --> 00:33:42,302
If you somehow survived
the perilous journey
596
00:33:42,336 --> 00:33:44,137
across the event horizon,
597
00:33:44,172 --> 00:33:45,706
you'd be able to look back out
598
00:33:45,740 --> 00:33:48,642
and see the entire future
history of the universe
599
00:33:48,726 --> 00:33:50,744
unfold before your eyes.
600
00:33:53,781 --> 00:33:55,315
How?
601
00:33:55,349 --> 00:33:57,084
Because when
spacetime is warped
602
00:33:57,118 --> 00:33:59,486
by the extreme gravity
of a black hole,
603
00:33:59,553 --> 00:34:02,255
time is stretched to the limit.
604
00:34:05,860 --> 00:34:08,161
But what would be
in front of you?
605
00:34:08,196 --> 00:34:10,831
Before we go there,
I should warn you
606
00:34:10,865 --> 00:34:13,667
that we're entering uncharted
scientific territory.
607
00:34:13,701 --> 00:34:17,604
For all we know, there may be
undiscovered laws of physics
608
00:34:17,638 --> 00:34:20,207
that govern events at the center
of a black hole.
609
00:34:22,360 --> 00:34:24,761
But until the next Einstein
comes along,
610
00:34:24,812 --> 00:34:27,380
let's perform
a thought experiment.
611
00:34:29,784 --> 00:34:32,018
That's how John Michell
first imagined dark stars
612
00:34:32,086 --> 00:34:34,187
in the 18th century,
613
00:34:34,222 --> 00:34:37,557
(voice distorting):
and how Einstein conceived
of his theory of rela...
614
00:35:00,648 --> 00:35:02,649
(rumbling, whooshing)
615
00:35:29,677 --> 00:35:33,079
JOHN HERSCHEL: Father, do you
believe in ghosts?
616
00:35:33,114 --> 00:35:36,316
WILLIAM HERSCHEL: Oh, no, not in
the human kind of ghosts.
617
00:35:36,350 --> 00:35:37,918
No, not at all.
618
00:35:37,952 --> 00:35:41,087
But look up, my boy,
619
00:35:41,122 --> 00:35:44,191
and see a sky full of them.
620
00:35:46,394 --> 00:35:49,429
DEGRASSE TYSON:
If you could survive the trip
into a black hole,
621
00:35:49,464 --> 00:35:50,997
you might emerge
in another place
622
00:35:51,032 --> 00:35:53,500
and time in our own universe,
623
00:35:53,534 --> 00:35:56,670
circumventing the first
commandment of relativity:
624
00:35:56,687 --> 00:35:59,639
thou shalt not travel
faster than light.
625
00:36:01,309 --> 00:36:04,544
Nothing can move through space
faster than light.
626
00:36:04,579 --> 00:36:07,013
But space is not
mere emptiness.
627
00:36:07,048 --> 00:36:11,618
Its properties can stretch
and shrink and can be deformed.
628
00:36:11,652 --> 00:36:15,889
And when that happens,
time is deformed, too.
629
00:36:19,060 --> 00:36:22,796
Einstein discovered that space
and time are just two aspects
630
00:36:22,830 --> 00:36:26,032
of the same thing: spacetime.
631
00:36:26,067 --> 00:36:28,635
Spacetime itself can
deform enough
632
00:36:28,669 --> 00:36:31,972
to carry you anywhere
at any speed.
633
00:36:32,006 --> 00:36:35,942
Black holes may very well
be tunnels through the universe.
634
00:36:50,791 --> 00:36:54,561
On this intergalactic subway
system, you could travel
635
00:36:54,595 --> 00:36:56,663
to the farthest reaches
of spacetime,
636
00:36:56,697 --> 00:37:00,100
or you might arrive in
someplace even more amazing.
637
00:37:02,703 --> 00:37:06,172
We might find ourselves in an
altogether different universe.
638
00:37:06,207 --> 00:37:09,009
But how can a whole universe fit
inside of a black hole,
639
00:37:09,043 --> 00:37:13,747
which is only a small part
of our universe?
640
00:37:13,781 --> 00:37:16,850
It's another magic trick
of spacetime.
641
00:37:16,884 --> 00:37:19,786
The phenomenal gravity
of a black hole
642
00:37:19,820 --> 00:37:23,890
can warp the space of
an entire universe inside it.
643
00:37:32,066 --> 00:37:34,434
Our local gravity might be
a drag to us,
644
00:37:34,468 --> 00:37:36,136
but it's really feeble compared
645
00:37:36,170 --> 00:37:38,605
with what goes on inside
a collapsed star.
646
00:37:38,639 --> 00:37:40,707
As far as we know,
647
00:37:40,741 --> 00:37:44,444
when a giant star collapses
to make a black hole,
648
00:37:44,478 --> 00:37:46,212
the extreme density
and pressure at the center
649
00:37:46,247 --> 00:37:50,984
mimic the Big Bang, which gave
rise to our universe.
650
00:37:51,018 --> 00:37:52,719
And a universe inside
a black hole
651
00:37:52,753 --> 00:37:55,288
might give rise
to its own black holes.
652
00:37:55,323 --> 00:37:57,857
And those could lead
to other universes.
653
00:38:01,095 --> 00:38:05,999
Maybe that's how
our cosmos came to be.
654
00:38:17,678 --> 00:38:19,479
For all we know,
655
00:38:19,513 --> 00:38:23,750
if you want to see what it's
like inside a black hole,
656
00:38:23,784 --> 00:38:26,052
just look around you.
657
00:38:30,691 --> 00:38:33,359
William Herschel went on
to discover that the sun
658
00:38:33,361 --> 00:38:36,830
and its planets are moving
through the Milky Way.
659
00:38:36,864 --> 00:38:39,432
And whatever became
of his son John?
660
00:38:39,467 --> 00:38:42,435
He grew up to become
a great scientist.
661
00:38:42,470 --> 00:38:46,106
His deep-space observations
built on those of his father
662
00:38:46,140 --> 00:38:48,775
to become the basis for the
standard catalog of galaxies
663
00:38:48,809 --> 00:38:50,710
we use today.
664
00:38:50,745 --> 00:38:53,947
When William was in failing
health, John stayed with him
665
00:38:53,981 --> 00:38:55,949
through the long nights
at his telescope
666
00:38:55,983 --> 00:38:57,784
to help him sweep the stars.
667
00:38:57,818 --> 00:39:02,789
And when his father died,
John wrote his epitaph:
668
00:39:02,823 --> 00:39:06,159
"He broke through
the walls of heaven."
669
00:39:17,505 --> 00:39:19,139
John often reminisced
670
00:39:19,173 --> 00:39:21,207
about those summer nights
with his father.
671
00:39:21,242 --> 00:39:25,845
Maybe that's why he sought
a way to preserve the past.
672
00:39:27,415 --> 00:39:29,149
John Herschel was one
of the founders
673
00:39:29,183 --> 00:39:31,217
of a new form of time travel,
674
00:39:31,252 --> 00:39:35,088
a means to capture
light and memories.
675
00:39:35,106 --> 00:39:37,557
He actually coined
a word for it:
676
00:39:37,591 --> 00:39:40,026
photography.
677
00:39:44,865 --> 00:39:46,399
When you think about it,
678
00:39:46,434 --> 00:39:49,235
photography is a form
of time travel.
679
00:39:49,270 --> 00:39:52,739
This man is staring at us
from across the centuries...
680
00:39:52,773 --> 00:39:55,175
a ghost preserved by light.
681
00:39:55,209 --> 00:39:58,011
It's not hard to imagine
that in the near future,
682
00:39:58,045 --> 00:39:59,779
we'll be able
to capture the past
683
00:39:59,814 --> 00:40:02,015
in all three dimensions.
684
00:40:02,049 --> 00:40:05,385
We'll be able
to step inside a memory.
685
00:40:10,124 --> 00:40:12,859
It may not be possible
to travel backward in time,
686
00:40:12,960 --> 00:40:16,896
but perhaps, one day,
we can bring the past to us.
687
00:40:18,499 --> 00:40:21,701
Here's a moment from my past.
688
00:40:21,736 --> 00:40:23,203
Like John Herschel,
689
00:40:23,237 --> 00:40:25,972
I'm remembering
a younger version of myself.
690
00:40:26,073 --> 00:40:28,575
December 20, 1975.
691
00:40:28,676 --> 00:40:31,144
A snowy day in Ithaca, New York.
692
00:40:31,178 --> 00:40:34,447
A branchpoint on the road
that brought me
693
00:40:34,482 --> 00:40:37,383
to this moment with you.
694
00:40:37,418 --> 00:40:40,186
It was the day
I met Carl Sagan.
695
00:40:42,022 --> 00:40:44,824
Reminds me of those
ghost stars in the sky...
696
00:40:47,695 --> 00:40:51,397
...you know, the ones that still
shine their light upon us
697
00:40:51,432 --> 00:40:53,666
long after they're gone.
698
00:41:10,050 --> 00:40:53,666
Captioned by
Media Access Group at WGBH
access.wgbh.org