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First there was light,
visible light.
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Then, we viewed the universe
in radio waves and X-rays.
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00:00:12,930 --> 00:00:15,500
Ever since
there's been astronomy,
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we've been looking at
different kinds of light
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and opening up the universe
a little bit more of the time.
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But then in 2015, like,
the roof came off.
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Something happened
that changed everything,
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the ability to see waves
in space and time itself.
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Gravitational waves.
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They help us roll back
the clock to the dawn of time,
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00:00:41,390 --> 00:00:44,890
discover epic cosmic collisions,
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on make Earth-shaking
discoveries.
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Gravitational waves
are the biggest game changer
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since the invention of
the telescope.
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We have a completely new
universe to view now.
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A new exploration of
space is just beginning.
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Long ago, 17 billion
light-years away,
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a cataclysmic showdown
plays out.
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Two black holes locked together
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in a deadly cosmic dance.
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Black holes are unimaginably
dense objects with gravity
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so intense that if you get
too close to them,
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you're gone.
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Their immense
gravitational pull causes
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them to spiral towards
each other.
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When black holes
collide, they don't just run
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into each other.
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They're in orbit about
each other.
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So what we're talking about is
an inspiralling orbit
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that goes faster and faster
and faster and faster.
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Until they finally
collide in a fatal embrace.
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But astronomers
don't see a thing.
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The problem with observing
colliding black holes is all
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about the name, black holes,
they give off no light.
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How can astronomers see
something that
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no telescope can detect?
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Across the universe,
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extraordinary events take place.
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But we sometimes miss them,
because we rely on light.
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Now, astronomers have
a new toolkit that's
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revealing the cosmos in
a totally different way...
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...using the very fabric of
our universe
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we call spacetime.
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Everything with mass,
like stars,
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planets, and black holes,
all curve this fabric.
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The more massive the object,
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the bigger the distortion
of spacetime.
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The classical analogy is
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this stretched rubber
sheet, right?
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And, like, a mass, like, the sun
is, like, a ball on this sheet,
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and it distorts and warps
the sheet
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into this valley, right?
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And if you roll a marble
across it like the marble is
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a planet, the marble will be
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pulled into orbit
around the ball because
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of the curvature of the sheet.
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But that's only half
the picture.
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If an object has mass and is
accelerating through
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spacetime, it creates ripples
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in that fabric of spacetime,
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and we call these
gravitational waves.
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Gravitational waves
give us vital clues
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about distant objects
that we can't see.
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The more massive the object that
produces them and the faster
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it's moving,
the bigger the ripples.
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These ripples pass through
planets, stars, and galaxies
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with ease.
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00:04:04,730 --> 00:04:07,930
When a gravitational wave
passes through an object like
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a star or a planet or a person,
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it stretches
and compresses them,
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like with this tennis ball.
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Now, if you're close to
a powerful source of
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gravitational waves,
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like merging supermassive
black holes,
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those waves are incredibly
strong, and they're capable of
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actually destroying a planet.
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But like the ripples on a pond,
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their strength and size
diminishes over distance.
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The farther away you are,
the weaker they get.
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And when they're hundreds of
millions of light-years away,
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they're actually smaller
than the size of an atom.
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So, to listen
for gravitational waves,
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scientists built the most
sensitive measuring device on
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the planet.
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This is LIGO,
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the Laser Interferometer
Gravitational-Wave Observatory,
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two enormous detectors
located almost 2,000 miles
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apart in Louisiana
and Washington state.
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Each sensor has L-shaped arms,
measuring 2.5 miles.
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Inside the LIGO detectors,
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inside these concrete tunnels,
there is a laser system.
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It's called an interferometer,
so light comes in from
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a laser beam and is split
into two paths.
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Normally, the lengths of
the two beams are the same.
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That changes when
gravitational waves
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hit the beams.
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When a gravitational wave
passes through,
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it changes the distance that
light travels along these arms,
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so one arm effectively gets
longer, and the other one
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gets shorter.
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The length of those two beams
varies just ever so slightly,
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and the very sensitive apparatus
in LIGO is able to pick that up.
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With this ultra-sensitive
laser system,
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LIGO picks up distortions in
spacetime, narrower than
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one millionth of the diameter
of an atom.
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Just that feat,
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just the fact that we were
able to build
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a detector to detect
gravitational waves
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is just mind-boggling.
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All of a sudden now,
we were listening
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to the faintest whispers
of the universe.
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In 2015, LIGO picked up
a whisper that had
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been traveling towards Earth
for over a billion years.
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Its source? Two colliding
stellar black holes.
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Watching two black holes
spiral in and merge...
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That's not something
we can do using optical
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telescopes or X-ray telescopes
or anything like that.
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But with LIGO, we could
actually detect that event.
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Now, scientists can paint
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accurate pictures of
invisible objects.
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You can tell you're looking
at black holes.
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You can get their masses,
you can get their distance.
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There's a phenomenal amount of
information in that wave.
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The colliding black holes are
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the most massive LIGO has
ever detected.
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One is 66 times
the mass of our sun,
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the other, 85 times
the mass of our sun.
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As two black holes
are spiraling in,
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they are moving faster
and faster
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as they get closer and closer.
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That means that
the gravitational waves
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they're emitting have
a higher and higher frequency.
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So as time goes on,
the pitch gets higher.
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- So it goes...
- ooop!
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Ooop!
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Zhhhrp!
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When they finally merge,
they create a giant.
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By analyzing that data,
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it's possible to establish
that the new black hole from
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the merger of these two original
black holes weighs as much as
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something like 140 times
the mass of our sun.
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It's really difficult
to overstate
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the importance of
gravitational wave detection.
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It's like adding on
an entirely new sense...
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All of a sudden,
there's a brand-new way
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to explore the rest
of the universe.
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Invisible cosmic
collisions are just
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the beginning of what
gravitational wave
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astronomy can reveal to us.
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Now, scientists are using
gravitational waves
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to revisit other
long-standing mysteries,
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like what causes
the brightest explosions
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in the cosmos?
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This is not
an everyday car crash.
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This is the most dramatic
event that
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you're ever gonna see
in our universe.
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Across the universe,
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strange bursts of light
puzzle astronomers.
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For just a fraction of a second,
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they shine more than
a trillion times brighter
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than the sun...
Then, they vanish.
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These brief flashes of light
are known
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as gamma-ray bursts
or GRBs for short,
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and they're such a mystery,
because they are
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insanely energetic, and we
don't know what causes them.
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For decades,
these short gamma-ray bursts
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have been an enigma.
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No explanation was off limits,
no matter how wild.
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Is it a supernova?
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Is it on alien civilization
saying hello?
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You know, we just don't know.
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In August 2017,
the Fermi Gamma-ray Telescope
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detected another
short gamma-ray burst,
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but this one was different,
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So a gamma-ray burst went off
130 million light-years away,
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and it actually produced
a ripple in space and time
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that LIGO could detect.
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Gravitational waves could help
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finally reveal what causes
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one of the brightest
explosions in the universe.
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LIGOS data suggests
the culprit could be two
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massive objects spiraling
towards each other
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and colliding.
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But based on the gravitational
wave data,
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these two objects were
too small to be black holes.
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They had to be something else.
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Not black holes,
but the ultra dense
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cores of collapsed stars
called neutron stars.
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A neutron star is
what's left over
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after a massive star
collapses in on itself.
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It's very, very dense, because
it took all, essentially,
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the mass of the core and
contracted it into a really,
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really small radius.
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As the dense neutron stars
spiral ever closer,
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the gravitational wave signal
gets stronger and stronger,
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until they collide, releasing
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an epic burst of
gravitational waves.
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Because they're not black holes,
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light can get out.
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And if you smash two things
together at these kind of
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absolutely massive speeds,
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there's a huge amount of
energy involved.
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Energy we detected both as
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invisible gravitational waves
and visible light.
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Could this light be a mysterious
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and ultra-powerful
gamma-ray burst?
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How could these colliding
dead stars be associated
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with gamma-ray bursts,
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which are in fact, the most
energetic explosions we see in
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the entire universe?
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Neutron stars have
powerful magnetic fields
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that trap particles of
gas and dust.
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During a collision,
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the swirling magnetic fields
twist up,
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building up more
and more energy.
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You have lots of little
particles of matter that are
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trying to keep up with these
rapidly spinning magnetic
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fields... that starts swooshing
them round until they reach
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pretty much the speed of light,
and eventually,
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they're kind of shot out of
the remnant in a tight beam.
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The beam is a gamma-ray burst,
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but they're not always
easy to detect.
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If the jet coming out
is pointed right at you,
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then you see this extremely
high energy event,
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the gamma-ray burst.
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If it's not pointed at us,
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we might miss it.
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Fortunately,
the gravitational waves
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show us where to look.
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Following the gamma-ray burst,
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we spotted a strange red cloud,
evidence of a heavy
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element factory.
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After the initial collision,
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there is a shell of debris
moving outwards,
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but then, high-energy neutrons
come slamming into this
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material and start to build
heavier elements,
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one after another.
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We can see the gold,
we can see the potassium,
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00:14:07,830 --> 00:14:11,630
we can see the plutonium
being created
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before our very eyes.
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The neutron star collision
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produced huge quantities
of heavy elements,
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blasting out enough gold and
platinum to weigh more than 10
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times the mass of the Earth,
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solving a long-standing mystery.
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00:14:30,420 --> 00:14:31,820
We knew that
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supernova explosions did
create some
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00:14:34,760 --> 00:14:35,890
of the heavier elements.
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But from everything
we've observed about supernova,
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00:14:39,490 --> 00:14:43,830
they don't happen often enough
to really populate a galaxy
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with all of the heavier
elements that we observed.
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This was the missing piece.
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The gold on your wedding ring,
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the gold in your jewelry,
255
00:14:55,280 --> 00:14:57,410
was formed and forged from
256
00:14:57,510 --> 00:15:00,950
a titanic collision before
the Earth even existed.
257
00:15:03,890 --> 00:15:06,150
The combination of
gravitational waves
258
00:15:06,250 --> 00:15:07,250
and telescopes
259
00:15:08,990 --> 00:15:12,030
proves that neutron star
collisions create
260
00:15:12,130 --> 00:15:14,430
precious metals
261
00:15:14,530 --> 00:15:17,400
and cause super-bright
gamma-ray bursts.
262
00:15:18,900 --> 00:15:23,440
When you can measure
a gravitational wave signal
263
00:15:23,540 --> 00:15:25,870
and a light signal
like a gamma-ray burst,
264
00:15:25,970 --> 00:15:29,040
you get a whole new way
to solve complicated,
265
00:15:29,140 --> 00:15:31,710
intertwined physical processes.
266
00:15:33,880 --> 00:15:36,550
It's like you're
watching a symphony on mute,
267
00:15:36,650 --> 00:15:38,920
and then you hit that button,
and the sound comes on,
268
00:15:39,020 --> 00:15:41,390
and it's just a completely
different picture.
269
00:15:45,160 --> 00:15:49,900
The sounds of the cosmos
don't just reveal collisions.
270
00:15:53,970 --> 00:15:57,440
It turns out, we can use
gravitational waves to help us
271
00:15:57,540 --> 00:16:00,610
understand some of the biggest
mysteries of the cosmos.
272
00:16:13,590 --> 00:16:15,720
Gravitational waves
are a new way
273
00:16:15,820 --> 00:16:19,590
to listen to the universe,
revealing unseen,
274
00:16:19,690 --> 00:16:24,130
epic cosmic events and adding
vital details to our picture
275
00:16:24,230 --> 00:16:25,430
of the cosmos.
276
00:16:28,200 --> 00:16:31,770
Every new way we figure
out to probe the universe is
277
00:16:31,870 --> 00:16:34,910
a good thing, and detecting
gravitational waves,
278
00:16:35,010 --> 00:16:37,410
it's a new dimension to being
able to study the universe.
279
00:16:37,510 --> 00:16:39,680
It's like... it's like
having a new sense.
280
00:16:42,520 --> 00:16:45,290
This new sense could be
just what astronomers need
281
00:16:45,390 --> 00:16:48,920
to answer some of the biggest
questions in physics,
282
00:16:49,020 --> 00:16:51,690
like, "What is the speed
of gravity?"
283
00:16:53,190 --> 00:16:56,130
And, "Does it travel at
the universe's speed limit?"
284
00:16:57,930 --> 00:17:00,300
One of the things we
learn early in science is that
285
00:17:00,400 --> 00:17:02,870
the universe has
an absolute speed limit,
286
00:17:02,970 --> 00:17:05,440
which is the speed of light in
a vacuum,
287
00:17:05,540 --> 00:17:08,710
which is
186,000 miles per second.
288
00:17:11,580 --> 00:17:13,480
Light from the sun
takes eight minutes
289
00:17:13,580 --> 00:17:15,850
and 20 seconds to reach Earth.
290
00:17:15,950 --> 00:17:19,390
So, if the sun disappeared,
291
00:17:19,490 --> 00:17:21,460
we wouldn't miss its
light immediately.
292
00:17:23,160 --> 00:17:25,960
But how quickly would we
notice its missing gravity?
293
00:17:27,560 --> 00:17:30,700
The first thing
that we'd notice is nothing.
294
00:17:30,800 --> 00:17:35,740
Things would seem very normal,
but then they wouldn't.
295
00:17:35,840 --> 00:17:40,570
There would be nothing curving
space where Earth is located,
296
00:17:40,680 --> 00:17:43,810
and so Earth would take off
in a straight line,
297
00:17:43,910 --> 00:17:46,480
moving at the same speed at
which it orbits the sun.
298
00:17:46,580 --> 00:17:52,250
And things will get cold
and lonely really, really fast.
299
00:17:54,020 --> 00:17:55,420
According to Albert Einstein,
300
00:17:55,520 --> 00:17:59,160
our skies would go dark, and
the earth would be flung into
301
00:17:59,260 --> 00:18:02,100
deep space at exactly
the same time.
302
00:18:03,800 --> 00:18:07,100
It's a foundation of
his famous Theory of Relativity,
303
00:18:07,200 --> 00:18:11,040
still the most complete theory
of how our universe works.
304
00:18:12,510 --> 00:18:14,370
Einstein's theory of
relativity has been
305
00:18:14,480 --> 00:18:15,940
a fantastic theory.
306
00:18:16,040 --> 00:18:19,680
It explains so many things
for us, including gravity.
307
00:18:19,780 --> 00:18:21,980
But when we look out
at the universe,
308
00:18:22,080 --> 00:18:25,090
there are many mysteries, there
are things that are quite hard
309
00:18:25,190 --> 00:18:26,190
to explain.
310
00:18:28,490 --> 00:18:30,560
At the top of the list...
311
00:18:30,660 --> 00:18:33,030
The mystery
of our expanding universe.
312
00:18:35,860 --> 00:18:39,100
There is something pushing
outward that is
313
00:18:39,200 --> 00:18:43,570
making that expansion rate
ever and ever faster.
314
00:18:43,670 --> 00:18:46,940
Astronomers call this
something dark energy.
315
00:18:49,340 --> 00:18:53,850
It accounts for 70% of the total
energy in the universe.
316
00:18:57,120 --> 00:19:00,020
Einstein's models of
the universe need dark energy
317
00:19:00,120 --> 00:19:02,920
to work, but we have no idea
318
00:19:03,020 --> 00:19:04,730
what it is.
319
00:19:06,490 --> 00:19:09,860
Dark energy is not something
we actually understand.
320
00:19:09,960 --> 00:19:12,170
It's kind of a placeholder term
321
00:19:12,270 --> 00:19:14,230
for something
we don't understand.
322
00:19:14,340 --> 00:19:17,910
And so people naturally are
looking for better theories,
323
00:19:18,010 --> 00:19:20,310
theories that are a bit like
Einstein's theory
324
00:19:20,410 --> 00:19:22,880
but just go that bit further
and explain
325
00:19:22,980 --> 00:19:26,550
some of these things that
we don't currently understand.
326
00:19:26,650 --> 00:19:30,220
One way to excise dark energy
327
00:19:30,320 --> 00:19:31,990
is with a new theory of gravity,
328
00:19:33,450 --> 00:19:35,820
one where the speed of
gravitational waves
329
00:19:35,920 --> 00:19:39,160
is different
from the speed of light.
330
00:19:39,260 --> 00:19:40,530
There are some so-called
331
00:19:40,630 --> 00:19:42,300
non-Einsteinian theories for
332
00:19:42,400 --> 00:19:45,400
the structure of spacetime
itself that don't actually
333
00:19:45,500 --> 00:19:46,870
require dark energy.
334
00:19:46,970 --> 00:19:49,670
For example, if gravity
doesn't propagate through
335
00:19:49,770 --> 00:19:52,770
spacetime at the same speed
that light does,
336
00:19:52,870 --> 00:19:55,340
you could find models
that don't actually require
337
00:19:55,440 --> 00:19:58,950
dark energy... it could be
a clean, simple, albeit very,
338
00:19:59,050 --> 00:20:02,150
very profound solution
to this underlying problem.
339
00:20:04,220 --> 00:20:06,220
In order to overthrow Einstein
340
00:20:06,320 --> 00:20:07,690
and eliminate dark energy,
341
00:20:07,790 --> 00:20:11,460
the speeds of light
and gravity must be different.
342
00:20:12,930 --> 00:20:14,760
We know the speed of light.
343
00:20:14,860 --> 00:20:18,370
So how do we test
the speed of gravity?
344
00:20:20,130 --> 00:20:21,970
In order to test
the speed of gravity,
345
00:20:22,070 --> 00:20:24,100
you need to have a system that
emits both
346
00:20:24,210 --> 00:20:26,140
gravitational waves and light.
347
00:20:28,240 --> 00:20:30,910
The colliding
neutron stars detected by LIGO
348
00:20:31,010 --> 00:20:33,750
in 2017 are part of
the solution.
349
00:20:35,480 --> 00:20:38,450
The collision released
a flash of light,
350
00:20:38,550 --> 00:20:40,820
along with a burst of
gravitational waves.
351
00:20:45,590 --> 00:20:49,300
But the universe
threw a curveball.
352
00:20:49,400 --> 00:20:52,470
The light signal arrived
1.7 seconds
353
00:20:52,570 --> 00:20:55,800
after the gravitational wave
signal.
354
00:20:55,900 --> 00:20:57,740
Does that mean
gravitational waves
355
00:20:57,840 --> 00:20:59,810
travel slightly faster
than light?
356
00:21:02,380 --> 00:21:05,480
Albert Einstein predicted
that gravitational waves
357
00:21:05,580 --> 00:21:07,410
would move
at the speed of light.
358
00:21:07,520 --> 00:21:10,180
So what if Albert Einstein
was wrong?
359
00:21:10,290 --> 00:21:11,920
I know, sounds crazy, right?
360
00:21:12,020 --> 00:21:15,020
That's like almost as crazy as
me being wrong, right?
361
00:21:15,120 --> 00:21:18,890
But if Einstein was wrong,
that's one thing.
362
00:21:18,990 --> 00:21:21,960
But a bigger problem is that
we'd have to rethink
363
00:21:22,060 --> 00:21:23,260
our physics.
364
00:21:25,530 --> 00:21:26,600
Before we do that,
365
00:21:26,700 --> 00:21:29,170
let's take a closer look
at the neutron star
366
00:21:29,270 --> 00:21:30,500
collision site.
367
00:21:32,110 --> 00:21:34,940
It's surrounded by a shroud
of gas and dust.
368
00:21:36,510 --> 00:21:39,380
Light is made of particles
called photons,
369
00:21:39,480 --> 00:21:42,680
which scatter when
they hit obstacles.
370
00:21:42,780 --> 00:21:46,220
But gravitational waves
pass through anything.
371
00:21:47,490 --> 00:21:50,260
They pass right through
everything like it's not there.
372
00:21:50,360 --> 00:21:51,930
Light, on the other hand,
373
00:21:52,030 --> 00:21:55,260
was slowed down by
interactions with that matter.
374
00:21:55,360 --> 00:21:57,500
It didn't just
escape immediately
375
00:21:57,600 --> 00:21:59,900
like the gravitational wave
signal did.
376
00:22:01,670 --> 00:22:03,870
The debris gave
the gravitational waves
377
00:22:03,970 --> 00:22:07,270
a head start by slowing
the light.
378
00:22:07,380 --> 00:22:10,140
So gravitational waves
and light do,
379
00:22:10,240 --> 00:22:13,250
in fact,
travel at the same speed.
380
00:22:13,310 --> 00:22:14,680
Einstein was right.
381
00:22:16,620 --> 00:22:19,420
This one event ruled
out the other theories of
382
00:22:19,520 --> 00:22:21,960
gravity that are competing
with Einstein's theory,
383
00:22:22,060 --> 00:22:25,060
things that people have been
working on all their life
384
00:22:25,160 --> 00:22:27,260
and overnight, it's gone.
385
00:22:29,130 --> 00:22:30,830
Thanks to gravitational waves,
386
00:22:30,930 --> 00:22:34,630
dark energy remains our best
explanation for why
387
00:22:34,740 --> 00:22:37,340
the universe's expansion
is accelerating.
388
00:22:38,840 --> 00:22:41,610
Maybe dark energy isn't what
we think it is, and maybe
389
00:22:41,710 --> 00:22:43,210
tomorrow, or maybe next year,
390
00:22:43,310 --> 00:22:44,880
or maybe next decade
or next century,
391
00:22:44,980 --> 00:22:49,150
- we will discover that.
- Gravitational waves are a huge
392
00:22:49,250 --> 00:22:52,620
step forward in our effort to
understand the universe,
393
00:22:52,720 --> 00:22:54,250
and I mean everything.
394
00:22:54,390 --> 00:22:57,490
Space, time, matter,
dark energy.
395
00:22:57,590 --> 00:23:00,990
We have a completely
new universe to view now.
396
00:23:03,770 --> 00:23:05,500
Now astronomers want to use
397
00:23:05,600 --> 00:23:08,570
gravitational waves
to answer another mystery.
398
00:23:10,170 --> 00:23:14,370
What happens when supermassive
black holes collide?
399
00:23:26,020 --> 00:23:30,720
We first detected
gravitational waves in 2015.
400
00:23:30,830 --> 00:23:33,460
Since then,
they've revealed colliding
401
00:23:33,560 --> 00:23:35,930
black holes across the universe.
402
00:23:38,130 --> 00:23:40,200
Prior to LIGO going online,
403
00:23:40,300 --> 00:23:43,840
we never witnessed black hole
collisions directly,
404
00:23:43,940 --> 00:23:47,370
but now that we can witness
them with our observatories,
405
00:23:47,480 --> 00:23:50,310
we're finding them
pretty regularly.
406
00:23:50,410 --> 00:23:53,280
We're seeing gravitational
waves come
407
00:23:53,380 --> 00:23:55,450
across the LIGO experiment
408
00:23:55,550 --> 00:23:57,380
left and right.
409
00:23:57,490 --> 00:24:00,220
But LIGO has only been
listening for gravitational
410
00:24:00,320 --> 00:24:02,020
waves from black holes
411
00:24:02,120 --> 00:24:04,490
on the smaller end
of the cosmic scale.
412
00:24:06,060 --> 00:24:09,860
When we look at the cosmic zoo
of black holes out there,
413
00:24:09,960 --> 00:24:14,300
we find small ones weighing,
you know, 10, maybe 30 times as
414
00:24:14,400 --> 00:24:18,040
much as the sun, and then large
all the way up to extra-large
415
00:24:18,140 --> 00:24:19,146
going from, like, a million
416
00:24:19,170 --> 00:24:20,870
to a billion times
as much as the sun.
417
00:24:22,340 --> 00:24:24,640
These supermassive black holes
418
00:24:24,750 --> 00:24:27,550
lurk at the hearts of galaxies.
419
00:24:27,650 --> 00:24:31,050
When Galaxies merge,
supermassive black holes
420
00:24:31,150 --> 00:24:33,320
should merge, too.
421
00:24:37,120 --> 00:24:39,560
But even though we see
galaxies colliding
422
00:24:39,660 --> 00:24:40,660
across the universe,
423
00:24:40,760 --> 00:24:45,000
we've never seen two
supermassive black holes
424
00:24:45,100 --> 00:24:47,930
collide, because they have too
425
00:24:48,040 --> 00:24:51,340
much orbital energy to get
close enough to merge.
426
00:24:56,010 --> 00:25:00,080
That orbital energy
has to go somewhere,
427
00:25:00,180 --> 00:25:03,120
and what supermassive black
holes do is they throw out
428
00:25:03,220 --> 00:25:06,220
stars that are around
the core of the galaxy.
429
00:25:06,320 --> 00:25:09,660
But when they get sufficiently
close, there are just no more
430
00:25:09,760 --> 00:25:11,190
stars to throw out,
431
00:25:11,290 --> 00:25:14,660
and so the idea is,
they can't merge.
432
00:25:14,760 --> 00:25:15,760
So there's a problem.
433
00:25:15,830 --> 00:25:19,070
How is it that they managed
to bridge that gap
434
00:25:19,170 --> 00:25:21,230
and finally spiral in?
435
00:25:21,340 --> 00:25:23,370
The only way to
understand if supermassive
436
00:25:23,470 --> 00:25:25,540
black holes merge is by looking
437
00:25:25,640 --> 00:25:28,640
at their gravitational
wave signal.
438
00:25:28,740 --> 00:25:30,810
Two supermassive black holes
439
00:25:30,910 --> 00:25:34,510
merging should release a burst
of gravitational waves
440
00:25:34,580 --> 00:25:36,420
millions of times more powerful
441
00:25:36,520 --> 00:25:38,820
than a stellar mass
black hole merger.
442
00:25:41,620 --> 00:25:43,960
But LIGO won't hear a thing.
443
00:25:45,190 --> 00:25:47,330
The problem with using LIGO
to detect the merger
444
00:25:47,430 --> 00:25:50,630
of supermassive black holes is
actually a scale of time.
445
00:25:54,140 --> 00:25:57,340
One wave, as these things move
around each other very slowly,
446
00:25:57,440 --> 00:26:02,280
would take over 10 years
to go by, just one wave.
447
00:26:02,380 --> 00:26:04,510
In order to detect
a gravitational wave with
448
00:26:04,610 --> 00:26:05,810
periods of decades,
449
00:26:05,910 --> 00:26:09,750
you also need an experiment
that can be extremely stable
450
00:26:09,850 --> 00:26:11,080
over that amount of time.
451
00:26:12,990 --> 00:26:15,460
Vibrations from earthquakes,
452
00:26:15,560 --> 00:26:17,420
weather, or even nearby traffic
453
00:26:17,530 --> 00:26:21,560
prevent LIGO from listening for
a decade, just to hear one wave.
454
00:26:23,860 --> 00:26:26,730
But there may be another way to
455
00:26:26,830 --> 00:26:30,540
detect gravitational waves
from supermassive black holes,
456
00:26:30,640 --> 00:26:35,510
using a strange type of dead
star called a pulsar.
457
00:26:35,610 --> 00:26:39,010
A pulsar is a kind of
458
00:26:39,110 --> 00:26:42,180
neutron star
that is rapidly spinning
459
00:26:42,280 --> 00:26:45,620
and has a beam of radiation
that makes
460
00:26:45,750 --> 00:26:48,390
wide circles across the sky.
461
00:26:48,490 --> 00:26:52,730
And when that flash of circle
washes over the planet Earth,
462
00:26:52,830 --> 00:26:55,160
we get a little beep,
a little beep.
463
00:26:55,260 --> 00:26:58,260
We get pulses of radiation,
hence pulsar.
464
00:27:00,430 --> 00:27:03,270
Pulsars are the best
timekeepers in the universe,
465
00:27:04,610 --> 00:27:09,180
but passing gravitational
waves make them miss a beat.
466
00:27:09,280 --> 00:27:12,010
What if we noticed that
the frequency of a pulsar was
467
00:27:12,110 --> 00:27:14,310
shifting very, very slowly,
468
00:27:14,420 --> 00:27:17,450
year to year to year,
over 10 years or more,
469
00:27:17,550 --> 00:27:21,590
just slightly getting a little
bit longer as space itself was
470
00:27:21,690 --> 00:27:23,360
changing between us
and the pulsar?
471
00:27:25,960 --> 00:27:28,400
By monitoring dozens of pulsars,
472
00:27:28,500 --> 00:27:31,200
Chiara Mingarelli
and a team of astronomers
473
00:27:31,300 --> 00:27:36,470
have created a galaxy-sized
gravitational wave detector.
474
00:27:36,570 --> 00:27:42,340
It's called
a pulsar timing array.
475
00:27:42,440 --> 00:27:45,880
You can really look for
deviations in those arrival
476
00:27:45,980 --> 00:27:47,350
times over decades,
477
00:27:47,450 --> 00:27:50,920
almost like a tsunami warning
system to show you when
478
00:27:51,020 --> 00:27:53,850
a gravitational wave
is passing by.
479
00:27:56,360 --> 00:27:58,530
After 12 years,
the team detected
480
00:27:58,630 --> 00:28:01,460
the same change
in a number of pulsars.
481
00:28:03,360 --> 00:28:04,630
These pulsars are all
482
00:28:04,730 --> 00:28:06,800
thousands of light-years apart.
483
00:28:06,870 --> 00:28:07,870
If you think about it,
484
00:28:07,940 --> 00:28:11,270
it's difficult to make
a signal that's the same
485
00:28:11,370 --> 00:28:12,970
in all of these pulsars.
486
00:28:13,070 --> 00:28:16,340
This has to be this common
signal from something like
487
00:28:16,440 --> 00:28:18,710
a gravitational wave event.
488
00:28:21,750 --> 00:28:24,480
The signal the team
detected wasn't created
489
00:28:24,590 --> 00:28:27,950
by just two supermassive
black holes colliding.
490
00:28:29,460 --> 00:28:33,830
It's evidence of gravitational
waves from hundreds of pairs of
491
00:28:33,930 --> 00:28:35,860
supermassive black holes,
492
00:28:35,960 --> 00:28:38,700
all in different stages
of merging.
493
00:28:41,070 --> 00:28:44,700
Because it takes so long for
one of these individual
494
00:28:44,810 --> 00:28:48,510
binary systems to merge,
there could be thousands,
495
00:28:48,580 --> 00:28:52,210
if not millions, of these
signals all being emitted at
496
00:28:52,310 --> 00:28:54,250
the same time, all of them.
497
00:28:54,350 --> 00:28:56,620
They all create this
gravitational wave background
498
00:28:56,720 --> 00:28:59,320
that we're just starting to see
the first signs of now.
499
00:29:02,490 --> 00:29:05,020
Astronomers predict
this gravitational
500
00:29:05,090 --> 00:29:07,730
wave background fills
our universe.
501
00:29:09,600 --> 00:29:12,230
If the signal the team
detected is confirmed,
502
00:29:12,330 --> 00:29:16,300
it's proof that supermassive
black holes do merge.
503
00:29:18,110 --> 00:29:21,840
The next step is to observe
that as it happens.
504
00:29:23,280 --> 00:29:26,010
It would be a dream
to see two supermassive
505
00:29:26,110 --> 00:29:27,310
black holes merging,
506
00:29:27,410 --> 00:29:30,150
emitting gravitational waves,
and also being able to point
507
00:29:30,250 --> 00:29:33,490
a telescope at them and to see
the physics of how they merge.
508
00:29:34,960 --> 00:29:37,620
Gravitational waves
reveal the hidden workings
509
00:29:37,730 --> 00:29:39,860
of the cosmos.
510
00:29:39,960 --> 00:29:43,230
They reach the farthest
corners of our universe.
511
00:29:44,570 --> 00:29:47,270
Now, astronomers are
using gravitational
512
00:29:47,370 --> 00:29:49,770
waves to look back in time.
513
00:29:51,070 --> 00:29:53,440
They'll let us see
all the way back
514
00:29:53,540 --> 00:29:55,380
to the earliest moments
of our Big Bang.
515
00:30:16,000 --> 00:30:18,330
13.8 billion years ago,
516
00:30:20,300 --> 00:30:22,470
the universe sparks into life.
517
00:30:24,270 --> 00:30:28,270
The tiny speck of energy
expands and cools.
518
00:30:28,380 --> 00:30:33,680
The infant cosmos is a fog of
tiny particles of matter.
519
00:30:33,780 --> 00:30:38,990
Over time, the particles form
atoms of hydrogen and helium.
520
00:30:40,450 --> 00:30:44,090
The fog clears, and the first
light races across
521
00:30:44,190 --> 00:30:45,660
the universe.
522
00:30:45,760 --> 00:30:49,660
We call that light the cosmic
microwave background.
523
00:30:50,870 --> 00:30:52,670
The cosmic microwave
background is simply
524
00:30:52,770 --> 00:30:54,570
the most distant light
we can see.
525
00:30:54,670 --> 00:30:57,640
So, looking at it give us
baby pictures of our universe
526
00:30:57,740 --> 00:31:00,440
the way it looked
400,000 years after a big bang.
527
00:31:02,210 --> 00:31:04,710
What happened before
these baby pictures
528
00:31:04,810 --> 00:31:07,010
remains a mystery.
529
00:31:08,220 --> 00:31:11,020
The leading theory is that
in the very first second
530
00:31:11,120 --> 00:31:12,850
of the Big Bang,
531
00:31:12,950 --> 00:31:15,590
our infant universe
had a growth spurt.
532
00:31:18,390 --> 00:31:21,230
Scientists call this
idea inflation.
533
00:31:22,400 --> 00:31:26,330
In a billionth of a billionth of
a billionth of a second,
534
00:31:26,430 --> 00:31:30,300
our universe grew a billion,
billion, billion,
535
00:31:30,400 --> 00:31:33,810
billion, billion, billion
times bigger.
536
00:31:33,910 --> 00:31:37,510
That is the mother of all
growth spurts... it laid
537
00:31:37,610 --> 00:31:41,580
the foundations for the entire
cosmos that we know today.
538
00:31:49,060 --> 00:31:51,160
Inflation is just a theory,
539
00:31:51,260 --> 00:31:54,860
but there may be a way
to prove it happened.
540
00:31:54,960 --> 00:31:57,500
Scientists think that during
that brief moment of
541
00:31:57,630 --> 00:31:58,870
cosmic expansion,
542
00:31:58,970 --> 00:32:02,800
inflation stretched tiny
fluctuations of gravity.
543
00:32:02,900 --> 00:32:06,840
That is such a violent process
that it actually causes ripples
544
00:32:06,940 --> 00:32:08,210
and distortions in the very
545
00:32:08,310 --> 00:32:10,710
shape and fabric of
space itself,
546
00:32:10,810 --> 00:32:13,110
which we can see today as
gravitational waves.
547
00:32:14,620 --> 00:32:17,450
Scientists call these
theoretical ripples through
548
00:32:17,550 --> 00:32:21,020
the early universe primordial
gravitational waves.
549
00:32:22,890 --> 00:32:25,120
When they were first released,
550
00:32:25,230 --> 00:32:27,690
these were deafening.
551
00:32:27,800 --> 00:32:30,760
But in the billions of years
since, our universe has grown
552
00:32:30,860 --> 00:32:32,300
bigger and colder,
553
00:32:32,400 --> 00:32:35,270
and these gravitational waves
have diluted
554
00:32:35,370 --> 00:32:38,670
so that they barely even
exist today.
555
00:32:38,770 --> 00:32:43,280
Scientists searched for
signs of these very weak,
556
00:32:43,380 --> 00:32:46,050
primordial gravitational waves
in the cosmic
557
00:32:46,150 --> 00:32:48,380
microwave background.
558
00:32:48,480 --> 00:32:50,950
And in 2014,
559
00:32:51,050 --> 00:32:54,620
a teen, using their
purpose-built microwave array
560
00:32:54,720 --> 00:32:56,920
in Antarctica called BICEP,
561
00:32:57,020 --> 00:32:59,660
found a strange
swirling pattern.
562
00:32:59,760 --> 00:33:03,960
When they saw those
swirls, they saw those patterns,
563
00:33:04,060 --> 00:33:05,100
they thought they had seen
564
00:33:05,200 --> 00:33:08,200
the signature of primordial
gravitational waves.
565
00:33:08,300 --> 00:33:11,970
Now this is really
the conclusive
566
00:33:12,070 --> 00:33:15,070
evidence that inflation
had to have happened.
567
00:33:15,180 --> 00:33:18,540
The results were exciting,
568
00:33:18,650 --> 00:33:21,510
but there was a glitch.
569
00:33:21,620 --> 00:33:23,580
This amazement lasted
570
00:33:23,650 --> 00:33:28,590
for a few months until cracks
started appearing in this,
571
00:33:28,690 --> 00:33:30,190
and gradually, it all collapsed.
572
00:33:33,160 --> 00:33:34,790
The signal,
thought to be proof of
573
00:33:34,900 --> 00:33:39,100
primordial gravitational waves
and the theory of inflation,
574
00:33:39,200 --> 00:33:42,000
turned out to be a case of
mistaken identity.
575
00:33:45,670 --> 00:33:47,710
As this light
from the ancient universe,
576
00:33:47,810 --> 00:33:50,510
from the cosmic microwave
background, travels
577
00:33:50,610 --> 00:33:53,210
through the universe, it had
to travel through dust
578
00:33:53,310 --> 00:33:58,480
before reaching our detectors,
and the dust itself can affect
579
00:33:58,590 --> 00:34:01,290
the light and mimic what
580
00:34:01,390 --> 00:34:03,720
the primordial gravitational
waves can do.
581
00:34:05,530 --> 00:34:07,890
The primordial
gravitational wave signal
582
00:34:08,000 --> 00:34:11,230
turned out to be
mainly clouds of dust
583
00:34:11,330 --> 00:34:12,800
floating through space.
584
00:34:15,400 --> 00:34:17,770
That's how BICEP bit the dust.
585
00:34:19,610 --> 00:34:21,410
BICEP failed to detect
586
00:34:21,510 --> 00:34:24,610
primordial gravitational waves.
587
00:34:24,710 --> 00:34:27,110
Can LIGO do any better?
588
00:34:27,210 --> 00:34:30,980
Unfortunately,
LIGO can't help us
589
00:34:31,090 --> 00:34:33,790
in observing primordial
gravitational waves.
590
00:34:33,890 --> 00:34:35,960
It can't even observe
supermassive black holes
591
00:34:36,060 --> 00:34:37,260
at the centers of galaxies.
592
00:34:37,360 --> 00:34:39,530
It is designed to
observe in a particular
593
00:34:39,630 --> 00:34:41,290
frequency range.
594
00:34:41,400 --> 00:34:44,200
Primordial gravitational waves
595
00:34:44,300 --> 00:34:46,370
are at such a low frequency in
596
00:34:46,470 --> 00:34:48,270
such a low amplitude
597
00:34:48,370 --> 00:34:53,010
that there is no hope of LIGO
being able to detect them.
598
00:34:55,580 --> 00:34:58,240
But scientists hope
that an ambitious project
599
00:34:58,350 --> 00:35:00,210
called LISA will.
600
00:35:01,750 --> 00:35:05,280
Not on Earth, but from
30 million miles above.
601
00:35:07,220 --> 00:35:10,490
LISA is like LIGO,
602
00:35:10,590 --> 00:35:12,490
but bigger and in space.
603
00:35:18,030 --> 00:35:22,070
Or the Laser Interferometer
Space Antenna, will be a system
604
00:35:22,170 --> 00:35:26,910
of three satellites arranged in
a giant triangular formation,
605
00:35:27,010 --> 00:35:29,910
1.5 million miles apart.
606
00:35:30,010 --> 00:35:32,280
If a gravitational
wave passes through them
607
00:35:32,380 --> 00:35:33,580
and changes that distance,
608
00:35:33,680 --> 00:35:36,080
they can detect that... because
the satellites are so much
609
00:35:36,180 --> 00:35:39,450
farther apart,
a very low frequency wave
610
00:35:39,550 --> 00:35:40,950
can make a detectable change.
611
00:35:41,050 --> 00:35:44,890
LIGO wouldn't be able to see
that, but LISA could.
612
00:35:44,990 --> 00:35:47,290
As well as listening
for low frequency
613
00:35:47,390 --> 00:35:50,530
gravitational wave sources,
like supermassive black
614
00:35:50,630 --> 00:35:51,960
hole mergers,
615
00:35:52,070 --> 00:35:55,330
LISA will listen for
primordial gravitational
616
00:35:55,440 --> 00:35:58,370
waves from the dawn of time.
617
00:35:58,470 --> 00:35:59,540
If it detects them,
618
00:35:59,640 --> 00:36:03,780
we will know that the infant
universe inflated.
619
00:36:04,950 --> 00:36:08,150
Inflation has explained
almost everything
620
00:36:08,250 --> 00:36:10,780
we measure in modern cosmology.
621
00:36:10,880 --> 00:36:12,650
It's an incredibly
successful theory.
622
00:36:12,750 --> 00:36:15,320
The icing on the cake
would be if we could
623
00:36:15,420 --> 00:36:17,990
also discover these
gravitational waves
624
00:36:18,090 --> 00:36:21,430
that it's supposed
to have created.
625
00:36:22,900 --> 00:36:26,000
From the Big Bang to
the most massive black holes,
626
00:36:27,100 --> 00:36:30,500
the universe talks to us
using gravitational waves.
627
00:36:32,770 --> 00:36:37,510
Just like with telescopes, we're
using gravitational waves to
628
00:36:37,610 --> 00:36:39,650
look at different types
of objects...
629
00:36:39,750 --> 00:36:44,320
Neutron star mergers and black
hole mergers... and learn more
630
00:36:44,380 --> 00:36:45,990
about the universe around us.
631
00:36:48,390 --> 00:36:49,660
They could even reveal
632
00:36:49,760 --> 00:36:53,060
the most elusive substance
in the universe...
633
00:36:53,160 --> 00:36:55,060
Dark matter.
634
00:36:55,160 --> 00:36:57,900
If anything's gonna
help us understand
635
00:36:58,000 --> 00:36:59,230
the nature of dark matter,
636
00:36:59,330 --> 00:37:01,670
it might just be
gravitational waves.
637
00:37:11,210 --> 00:37:12,580
Across the universe,
638
00:37:12,680 --> 00:37:15,850
an invisible substance holds
galaxies together.
639
00:37:15,920 --> 00:37:20,090
Without it,
they would fly apart.
640
00:37:20,190 --> 00:37:23,990
The Milky Way
should've dispersed long ago,
641
00:37:24,090 --> 00:37:26,160
and the Magellanic clouds
right in front of us are
642
00:37:26,260 --> 00:37:27,590
exactly the same.
643
00:37:27,690 --> 00:37:29,500
These things should be
just shedding stars
644
00:37:29,600 --> 00:37:32,230
left and right as they fly off
this rotating galaxy.
645
00:37:32,330 --> 00:37:34,770
Instead, they're not.
They're holding together.
646
00:37:34,870 --> 00:37:37,340
There are motions in the stars
that we just cannot
647
00:37:37,440 --> 00:37:39,740
account for unless there's
something holding
648
00:37:39,840 --> 00:37:40,840
the whole thing together.
649
00:37:42,580 --> 00:37:45,710
We call this mysterious
substance dark matter.
650
00:37:45,810 --> 00:37:50,280
It doesn't interact with
light, so we can't see it.
651
00:37:50,380 --> 00:37:52,350
But we cannot ignore it.
652
00:37:54,320 --> 00:37:57,760
From the motions of stars
inside of galaxies to
653
00:37:57,860 --> 00:38:00,630
the motions of galaxies
inside of clusters
654
00:38:00,730 --> 00:38:04,500
to the very structure of
the universe itself,
655
00:38:04,600 --> 00:38:08,730
we see evidence for dark
matter everywhere we look.
656
00:38:11,410 --> 00:38:13,710
We think dark matter makes up
657
00:38:13,810 --> 00:38:17,780
85% of the matter
in the universe.
658
00:38:17,880 --> 00:38:22,080
But because we can't see
dark matter with telescopes,
659
00:38:22,180 --> 00:38:23,880
we know very little about it.
660
00:38:25,650 --> 00:38:27,920
While we know that it's there,
661
00:38:28,020 --> 00:38:31,560
we haven't actually answered
the question of what it is
662
00:38:31,660 --> 00:38:36,000
or how it interacts or why it's
there or how it's created.
663
00:38:36,100 --> 00:38:37,860
So you have to be
really creative
664
00:38:37,960 --> 00:38:39,280
if you want to go after
this stuff
665
00:38:39,370 --> 00:38:42,200
and really understand what's
it made out of?
666
00:38:45,570 --> 00:38:48,270
One creative theory
suggests that black holes
667
00:38:48,380 --> 00:38:50,580
make up dark matter,
668
00:38:50,680 --> 00:38:54,580
not the regular stellar mass
black holes that LIGO detects,
669
00:38:56,020 --> 00:38:58,420
or the supermassive black
holes that
670
00:38:58,520 --> 00:39:00,720
lurk at the center of galaxies
671
00:39:00,820 --> 00:39:05,460
but tiny, primordial black
holes born during the period of
672
00:39:05,590 --> 00:39:09,430
rapid expansion in the first
moments of the Big Bang.
673
00:39:10,900 --> 00:39:13,370
Primordial black holes could be
674
00:39:13,470 --> 00:39:16,940
potential explanations
for what we call dark matter.
675
00:39:17,040 --> 00:39:19,240
And if there's enough of them,
they can hold an entire
676
00:39:19,340 --> 00:39:21,070
galaxy together.
677
00:39:21,170 --> 00:39:24,980
We don't know if
primordial black holes exist,
678
00:39:25,080 --> 00:39:30,050
but gravitational waves
could change that.
679
00:39:30,180 --> 00:39:32,550
When you form
a primordial black hole,
680
00:39:32,650 --> 00:39:35,220
you send out a burst of
gravitational waves
681
00:39:35,320 --> 00:39:38,190
that, in principle, carries on
traveling through the universe,
682
00:39:38,290 --> 00:39:39,830
and you might be able to
detect it
683
00:39:39,930 --> 00:39:41,190
still today.
684
00:39:41,290 --> 00:39:43,900
The problem is that
these things would have emitted
685
00:39:44,000 --> 00:39:46,170
gravitational waves at
a frequency that is not
686
00:39:46,270 --> 00:39:47,630
detectable by LIGO.
687
00:39:47,730 --> 00:39:51,000
And so it's very hard to
discern whether or not they
688
00:39:51,100 --> 00:39:54,370
are plentiful enough to
actually serve as a compelling
689
00:39:54,470 --> 00:39:55,470
dark matter candidate.
690
00:39:58,040 --> 00:40:00,450
If primordial
black holes do exist,
691
00:40:00,550 --> 00:40:03,650
they still might not explain
all the dark matter in
692
00:40:03,750 --> 00:40:05,350
the universe.
693
00:40:05,450 --> 00:40:07,750
They might be working
with another type
694
00:40:07,850 --> 00:40:10,420
of dark matter to hold
galaxies together.
695
00:40:11,830 --> 00:40:14,890
The upcoming LISA mission
may fill in the blanks.
696
00:40:16,530 --> 00:40:18,760
What we call dark matter
could be simple.
697
00:40:18,870 --> 00:40:22,470
It could just be made of one
thing that absolutely floods
698
00:40:22,570 --> 00:40:24,700
the universe,
or it can be made of
699
00:40:24,810 --> 00:40:28,310
many different things that
all work together to combine
700
00:40:28,410 --> 00:40:29,910
to make this effect.
701
00:40:30,010 --> 00:40:32,110
Is dark matter all
primordial black holes?
702
00:40:32,210 --> 00:40:34,910
Is it something else that
we haven't thought of yet?
703
00:40:35,020 --> 00:40:38,280
Gravitational waves could
provide those answers.
704
00:40:40,490 --> 00:40:43,720
The detection of tiny
gravitational waves generated
705
00:40:43,820 --> 00:40:45,560
by primordial black holes
706
00:40:45,660 --> 00:40:49,900
will be a huge advance in our
understanding of dark matter.
707
00:40:50,000 --> 00:40:51,600
With gravitational
wave astronomy,
708
00:40:51,700 --> 00:40:54,930
we're seeing things that
we have never seen before.
709
00:40:55,040 --> 00:40:56,540
So who knows
710
00:40:56,640 --> 00:40:59,100
what we're gonna see as we
continue to look out into space?
711
00:41:01,840 --> 00:41:04,880
We've been able
to see dozens of black holes
712
00:41:04,980 --> 00:41:07,610
merge,
two neutron stars merging,
713
00:41:07,710 --> 00:41:10,650
and discovered from
that merger that neutron stars
714
00:41:10,750 --> 00:41:12,750
can make platinum and gold.
715
00:41:15,820 --> 00:41:18,390
From thinking that we would
never be able to see
716
00:41:18,490 --> 00:41:22,730
gravitational waves to seeing
gravitational wave signals
717
00:41:22,830 --> 00:41:25,830
happen on the regular...
It's just crazy.
718
00:41:27,530 --> 00:41:30,740
Already,
we've heard epic explosions.
719
00:41:32,310 --> 00:41:35,840
We've identified the brightest
lights in the cosmos,
720
00:41:37,340 --> 00:41:40,510
and we have solved some of
the biggest mysteries
721
00:41:40,610 --> 00:41:42,580
in astronomy.
722
00:41:42,680 --> 00:41:46,090
But that is just the beginning.
723
00:41:46,190 --> 00:41:49,190
Right now is
a golden age in astronomy.
724
00:41:49,290 --> 00:41:51,666
Think of the time that you're
living in... the first detection
725
00:41:51,690 --> 00:41:54,830
of gravitational waves by LIGO
was only a couple of years ago.
726
00:41:54,930 --> 00:41:56,700
You were here of the birth of
727
00:41:56,800 --> 00:41:58,700
this entirely new view of
the universe.
728
00:41:58,750 --> 00:42:03,300
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