How the Universe Works

First there was light, visible light.

Then, we viewed the universe in radio waves and X-rays.

Ever since there's been astronomy,

we've been looking at different kinds of light

and opening up the universe a little bit more of the time.

But then in 2015, like, the roof came off.

Something happened that changed everything,

the ability to see waves in space and time itself.

Gravitational waves.

They help us roll back the clock to the dawn of time,

discover epic cosmic collisions,

on make Earth-shaking discoveries.

Gravitational waves are the biggest game changer

since the invention of the telescope.

We have a completely new universe to view now.

A new exploration of space is just beginning.

Long ago, 17 billion light-years away,

a cataclysmic showdown plays out.

Two black holes locked together

in a deadly cosmic dance.

Black holes are unimaginably dense objects with gravity

so intense that if you get too close to them,

you're gone.

Their immense gravitational pull causes

them to spiral towards each other.

When black holes collide, they don't just run

into each other.

They're in orbit about each other.

So what we're talking about is an inspiralling orbit

that goes faster and faster and faster and faster.

Until they finally collide in a fatal embrace.

But astronomers don't see a thing.

The problem with observing colliding black holes is all

about the name, black holes, they give off no light.

How can astronomers see something that

no telescope can detect?

Across the universe,

extraordinary events take place.

But we sometimes miss them, because we rely on light.

Now, astronomers have a new toolkit that's

revealing the cosmos in a totally different way...

...using the very fabric of our universe

we call spacetime.

Everything with mass, like stars,

planets, and black holes, all curve this fabric.

The more massive the object,

the bigger the distortion of spacetime.

The classical analogy is

this stretched rubber sheet, right?

And, like, a mass, like, the sun is, like, a ball on this sheet,

and it distorts and warps the sheet

into this valley, right?

And if you roll a marble across it like the marble is

a planet, the marble will be

pulled into orbit around the ball because

of the curvature of the sheet.

But that's only half the picture.

If an object has mass and is accelerating through

spacetime, it creates ripples

in that fabric of spacetime,

and we call these gravitational waves.

Gravitational waves give us vital clues

about distant objects that we can't see.

The more massive the object that produces them and the faster

it's moving, the bigger the ripples.

These ripples pass through planets, stars, and galaxies

with ease.

When a gravitational wave passes through an object like

a star or a planet or a person,

it stretches and compresses them,

like with this tennis ball.

Now, if you're close to a powerful source of

gravitational waves,

like merging supermassive black holes,

those waves are incredibly strong, and they're capable of

actually destroying a planet.

But like the ripples on a pond,

their strength and size diminishes over distance.

The farther away you are, the weaker they get.

And when they're hundreds of millions of light-years away,

they're actually smaller than the size of an atom.

So, to listen for gravitational waves,

scientists built the most sensitive measuring device on

the planet.

This is LIGO,

the Laser Interferometer Gravitational-Wave Observatory,

two enormous detectors located almost 2,000 miles

apart in Louisiana and Washington state.

Each sensor has L-shaped arms, measuring 2.5 miles.

Inside the LIGO detectors,

inside these concrete tunnels, there is a laser system.

It's called an interferometer, so light comes in from

a laser beam and is split into two paths.

Normally, the lengths of the two beams are the same.

That changes when gravitational waves

hit the beams.

When a gravitational wave passes through,

it changes the distance that light travels along these arms,

so one arm effectively gets longer, and the other one

gets shorter.

The length of those two beams varies just ever so slightly,

and the very sensitive apparatus in LIGO is able to pick that up.

With this ultra-sensitive laser system,

LIGO picks up distortions in spacetime, narrower than

one millionth of the diameter of an atom.

Just that feat,

just the fact that we were able to build

a detector to detect gravitational waves

is just mind-boggling.

All of a sudden now, we were listening

to the faintest whispers of the universe.

In 2015, LIGO picked up a whisper that had

been traveling towards Earth for over a billion years.

Its source? Two colliding stellar black holes.

Watching two black holes spiral in and merge...

That's not something we can do using optical

telescopes or X-ray telescopes or anything like that.

But with LIGO, we could actually detect that event.

Now, scientists can paint

accurate pictures of invisible objects.

You can tell you're looking at black holes.

You can get their masses, you can get their distance.

There's a phenomenal amount of information in that wave.

The colliding black holes are

the most massive LIGO has ever detected.

One is 66 times the mass of our sun,

the other, 85 times the mass of our sun.

As two black holes are spiraling in,

they are moving faster and faster

as they get closer and closer.

That means that the gravitational waves

they're emitting have a higher and higher frequency.

So as time goes on, the pitch gets higher.

- So it goes... - ooop!

Ooop!

Zhhhrp!

When they finally merge, they create a giant.

By analyzing that data,

it's possible to establish that the new black hole from

the merger of these two original black holes weighs as much as

something like 140 times the mass of our sun.

It's really difficult to overstate

the importance of gravitational wave detection.

It's like adding on an entirely new sense...

All of a sudden, there's a brand-new way

to explore the rest of the universe.

Invisible cosmic collisions are just

the beginning of what gravitational wave

astronomy can reveal to us.

Now, scientists are using gravitational waves

to revisit other long-standing mysteries,

like what causes the brightest explosions

in the cosmos?

This is not an everyday car crash.

This is the most dramatic event that

you're ever gonna see in our universe.

Across the universe,

strange bursts of light puzzle astronomers.

For just a fraction of a second,

they shine more than a trillion times brighter

than the sun... Then, they vanish.

These brief flashes of light are known

as gamma-ray bursts or GRBs for short,

and they're such a mystery, because they are

insanely energetic, and we don't know what causes them.

For decades, these short gamma-ray bursts

have been an enigma.

No explanation was off limits, no matter how wild.

Is it a supernova?

Is it on alien civilization saying hello?

You know, we just don't know.

In August 2017, the Fermi Gamma-ray Telescope

detected another short gamma-ray burst,

but this one was different,

So a gamma-ray burst went off 130 million light-years away,

and it actually produced a ripple in space and time

that LIGO could detect.

Gravitational waves could help

finally reveal what causes

one of the brightest explosions in the universe.

LIGOS data suggests the culprit could be two

massive objects spiraling towards each other

and colliding.

But based on the gravitational wave data,

these two objects were too small to be black holes.

They had to be something else.

Not black holes, but the ultra dense

cores of collapsed stars called neutron stars.

A neutron star is what's left over

after a massive star collapses in on itself.

It's very, very dense, because it took all, essentially,

the mass of the core and contracted it into a really,

really small radius.

As the dense neutron stars spiral ever closer,

the gravitational wave signal gets stronger and stronger,

until they collide, releasing

an epic burst of gravitational waves.

Because they're not black holes,

light can get out.

And if you smash two things together at these kind of