Here’s a ridiculous idea: Imagine I snapped a twig in an isolated forest.

Then a listener on the opposite side of the planet hears the sound vibrations created by the snap, and just by hearing it, can tell you exactly where I was, at what time I snapped it, and even what kind of tree the twig came from.

Of course this is impossible.

But what if I told you that human beings could do something similar, but even more impressive?

Instead of a snapped twig, the source of the vibration is a violent event in space, an event that we can detect here on Earth even though it occurred billions of light years away and billions of years in the past.

All we had to do was build one of the largest and most sensitive devices ever created, the Laser Interferometer Gravitational-Wave Observatory.

LIGO.

I'm driving along one of the 4 kilometer long arms of LIGO.

The first ever observatory to observe gravitational waves.

The arm is so long that they actually needed to correct for the curvature of the Earth and we're about to go inside to see all the instrumentation.

I'm so excited.

We're putting on some of our clean room garb.

This is a class 10,000 clean room.

There's more classes, but this is pretty clean.

Yes.

Yeah this is cool.

So this is like Laser starts over there, then there's mirrors here, beam splitter, laser splits You've got arm going this way.

Another arm going that way.

It's all in this room.

This is like the brain the guts.

This is everything.

Up until extremely recently, astronomy was based almost exclusively on studying light.

But LIGO is not like other observatories.

It’s blind.

It senses no part of the electromagnetic spectrum like visible light, microwaves, or radio waves.

Instead, it detects gravitational waves, which are a completely separate phenomenon from light.

In 1916, when Albert Einstein published his theory of general relativity, he predicted that masses accelerating in space would create ripples in the fabric of spacetime itself.

For decades, no one knew if these ripples, called gravitational waves, actually existed.

But 99 years later in 2015, the LIGO team finally sensed a series of extremely tiny vibrations a thousandth the width of a proton.

To get this level of sensitivity, they needed to create an observatory that was unlike anything else on Earth.

Rather than pointing a mirrored sensor at the stars, they built an L-shaped detector with two 4-kilometer long vacuum tubes.

The arms are covered in concrete so that they are completely isolated from all vibrations.

They then use lasers and mirrors to detect the compression of gravity, with special suspensions that actively cancel out any vibrations generated from activity on the Earth.

And they needed to build two of them 3000 miles apart to operate in complete coordination, because a random vibration at one location could create a false signal that looks like a gravitational wave but isn't.

If a detection is made at both places, you eliminate the possibility of it being just a local disturbance.

The beam splitter itself is in this large chamber right here.

Oh cool.

But part of the reason that's so large as well.

Is not just the mirror, you've got get that quad suspension system and that quad suspension system is really where the magic is in reducing the ground vibration.

Because if you don't reduce those ground vibrations you don't get detections.

And you have that quad suspension system on .

.

.

on the four main mirrors as well as the beam splitter.

So the five largest optics that we have that is in the suspension system.

We really say every vibration bothers us and we mean it.

From the thermal effects on the mirrors to earthquakes in Australia they all can cause us problems.

After months of analysis, scientists confirmed that the vibrations they detected in 2015 were the signature of two black holes merging.

It was a violent event that had occurred 1.3 billion light years away and 1.3 billion years in the past.

It also confirmed that an entirely new type of astronomy was possible.

Wow.

So this is it.

So that's where the lasers exit the enclosure area and go directly into the vacuum system.

There's laser beams going through these tubes.

That is very exciting.

They're in infrared so we wouldn't be able to see them, if there were no protective tubes there.

But still.

There's this laser beam that comes out, but it's a bit messy.

It's got different types of spatial patterns and they clean it before it gets sent out into the arms, but then you're telling me now that they also clean the input that comes back in.

The output.

Yeah.

So you're getting rid of patterns of light you don't want and then you do that before it goes in and you also do that on the way back out.

So yeah the light comes in, recombines and then it goes down here for detection.

And then detection happens here.

So this part we're going to walk up to is where the detection happened?

Yeah.

I hate to say where the magic happened because the magic happened all over, but this is where the actual detections are made.

This is amazing.

You have to see this.

The beam makes it here where you've got the detector.

That's where the detection happens.

And I actually have the photodiode in my office that saw the first gravitational wave ever.

How did you get that?

Well cause I have to put it out.

We're putting googles on to protect from the infrared laser, which you wouldn't be able to see, but you would be able to feel it.

LIGO is allowing researchers to explore areas of the universe that were previously hidden because their energy is emitted by objects that are inherently dark, like the collision of black holes.

We now know that over millions of pairs of black holes revolve around each other in binary systems.

As they orbit, these pairs emit gravitational waves, which removes some of the system's energy and forces the pair to rotate faster and closer together.

But because this energy isn't released as light, prior to the detection of gravitational waves, we were never able to observe them and the very existence of such black hole binaries was contested.

But perhaps the most exciting possibility of gravity waves is yet to come.

In the very first moments of the universe, the electromagnetic force and the weak nuclear force, two of the fundamental forces, were indistinguishable.

But when these forces separated, they are believed to have produced gravitational waves.

And unlike light waves, gravitational waves interact extremely weakly with matter.

So much so that gravitational waves could still be traveling from a time when the universe was a billionth of a billionth of a billionth of a billionth of a second old.

If we can build a detector with enough sensitivity to observe these waves, they might give us some information about the very first moments of our universe, which is very exciting.

And while this degree of accuracy might be far off, the stunning success of LIGO so far has empowered the advancement of bigger, more sensitive gravitational wave detectors around the world and even in space.

Ultimately, gravitational waves tell us how stars live and die.

They give us insight into the lifespan of ravenous black holes They may reveal what happened at the very beginning of time.

Gravitational waves are a profound expansion of our human senses, broadening our vision and bringing us one big step closer to understanding the inner workings of the universe.

Now that you know all about gravitational waves, keep watching as Joe Hanson explores how scientists created humanity’s first image of a black hole with a telescope the size of the Earth.

Thanks to Draper and their Hack The Moon Initiative for supporting PBS Digital Studios.

You can learn more about Hack the Moon at wehackthemoon.com PBS is bringing you the universe with the SUMMER OF SPACE, which includes six incredible new science and history shows airing on PBS and streaming on PBS.org and the PBS Video app.