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Two large,Watch Kill Bill: Vol. 1 Online "L"-shaped instruments separated by 1,865 miles are eavesdropping on the most extreme events in the universe.

The two Laser Interferometer Gravitational-Wave Observatory (LIGO) sites, located in Louisiana and Washington, have been tasked with picking up the faint signs sent out into the universe by black holes colliding in deep space.

These collisions are so extreme that they literally warp the fabric of space and time around them, like a rock being dropped into a stream. However, the ripples -- known as gravitational waves -- are extremely difficult to detect.

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"Gravitational waves are so difficult to detect because they are extremely faint," Nergis Mavalvala, a LIGO scientist working at MIT, said via email. "Even though a huge amount of energy is released when black holes collide, the effect of the waves here on Earth, a billion years later, is tiny."

In spite of having the odds against them, scientists have already used LIGO to detect the collisions of two different pairs of black holes billions of light-years from Earth. In doing so, they confirmed one of the last bits of a theory first put forth by Albert Einstein more than a century ago.

So, how does LIGO do it?

How LIGO works

When a gravitational wave passes through Earth, it stretches and shrinks the arms of the observatories by just a fraction of a proton, but because of the extreme sensitivity of LIGO's instruments, even that small movement is enough for scientists to know that a gravitational wave was there.

Because the speed of light is constant, it is unaffected by the warping of space-time around it, meaning that no matter if a gravitational wave is passing through or not, light will just keep on moving at its regular speed.

If matter is warped around light -- as is the case when a gravitational wave passes through Earth's part of space -- then it would change the distance the light needs to travel in order to get to any given point.

Original image has been replaced. Credit: Mashable

Scientists took this into account in designing LIGO by constructing two identical arms with a laser running through each.

Both arms of each observatory are 4 kilometers, or 2.5 miles, long, and while it may sound wasteful to have two identical observatories in two different parts of the country, it's actually necessary for gravitational wave research.

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If only one LIGO instrument picks up a wave signal, it might not truly be from a gravitational wave, but if both see the same thing at the same time, it's more likely that a gravitational wave did pass through.

The researchers running the observatories know exactly when the end of each laser will bounce against the end of the arm and head back toward the center. If the two arms of the laser don't match up exactly, that means they caught sight of a gravitational wave.

This happens because the wave expands and contracts the arms ever so slightly as it passes through Earth, meaning that the laser won't precisely match up back in the middle.

"What LIGO had to do to detect the waves was to measure the motion of mirrors (due to the passing gravitational wave) that was smaller than a single proton," Mavalvala said.

"Imagine that, put mirrors 4 kilometers [2.5 miles] apart and watch them get closer or farther to each other by a distance one-one thousandth of the size of a proton. That was the technical feat of LIGO, necessitated by the paucity [and] the weakness of the waves."

Ripples through you

Original image has been replaced. Credit: Mashable

The most mind-bending part of the whole operation is probably the fact that those gravitational waves pass through our bodies as well.

As gravitational waves ripple out through the cosmos, they warp the fabric of space and time, so they don't just change the arms of the observatories, they change all of the other matter surrounding it as well.

Now, it's not as if you'll be able to feel a gravitational wave as it passes through Earth's part of space. The ripples only slightly change the fabric of space and time, so it's not something that we can perceive as it's happening.

In other words, ripples shot into space by black holes colliding in massive, Earth-shattering explosions pass through our bodies and we don't even notice.

LIGO first captured hints of these black hole collisions on Sept. 14, 2015, when the detectors picked up their first gravitational waves. When those waves are translated into sound, the collision sounds something like a chirp.

On that day, space-time warped around us, allowing the sensitive instruments to find their waves rippling through the cosmos.

LIGO team members figured out that the wave was created by two black holes that were about 30 times the mass of the sun colliding about 1.3 billion years ago.

Scientists want to learn more about these gravitational in order to figure out how the objects that create them work.

Researchers still don't know much of anything about the inner-workings of black holes, so by studying the ways they interact with gravity, they can piece together a little more about how these mysterious objects -- and others like them -- fit in the fabric of our universe.

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