Thursday, February 11, 2016

Scientists at LIGO prove Einstein's theory of gravitational waves

Scientists have for the first time found telltale ripples in the fabric of space and time, confirming one of Albert Einstein’s strangest, yet most elegant, predictions.

This discovery, announced Thursday, comes 100 years after Einstein first theorized gravitational waves. It will usher in a new age of research and discovery for physicists around the world.

Researchers working with the recently upgraded Laser Interferometer Gravitational-Wave Observatory (LIGO) — a set of two identical observatories located in Washington and Louisiana — revealed that they have directly seen the ripples in space-time, which are known as gravitational waves, created by two black holes that collided 1.3 billion years ago.

Each black hole was 30 times the mass of the sun.

"We have detected gravitational waves. We did it,” said David Reitzecaltech, the executive director of LIGO, at a press conference in Washington, D.C.. The waves were detected on Sept. 14, 2015, scientists said.

Gravitational waves are produced when two black holes, two neutron stars or a black hole and a neutron star collide, distorting the fabric of space-time around them in the process. Those ripples propagate out into the universe, distorting the space-time, around other objects, including Earth.

These three GIFs, each 10 seconds, are a simulation of the merger of two black holes and the resulting emission of gravitational radiation.
The colored fields represent a component of the curvature of space-time. The outer red sheets correspond directly to the outgoing gravitational radiation.
The brighter yellow areas near the black holes do not correspond to physical structures but general indicate where the strong non-linear gravitational-field interactions are in play.

“When we find gravitational waves, that will change everything because finding gravitational waves will not only be discovering something, but it will open a door to millions of new discoveries,” LIGO scientist Szabolcs Marka said ahead of the announcement.

The new discovery also confirms one of Einstein’s strangest predictions put forth as part of his general theory of relativity 100 years ago.

General relativity combines two different ways of understanding how the universe works. Einstein’s special relativity showed how mass and energy are related, while Newton’s law of universal gravity explains how objects in space are attracted to one another.

After Einstein developed general relativity, he posited that gravitational waves should exist when two extremely massive objects collide, because space-time can become warped within the context of the newly unified theory.

This isn’t general relativity’s last test, however. Scientists still aren’t sure how to reconcile quantum theory with general relativity, and it’s possible Einstein’s theory breaks down in other ways yet to be found.

Imagine space-time as a web stretched out and held taut like the mesh on a trampoline. When you place massive objects like stars, black holes and planets on that fabric of space-time, it stretches and distorts it. That distortion creates the force we feel as gravity.

“We don’t think of space as a thing, as a stuff, but everything that happens in the universe is happening in this framework which we call space-time,” astrophysicist Katie Mack, who is unaffiliated with LIGO, said in an interview.

“You can visualize it as a malleable thing. It can stretch; it can squeeze; it can have waves propagating through it. And it’s a very weird thing to think about, but everything that happens in the universe — a bit of light traveling from one place to another, or mass moving around — that’s happening in space-time, and it’s affecting space-time and it’s being affected by space-time.”

The space-time around various objects can also be changed by other massive cosmic objects nearby — much as the sun affects the orbits of the planets in the solar system.

As black holes or neutron stars collide, they create ripples in space-time, which are sent out into the universe.

Models of what these waves might look like show them behaving similarly to sound waves, oscillating up and down, creating valleys and peaks. As the two colliding objects get closer to one another, the peaks and valleys become higher and lower as well as closer together.

This movement eventually creates what’s known as a “chirp,” which looks like a high-frequency blast as the two objects reach the moment when they merge, Mack said.

Some scientists have converted those “waveforms” into actual sound to demonstrate, and in anticipation of Thursday’s announcement, some even posted videos on themselves trying to mimic a chirp.

Although these ripples are created by some of the most extreme collisions in the universe, they can only be measured with the most sensitive instrumentation.

The first studies looking at the feasibility of detecting gravitational waves with something like LIGO were published in the 1970s, and the instrument was funded by the National Science Foundation in 1979.

Some people thought that these waves were too hard to detect, that the sensitivity needed to detect the waves wasn’t possible with instruments on Earth.

There have also been some false alarms. In 2014, scientists working with a different instrument called BICEP2 announced what they thought was the discovery of primordial gravitational waves — ripples in the fabric of space-time created when the Big Bang happened.

After further analysis, it was later found that the signal was actually just dust masquerading as a gravitational wave signature, overturning the hope that scientists had found the first direct hint of the ripples.

The LIGO detectors in Louisiana and Washington look like two giant “L’s” from above. When the twin detectors are used together, they can see tiny changes in space-time.

Each arm of the L is 4 kilometers, 2.5 miles, long and has a powerful laser running through it.

Once the light of the laser makes it to each end of an arm, a mirror bounces it back toward the bend in the detector. Because the two halves of the lasers are exactly the same length and each move at the speed of light, they should make it back to the bend at the same time, however, if a gravitational wave passes through, the arms won’t match up, meaning that the light won’t reach the bend in the detector at precisely the same moment.

“If a gravitational wave were to slightly (about 1/1000 the diameter of a proton) stretch one arm and compress the other, the two light beams would no longer completely subtract each other, yielding light patterns at the detector output," LIGO says on its website.

Scientists need two detectors in order to be sure that whatever signal they observe is actually caused by a gravitational wave. If both detectors catch sight of the wave and are consistent with one another, then there’s a higher chance that it’s a real signal as opposed to just some fluke in the system.

LIGO works because the speed of light is incredibly constant, Mack said.

“The fascinating thing — it almost gives you goosebumps — is that the reason you can measure this stuff is that the speed of light is so constant that it cannot change even when space-time is stretched,” Mack added.

“The light doesn’t care that the space-time has warped. The light doesn’t care that the distance is longer. It’s still going to go the same speed, and so it’s going to take longer to get to the end,” she said.

LIGO also recently completed its first science run as a newly upgraded instrument with higher sensitivity than ever before.

The two detectors — inaugurated in 1999 — were online from 2002 to 2010, but since the end of that run, the instrument was shut down for the redesign, which ultimately made it at least three times more sensitive than the first iteration.

By detecting gravitational waves, scientists will also “discover a way to listen to the death cry of stars, neutron stars, black holes, what we could not hear before,” Marka said.

“When we observe them [gravitational waves] regularly we will actually observe things which are specifically not available through any other means. It’s like how X-ray changed medicine,” he added.

Scientists will now be able to learn more about the black holes and stars that sent out the gravitational waves.

The waves carry with them information about what the objects that created them were like. Because scientists have been modeling what a signal from one of the waves could look like they have a sense of what kinds of ripples a cosmic collision would create.

By revealing these ripples in the universe, it can also tell us something about how many black holes there are out there while also teaching scientists more about how galaxies grow, helping to refine our understanding of our place in space.

The discovery also likely marks a shift in the way physicists explore the universe.

Instead of just viewing cosmos in visible, infrared and ultraviolet wavelengths of light, by investigating gravitational waves, scientists will have a new way of probing the universe for information.

“Imagine that you have all of your senses,” Marka said.

“But you can’t hear, and the first day you gain hearing, you get a new life. Imagine how your life would change if you can actually observe, sense your surroundings in a very different way. This is what we actually gain with gravitational waves.”

It's that big of a deal

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