Scientists Detect Gravitational Ripples in Space-Time

merging black holes
The collision of two black holes holes—a tremendously powerful event detected for the first time ever by the Laser Interferometer Gravitational-Wave Observatory, or LIGO—is seen in this still from a computer simulation. LIGO detected gravitational waves, or ripples in space and time generated as the black holes spiraled in toward each other, collided, and merged. This simulation shows how the merger would appear to our eyes if we could somehow travel in a spaceship for a closer look. It was created by solving equations from Albert Einstein's general theory of relativity using the LIGO data. LIGO/CalTech

Sometimes the cosmos surprises us with unusual events we never knew could occur! About 1.3 billion years ago (back when the first plants were showing up on Earth's surface), two black holes collided in a titanic event. They eventually merged to become one very massive black hole with the mass of about 62 suns. It was an unimaginable event and created ripples in the fabric of space-time. They showed up as gravitational waves, first detected in 2015, by the Laser Interferometer Gravitational Wave Observatory (LIGO) observatories in Hanford, WA and Livingston, LA.

At first, physicists were very cautious about what that "signal" meant. Could it really be evidence of a gravitational wave from a black hole collision or something more mundane? After months of very careful analysis, they announced that the signals the detectors "heard" were the "chirp" of gravitational waves passing by and through our planet. The details of that "chirp" told them that the signal originated from the merging black holes. It is a huge discovery. 

Opening a Window on a New Science: Gravitational Astronomy

To understand the big hoopla about detecting gravitational waves, you have to know a little about the objects and processes that create them. Back in the early part of the 20th century, the scientist Albert Einstein was developing his theory of relativity and predicted that the mass of an object distorts the fabric of space and time (space-time). A very massive object distorts it a lot and could, in Einstein's view, generate gravitational waves in the space-time continuum.

 

So, if you take two really massive objects and put them on a collision course, the distortion of space-time would be enough to create gravitational waves that work their way out (propagate) across space. That is, in fact, what happened with the detection of gravitational waves and this detection fulfills Einstein's 100-year-old prediction.

How Do Scientists Detect Find These Waves?

Because the gravitational wave "signal" is very difficult to pick up, physicists have come up with some clever ways to detect them. LIGO is just one way to do it. Its detectors measure the wiggles of gravitational waves. They each have two "arms" that allow laser light to pass along them them. The arms are four kilometers (almost 2.5 miles) long and are placed at right angles to each other. The light "guides" inside them are vacuum tubes through which laser beams travel and eventually bounce off mirrors.  When a gravitational wave passes by, it stretches one arm just a small amount, and the other arm shortens by the same amount. Scientists measure the change in the lengths using the laser beams. Both LIGO facilities operate together to get the best possible measurements of gravitational waves. 

There are more ground-based gravitational wave detectors on tap. In the future, LIGO is partnering with India's Initiative in Gravitational Observation (IndIGO) to create an advanced detector in India. This sort of collaborative is a big first step toward a global initiative to search out gravitational waves. There are also facilities in Britain and Italy, and a new installation in Japan in the Kamiokande Mine is underway.

 

Heading to Space to Find Gravitational Waves

To avoid any possible Earth-type contamination or interference in gravitational wave detections, the best place to go is to space. Two space missions called LISA and DECIGO are under development. LISA Pathfinder  was launched by the European Space Agency in late 2015. It is really a testbed for gravitational wave detectors in space as well as other technologies. Eventually, an "expanded" LISA, called eLISA, will be launched to do a full hunt for gravitational waves. 

DECIGO is a Japan-based project that will seek to detect gravitational waves from the earliest moments of the universe.

Opening a New Cosmic Window

So, what other types of objects and events excite gravitational wave astronomers?  The biggest, splashiest, most catastrophic events, such as black hole mergers, are still prime candidates.

While astronomers know that black holes collide, or that neutron stars can mesh together, the actual details are difficult to monitor. The gravitational fields around such events distort the view, making it tough to "see" details. Also, these actions can occur at great distances. The light they emit appears dim and we don't get a lot of high-resolution imagery. But, gravitational waves open up another way to look at those events and objects, giving astronomers a new method for studying dim, distant, yet powerful and downright weird events in the cosmos.