Observatory Detects Gravitational Waves

Gravitational waves resulting from the coalescing of a pair of black holes 1.3 billion light-years from Earth.
(Courtesy: ESA)

On February 11, 2016 the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravity waves. The discovery confirms Albert Einstein’s prediction that such waves exist.

In 1918 Einstein proposed that accelerated, massive bodies produce tiny fluctuations in the fabric of spacetime. These fluctuations are referred to as “gravitational waves.”

Trying to detect gravitational waves is very difficult. In fact, scientists have been trying for 100 years.

LIGO was able to do it using two gravitational wave detectors – one in Livingston, Louisiana and the other in Hanford, Washington. The process of discovery involved more than a thousand scientists from all over the world.

The experiment that detected the waves used laser beams to monitor two perpendicular arms, each extending four kilometers to look for tiny changes in their length that might be caused by passing gravitational waves.

The infrastructure used to detect the waves was recently upgraded into an advanced state and the LIGO experiment obtained the image shown here that was the result of the experiment during the first observation run in the new configuration. The data was collected between September 2015 and January 2016.

The first indirect confirmation that gravitational waves exist came in the late 1970s, with observations of a pair of neutron stars – the dead cores of massive stars – rapidly orbiting each other.

One of the two neutron stars appears as a pulsating radio source, or pulsar, which allowed precise timing measurements of the system. As the two stellar remnants circle each other, scientists noticed that they move into tighter and faster orbits, and the rate of acceleration was just as it was expected if they were to lose energy by emitting gravitational waves.

However, the LIGO experiment provides the first direct detection of gravitational waves that scientists had been seeking for decades.

The recorded signal is very strong, and appears to come from a pair of coalescing black holes about 1.3 billion light-years away from Earth. The two monstrous bodies, with masses equivalent to 36 and 29 times the mass of the Sun, respectively, merged to form a single, even more gigantic black hole of 62 solar masses, releasing the remaining three solar masses in gravitational waves.

“Now that gravitational waves have been found, we can start delving into the physics of their sources. That’s where the move to space will make the difference,” explained Oliver Jennrich, LISA Pathfinder deputy project scientist at ESA.

Like light, gravitational waves also span a broad spectrum of frequencies, and different astronomical objects are expected to emit these waves all across the spectrum. Ground-based experiments like LIGO are sensitive to high-frequency waves, like those coming from coalescing pairs of black holes or neutron stars, with frequencies of 10-1,000 Hz.

To detect gravitational waves with lower frequencies, such as those from the merging of supermassive black holes at the center of massive galaxies, scientists need to investigate changes in length of much longer arms – about one million kilometers. This can only be achieved in space, using laser beams to monitor the distance between three freely falling masses separated by much larger distances that can be achieved on Earth.

The European Space Agency (ESA) is currently involved in an experiment with the LISA Pathfinder to measure gravitational waves in space. Spacehacker recently published a story on their experiment.


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