LIGO detects gravitational waves for third time

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LIGO scientists, including those from India, have for the third time successfully detected gravitational waves - ripples in space and time - generated by a merger of two massive black holes three billion light years away from Earth.
The new black hole, formed by the merger, has a mass about 49 times that of our Sun.
The finding confirms predictions made by the general theory of relativity which German scientist Albert Einstein formulated over 100 years ago.
The detection fills in a gap between the masses of the two merged black holes detected previously by The Laser Interferometer Gravitational-wave Observatory (LIGO), with solar masses of 62 (first detection) and 21 (second detection).

"We have further confirmation of the existence of stellar-mass black holes that are larger than 20 solar masses - these are objects we didn't know existed before LIGO detected them," said David Shoemaker, spokesperson for the LIGO Scientific Collaboration (LSC), a body of more than 1,000 international scientists.
"It is remarkable that humans can put together a story, and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us," said Shoemakers from the Massachusetts Institute of Technology (MIT) in the US.

The new detection occurred during LIGO's current observing run, which began November 30 last year.

LIGO observations are carried out by twin detectors located in Washington and Louisiana in the US.
Sixty-seven scientists from 13 Indian institutions are part of the LIGO Scientific Collaboration, under the umbrella of the Indian Initiative in Gravitational-Wave Observations (IndIGO).
The Indian team in LIGO includes scientists from the Chennai Mathematical Institute, Tata Institute of Fundamental Research (TIFR) Bengaluru, TIFR Mumbai, Indian Institute of Science Education and Research (IISER) Kolkata, IISER Trivandrum, Indian Institute of Technology (IIT) Bombay, IIT Madras, IIT Gandhinagar, and IIT Hyderabad.
LIGO made the first-ever direct observation of gravitational waves in September 2015 during its first observing run since undergoing major upgrades in a programme called Advanced LIGO.

The second detection was made in December 2015. The third detection, called GW170104 was made on January 4 this year.
In all three cases, each of the twin detectors of LIGO detected gravitational waves from the tremendously energetic mergers of black hole pairs.
These are collisions that produce more power than is radiated as light by all the stars and galaxies in the universe at any given time.
The recent detection appears to be the farthest yet, with the black holes located about 3 billion light-years away.
The black holes in the first and second detections are located 1.3 and 1.4 billion light-years away, respectively.

The study also puts Albert Einstein's theories to the test. For example, the researchers looked for an effect called dispersion, which occurs when light waves in a physical medium such as glass travel at different speeds depending on their wavelength; this is how a prism creates a rainbow.
Einstein's general theory of relativity forbids dispersion from happening in gravitational waves as they propagate from their source to Earth. LIGO did not find evidence for this effect.
"It looks like Einstein was right - even for this new event, which is about two times farther away than our first detection," said Laura Cadonati of Georgia Institute of Technology in the US.

"We can see no deviation from the predictions of general relativity, and this greater distance helps us to make that statement with more confidence," said Cadonati.
The finding will be published in the journal Physical Review Letters.