Friday, June 17, 2016

Scientists Detect Gravitational Waves For The Second Time

An artist's animation shows the merger and the gravitational waves that ripple outward. Credit: LIGO/T. Pyle

For the second time in less than a year, scientists have detected gravitational waves -- ripples in the fabric of spacetime --  from a second pair of colliding black holes, validating the first gravitational wave detection in September 2015 that proved Einstein's 1915 general theory of relativity.  

The second gravitational waves were detected by both of the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington on December. 26, 2015 at 03:38:53 UTC.

According to general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes. During the final fraction of a second, the two black holes collide into each other at nearly one-half the speed of light and form a single more massive black hole, converting a portion of the combined black holes’ mass to energy, according to Einstein’s formula E=mc2. This energy is emitted as a final strong burst of gravitational waves. It is these gravitational waves that LIGO has observed on two separate occasions.


Gravitational waves carry information about the origins of black holes and about the nature of gravity that cannot otherwise be obtained. Physicists have concluded that these gravitational waves were produced during the final moments of the merger of two black holes -- 14 and 8 times the mass of the sun -- to produce a single, more massive spinning black hole 21 times the mass of the sun (In comparison, the black holes detected Sept. 14, 2015, were 36 and 29 times the sun's mass, merging into a black hole of 62 solar masses.)

This time, the gravitational waves released by the violent black hole merger resulted in a longer signal, or chirp, providing more data. The December chirp lasted one second; the September chirp lasted just one-fifth of a second. The higher-frequency gravitational waves from the lower-mass black holes better spread across the LIGO detectors' sweet spot of sensitivity.

"It is very significant that these black holes were much less massive than those in the first detection," said Gabriela Gonzalez, spokesperson of the LIGO Scientific Collaboration and professor of physics and astronomy at Louisiana State University. 

Scientists now have a small population of black holes from which to learn more about the universe.

"Because of their lighter mass, they spent more time -- about one second -- in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our universe," she said.

Gravitational waves are not sound waves, but researchers have converted the gravitational wave's oscillation and frequency to a sound wave with the same frequency, producing a "chirp" people can hear.