Laser probers sniff more gravitational waves from mega black hole smash

An international team of physicists has announced that the Laser Interferometer Gravitational Wave Observatory (LIGO) has detected gravitational waves from a second pair of colliding black holes.

The genius of Einstein

At the start of the year, the scientific community was buzzing with excitement after rumours started circling that LIGO had detected gravitational waves. In February, scientists confirmed that the rumours were true.

They had managed to detect gravitational waves for the first time, which provided further proof that Einstein’s theory of General Relativity was correct.

Einstein envisioned that spacetime – where all events in the universe take place – is like a fabric. Objects with larger masses warp the fabric to a greater extent than objects with less mass, causing smaller, neighbouring objects to fall towards them. The force of attraction between the objects is gravity. It causes ripples in spacetime and the energy takes the form of gravitational waves.

The amplitude of the wave decreases as it travels through space, so by the time they have reached the Earth many signals are too faint to detect. So far, only incredibly large gravitational waves produced from incredibly violent events – such as two massive black holes smashing into one another – have been detected by Advanced LIGO.

Second black hole merger event

The latest signal comes from the final moments when two black holes – one 14 times the mass of the Sun and the other eight times the size – merged to create a massive, spinning black hole 21 times the mass of the Sun. Mass equivalent to one solar mass (M_☉) was converted into energy and released as gravitational waves.

Physicists have analysed the signal which came from the last 27 orbits of the black hole before they spiralled into each other 1.4 billion years ago. The position of the source can be determined from the arrival time of the signals – which were 1.1 milliseconds apart – between the detector at Livingston and the detector at Hanford.

"It is very significant that these black holes were much less massive than those observed in the first detection," said Gabriela González, LIGO Scientific Collaboration (LSC) spokesperson and professor of physics and astronomy at Louisiana State University. "Because of their lighter masses compared to the first detection, 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."

The second discovery "has truly put the 'O' for Observatory in LIGO," said Caltech's Albert Lazzarini, deputy director of the LIGO Laboratory. "With detections of two strong events in the four months of our first observing run, we can begin to make predictions about how often we might be hearing gravitational waves in the future. LIGO is bringing us a new way to observe some of the darkest yet most energetic events in our universe."

The signal was first detected in the early hours of boxing day last year but the paper has only just been published in the Physical Review Letters today.

Due to the large international team analysing signals from two detectors, it took longer to write the paper than confirm the signal, Professor Ken Strain, who was involved in building the Advanced LIGO hardware told The Register. Strain is also the Deputy Director of the Institute of Gravitational Research at the University of Glasgow.

It has to be an extremely good signal, Strain explained. The signal has to be reproducible only once in every ten million times when scientists run a series of statistical analyses before they believe that the signal has come from a gravitational wave.

LIGO started operating in 2002 but did not detect any gravitational waves for years until the detector was made more sensitive. Advanced LIGO is roughly four times more sensitive than the first generation of LIGO detectors, allowing more distant signals to be detected.

More than 1,000 scientists from 15 countries, across 90 universities and research institutes take part in the LIGO Scientific Collaboration (LSC) group. The LSC carry out all LIGO research and continue to work together with the Virgo Collaboration, to analyse future signals from Virgo, another gravitational wave detector based in Italy. ®