Second Gravitational Waves Detection for LIGO
When two black holes merge, gravitational waves ripple outward as the black holes spiral toward each other. The black holes — which represent those detected by LIGO on Dec. 26, 2015, formed a single black hole 21 times the mass of the Sun. In reality, the area near the black holes would appear highly warped, and the gravitational waves would be too small to see.
Artist’s concept by T Pyle/LIGO
They’ve done it again over at LIGO. Scientists in the LIGO Scientific Collaboration and the Virgo Collaboration detected the faint, almost imperceptible gravitational waves from yet another black hole merger. Rumors have been flying the past few days about this (the first detection was announced earlier this year). Today, scientists using the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Hanford, Washington and Livingston, Louisiana made the news public: they detected a second gravitational wave signal on Christmas Day, 2015. It was a faint chirp generated by the cataclysmic collision of two massive black holes lying some 1.4 billion light-years away.
Here’s what the folks at Massachusetts Institute of Technology (who participated in the study) had to say about the discovery (which was formally announced today at the American Astronomical Society meeting in San Diego):
The researchers calculated that the gravitational wave arose from the collision of two black holes with masses of 14.2 and 7.5 times the mass of the Sun. The signal picked up by LIGO’s detectors encompasses the final moments before the black holes merged: For roughly the final second, while the signal was detectable, the black holes spun around each other 55 times, approaching half the speed of light, before merging in a collision that released a huge amount of energy in the form of gravitational waves, equivalent to the mass of the sun. This cataclysm, occurring 1.4 billion years ago, produced a more massive spinning black hole that is 20.8 times the mass of the Sun.
This second detection of gravitational waves, which once again confirms Einstein’s theory of general relativity, successfully tested LIGO’s ability to detect incredibly subtle gravitational signals.
The signal the scientists detected wasn’t like a loud “belch” of data. Indeed, it was the opposite: very faint — almost a “blip” in the data. It took quite a bit of data analysis to tease the signal out of the noise, but once the researchers figured it out, they knew they had another notch in their gravitational wave belts.
LIGO’s Gravitational Wave Mission
I got the chance to visit the Livingston, LA facility some years ago during a meeting. I was amazed at the painstaking care the scientists take to build an instrument capable of detecting these signals. Sorting them out from all the other “noise” in the universe is even more precise. They also have to deal withthe “noise” of things like gravel trucks and trains passing by here the facilities. That requires a lot of data analysis and a clear understanding of what they’re looking for in the data.
LIGO’s two interferometers, each four kilometers long (2.5 miles) are designed so that each detector should stretch by an infinitesimal amount if a gravitational wave were to pass through. On Sept. 14, 2015, the detectors picked up the very first signal of a gravitational wave, It, too, was caused by a black hole merger. It stretched each detector by as little as a fraction of a proton’s diameter. Just four months later, on Dec. 26, LIGO recorded a second signal, which stretched the detectors by an even smaller amount. That may seem small, particularly when you think about the tremendous cataclysm that created the waves, but tiny wiggles of data such as these often provide some of the most profound insights into how objects work in the universe.
In particular, the detection of gravitational waves will help answer some important questions about how black holes merge, and even how the massive ones form in the first place. There are many theoretical scenarios, and data from LIGO will help scientists sort out which ones really help explain what we observe and know about these cosmic monsters.
Gravitational wave science is really just at the starting gate of what we will be able to discover about the universe when events and objects do interesting things. I look forward to further detections as the LIGO folk continue their search for their once-elusive prey. (If you want to read more details about this second discovery, check out the MIT News source story and there’s a Youtube video of the “chirp” here and a very cool video animation of the merger, as well.)