Researchers at the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) have discovered gravitational waves for the second time, just four months after they first heard the signals Einstein predicted in his General Theory of Relativity.
The event, called GW151226 and discovered on Boxing Day 2015, came from the collision of two black holes significantly smaller than September’s discovery at 14 and eight times the mass of our Sun, and produced a single spinning black hole 21 times heavier than the Sun. The collision occurred approximately 1.4 billion years ago and released around the same amount of gravitational energy equivalent to the mass of our Sun.
“It is very significant that these black holes were much less massive than those observed in the first detection,” says Gabriela González, 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.”
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Gravitational waves are caused by the collision of two objects, but the effects are so small that it takes the collision of massive objects, such as black holes, to create the faintest detectible signal. The mass of an object distorts the fabric of spacetime in a way similar to the material of a trampoline when you stand on it. When two black holes get closer they begin to swirl together with increasing speed until they collide, producing gravitational waves that send ripples though the fabric of spacetime like ripples in water.
LIGO works by firing two laser beams with exactly the same frequency down two 4km tubes at 90 degrees to each other, which are then reflected back again. The system is designed so that the signals returned are precisely matched to cancel each other out, and so nothing is captured by a sensitive light meter. However, if a gravitational wave passes across the path of these lasers their frequency is altered slightly, meaning the frequencies do not cancel each other out and a light signal is detected.
But the system is too sensitive to work in isolation, even a car driving past can create a false signal, so a second detector with exactly the same experiment has to be built a significant distance away. That’s why LIGO in Louisiana detected a signal 1.1 milliseconds before the detector in Washington, confirming they were in fact gravitational waves.
Where do they come from?
It is still too early to pinpoint where the collision event happened, but the European interferometer Virgo, which is scheduled to open later this year, will open up a network of ground-based detectors that will significantly improve the localisation of the signals.
LIGO will resume capturing data this autumn and improvements in sensitivity means researchers hope to double the amount of the Universe covered.
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