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Gravity Wave detected

One hundred years after Einstein predicted their existence, gravitational waves have been detected at LIGO

One of the predictions of Einstein's General Theory of Relativity is the existence of gravity waves. When a large amount of energy is generated, waves of space-time disturbance should be generated, like ripples in water. These ripples would alternately compress and stretch space as it progresses at the speed of light. In 1993, Russell Hulse and Joseph Taylor were awarded the Nobel Prize for their 1974 discovery of energy loss due to gravitational radiation in a binary pulsar (PSR 1913+16), the first evidence which provided some confirmation of Einstein's prediction.

LIGO interferometer prototype, showing the laser and the hub which contains the beam splitting mirror

However, direct evidence would need to wait another 41 years. The amount of distortion would be very small, so an extremely sensitive detection system is needed to detect it. Such a detector was built in the first decade of the 21st century in the US.

At the LIGO detector, Livingston, Louisiana, a gravitational wave detector was built between 2002 and 2010. The enormous interferometer has arms 4 kilometers long. It is funded (620 million US dollars) by the National Science Foundation (USA), the Max Planck Society (Germany), the UK Science and Technology Facilities Council and the Australian Research Council.

The first phase produced no positive results, and the facility was shut down in 2010 for an overhaul. In September 2015, the detector had a sensitivity four times that of the first phase, and will be enhanced again to its design sensitivity by 2021. The interferometers use a laser and a semi-transparent mirror at an angle of 45°. The light is thereby divided into two perpendicular beams which are then reflected back to reform a single beam. If the two beam parts are synchronised (wave crest matching wave crest), observers know that no spatial distortion has occurred. A gravity wave would distort the space unequally between the two arms of the interferometer, in a beat pattern as the space expands and contracts. Having two detectors so far apart assures the scientists that the phenomenon is not caused by a local effect, such as a seismic event, or, given the sensitivities involved, even a passing truck. The expected distortion is expected to be of the order of $10^{-21}$ m, which is a billionth the width of a proton.

LIGO mirror
Technicians adjusting a mirror on the interferometer at LIGO

The first positive result was announced on 11 February 2016. Two black holes with masses of the order of 30 times the Sun collided 1.3 billion light-years away, and the gravitational ripple it caused was detected in September 2015, 100 years after Einstein predicted gravitational waves!

The LIGO project utilises two sites: the Hanford Observatory (Washington State) and the Livingston Observatory (Louisiana), a separating distance of 3,002 km. Since light travels at 300 thousand km per second, it would take light 100th of a second (10 milliseconds) to travel between the two detectors directly. Triangulation therefore allows the direction of the wave to be determined by the difference in arrival times at the two detectors.

Article by Andrew Bone, February 17, 2016

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