Gravitational Waves Detected: Einstein Proven Right

Scientists from the Laser Interferometer Gravitational Wave Observatory (LIGO) including a team from the ANU Research School of Physics and Engineering have announced the world first detection of gravitational waves by the LIGO detectors. The detection of gravitational waves confirms a prediction made by Albert Einstein’s Theory of General Relativity 100 years ago.

The gravitational waves detected by LIGO were emitted by the most violent explosion ever recorded by humankind, the inspiral and merger of two black holes. Occurring 1.3 billion years ago, each of the black holes contained the mass of around 30 suns, resulting in a final black hole containing the mass of the over 60 suns.

To detect gravitational waves LIGO bounces laser light between mirrors attached to two masses placed 4km apart. This laser light is used to measure the distance between the mirrors, which enables the detection of the changes induced by passing gravitational waves.

Dr Robert Ward from the Centre for Gravitational Physics elaborates “Our system to measure the separations, while incredibly precise, only works when the mirrors are very near (closer than the size of a single atom) to their ideal ‘operating’ positions. Since the ground, and indeed everything around, is already moving much more than that, we must actively, gently hold the masses in place such that we can measure their movements while letting them remain free enough to respond to gravitational waves”

“In fact the problem is much harder than this. If the masses are not already very very near their ‘operating’ positions, our system to measure where they are, and which we would use to hold them in place, doesn’t work at all!  Initially the masses are nowhere near their ‘operating’ positions, and so we must find a way to locate them precisely in order to bring them to their operating point.  Getting the mirrors to the operating point is called lock acquisition.”

The Department of Quantum Science team, led by Professor David McClelland, contributed a critical piece of hardware to LIGO called the Arm Length Stabilisation (ALS) system. This system utilises a second green laser for “lock acquisition” and is thus used to “turn the detector on”. The ALS system automates the complicated “reboot” process of the system. Because ‘locked state’ can be achieved quickly, this maximises the number hours a day the interferometer can be searching for gravitational waves.

The ALS system was designed, constructed and tested at the Department of Quantum Science facility. ANU researchers Bram Slagmolen and Robert Ward working with onsite staff installed and commissioned this system on the LIGO interferometers in the USA, culminating in the first lock being achieved in May 2014, with the first automated lock shortly thereafter in June 2014.

The Department also designed, built, installed and commissioned 30 small optics steering mirrors for routing the signal laser beam around inside the detector.

Professor Susan Scott from the Centre for Gravitational Physics also took a central role in the LOOCUP project, which has now evolved into the electromagnetic follow-up program, comprising the LIGO Scientific Collaboration (LSC) and Virgo, a European gravitational wave detector, with international collaborators. The aim is to provide electromagnetic confirmation of a gravitational wave event. Their emphasis is on using the SkyMapper optical transient telescope which played an important role in the recent first observing run of Advanced LIGO.

This ANU research was funded by Australian Research Council Grants.  LIGO is funded by the U.S. National Science Foundation, the Science and Technology Facilities Council (STFC) of the United Kingdom, Max-Planck-Society (MPS) in Germany, and the Australian Research Council.

Find out more about the contributions of the ANU, and other Australian partners in the LIGO project, at The Centre for Gravitational Physics.

The research has been published in the journal Physical Review Letters.

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Professor David McClelland
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