Gravitational waves detected: Einstein proven right

Scientists from the Laser Interferometer Gravitational Wave Observatory (LIGO) Collaboration, which includes The Australian Consortium for Interferometric Gravitational Wave Astronomy (ACIGA), have announced the detection of gravitational waves by the LIGO detectors. ACIGA includes 56 scientists at six Australian universities including The Australian National University, the University of Western Australia, the University of Adelaide, the University of Melbourne, Monash University, and Charles Sturt University. The gravitational waves were emitted by the inspiral and merger of two black holes, each of which contained the mass of around 30 suns. The final black hole contains the mass of the over 60 suns. Occurring 1.3 billion years ago, this was the most violent explosion ever recorded by humankind.

ANU Centre for Gravitational Physics researchers played a crucial role in the discovery. The CGP, led by Professor David McClelland, contributed a critical piece of hardware for the Advanced LIGO detectors called the Arm Length Stabilisation system. This system is required to “turn the detector on”. To sense the gravitational waves we attach mirrors to two masses separated by 4 km and bounce laser light between them. We use the laser light to measure where the mirrors are with respect to each other, and this lets us record any changes in their separation induced by a gravitational wave. There is a problem though, in that 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. In LIGO there are five relevant mirror separations to measure, so this is a very complex and coupled control problem. The technical term for this is lock acquisition. At ANU we solved this by introducing a second, different coloured laser beam. The main (science) laser used to record the signal is infrared (1 micron). The laser we use to determine the position of all the optics in the first place is green (532 nm). The green laser system is not precise enough to measure gravitational waves directly, but it allows us to lock mirrors in groups of 2 and then deterministically move them all into ‘resonance’ with the science laser. This process has been automated so that now should something like a major earthquake cause the mirrors to drop out of this ‘locked state’, our green system clicks into action and sets up the system again. Because we can get back into the ‘locked state’ quickly, this maximises the ‘duty’ cycle, ie how many hours a day our interferometer is searching for gravitational waves. This was also one of the reasons why the commissioning of aLIGO has been so much quicker than its predecessor initial LIGO – knowing that the complicated lock acquisition process is fully automated makes it easier to perform the necessary tests to improve sensitivity.

Funded by the Australian Research Council (ARC), we designed, constructed and tested the system in the ANU 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.

Also funded by the ARC, the ANU CGP also designed, built, installed and commissioned 30 small optics steering mirrors for routing the signal laser beam around inside the detector.

The CGP data analysis group, led by Professor Susan Scott, took a central role in the LOOCUP project, which has now evolved into the electromagnetic follow-up program, comprising the LSC and Virgo with international collaborators. The aim is to search for burst gravitational wave sources using a multi-messenger astronomy approach, providing electromagnetic confirmation of a gravitational wave event. The group has been involved in the important quest to analyse coincident event triggers from the LIGO-Virgo interferometer network in close to real-time and has contributed to the construction of the data pipeline. Their emphasis is on using the SkyMapper optical transient telescope which played an important role in the recent first observing run of Advanced LIGO.

Now that we have arrived at the dawn of gravitational wave astronomy, we at the ANU CGP are continuing to work to improve the sensitivity of the detectors, focusing on the quantum noise. Advanced LIGO is now close to a level of performance where how well we can measure a GW induced change will be limited by inherent quantum noise accompanying the laser light used for sensing! In the quantum world, trying to measure or observe an object disturbs its state reducing the accuracy of the measurement. Again funded by the ARC, we at the ANU CGP are in the process of developing a system called a ‘squeezed light generator’, which can be installed on Advanced LIGO in the future. With this system we can beat even these quantum limits, opening up the observable GW universe and thus the GW event rate by another factor of 30.

The ANU Centre for Gravitational Physics is an active partner in the Australian Consortium for Interferometric Gravitational Wave Astronomy (ACIGA). Our ACIGA partners also made significant contributions to Advanced LIGO. For more information about ACIGA, including details about the contributions made by our Australian partners to Advanced LIGO, please visit the ACIGA website.

More details about the event and the detector, including links to the published papers, can be found on the LIGO Collaboration website.

Updated:  15 January 2019/ Responsible Officer:  Head of Department/ Page Contact:  Physics Webmaster