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Listening To The Dark Side Of The Universe At LIGO

by Julie Boyle

Julie Boyle Summer 2010This summer I spent a month working in Louisiana at LIGO, CalTech and MIT's Laser Interferometer Gravitational Waves Observatory.

First predicted in 1916 by Albert Einstein's General Theory of Relativity, and often referred to as ripples in space time, gravitational waves are generated during extremely violent astrophysical events in which the velocities of objects such as neutron stars or black holes change by substantial fractions of the speed of light over a very brief period of time. They are so weak that scientists have been unsuccessfully searching with more and more sensitive instruments for over forty years, but now, with recent developments in technology, colossal instruments like LIGO are showing great promise. Detecting the elusive waves will provide totally new insights and open up a whole new exciting window on the Universe, leading to a new kind of astronomy. Information about the universe is currently restricted to that which can be seen using electromagnetic radiation such as light, radio waves and X-rays, but vast parts of the Universe are obscured by dark clouds and cannot be observed by means of conventional astronomical techniques. Gravitational waves can pass through these clouds unhindered. The Big Bang is believed to have created a flood of gravitational waves which still fill the Universe today and so LIGO will even allow us to study the creation, composition, development, and fate of the Universe.

Julie Boyle Summer 2010 4The perpendicular interferometer arms are each an impressive 4 000 m long and beams of light travel down them, bounce off mirrors at the far end and then return to interfere with one another. The idea is that a passing gravitational wave should change the interference pattern in a characteristic way. Since gravitational waves only interact very weakly with particles, LIGO requires ultra-high precision engineering. The researchers manage to adjust the position of the laser beam at the mirrors to within the range of a tenth of a human hair. Moreover, the laser interferometer is constructed in such a way that it can detect a change of less than one thousandth of the diameter of an atomic nucleus in the relative lengths of its 4 km arms. Put another way, that's equivalent to measuring a change of one hydrogen atom diameter in the distance from the Earth to the Sun.

Julie Boyle Summer 2010 6This summer was a particularly exciting time to visit as they're currently upgrading to Advanced LIGO, which will be able to detect events at 10-times-greater distances and will be sensitive to sources occupying a volume 1,000 times larger than we can see at present. The main way they hope to achieve this improved performance is by reducing stray noise. As the detector is sensitive to movements a hundred million times smaller than an atom, noise could mimic or hide a gravitational wave signal. Examples of sources are earthquakes, thermal noise from the Brownian motion of the molecules, traffic and the tree logging in the surrounding forests. It was really fascinating to see on LIGO's monitors the exact time that a train passed the local rail road crossing and the times of the day when the nearby highway was at its busiest. For the same reason, there was a strict 10 mph speed limit in LIGO's car park and people had to shuffle when passing certain areas!

Julie Boyle Summer 2010 3Although gravitational waves have not been directly observed, the study of pulsars has provided very strong indirect evidence. Pulsars are rotating neutron stars and they were first discovered by Dame Jocelyn Bell Burnell, a graduate of Glasgow University. Physicists at Glasgow University have also played a major role in designing the suspension systems for the silica mirrors. I spent most of my summer accompanying an Engineer and a Physicist at LIGO who were working together to perfect and test a new prototype suspension system. This suspension cushions the mirrors from any vibrations coming through the ground by basically hanging it on the end of several pendulums attached to steel blades. If noise causes parts of the suspension to move then this will cause the light intensity striking various photodiodes to change and the signal they produce is inverted and fed back to electromagnets, so that the motion is damped.  Light scattered by dust particles can interfere with the main laser beam and fake a gravitational wave signal, so we had to work in a clean room which blows air down towards the ground and we wore lab coats, gloves and masks.  Delicately attaching the electromagnets with screws to the suspension and testing their performance did at times feel very much like playing with a giant version of meccano. A lot of their time was spent keeping their electronic logbook up to date and communicating and explaining what they did, so that other scientists could understand and take over at a later stage. I also spent some of my days working with staff in LIGO's Science Education Centre. Trying their huge range of exhibits and meeting the visiting school teachers and students was a really fascinating experience.

It was interesting to learn that technology developed in the UK to improve the detector has already proved useful outside the field of gravitational waves. It has led to improved coatings for highly reflecting mirrors, improving navigation and time measurements. Certain techniques to treat the materials are now being used in the optics and engineering industries and a US company is now applying the suspension technology to the oil industry.

I found it ironic that I, like a billion others, was depending on Einstein every day to keep me safe. Trying to navigate my way from my accommodation in downtown Baton Rouge to the swamp land where the lab was located whilst driving on the wrong side of the road in unfamiliar territory in some pretty exceptional weather conditions, I was especially thankful that Einstein's theories allowed my GPS system to work accurately!

JB Summer Lisa gravitational waves 34469In addition to LIGO (USA), there is VIRGO (Italy/France), TAMA 300 (Japan) and GEO 600 (Germany/UK). By 2020 they will also be joined by LISA (The Laser Interferometer Space Antenna), a joint NASA/European Space Agency space-borne interferometer. This mission will use three spacecraft orbiting the sun and flying in formation three million miles apart, each housing floating cubes of metal. Laser beams fired between the spacecraft will allow minute changes in the distance between each of the cubes to be measured, accurate to 40 trillionths of a metre. The huge distance between the three spacecraft means that LISA will be able to detect gravitational waves of very low frequencies and so will be able to study signals that even Advanced LIGO cannot.

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