Gravitational Waves: Five Detections On

“We did it!” came the proclamation from David Reitze at the first Gravitational Waves general press announcement on the 11th of February 2016. This marked the first detection of Gravitational Waves in human history on the 14th of September 2014. Using the Laser Interferometer Gravitational-Wave Observatory (or LIGO) facilities, the merging of two black holes was found to cause an event so energetic and cataclysmic that it caused space itself to oscillate; space literally contracting and relaxing as the shock waves passed through the Universe.

Now, three years on, four more detections of gravitational waves have been made. Almost all of these have been due to black hole mergers, but as of the most recent announcement a new type of merger was observed: the merging of two neutron stars. The latest detection is even more exciting not only because the gravitational waves were observed, but due to good timing and the addition of the VIRGO detector to the LIGO network, its source was actually located and other instruments were used to measure it. And what we saw was astonishing.

First of all, what is a gravitational wave? A gravitational wave is one of the key predictions of Einstein’s theory of General Relativity – the theory which has been tested through again and again, and yet still stands. Simply put, this theory sees space as a fabric – named spacetime. This fabric is easy to visualise in 2D: imagine a flat sheet of elastic rubber which curves when it is pressed down on. This curvature represents the effect that mass has on spacetime: it curves it and forms dips in it. Now, if a powerful event occurs, in our scenario let’s say you flick the elastic, it causes the rest of the elastic sheet to vibrate very briefly before it settles again. This is analogous to a gravitational wave and its effect.

As this is the key prediction of Einstein’s theory – a theory which seems to be working so well in many other scenarios – we should be able to verify this important prediction as well. The only challenge, however, is that the change in spacetime due to the wave is tiny. How tiny? The wave that was detected in September 2014 changed the length of the 4km long arms of LIGO by a ten thousandth the width of a proton. An easier way to imagine this is that it changed the total length of the Solar System by the width of a human hair.

So, how did we measure this? Essentially, using colossal interferometers. Interferometers are actually very simple to make (in a lab, I mean): they involve two “arms” at right angles to each other which split a laser at their intersection. The separate beams then travel down those arms, reflect back when they reach the end, recombine at the intersection and finally go into a detector. The distances involved have to be precisely measured to allow the light to get back to the detector perfectly in “anti-phase”. Without going too much into detail of the behaviour of light, this essentially means that they cancel out their intensity (or brightness), so no light should be incident on the detector. However, since we know that gravitational waves change the shape of space itself, this means that when they pass through the interferometer, they change the lengths of its arms. The light will therefore reach the detector in a combination that is no longer perfectly “out of phase”, and so its intensity will change. Thus, one can say there has been a detection.

This project took the work of literally thousands of scientists. It was, and still is, a worldwide collaboration. The two LIGO detectors are found in the United States and the VIRGO detector is located in Italy. The parts for all of these detectors, however, were designed, manufactured and tested in many universities from Glasgow to MIT to Taipei to Perth in Australia, along with the data analysis of the results taking place in even more universities. This most recent detection was an even bigger collaboration. The team at LIGO and VIRGO instituted the help from another seventy observatories from across the Southern Hemisphere. And because of this worldwide network of trust, collaboration and common goal, the first images of the source of a gravitational wave were found less than twenty-four hours after the detection.

But, what are the bigger achievements here? What does this mean for Astronomy, science and the world as a whole? While the practical application of research such as this can be hard to see, I would argue that this is some of the most important research we can do with our current knowledge and technology. It helps us discover the true nature of the Universe. The first detection not only proved the existence of gravitational waves, but was also one of the first direct observations of black holes. It even raised further questions about them: Where did these black holes come from? Were they primordial? Did the stars which formed them orbit each other before collapsing and becoming black holes? If that’s the case, how is that even possible? These are questions which we still need to work on, even three years after the detection.

In my opinion, the most important discovery made so far comes from the latest detection. As we discovered the source of the gravitational waves, an ensemble of observatories pointed their instruments at it and detected a whole range of data. This allowed us, for the first time ever, to see the formation of platinum, gold, uranium and other heavier elements. This means that we have proved that heavier materials – that make up our planet, and part of our everyday lives – were formed in such colossal and destructive processes as these. And I think that connects us more to this research than we give it credit for.

Special thanks to Professor Sheila Rowan, who gave us a deeper insight into this research and its growing field.

[David O’Ryan]

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