The year 2016 heralded the arrival of a new probe of nature – the first direct observation of gravitational waves that earned scientists the 2017 Nobel Prize in Physics for this groundbreaking discovery.

Sure, we had seen hints before; in fact, the careful observation of the orbits of a binary star system agreed with Einstein’s prediction of gravitational waves. But this was the first time that the ripples of the very fabric of space-time were observed.

Measuring these ripples is no mean feat as the precision required is phenomenal – imagine measuring the distance between the earth and the moon to the nearest human hair. This is what the LIGO consortium achieved throughout more than half a decade of experimentation.

Such is the impact of this discovery that the Institute of Space Sciences and Astronomy (ISSA) has invited more than 130 scientists from all over the globe to Malta at the end of January. These great minds will descend on the island to discuss the meaning of these observations in the context of our understanding of fundamental physics and how to use these gravitational waves to push the boundaries of our current theories of gravity.

What are these mysterious ripples in space-time? Predicted in 1915 by Albert Einstein through the introduction of general relativity, gravitational waves really represent the oscillations of space-time itself. Einstein correctly predicted that anybody with mass will bend the fabric of space-time and that gravity, rather than acting like electromagnetic forces, arises from the bending of this space-time fabric. Einstein predicted that light passing close to a heavy mass, such as a star or even a galaxy, would be bent due to the presence of this mass. (see Figure 1)

First observed by Sir Arthur Eddington in 1919 during a solar eclipse, this light-bending phenomenon spelt the end of Newtonian gravity and the dawn of a new theory of gravity. The implications of this new theory were far-reaching, so much so that even today people are devising new tests to push Einstein’s theory to its limits – one of the most challenging being gravitational waves.

In the same way that when one drops a pebble into a pond ripples on the surface of the water can be seen, astronomical cataclysmic events, such as the collision of two black holes or the merger of two very dense neutron stars, cause gravitational ripples in the fabric of space-time (as shown in Figure 2).

Figure 2: Ripples in the space-time fabric being created by the merging of two neutron stars.Figure 2: Ripples in the space-time fabric being created by the merging of two neutron stars.

So how do we catch these gravitational waves? By carefully combining two laser beams over four kilometres of distance and carefully measuring how the interference pattern changes, as a gravitational wave distorts the space-time through which lasers are passing (see Figure 3). This experiment is extremely sensitive as the tiny gravitational wave signals are drowned out by more common effects, such as tides and seismic activity. Scientists have to carefully remove these effects, often more than a billion times stronger than the gravitational wave signal, to reveal the true nature of these gravitational waves.

Having detectors both in the US and in Italy, scientists are now capable of not only detecting these waves but determine the nature of the event that caused these waves and pinpoint a time and a place in the universe where these events happened.

The next step is space! That’s right, the European Space Agency is planning to put one of these experiments in space. Called eLISA (Laser Interferometer Space Antenna) and costing over €2.4 billion, this experiment will be able to give scientists unprecedented resolution and sensitivity to gravitational waves, allowing them to study the gravitational waves from the universe’s most catastrophic event – the Big Bang itself!

ISSA at the University of Malta is carefully studying the nature of these events and devising clever computer simulations to predict what these gravitational wave events will look like to the experiments of the future.

In collaboration with Spazju Kreattiv and Malta Café Scientifique, ISSA is also organising a public event discussing the nature of gravitational waves on January 24 at 7pm at the Aula Magna in the Valletta Campus of the University of Malta. To register, follow the link http://bit.ly/2zmHuwe.

Prof. Kristian Zarb Adami is director of the Institute of Space Sciences and Astronomy and Dr Jackson Levi Said is lecturer at the same institute.

Figure 3: LIGO experiments in the US were the first to detect gravitational wavesFigure 3: LIGO experiments in the US were the first to detect gravitational waves

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