Riding the gravitational wave

Graeme Eddols explores the significance of the University’s contribution to gravitational wave research.

Gravitational waves are caused by some of the most energetic events in the universe. Their existence was predicted by Albert Einstein over 100 years ago in his famous General Theory of Relativity, and it has taken almost the same length of time for scientists and engineers to detect them. The Nobel Prize winning detection in 2015 opened an entirely new window into the universe that all previous forms of astronomy were blind to.

So, what are these mysterious waves, why don’t we feel them, and why are they so hard to detect? Black holes and neutron stars are the objects that got many astronomers and physicists like myself excited about studying space in the first place, and they are some of the main producers of this gravitational radiation. Black holes are typically formed from supermassive stars that have collapsed under their own gravity. Some lighter stars, still significantly heavier than our own sun, may instead collapse into neutron stars. If you took the sun and squashed it down to the size of Glasgow, you’d get a neutron star. These stars are so dense that a teaspoon’s worth of one would weigh as much as Mount Everest. 

Black holes and neutron stars are found orbiting around one another, in a variety of combinations, throughout the universe. As these large masses whirl around at around 60-70% of the speed of light, they send ripples through the fabric of spacetime itself. If you imagine two corks orbiting each other on the surface of a still pond, as they rotate around one another they will send out ripples in all directions. This is analogous to gravitational waves, except that they exist in four dimensions instead of two. 

The very first detection of gravitational waves was due to the collision of two black holes. This collision event released so much energy that, if it could have been observed in visible light, it would have outshone all the stars in the observable universe in that instant. However, even with this amount of energy, by the time the waves reach Earth they only disturb spacetime by about one thousandth of the diameter of a proton – this is why gravitational wave detectors have to be the most sensitive scientific instruments in the world. 

At the time of writing this article, there have been 61 publicly announced gravitational wave detections since the upgraded Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors came online in 2015. Further upgrades to the LIGO detectors, combined with the Italian Virgo detector (which will hopefully soon be joined by the Japanese KAGRA detector), have afforded the international collaboration of scientists greater sensitivity to gravitational waves, as well as better sky localisation. The latter is important for when two neutron stars collide, or a neutron-star merges with a black hole, since it signals the world’s astronomers to point their telescopes towards the source of the gravitational waves. 

Two neutron stars smashing together were first detected on 17 August 2017, in an event known as GW170817. This led to the dawn of what we now call “multi-messenger” astronomy, allowing gravitational wave scientists to work in unison with their electromagnetic and neutrino counterparts. Up until now, astronomy has always operated as part of the electromagnetic (optical, infrared, radio, etc) and neutrino spectrum, but gravitational waves are a completely separate and new way to unlock the secrets of the universe. This detection was significant, particularly in relation to understanding where heavier elements like gold and platinum come from. It also proved yet another one of Einstein’s hypotheses correct, that gravitational waves travel at the speed of light. This was discovered because the light from the neutron star collision, or “kilonova” as they are known, arrived on Earth at almost exactly the same time as the gravitational wave signal, a mere 1.7 seconds apart even after travelling for 130m light years.

The University of Glasgow has been instrumental in the detection of gravitational waves. For over 50 years, scientists have been working at Glasgow to develop the technologies required to make the current generation of detectors work. Professor Sir Jim Hough and his PhD supervisor, the late professor Ronald Drever, dedicated their entire lives to this pursuit. To this day, Hough imparts his vast knowledge and expertise to PhD students like myself, who conduct research as part of the Institute for Gravitational Research (IGR) in the University’s Kelvin Building. The IGR is not only one of the University’s largest research groups, but is also one of the largest Gravitational Research groups in the world, with a membership of approximately 70 scientists and doctoral students. The research group is split, with one half conducting research into the astrophysics and data analysis side, and the other half covering the experimental aspects of the detector design. 

Nobel laureate professor Rainer Weiss, who recently visited the University of Glasgow to celebrate Hough’s longstanding efforts in the field, mentioned Glasgow’s vital role in his Nobel Prize lecture. On the development of the suspension technology used to hold the mirrors in the LIGO project, he said “that four-stage suspension is a critical part of LIGO, and especially a critical part [of the fact] that we made a detection [of gravitational waves]”. This was in reference to the laser-pulled ultra-pure glass fibres which were developed in Glasgow. These glass fibres, each only around 0.4 millimetres thick, could theoretically hold an average person’s weight. They are designed to reduce unwanted noise, which arises from tiny fluctuations in temperature on atomic scales being transmitted into LIGO’s mirrors. These mirrors reflect a laser beam and LIGO measures the distance in each 4km long interferometer arm. When a gravitational wave passes by, the distance the light travels in each arm changes as the mirrors move. One arm gets stretched and one arm gets squashed, due to the polarised nature of gravitational wave radiation. This seemingly tiny noise can have a huge impact on the detector sensitivity. In fact, LIGO is sensitive enough to have its observing runs interrupted by earthquakes on the other side of the planet, the gravitational pull from aircraft flying overhead, and even wind vibrating buildings hundreds of miles away and transmitting those vibrations through the ground. 

Today, Glasgow is at the heart of research and advancement of the current and next generation of gravitational wave detectors, alongside colleagues all over the world. It is an incredibly exciting time to be working in such a scientifically diverse field, and Glasgow’s IGR group is no exception. Developments in new suspension technology, cryogenics, mirror coatings, laser interferometry, data analysis, and machine learning techniques will one day allow us to see smaller gravitational wave signals and hopefully allow us to observe right up to the edge of the observable universe.

For more updates from the Institute for Gravitational Waves, you can visit their Twitter @UofGravity.