In 2015 history was made when LIGO (Laser Interferometer Gravitational-Wave Observatory) detected the first ever gravitational wave signal. This was an incredible technological achievement and the beginning of a completely new way of investigating the cosmos.
The collision of two massive objects shakes the fabric of space, making it ring like a bell and producing ripples that travel unhindered through space. For several decades astronomers and physicists worked on the construction of LIGO with the goal of detecting these ripples. LIGO is the most sensitive instrument ever devised. It consists of two laboratories, one located in Hanford, Washington, and the other in Livingston, Louisiana. Each houses an L-shaped interferometer whose arms extend for 4 kilometres (2.5 miles). Within these arms, a powerful laser beam travels back and forth, bouncing between mirrors before recombining to form an interference pattern. As a gravitational wave passes by, the fabric of space is pulled and pushed and this alters the distance between the mirrors and these tiny disturbances change the interference pattern. LIGO’s sensitivity is truly astonishing. It can detect changes in distance of around one billionth of the size of an atom. Having two observatories is important; like listening in stereo, it helps to determine the direction from which the waves arrive. It also ensures that a signal came from deep space and not a local disturbance.
“LIGO has provided the most direct evidence that we have for black holes and their properties.”
By comparing the data captured by LIGO to computer models, physicists can determine how each gravitational wave signal was created. It is possible to deduce the masses of the colliding bodies, the rate at which they were spinning, the energy released in the collision and how far away they are. LIGO’s first signal arrived from the collision and merger of two black holes located around 1.3 billion light years away. In the subsequent five years, LIGO received close to one hundred signals. Almost all of them came from collisions between pairs of black holes. The most epic was the collision and merger of black holes with 85 and 66 times the mass of the Sun that produced a black hole of 142 solar masses. During this collision, a mind-boggling nine solar masses were converted into pure energy in the form of gravitational waves.
In 2017, the Italian gravitational wave observatory Virgo also achieved the exquisite sensitivity necessary for the detection of gravitational waves and joined the LIGO observatories in their quest for distant cosmic dramas. Later that year on 17 August one of the most spectacular duly arrived. This event, named GW170817, was the first detected signal to come from the merger of two neutron stars rather than two black holes. Neutron stars are bizarre objects formed from the collapsed cores of stars that have run out of nuclear fuel. They are just 20 kilometres in diameter but contain at least one and a half times the mass of the Sun. In many ways they are like gigantic atomic nuclei. This was the first time and, so far, the only time that the source of a gravitational wave signal has been located with optical instruments, heralding the dawn of multi-messenger astronomy. The combination of optical and gravitational data has greatly advanced our understanding of what happens when two neutron stars collide. It is like being able to both see the lightning and hear the thunderclap. These observations lent support to the idea that many of the heavier chemical elements such as gold are created and dispersed in neutron star collisions.
“LIGO offers wonderful new tests of our best theory of gravity, Einstein’s theory of general relativity.”
In 2020, LIGO’s operations were suspended to allow for a major upgrade of the system. Now, after a three-year hiatus, LIGO is back up and running. On 24 May LIGO started a new observing run with refined instruments. With its enhanced sensitivity, it is expected to detect a gravitational wave signal every two to three days. LIGO is the lynchpin of the LIGO-Virgo-KAGRA collaboration—a partnership with the world’s other two gravitational wave observatories: Virgo in Italy and KAGRA in Japan. The construction of a third LIGO detector in India has also recently been approved. This expansion of the global network of gravitational wave observatories will help to pinpoint the location of gravitational wave sources so that they can also be studied optically.
LIGO has provided the most direct evidence that we have for black holes and their properties, and offers wonderful new tests of our best theory of gravity, Einstein’s theory of general relativity. By observing and studying the mergers of black holes and neutron stars, scientists are gaining new insights into fundamental physics, the nature of gravity, and the evolution of the universe itself. The restart of LIGO and the global gravitational wave research network launches a new phase of deep space exploration. We can look forward to more incredible discoveries in the near future.