Newton's Law of Universal Gravitation

Sir Isaac Newtown revolutionised our understanding of physical forces when in 1687, via his famous work ‘Principia’ he introduced his ‘law of gravity’ (Newton, 1687), presenting an equation which showed a gravitational attraction between any two objects. Until that time, all forces arose when two objects pushed or pulled each other using physical contact, yet Newton’s new force operated across empty space, for example, between the earth and the moon, which Newton himself described as “action at a distance” (Rothman, 2018).

Over time, the scientific notion of ‘fields’ was developed by a range of scientists in order to better understand how forces could be transmitted instantaneously from one object to another, be it electromagnetic fields, through which light can travel, or gravitational fields, through which the weak force of gravity may travel also at the speed of light and, interestingly, varying to the degree by which the objects may be moving.

(Rothman, 2018).

In his 1916 treatise on the general theory of relativity, Albert Einstein predicted the existence of ‘gravitational waves’, or invisible ‘ripples’ in the space-time continuum, not dissimilar to the ripples created in water when a heavy object such as a rock is thrown into a pond (Einstein, 1920).

All mass exerts gravity, yet it requires cosmic bodies of enormous mass travelling at enormous velocities to create gravitational waves large enough to travel at the speed of light and potentially be detected, thus providing information about where these bodies are located and the nature of gravity and the cosmos itself. The larger the waves the easier they may potentially be to detect, and so the best source of gravitational waves emanate from the catastrophic collisions of huge stellar bodies, such as collapsing neutron stars or, better still, when two black holes in orbit of each other coalesce into one body (LIGO Caltech, n.

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d.). In these situations, enormous bursts of waves explode out from the source and travel lightyears in all four dimensions for millions upon millions of years.

The act of detecting gravitational waves has, until recently, been impossible. Despite gravitational waves being generated by some of the most cataclysmic events in the universe, the sizes of these gravitational ripples are relatively small. It requires highly sensitive instrumentation to perceive them in any way. One of the first ideas for detecting the waves was to use Michelson ‘laser interferometry’, which is a technique involving the merging of two lasers into an ‘L-shaped’ pattern to create an ‘interference pattern’, known as ‘fringes’, by utilising mirrors at the end of each arm of the ‘L’. The fringes are detected using a device known as a ‘photodetector’, which can The length of the lasers change as gravitational waves stretch and compress space itself, and the interference pattern changes by measurable amounts, albeit minute amounts, which are able to be recorded by computers (LIGO Caltech, n.d.).

The first detectors were built in the 1980s in various universities around the world, proving that all of the major engineering challenges of building a laser interferometer were solvable. Construction of the ‘Laser Interferometer Gravitational-Wave Observatory’, otherwise known as ‘LIGO’, began in 1994 at duel sites in Washington State and Louisiana within the United States of America. LIGO’s interferometers are the largest ever built, with arms reaching 4km long. Despite this large length, the detectors were still not long enough for the sensitivity required to detect gravitational waves. As a result, devices known as ‘Fabry Perot Cavities’ were introduced on each arm near the beam splitter in the centre where the two arms cross. By using these cavities, the laser in each arm is bounced upwards of three-hundred times before combining with the beam from the other arm. (LIGO Caltech, n.d.). As longer is better when it comes to sensitivity, these increases have vastly improved LIGO’s detection capabilities.

In 2002, the observatories began scanning the heavens for gravitation waves. For the first eight years of its operation the program had no success and was eventually shut down in 2010 in order to undergo major equipment upgrades. The result of these upgrades became known as ‘Advanced LIGO’.

In February of 2016 scientists discovered the first tell-tale indicators of these gravitational wave ripples sweeping over the earth. It represented the very first direct detection of gravitational waves generated by the collision of two colossal black holes, both of which were ’30 solar mass’ or thirty times larger than our sun, and which were located approximately 1.3 billion light years away. This discovery was the culmination of decades of research and construction of huge instruments, known as ‘interferometers’, to detect the ‘warping’ of space-time caused by gravitational waves.

Additional upgrades to the LIGO detectors were complemented by a 2019 collaboration with the Virgo Interferometer, located at the European Gravitational Observatory, in Pisa, Italy (LIGO Caltech, n.d.). The result of this collaboration and upgrades has been an increased sensitivity that has allowed the detection of many more gravitational events that previously.

It appears that a completely new and revolutionary science has evolved, providing scientists and astrophysicists with the ability to plan even further experiments to detect far greater amounts of gravitational waves with far higher levels of equipment sensitivity. Soon Japan will join the LIGO-Virgo collaboration via its KAGRA facility (LIGO Caltech, n.d.), adding a fourth detector to the network, facilitating the ability to track gravitational waves to their original location with far greater accuracy. India is also constructing a detector, the LIGO-India instrument, adding once again to the growing global network (LIGO Caltech, n.d.).

Future advancements to the current interferometer technology available include the ‘Einstein Telescope’ in Europe, comprising three underground arms of ten kilometres each, and the ‘LIGO Cosmic Explorer’, a version of the current LIGO but with arms approximately ten times longer than current LIGO instruments and able to make millions of wave detections each year. Using these advancements, astronomers may be able to detects nearly all stellar black-hole mergers in the entire observable universe. There is also the possibility of finding rare forms of mergers which have not been detected so far, for example, a neutron star collapsing into a black hole, or binary black holes with enough surrounding material to be currently viewed by telescopes. These new instruments may act as ‘finder-scopes’ to the far reaches of the universe.

LIGO’s Cosmic explorer may also be able to probe the interior of neutron stars, which contain the densest matter in the universe, thus better understanding the fundamental structure of these exotic phenomenon. One of the most exciting capabilities of future interferometer advancements is the ability to provide an independent measurement of the geometry and expansion of the universe, thus providing astronomers with a completely new way to measure the ‘hubble parameter’ and also provide a method of calculating the amount of ‘dark matter’ and ‘dark energy’ in the universe, and in so doing perhaps help us understand what exactly is the true nature of these dark materials.

Today, the most advanced detectors, the ‘LIGO-Virgo’ collaboration, have found over fifty gravitational wave discoveries, on average one every week, allowing astronomers to perceive the universe in a completely different way.