As far as we know, there are four fundamental forces in the universe: Strong Nuclear, Electromagnetic, Weak Nuclear and Gravitational. The strongest force is the Strong Nuclear force, which keeps the nuclei of atoms together.
Every day we experience the Electromagnetic Force, as the electrons in our skin repel electrons in the objects we touch. This is about a hundred times weaker than the Strong Nuclear Force.
The Weak Nuclear Force is about ten billion times weaker than the Electromagnetic Force. This force is responsible for particle and nuclei decay, which produces radioactivity.
Gravitational Force is the weakest of all four fundamental forces, approximately 1038 times weaker than the Strong Force, 1036 times weaker than the Electromagnetic Force and 1029 times weaker than the Weak Force. Because of this, Gravitational Force is of little importance on a subatomic scale – but is nonetheless the dominant force at the macroscopic scale: Gravitational Force forms, shapes, and puts astronomical bodies like planets, stars, galaxies and galaxy clusters into their respective orbits.
Gravitational Force is the phenomenon by which things that have a mass are attracted towards each other. We experience it as gravity on our home planet and we have to fight against it whenever we want to travel into space. But despite how intimate we are with its effects, it has only been recently that we have began to understand the nature of Gravitational Force better.
In 1916, Albert Einstein predicted that the Gravitational Force is a consequence of the curvature of space-time. His theory predicted that catastrophically violent events could create sudden changes in the curvature of space-time, similar dropping a stone in to a pond. The effect of this event would then radiate out as waves.
Einstein showed that massive, compact objects like neutron stars or black holes orbiting each other would produce gravitational waves, losing energy and causing them to orbit closer and faster until they coalesce, producing a huge final wave. These ripples would travel at the speed of light throughout the entire Universe, and would carry information about their origins, but also clues to the nature of gravity itself.
The problem with detecting gravitational waves is that they are normally too weak for our sensors. That means that the only ones that can be detected are those produced by colossal catastrophic events. These would include colliding black holes, , coalescing neutron stars or white dwarf stars, and even the wobbly rotation of neutron stars that are not perfect spheres.
The first proof for gravitational waves came in 1974, when two astronomers from the Arecibo Radio Observatory in Puerto Rico discovered two extremely dense and heavy stars in orbit around each other – what is known as a “binary pulsar”. After eight years of measuring how the period of the stars' orbits changed over time, the astronomers found that the two stars were drawing closer to each other at precisely the rate predicted by Einstein’s general relativity.
The binary pulsar has now been monitored for over 40 years, leaving no doubt that it is emitting gravitational waves. As such, timing of pulsar radio emissions became a kind of standard method for hunting gravitational waves among astronomers: and found similar effects, further confirming the existence of gravitational waves. But the problem is that these are indirect measurements that show the waves mathematically.
The first real, direct observation of gravitational waves came on 14 September 2015, when the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of ground-based observatories in the US (Washington and Louisiana), announced the observation of distortions in space-time caused by passing gravitational waves generated by two colliding black holes nearly 1.3 billion light years from Earth. A second observation followed in December of that year, and the third and latest observation came in January 2017.
The ability to detect gravitational waves is like having a whole new sense, so can it help us with anything else? The answer is gravitational-wave astronomy. This is an emerging field where astronomers can use gravitational waves to study areas the Universe they could not “see” before.
One example are black holes, which are normally invisible, and can only be detected when they’re enormous mass bends light passing behind them. But through the “lens” of gravitational waves, black holes can be seen moving across the universe, leaving ripples of distorted space-time behind them.