Very fast yet invisible ripples in space time caused by very energetic processes in the universe, like a Black Hole collision or a supernova, are called Gravitational Waves.
These waves squeeze and stretch anything as they pass on. They travel at a speed of 299337.984 Km/s. Gravity (Gravitational Force) from bodies in the universe cause these Gravitational waves. But these types of objects that create gravitational waves are far away. And sometimes these events cause small, weak gravitational waves. The waves become very weak by the time they reach Earth making gravitational waves hard to detect.
Albert Einstein first predicted the existence of gravitational waves in the theory of relativity. In layman’s language, he pointed out (predicted) that something special happens when two astronomical bodies orbit each other, according to him these generate ripples in space and these ripples spread in space like in a pond when a stone is dropped.
The first proof of two astronomical objects orbiting each other - a binary pulsar (where a pulsar is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles) was discovered by two astronomers using the Arecibo Radio Observatory in Puerto Rico 20 years after Albert Einstein’s death.
Existence of gravitational waves was physically discovered for the very first time on September 14, 2015 using a very sensitive instrument called LIGO (Laser Interferometer Gravitational-Wave Observatory). The observed wave was generated by a collision of two black holes which happened 1.3 billion years ago, but the waves reached earth in 2015.
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