### Gravitational Waves

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In 1950, German physicist Albert Einstein developed a theory known as the General Theory of Relativity, in which he looked at gravity from a different perspective. This theory is based on a mathematical model that combines the time dimension with the three dimensions of space, forming a four-dimensional continuum known as the space-time fabric. According to Einstein’s theory, gravity is nothing but warps and curves in the space-time fabric, and these curves are the reason why gravity exists.

Einstein pushed his ideas one step further when he realized that the space-time fabric is flexible and can fluctuate. So, if any luminary swimming in space fluctuates, it jostles the space-time fabric creating ripples; these ripples are what we call gravitational waves. Although any object with mass can make these ripples in the fabric, the effects of these ripples depend on the magnitude of the mass. In other words, the more mass the object has, the more severe the ripples in the fabric are, and accordingly the more noticeable the gravitational waves are.

The objects that have the biggest mass and are able to make the strongest gravitational waves are incontrovertibly black holes. Black holes are regions in space with a great amount of matter concentrated in a small area. Therefore, a black hole might have a very small size, but a huge mass; besides great mass, black holes exhibit very strong gravity that nothing, even light, can escape.

In September 2015, scientists managed to pick up the first gravitational waves resulting from the vibration of two black holes while colliding and merging together. It is worth mentioning that the mass of these black holes is 30 times greater than the mass of the Sun. Originally, the two black holes orbited each other for millions of years, before getting gradually closer and ending up circling each other at tremendously high speed; at the end, they collided and merged to form a bigger black hole. The huge collision made frequent ripples in the space-time fabric, which started to fluctuate, generating gravitational waves that were noticeable enough to be picked up by a detector known as the Laser Interferometer Gravitational-Wave Observatory “LIGO”.

In spite of the fact that the signal picked up by LIGO had a very short existence period of only one-fifth of a second, it is still considered a fabulous scientific discovery to prove the existence of gravitational waves for the first time. It is worth mentioning that not only can black holes generate gravitational waves, but also other luminaries such as neutron stars can. Taking into consideration that every object generates its distinct gravitational waves, gravitational waves can be considered fingerprints for different objects. By analyzing the gravitational waves picked up by LIGO, scientists can specify their source and determine whether they were generated by a black hole, neutron star or any other luminary.

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Cover image: nationalgeographic.com

The original article was published in SCIplanet, Spring 2020 issue "Dualities of Life: The Earth and The Sky".