Gravitational Waves: A great discovery of the decade

Gravitational waves are some kind of waves in the fabric of space-time proposed by Einstein. So, to understand gravitational waves, first we have to know about the space-time.

What is space-time:

To understand the space-time concept we have to understand about space and time separately. So, what is space? Simply, space is the 3 dimensional coordinate in which we are living. To locate an object in space, we use three coordinates - length, breadth and height. For example, in a coordinate system, to locate a point from the center, we use X,Y and Z axis denoted by (x,y,z). In general also, we can use this technique to locate any point in the space. In this world, we can feel only these three directions which are also called dimensions. This is space. Simply, the coordinate of the space is (x,y,z).

Now, what is time? Time is also a dimension. We cannot see this like the other three dimensions, but we can feel it. Newton said that time is absolute. It does not depend on anything, it flows like water in waterfall. Once you passed it, you can never get back that time. But, Einstein suggested two theories of relativity, in where he explained about time and space. According to him, time is not absolute like the other things. Everything in the universe is working with the principle of relativity, To locate any point in the hyperspace, we need a four dimensional coordinate. Because, in space, every object changes its position rapidly with time. For example, an asteroid was at the position of coordinate (x,y,z) in T time with respect to earth.

Now, by combining the space (3 dimensional) and time (4th dimension), we will get a 4 dimensional space-time. In the theory of general relativity, Einstein imagined about this 4 dimensional space-time at the very first time. According to him space and time do not exist alone, rather they exist together by combining a network of space-time. Space-time has many properties like - it can be bended, twisted or fluctuated, massive objects can warp the space-time fabric, etc.

What is Gravitation:

With the help of Newtonian mechanics, Newton described about gravity and gravitational force. He said that, gravitational force was a force which binds every celestial objects with one another like strings. But, Einstein proved with his theory of general relativity that gravitational force was nothing but the curvature of space-time. After the revolution of quantum mechanics, gravitation was also included in the Standard Model of particle physics and with the help of Quantum gravity It was described in the quantum level. Quantum gravity explained about graviton which carries the characteristics of gravitation.

What are gravitational waves:

Gravitational waves are nothing but some ripples in the fabric of space-time. Like, when we through a piece of rock in a pond, then ripples of water forms in a concentric circular pattern. Gravitational waves are some kind of ripples of space-time. They are formed when two or more massive objects collide with each other, explosion or something like that. For example; when two black holes collide or merge to each other, supernova explosion or collision of two or more celestial objects; some kinds of ripples or waves form in the fabric of space-time. Those waves are called Gravitational waves.

What is LIGO:

The full form of LIGO is The Laser Interferometer Gravitational Wave Observatory which is located in the United States. Actually, LIGO is separated in two parts in the United States- one is in the Hanford Washington and other is in the Livingston, Louisiana.

The LIGO is like "L" in shape. Each arm of LIGO is 4 km long. It uses laser beam to detect the gravitational waves. This is the world's largest gravitational wave observatory. Actually, it is an interferometer in which laser beams are applied to measure the gravitational waves. In 14 September, 2015, LIGO detected gravitational waves at very first time.

How LIGO works:

LIGO is basically an interferometer. The main parts of LIGO is LASER chamber, beam splitter, highly reflecting mirrors and photodetector.

There was a LASER chamber in where laser beam was stored. In front of that chamber a beam splitter was placed in a straight line of the chamber. Two highly reflecting mirrors were placed perpendicular to each other at an equal length (4 km) from the beam splitter. Behind the beam splitter, a photodetector was placed.

A laser pulse is shooted from the LASER chamber which is headed to the beam splitter. The beam splitter splits the laser beam into two separate beams perpendicular (90⁰) to each other. As the two arms are equal in length (4 km), that is why the beams travel equal length (4 km). Due to the highly reflecting mirrors, the beams are reflected back and they again travel equal length to the beam splitter. As the beams arrive to the beam splitter at the same time, their phases are equal and opposite to each other, so they cancel out each other. Hence, there are no detection of any light in the photodetector. That means, in normal operation, the photodetector does not detect any light.

But, when gravitational waves pass through the earth, the space-time of earth is distorted for sometime. Obviously, we cannot feel that distortion, but the equipments of LIGO is so sensitive that it can detect the difference between the two vacuum tubes of one-thousandth of charge diameter of a proton. When gravitational waves pass through it, then it cause a difference in length between the two vacuum tubes. Due to this difference, the laser beams travel different distances with respect to the other. So, they do not superimpose in the beam splitter at the same time and hence, they cannot cancel out each other. Then the photodetector detects light behind it. Thus, gravitational waves are detected. But, this is not such simple as stated above. LIGO owned very sensitive equipments which can detect a very small amount of difference of length. The two arms of LIGO were made ultra high vacuum, measuring 4 km each.

How did LIGO detect it:

Now, here a question may arise,"LIGO detected gravitational waves which were caused by merging two black holes which were 1.3 billion light years away from us, then that incident did not happen in 2015. But, how LIGO detected those waves in 2015?"

Well, gravitational waves are created when massive objects collide with each other, two or more black holes or neutron stars collide or merge with each other. The two black holes of almost 30 solar mass merged with each other in about 1.3 billion years ago. As, gravitational waves travel with the speed of light, that is why it took almost 1.3 billion years to reach the earth's atmosphere. Hence, LIGO had detected these waves at that exact time, when it arrived. That means, these gravitational waves, created by those two black holes arrived to earth after almost 1.3 billion years later. On 16 June, 2016, LIGO announced that they had detected another second signal of gravitational waves after merging two black holes with 14.2 and 7.5 times the mass of sun. Like that, LIGO had detected gravitational waves many times. For such a great discovery, the three physicists were awarded with Nobel prize of physics in 2015, they were - Rainer Weiss, Kip Thorne and Barry Barish.

Effect on earth??

Another question may arise," If these gravitational waves are too strong then why did they not destroy the earth?" Actually, gravitational waves are too strong. After merging two black holes or neutron stars, they create powerful ripples in the fabric of space-time. Of course, if there were some source of gravitational waves near earth, then the earth would be destroyed due to the gravitational waves. But, that did not happen. Because, those black holes or neutron stars are too far away from us, so the effect of gravitational waves becomes too negligible after reaching earth. Though it is negligible for us, but LIGO can detect it due to sensitivity of the equipements.

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