There are many things in this universe that we can easily see or observe, such as planets, stars, galaxies, etc. On the other hand, there are some things that we have neither seen nor directly observed. But still, we believe that they definitely exist. We only know about them because our mathematical equations predict them. One such mystery was the Gravitational Wave.
Albert Einstein predicted the existence of Gravitational Waves in 1916, based on the equation of his General Theory of Relativity. But prior to 2015, Gravitational Waves used to exist only in theory and equations. On 14 September 2015, LIGO first detected gravitational waves, which was created due to the merging of two black holes that existed billions of light-years away from the earth.
This discovery not only proved Albert Einstein's theory that is more than 100 years old but also gave us a new tool to understand this universe better. You can guess how important it was to detect Gravitational Waves that the three physicists ( Rainer Weiss, Kip Thorne, and Barry Barish ) involved in this discovery were awarded by Nobel Prize. If you do not know what is gravitational wave then this article is for you. In this article, you will know -
1. What is Gravitational Wave?
2. How Gravitational Waves are produced?
3. How do scientists detect Gravitational Wave?
4. How does LIGO Observatory work?
What is Gravitational Wave?
The year was 1915 when Albert Einstein published his General Theory of Relativity. In this theory, through 4-dimension of space-time, he told what gravity really is and how does it work. He believed that any kind of mass distorts the space-time fabric. If the mass of an object is high, it will distort the space-time more and if its mass is less then it will distort the space-time less.
It would be such that objects with less mass will begin to move toward larger objects. This is what we know by the name of gravity. If you have not yet understood this, then let us understand this by the example of a sheet and a ball placed on it.
Before I start explaining this, I want to make it clear that the example of sheet and ball does not completely represent the behavior of space-time. Because the space-time is 4 dimensional ( length, width, height, and time ) and the sheet is 2 dimensional ( length and width ). But to easily understand the behavior of space-time, this example is considered best.
Suppose you have a sheet, a big ball, and two small-sized balls. What will happen when you put the big ball on it, it will push the sheet inwards. Now if you put two small-sized balls on that sheet, then you will see that both those balls also push the sheet inward but less than the big one. Since the big ball has pushed the sheet more inward, both the smaller balls will start moving towards the big ball. This is what we know by the name of gravity.
This means that massive objects such as neutron stars or black holes distort space-time more than smaller mass objects such as planets, satellites, etc.
In such a situation, if these massive objects ( Neutron stars or Black holes ) start moving from their places, then in space-time, ripples or waves start forming. It is like when a steamer moves in a pond or river, some ripples or waves are formed in the water. These ripples formed in space-time are called Gravitational Waves. This is the introduction of Gravitational Waves. Now let us know how exactly they are produced.
How gravitational waves are produced?
When two neutron stars or black holes are close together, they first revolve around each other and after that, they become one. When they start circling each other, ripples start to form in space. But when they merge together and the two become one, a lot of energy is released at that time.
For example, when two black holes collide with each other and become one, at that time, in just seconds, so much energy is released that even the energy of the stars of the entire universe cannot be combined to generate this much energy. The ripples that are formed in space-time are the most powerful at that time.
This means Gravitational Waves are generated when two massive objects either move from their place or collide with each other. These gravitational waves travel in the universe at the speed of light.
How do scientists detect Gravitational Wave?
In 1916, Albert Einstein predicted, based on his General Theory of Relativity, that when two large objects move toward each other, the distance between them would decrease and generate Gravitational Waves.
But until the year 1974, it remained only a theory. Because we had no way to test it. But in the year 1974, two astronomers discovered a binary pulsar in which two extremely dense and heavy stars were circling each other.
It was a perfect system to test Albert Einstein's theory of gravitational waves. If the distance between them reduced with passing time, it would have been proved that Einstein was right.
After observing this binary system continuously for 40 years, it was found that the two stars were really coming close to each other i.e Einstein was right. It also means that these stars were generating the Gravitational Waves.
But we were not able to detect those gravitational waves before 2015. The day was 14 September 2015, when LIGO first detected the distortion in space-time. This distortion was created by the collision of two black holes 1.3 billion light-years away from earth.
This was the first time the gravitational waves were directly detected. Since then, the gravitational waves have been detected twice more and will continue to be detected even further.
Since the gravitational waves are only a ripple made in space-time, if there was a source of these waves close to the earth, then their impact would have erased the life on earth.
But it is not so, gravitational waves come to us from such a distance that their impact on the earth is so diminished that they are able to affect the earth at a thousand times smaller levels than the nucleus inside an atom. This is why we are not able to feel the effect of Gravitational waves.
Now the next question comes when the gravitational wave affects us at such a small level, how did scientists detect them through LIGO. So let's know what LIGO is and how does it work?
How does LIGO Observatory work?
LIGO ( Laser Interferometer Gravitational-Wave Observatory ) is the world's largest gravitational wave observatory. In technical terms, it is an interferometer.
It works like this:
A laser beam is fired from the central building which splits into the base ( or beamsplitter ) and goes into two long perpendicular tubes. The length of these two perpendicular tubes is exactly equal and there is a mirror at the end.
When the laser beams collide with the mirror and return to the base, the light wave aligns in such a way that they cancel out each other. Because of this, nothing is detected on the rear photodetector. That means normally the photodetectors present in the LIGO do not detect anything.
But when Gravitational Waves arrives on earth, it disturbs the space. Due to which there is a continuous difference happening in the length of both perpendicular tubes for some time.
Sometimes the first tube becomes slightly longer and the second shorter, sometimes the second tube becomes slightly longer and the first slightly shorter. This goes on until the gravitational wave passes through the earth and transcends it.
The instrument of LIGO is so powerful that if there is any difference between the one thousand parts of a proton in both perpendicular tubes, then they also detect it.
Due to changes in the length of both perpendicular tubes, the alignments of light waves are no longer the same, for which they cannot cancel out each other. As a result, some light reaches in the photodetector and they are detected.
The intensity of this light depends on the difference in the length of the perpendicular tubes. Measuring this intensity means measuring the Gravitational Waves.
In this way, scientists measure gravitational waves. The gravitational wave that the LIGO has the ability to detect originates from events where large amounts of energy are released.
For example - the collision between two black holes, the explosion of stars, and even the birth of the universe itself. That is, by studying the gravitational waves, we can know a lot about the universe.
Till now we have been using only Visible lights, X-rays, Radio waves, Microwaves, etc to uncover the mystery of this universe. However, for some time, we have also started using sub-atomic particles such as neutrinos to solve the mystery of this universe.
In such a situation, gravitational waves can prove to be a very unique discovery. Because gravitational waves are not electromagnetic radiation, a completely different phenomenon that not only helps us to understand gravity better but also tells us about the behavior of the black hole and other objects in this universe that is still mysterious to us.