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Gravitational Waves Demystified

Those of us who have heard the news about the discovery of gravitational waves may have wondered, what’s the big deal? Why has this news gone viral on social media? What are these gravitational waves in the first place and what difference has their discovery made in the field of science? Why has it taken so long to detect them, even if they were known to exist, theoretically, 100 years ago?

I will try to answer all these questions in this article one by one. Let’s start off by understanding the fundamentals of gravitational waves.

By Rajwinder Singh

Waves:

Waves can be described as some sort of disturbance travelling through space (you can consider space as your surroundings) as time passes. Let’s understand this definition with some examples: Sound is a kind of wave which is created by the rapid back and forth vibration of an object (e.g. drum). When an object moves back, a low pressure area is created in front of it, and when it moves forward, a high pressure area is created. This phenomena continues and packets of different air pressure travel through space from that object to our ears, which are called sound waves. Fig 1 illustrates this phenomena physically. Here the disturbance is ‘pressure’. Another example is water waves. When we throw a stone in a still pond, we see that ripples start from the point where the stone created an impact. The disturbance being transferred from one place to another are water waves.

Science. Figure 1
Figure 1

Forces in nature:

I hope all of you have an idea about what a force is. If not, a force is a push or a pull that tends to change the state of rest or motion of any object. There are four kind of fundamental forces in nature. Electromagnetic (EM) force, Gravitational force, Weak force, and Strong force. We are able to stand up straight on the ground due to both EM, as well as Gravitational forces. Gravitational force pull us toward the centre of the earth. As we are in contact with the ground, atoms on the surface of the earth and the atoms of our feet touch each other. Atoms contain electrons, which are negatively charged, and if two atoms come close to each other, they repel each other due to the overlap of electrons. Hence, an EM force acts against gravitational forces and balance each other. The other two forces that I mentioned, we do not need to concern ourselves with, as they act inside the nucleus of an atom. If you stand on a scale that reads 60 kilograms, for example, then this is the amount of gravitational force between you and the earth. For the purposes of this discussion, we will only be concerned with gravitational force.

Gravitational force/Gravity:

It all started when an apple hit Isaac Newton’s head while sitting under an apple tree. He thought, why did the apple fall down? Why not fly away in the air? He developed a theory of gravity, which explained why things always fall down on earth. This theory was not only applicable to earth, but was also a universal theory. According to Newton, every mass in the universe attracts another mass. This attraction is proportional to the product of both masses (in this case, the mass of the apple and mass of the earth). Simply put, attraction is greater if the mass of both objects multiplied together is larger. This attraction also decreases if the objects are far away and increases if they are they come closer together. More accurately, gravitational force is inversely proportional to the square of the distance between objects. The stability of the universe is only possible due to this gravitational force. Planets revolve around their sun due to gravity!

Einstein and his theory about Gravity:

In 1905, Einstein proposed ‘The Special theory of relativity’. Imagine that a police car is chasing the car of a serial killer and they want to shoot him! What should they do? They must move parallel to the vehicle of the killer at the same speed! That should make the relative speed of both of the cars zero, so that they are ‘at rest’ with respect to each other in order to make sure the bullet would hit the culprit. What if light was the serial killer the police were chasing? Einstein said that you cannot chase light! The speed of light is 300,000,000 m/s. Even if you achieve a speed equal to the speed of light (which you can’t according to this theory), light will still be moving at its own speed with respect you. This has some serious implications in the structure of space and time. Speed is equal to the distance moved in space, per unit of time. Mathematically, it can be shown that space can be contracted and that time can be stretched! This must sound absurd, but is what it is. If we combine space and time into a single entity, ‘spacetime’, we can say that now that the structure of spacetime can be altered. Think of everything immersed in this spacetime, which we can’t feel, but is there!

Figure 2.
Figure 2.

In 1916, Einstein published another theory, the ‘General Theory of Relativity’. One of the things this theory suggests is that masses in space distort the geometry of spacetime. For more clarity, see the figure 2, in which the earth distorts the fabric of spacetime. It is only the imagining of the earth distorting two dimensional spacetime. But in reality, spacetime is four dimensional – the three dimensions in which we live in, plus a time dimension. Now, you might ask, do I alter the structure of spacetime? The answer is yes, but your mass is so small that it’s negligible. To detect any change, one has to look at an astrophysical event which involve massive objects, such as the collision of two stars. This kind of event creates ripples in spacetime, just as a stone would create waves in water. These ripples travel though spacetime just like waves and are known as Gravitational waves.

Detection of Gravitational waves at LIGO:

A few weeks ago, scientists at LIGO (Laser Interferometer Gravitational-Wave Observatory) announced that they have confirmed the detection of gravitational waves. These waves originated from an event in which two black holes merged together. Black holes can be created from dying stars, which are collapsing on themselves, creating such strong gravitational pull that that nothing can escape from its effective gravitational zone (the event horizon). Not even light! The mass of each black hole was more than 25 times that of the sun. When they merged, they emitted gravitational waves in all directions. Fig 3 shows a numerical simulation of the gravitational waves emitted by the merger of two black holes. The coloured contours around each black hole represent the amplitude of the gravitational radiation. When these waves travel though space, anything the waves hit, will vibrate in one of the directions.

Science. Figure 3
Figure 3.

Experimental setup for detection.

The principle of detection of these waves is really simple, but the engineering used to construct this setup is mind blowing. Computational techniques required to detect such a minute signal was another challenge. To build this required huge amounts of money, extremely skilled personnel and advanced technology which is why it took so long to prove Einstein’s theory. To confirm the detection, two similar setups located in Livingston, Louisiana, and Hanford, Washington, in the United States, were also built. At each observatory, a 4-km long L-shaped LIGO interferometer using laser light split into two beams that travel back and forth down the arms of four-foot diameter tubes kept in a near-perfect vacuum. The beams were used to monitor the distance between mirrors precisely positioned at the end of each of the arms. As discussed earlier, according to Einstein’s theory, the distance between the mirrors will change by an infinitesimal amount when a gravitational wave passes by the detector. A change in the length of the arms smaller than one-ten-thousandth the diameter of a proton can be detected. Figure 4 shows the signal of gravitational waves picked up at LIGO. This confirmed the existence of gravitational waves.

Figure 4.
Figure 4.

How can gravitational waves help mankind?

The simple answer is to understand our universe better! There are many things that do not emit light in our universe, such as black holes, so they can’t be observed using light telescopes. We can use the gravitational waves emitted by these objects to study them. In summary, I can say that this is just the beginning and it has opened a whole new field of science. There are many more applications to be figured out!