If you are following the news then you might have heard the term “Gravitational Waves” and wondered what they might be or whats their significance. In this article we would talk about how these waves were discovered and how they were theorised.
What are Gravitational Waves?
Gravitational waves are the “Ripples”, that stretch and contract spacetime, in the fabric of space.
Spacetime has been described in the most simplistic way by H.G. Wells in his novel “The Time Machine” where he wrote, “There is no difference between time and any of the three dimensions of space except that our consciousness moves along it”.
If we take time as the fourth dimension and then combine it with space, the other three dimensions i.e. (length, breadth, height), then we call that representation as spacetime.
To understand Gravitational Waves we must first understand what is Gravity in respect to spacetime. When we describe Gravity in spacetime we say, it is the distortion in the fabric of space created by massive objects.
Think of Gravity as a dimple/bulge in the curvature of spacetime created by massive objects that press down onto the fabric of spacetime.
We have used the word ‘fabric of spacetime’ quite a lot in the last few explanations but never defined what it is?
What is the “Fabric of Spacetime”?
According to the standard definition – “Spacetime is any mathematical model that combines space and time into a single interwoven continuum”.
But that only defines spacetime. If we want to understand what it is more simply we can think of it in this way.
Imagine a trampoline or if you can’t imagine a trampoline imagine a very fluffy/soft bed or a waterbed if you have lied on one (lucky you). Now what do you think would happen if you put a bowling ball in the middle of the bed or trampoline?
It would get squeezed at the point where the bowling ball is kept and it would create a type of dimple/bulge in the bed/trampoline.
This type of surface that bends when we put massive objects on it can be imagined as the fabric of spacetime. Albert Einstein thought of spacetime in a similar way, which helped him form the Theory of General Relativity.
What is Gravity?
Earlier we talked a little bit about what is gravity but not how it works.
To jog your memory we defined gravity as – ‘ A dimple/bulge in the curvature of spacetime created by massive objects that press down onto the fabric of spacetime.’
This type of distortion in the curvature of spacetime is called Gravity as Einstein defined it.
Of course we can’t think as deeply as Einstein did (’cause for all we know we are merely doing his homework for him ) but we can simplify what he said.
If you have ever jumped on a trampoline or a bed (which we all did when we were young) you might have seen that when we jump it creates a bulge in the bed. Some of you might have seen that if you have things lying around on the bed when jumping they tend to move towards the centre of the bulge/dimple/distortion. If you magnify this effect billion of times we can create a similar type of bulge in the spacetime fabric.
Thats all what the Earth does, it creates a bulge in spacetime thereby curving it.
If you want you can think of Earth rolling around a valley in space created by the sun due to its mass i.e. very big. It distorts the spacetime around it and that is also the reason why planets revolve around it. So in short planets behave like the marble around the sun which is itself behaving like the bowling ball on the spacetime fabric.
Now how does this distortion effects the movement of earth?
Think of the bed with the bowling ball lying in the centre from the earlier example. Now imagine what would happen if you slid/glided a marble on the bed just aside from the bulge,in the curved part of the bed?
The marble would not follow the straight path but would curve along with the curve, the same thing happens with our earth. As it is very small as compared to the sun it just follows the curve of the bulge created by the sun thus revolving around the sun. The Earth does not stop like the marble as there are not external forces like friction to stop its movement.
Now we know what Gravity is and we can now understand more about Gravitational Waves.
Imagine the soft bed that was having a bowling ball in the middle with a marble revolving around it about which we talked about earlier. For now we can ignore the marble as it has no use and can focus on the bowling ball.
Imagine a bed sheet covering the entire bed. We have to imagine the bowling ball and the bed being a part of the sheet or we could imagine the bed to be a water bed and the bowling ball a part of the waterbed and must remember what would happen with them would also happen with the bowling ball.
Now think what would happen if anyone were to move the sheet up and down rapidly or a boxer would use it as a punching bag?
This would create ripples or waves in the sheet or the water bed respectively.These ripples or waves would expand and contract the bowling ball, bed and the sheet thus deforming their sizes as they are part of one system.
In reality the gravitational waves stretch and contract the space between the atoms which stretches or contracts the matter made of these atoms.
You do not realise but this is happening with you at this moment, you are being stretched and compressed by these Gravitational Waves yet you do not feel their effect. Why?
It is mainly because as these waves produce changes of a very small magnitude. These ripples would stretch and compress the earth by less than an atomic diameter (that’s extremely small). It would be less than 10 to the power -9 metres.
If Gravitational waves bring very small changes in matter then how were they discovered by us?
Our hunt for gravitational waves began when Einstein submitted his paper for Physical Review which was titled, “Do Gravitational Waves exist?”, at first his paper was rejected.
It was rejected because when the committee gave the paper to be reviewed by Howard P. Robertson, he criticized the paper their as he believed that they could exist.
After the criticism Einstein worked on the paper where he in the end published it with possibility that Gravitational Waves existed.
On 14th September 2015 these ripples in spacetime were discovered by LIGO. Then to our astonishment they were discovered again on 25th December.
Now you must be thinking what is LIGO?
Vacuum Tubes about 2.5 mile long
What is LIGO and how it helped in the detection of these Gravitational Waves?
LIGO stands for Laser Interferometer Gravitational-wave Observatory. It has special instruments which are equipped for the detection of Gravitational Waves. The project has been going on for about 45-50 years with the prediction of existence of Gravitational Waves being done by Albert Einstein after creating his General Theory of Relativity in 1916.
How does LIGO work?
Working of LIGO.
We know that Gravitational waves produce extremely small changes when they distort matter. So how do we measure these changes?
As we get from the name it involve lasers.
The device involves a laser and two mirrors which are placed at a very large apart from each other. The laser hits a splitter which splits the laser in two directions as we can see in the image.
The laser travel the distance, which is about 2.5 miles simultaneously in two different directions and are reflected back by the mirrors. The reflected beams of the laser then meet at the splitter where they work together forming an interference pattern. An image is provided below showing how an interference pattern looks.
All this work has been done to obtain this interference pattern that helps in detection of the waves.
The lasers are fired in a vacuum tube which ensures that movement of air particles do not interfere the observations as the Gravitational Waves are of extremely low wavelength and any movement would give false results.
What happens in the vacuum tubes?
While the lasers are creating the interference pattern and a Gravitational wave passes it disturbs the interference pattern and it starts to wobble.
This happens because when the gravitational wave passes it stretches the distance of one reflected laser beam relative to the other which changes the movement of one of the beam with respect to other, which changes the interference pattern thereby helping us detect the passage of wave.
To make this work scientists worked on this for about the last 40-50 yrs.
Interference Pattern representation.
How are Gravitational Waves produced?
To understand how they are produced we have to use the most powerful computer i.e. our imagination.
Knowledge will get you from point A to B. Imagination will take you everywhere.
Imagine a very soft/fluffy bed or a waterbed, your phone lying on the edge of the bed and a child is jumping on the bed again and again. What do you think would happen with your phone?
It would fall down as the bed would first bulge downwards as the child jumps on it and then go back up thus creating a wave. This wave would travel across the bed and would knock your phone off the edge.
A similar type of wave is produced by blackholes but their way of producing this wave is a little different.
To understand how they produce these waves we need to think of a boat moving/accelerating through water.
When a boat moves though water it creates ripples behind it. Similarly when two blackholes revolve around each other, they move through spacetime thereby creating ripples behind them. These ripples are what we are calling and what Einstein called gravitational waves.
The difference between them is that the waves created by the boat are in water and the waves created by blackholes are produced in the fabric of spacetime.
Now we know so much about these waves but why are they so important?
First of all these are a huge discovery as they confirm Einstein’s ideas and opens a whole new way of seeing the Universe. (We are respecting Einstein even more after this has been confirmed as he literally predicted what happened about 1.4 billion years ago.)
Their importance is described beautifully by Brian Greene as:
“We know light can be blocked, take for example my face can be blocked by hand and now is not visible to you but you can’t block Gravity. Which means gravity can go into places in the universe that light can’t penetrate. Using these Gravitational Waves we may be able to study how black holes combine and study even the big bang itself. They can help us map things we can’t see.”
They have the capability to provide us with insights about how the universe works, how galaxies are formed and things we can’t possibly imagine. (Actually we can, at least Einstein would have.)
We have just begun exploring the universe with the help of Gravitational Waves and who knows what mysteries of the universe they might help unfold.
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