What are gravitational waves?

Gravitational Waves: The Ripples In Space-time

In 2015, when LIGO detected gravitational waves directly for the first time, it was nothing less than a milestone in the history of physics. Einstein’s predictions were proved true, which was a big deal in itself. But, what are these gravitational waves? What do they symbolize and why did their detection become a reason for celebration? Let’s try to find an answer to all these questions in our today’s article of the Basics of Astrophysics series.

You can also watch this short video on gravitational waves:

What Are Gravitational Waves?

In the sixteenth century, Newton stated that each particle in this universe attracts every other particle with an invisible force, the force of gravity. However, he was not sure about the mechanism of this magical entity.

So, in the twentieth century, Einstein came up with his explanation of gravity, the General Theory of Relativity. Einstein stated that massive objects distort the fabric of spacetime. And the effect of this distortion is what is felt as gravity. More is the mass of an object, more is the distortion produced by it.

Distortion of the curvature of space time by objects of different masses.
Image: esa.int
Distortion of the curvature of space time by objects of different masses.
Image: esa.int

Now, when two massive objects spiral toward each other, they wrinkle the spacetime and send those distortions and energy across the cosmos. These disturbances propagate through the cosmos just like the ripples spread across a pond at the speed of light. These ripples in the fabric of spacetime are what we call the gravitational waves.

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In technical terms, according to the astrophysicist Shane Larson, gravitational waves “are propagating disturbances of the shape of spacetime,”. These waves transport energy in the form of gravitational radiations, a form of radiant energy similar to the electromagnetic radiations.

Two-dimensional representation of gravitational waves generated by two neutron stars orbiting each other.
Courtresy: wikipedia
Two-dimensional representation of gravitational waves generated by two neutron stars orbiting each other.
Courtesy: Wikipedia

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What causes Gravitational waves?

The most powerful gravitational waves are created when massive objects move at very high speeds. This means the significant ripples in the curvature of spacetime are produced when two black holes orbit each other and merge. Moreover, two big stars, especially the neutron stars also give rise to gravitational waves while orbiting each other. Apart from these, another important event that can shoot out strong gravitational waves is the asymmetric explosion of a star, that is, a supernova.

How to detect these ripples?

We have compared the gravitational waves to the ripples in water. But, we can easily see the ripples in water, so can we directly see these ripples in the fabric of spacetime as well? Sadly, the answer is NO! Gravitational waves are invisible. But don’t get disheartened, we can still detect them! How?

Well, the gravitational waves are incredibly fast. They travel at the speed of light, which is 186,000 miles per second. So, as they travel, they squeeze and stretch everything in their path as they pass by.

Imagine having an elastic band in our hand, when you pull it, what happens? It gets stretched in one direction and get squeezed in the other direction. Now, this is exactly what gravitational waves do with the fabric of spacetime! So, we measure this distortion in spacetime in order to indirectly detect the gravitational waves.

Related articles in this series:
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LIGO And The First Detection Of Gravitational Waves

As mentioned earlier as well, the gravitational waves originate from events that occur far away from earth. So, as they travel through the fabric of space-time, they get quite weak in intensity which makes them quite difficult to detect. And, this is the reason why these took about 100 years to finally get detected after having been proposed theoretically. Even when Einstein proposed his General Theory of Relativity, he said that detecting these waves is beyond human ability.

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But the visionaries at Caltech and MIT: Kip Thorne, Rainer Weiss, and Ron Drever proved his statement wrong. They came up with an idea to detect these waves using lasers that measured distances between mirrors kilometers apart. But here, they were talking about measuring distances such as one-tenth of a nucleus of an atom, using lasers. Yes, you read it right! People considered them crazy, but, somehow their idea was accepted, and finally, the work began in 1994. This project was named LIGO that stands for Laser Interferometer Gravitational Waves Observatory.

LIGO is a system of two observatories built in the US, one in Livingston (Lousiana), and other in Hanford (Washington). After the work of hundreds and thousands of scientists, the LIGO came into working in 2015. And finally, on September 14, 2015, LIGO saw on two detectors, a gravitational wave. A signal was observed on both these detectors. After decoding these waves, scientists concluded that they came from two black holes fusing together to become one, a billion years ago.

Simplified diagram of an Advanced LIGO detector (not to scale)
Courtesy : wikipedia

Simplified diagram of an Advanced LIGO detector (not to scale)
Courtesy : wikipedia

This wave produced a difference in distance of about four-thousandths of a proton over four kilometers. So you can very well imagine the precision of this instrument! And this is how, the recipient of the Nobel Prize in Physics, the first successful detection of gravitational waves finally took place! And since then, there has been no looking back.

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Why Are These Waves So Important?

We are simply investing billions of dollars and hundreds of years to detect the gravitational waves. But, are they really worth it? Well, the answer is yes!

Gravitational waves act as storytellers and can tell us a lot about the journey of our universe. They carry the record of all the catastrophic events that have ever occurred in our universe. These waves can reveal a lot about the origin and cause of these events, which will eventually help us understand the universe and the big bang. In a literal sense, gravitational waves can indeed help us to hear the first cry of our universe! Yeah, I’m not kidding!

Authors message:

The gravitational waves can indeed tell us a lot about our universe. And maybe this is the reason why the world is ready to invest a lot of money and labor in designing the most precise instruments for their detection.

Well, talking about me, I came to read about gravitational waves for the first time in the first year of my undergraduate degree, when my university had organized a poster presentation on gravitational waves and I volunteered to participate in it. Since then, the concept of Gravitational waves has always amused me. I must say, that this has been one of the key concepts that drove me closer to the field of astrophysics and cosmology.

Read all the articles of Basics of Astrophysics here

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