Gravitational Waves

1.3 billion years ago somewhere far in the universe two black holes revolving around each other at high speed came together to merge to form a single black hole and thus creating the ripples in space-time called the gravitational waves. These waves traveled with the speed of light across the universe and reached Milky Way galaxy 50 thousand years ago. These waves then traveled across the Milky Way Galaxy for those 50 thousand years and finally reached our earth on 14 September 2015.

How do we know about this event that happened so far from us and so many years ago? This post will give an insight on gravitational waves and how it helps in exploring the universe.

Also check out Gravity Assist Maneuver: Nature wants to be explored!

What are Gravitational Waves?

Gravitational waves are the ripples in space time caused by accelerated masses that propagate outward from the source. Gravitational waves travel at the speed of light and they interact weakly with the matter. This is the reason why gravitational waves can travel very far across the universe.

Gravitational waves

It was the famous scientist Albert Einstein who predicted the gravitational waves back in 1916.Einstein’s theory of relativity introduced the concept of space-time and how mass could cause the curvature in space-time. Einstein thus predicted if two huge masses like black holes or neutron stars rotate about each other then they would cause disturbances in the curvature of space-time. The strongest waves are originated through collision and merging of these binary stars.

Space-time curvature
A picture showing Space-time curvature

The idea of gravitational waves was then studied for many years. Finally, after a century, gravitational waves were detected for the first time using two LIGO detectors, one at Hanford Site, Washington and the other at Livingston, Louisiana.

Exploring the Universe Using these Waves

 We have been exploring the universe with the help of electromagnetic radiations for quite a long time. By detecting the electromagnetic radiations coming out of astronomical objects and events we learn about the existence and positions of those objects. However, electromagnetic radiations interact with the matter and that’s why radiations far away from us may not reach the earth. As mentioned earlier, gravitational waves interact very weakly with the matter and thus they travel very far distance. In essence, gravitational waves are bound to enable us to make astronomical research even farther than what was possible using electromagnetic waves.

Thus, detecting gravitational waves coming far from the universe is going to help us know more about the origin of the universe and how the universe works. For example, the first gravitational wave observation detected the waves coming from the source 1.3 billion light years far from the earth. If we think about it the other way, we recorded the event that happened 1.3 billion years ago in the universe. Therefore, detecting the gravitational waves farther and farther from us will let us know about the events that occurred at the beginning of the universe and eventually we might found out about the origin of the universe.

How to detect Gravitational waves?

By the time the gravitational waves reach the earth they become very weak. The space-time ripples they cause is about 10,000 times smaller than the size of nucleus of an atom. Therefore, detecting the gravitational waves was one of the greatest challenges to scientists.

Effect of Gravitational waves on matter
Effect of Gravitational waves on matter

 As a gravitational wave passes an observer, that observer will find space-time distorted by the effects of strain. Distances between objects increase and decrease rhythmically as the wave passes, at a frequency equal to that of the wave. Thus, we have to measure this change in length to detect the gravitational waves.

However, due to the astronomical distances to these sources, the effects when measured on Earth are predicted to be very small, having strains of less than 1 part in . This shows that we must precisely measure such a small change in length to detect a gravitational wave. Similarly, we must make sure there is no intervention of other local waves and disturbances. To sum up, we need a very precise and sensitive apparatus.

After years of research, scientists and engineers finally completed the LIGO project.

What is LIGO?

LIGO stands for Laser Interferometer Gravitational Wave Observatory. Now you might wonder what is an interferometer? Interferometers are used in many scientific fields and they merge two or more sources of light in order to create an interference pattern. Such patterns result from overlapping waves of light. Studying the interference pattern helps to get information about the source that emitted the waves.

LIGO is the world’s largest gravitational wave observatory. It consists of two detectors situated 1,865 miles (3,002 kilometers) apart in isolated regions within the states of Washington and Louisiana. Each L-shaped facility has two arms positioned at right angles to each other and running 2.5 miles (4 kilometers) from a central building. Lasers are beamed down each arm and bounced back by mirrors, essentially acting as a ruler for the arm. Sensitive detectors can tell if the length of the arms of a LIGO detector varies by as little as 1/10,000 the width of a proton, representing the incredibly small scale of the consequences imparted by passing gravitational waves.

LIGO has two observatories to act as a check on the other to rule out that a possible gravitational-wave detection isn’t due to a local, terrestrial disturbance; both facilities will detect a real gravitational wave moving at the speed of light nearly simultaneously.

Let me explain you the process of detection a little. Within LIGO, the lasers beamed down its arms return back and are set to cancel one another out completely. As a result, no light reaches another LIGO component called a photodetector . If, however, a gravitational wave reaches the LIGO facility, it is going to stretch one detector arm and compress the other, throwing off this perfect destructive interference. Some light would then reach the photodetector. The pattern of this light would offer information about the changes the arms underwent, and thus reveal properties about the incident gravitational waves and their source.

Basic Michelson Interferometer

In 2017, the Nobel Prize in Physics was awarded to Rainer WeissKip Thorne and Barry Barish for their role in the direct detection of gravitational waves.

Other Detectors

At present there are three gravitational wave detectors in function. There is another Gravitational Wave detectot called VIRGO located in Santo Stefano a Macerata, near the city of Pisa, Italy. Why do we need three detectors? This is because more detectors will help to know the position of the stars more precisely. Greater the number of detectors, greater the precision. Therefore, another detector called KAGRA is already developed in Japan and supposed to function in late 2020, however, the date might be affected due to the Coronavirus Pandemic. Similarly, LIGO- India is another detector in development that might be functional by 2024-2026.

There have been more than 90 gravitational wave detections. Click here to get more information . Here is a picture of first gravitational wave observation on 14 September 2015.

First gravitational wave observation

Thank you for reading this. I tried to explain everything I know about the gravitational waves. I hope to see your feedback in the comments. Please feel free to point out my mistakes if any.

By Nishchal Poudel

Nishchal is currently studying Bachelor in Aerospace Engineering at IOE Pulchowk Campus, Nepal.

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