Gravitational Waves

Albert Einstein, in his theory of General Relativity, proposed that Gravity is not just a force that attracts the object. Gravity is responsible for making ripples, called gravitational waves, in the spacetime fabric. These waves can be a great medium to unveil various things in space that we think don't exist.  

 Let's dive deep into this topic and understand what exactly gravitational waves are and how they work.

What Are Gravitational Waves? 

Two massive bodies creating ripples in spacetime fabric| Source: Scitechdaily.com

Gravitational waves are invisible ripples or disturbances in spacetime fabric when objects move at very high speeds. But what are these ripples?

The answer is directly linked to gravity. Gravity is a force that bends the spacetime curvature. Imagine that space is a rubber sheet and every massive object will cause that rubber sheet to bend. The larger the mass of the body, the larger the depression, and hence, the larger the gravitational pull. If every mass in this universe attracts every other mass, then changes in gravity can tell us about what these bodies are doing in space. Fluctuations in gravity cause ripples in space, which are exactly like rings in water when a pebble is thrown into it. 

You may now wonder, "what causes gravity to fluctuate and produce waves?"

Not one activity is responsible for this distortion. Whenever masses accelerate, anything with mass and energy passes through a region where spacetime curvature changes. If two balls swirl around each other on a trampoline, they would certainly create distortion. In the same way, ripples(or gravitational waves) in the space fabric are produced if two bodies rotate around each other. This applies to:

• Two celestial bodies orbiting each other.

• Rapid changes in spinning and collapsing objects.

• Merging of two massive bodies(like black holes)

In all these cases, there is a rapid change in the energy distribution of a particular region which produces a kind of radiation, called gravitational waves. These waves travel at the speed of light.

Their Discovery 

In 1916, Albert Einstein predicted gravitational waves in his Theory of General Relativity. According to his theory, orbits would decay over a long period. For instance, Earth would spiral into the Sun almost after 10^150 years(we would not be able to experience it), but we can witness this decay over time in more extreme systems like neutron stars. To keep up with the law of conservation of energy, Einstein predicted that this energy must be carried away in the form of gravitational waves.

However, the existence of gravitational waves was not proved until 1974, 20 years after Einstein's death. The two astronomers discovered a binary pulsar using the Arecibo Radio Observatory in Puerto Rico. They knew that this type of system could radiate gravitational waves and hence is the best to test Einstein's prediction of these waves. Eight years of observation culminated in the discovery that the stars were getting closer to each other at the rate predicted by Einstein's theory if they were emitting gravitational waves.

According to Einstein, we would never be able to detect these waves due to the sounds and vibrations present on Earth. But now, almost after a century, scientists proved him wrong by directly detecting the gravitational waves produced by two black holes merging. 

Gravitational Waves Detected 

How would you observe a ripple in space? 

Gravitational waves are too small to be detected. Big ripples are produced by the bodies that are very massive moving at tremendously high speed, like- a pair of black holes, a pair of neutron stars, a black hole, and a neutron star. 

Now the question is "How would you observe a ripple in space?"

Gravitational waves compress the space in one direction and expand in the perpendicular direction| Source: forbes.com

The region from where the crest and troughs of gravitational waves pass are compressed and rarefied in mutually perpendicular directions. This means that if our bodies were sensitive enough to feel these waves, we would experience being stretched sideways while being compressed vertically and vice versa in the next instant. But these changes happen on a very small scale. So, scientists developed an instrument called Laser Interferometer Gravitational-Wave Observatory (LIGO) to detect these waves.

Passing gravitational waves causes the length of arms to change slightly | Source: spaceplace.nasa.gov

It is an L-shaped instrument that has 4km long arms. It uses lasers to measure the change in the distance between the end of the tunnels. Lasers, mirrors, and other sensitive instruments of LIGO are capable of measuring the change in the distance which is 1,000 times smaller than the width of a proton. When a gravitational wave passes through it, it stretches space in one direction and squeezes in another, which increases the length of one arm and decreases the other. By measuring the interference of the lasers, as they bounce between different points, it can be measured whether the space in between has stretched or compressed. 

On September 14, 2015, the LIGO created history by sensing undulations in spacetime caused by gravitational waves produced by two colliding black holes for the first time. The signal also matched with that calculated using supercomputers.

Once a gravitational wave is detected, scientists can extract information about the wave source that was 1.3 billion light-years away from the Earth in this case. Scientists also calculated the mass of the merging black holes- 29 and 36 solar masses resulting in a black hole of 62 solar masses.

This discovery marks the beginning of a new era that will explore the universe using waves of gravity. Every time a new method of discovery is found, various bizarre things get revealed.

Sound Of Black Holes 

"Can it be possible that there is no sound in space?"

"Is it possible for violent events like black holes' collision to go into utter silence?"

"I don't think so."

Yes, there is sound in space and scientists have proved it. Sound requires a medium to travel but space doesn't have so many particles to propagate the sound waves. Even supernovae, black hole mergers, solar flares, and other cosmic catastrophes would go silent before they are ever heard. But scientists have found something else to help us hear the sound of the universe. Gravitational waves have a kind of compression and rarefaction that does not need any medium to travel.

For the first time, two massive black holes' inspiral and merger were witnessed by the gravitational waves which were detected by LIGO. Gravitational waves were converted into sound waves by LIGO.

New detectors of gravitational waves will open up our ears to the sounds of the universe that are completely unknown as we have just started listening.