Black hole merger

Listening To The Music Of The Universe Through Gravitational Waves.

This is a guest article by Sumeet, a student from Guru Nanak Dev University, pursuing Master's in Physics. She has a deep interest in cosmology and Theory of Relativity. Currently, she is working in the field of Atomic Physics.


Our Universe has some strange, some extraordinary, some weird and some wonderful stuff. But unfortunately, we know very little of this magnificent universe. About 5% of the universe is observable; the rest 95% is dark matter, of which we know nothing. The key to understand the Universe is through GRAVITY.

What is Gravity?

Space time curvature
Einstein's view of Gravity

Einstein published his General Theory of Relativity, in 1916. This theory explains gravity. Gravity, as this theory describes is the force that arises due to the curvature in the space-time. Just like when you sit on a mattress, its contours distort, the surface beneath you gets curved. Similar is the force of gravity. Gravity in the same manner distorts the contour of the space-time fabric. Einstein in this theory predicted the Gravitational waves.

What are Gravitational Waves?

If we put it into simple words, gravitational waves are the ripples in the space-time fabric. These waves stretch the fabric of space-time in one direction and compress in the other. What causes these ripples? These waves are produced when masses move such as when stars move around one another. With the motion of the bodies, the energy emitted, travels in the form of waves. Once emitted, it keeps on travelling through the space.

The Origin of Gravitational Waves

The origin of gravitational waves

About 1.3 billion years ago, two black holes, one with the mass of 29 suns and other with the mass of 36 suns, locked into a spiral, were moving towards each other, just like “dancing in tango”. Just when they approached the speed of light, they fused into one, a single black hole with the mass of 60 suns compressed in the space of 360 kilometers. This fusion produced 3 suns worth of star stuff into pure energy. This has been the most energetic event known so far. Now, since they are black holes, the energy emitted was not in the form of light, but in the form of the Gravitational Waves. The Gravitational waves so produced carried the news of cosmic hug to the rest of the universe.

Also Read: What does E = mc^2 really mean?

A crazy idea to detect these waves

The LIGO Observatory

When Einstein gave the General Theory of Relativity, said that detecting these waves is beyond human ability. But the visionaries at Caltech and MIT: Kip Thorne, Rai Weiss and Ron Drever gave an idea to detect these gravitational waves. They presented how these waves could be measured using lasers that measured distances between mirrors kilometres apart. But here they were talking about measuring distances such as one tenth of a nucleus of an atom, using lasers. People thought they were nuts, but brilliant nuts. Their idea was somehow accepted and funded by US National Science Foundation. 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 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. On September 14, 2015, LIGO saw on two detectors, a Gravitational Wave. A signal on both the detectors was observed i.e., waves with increased frequency and amplitude. Decoding these waves told that they came from two black holes fusing together to become one, a billion years ago. Another signal was detected in December of the same year. The first wave produced difference in distance of about four thousandth of a proton over four kilometres. So you can imagine the precision of this instrument. The second wave was rather smaller but still convincing.

Also Read: Top 5 Unsolved Problems in Particle Physics

Now this might come to your mind that why we made two observatories. We could simply make one and save billions of dollars. But making two provided the difference in the local effects of gravitational waves. Fortunately, the signals on both detectors were the same.

What happened when these waves passed through the Earth?

When these waves passed through the earth, every distance on the earth changed a bit and very human stretched and compressed by a small amount (about one part in 10^20). But these changes in distances created were very minute, for example, if the distance between the sun the earth changed by one atomic diameter. This is why it took us so long to measure them. These waves being so weak were extremely hard to detect.

Where is the music?

The Music is in the Universe. LIGO acts more like an ear than like an eye. We can actually listen to these waves, by converting them into pressure waves in the air. Fortunately, they lie in the audible range and hence we can hear the universe singing to us. This is what we call “The Music of the Universe”. It is the rustle and chirp of the cosmos that makes it way all through the space.

Also Read: What is the Schwarzchild Radius for Black Holes?

Why do we need Gravitational Waves?

Gravitational Waves carry the record of all the cataclysmic events of the universe. These waves can be regarded as story telling waves. When detected, they tell about their origin and cause which will help us understand the universe and the Big Bang. We could literally hear the first cry of the universe. What could be more glorious!? Using these waves we can also discover the parts of the universe not yet known. And Why not?! “We are the cosmos made conscious and life is the means by which the universe understands itself” So, let’s understand and unravel The Secrets of the Universe.

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