Gravity Got You Down?

One morning, the world woke up to find that the internet was bursting with news of the detection of Einstein’s long predicted gravitational waves. Well, hooray for Mr. Einstein, but few people truly understand the significance of this discovery.

To find out, we must go all the way back to the history of gravity itself…

An apple falls on Isaac Newton’s head. (1600s)

Until the seventeenth century, people had never really stopped to think about why stuff just fell down instead of floating around. The legendary apple chose the right head to fall on, because out of that head popped the first definition of what we now know to be gravity. Sir Isaac Newton defined gravity as an attractive force between any two objects in the universe that depends only on the mass of each object and the distance between them.

This definition of gravity, along with his laws of motion, became the foundation of physics for the next two hundred years, solving innumerable problems in various fields, with a vast variety of applications.

A patent office clerk delves into physics to pass the time. (Early 1900s

For over two hundred years by now, Newton’s definition of gravity had been accepted without question as one of the fundamental truths of science.

But Albert Einstein, then just a patent office clerk in Switzerland, found a problem with it. Einstein had just proved that the speed of light was a constant, and that nothing in the universe could travel faster than that. This meant that even the effects of gravity had to ‘travel’ from the source to its destination. For example, in our solar system, if the sun suddenly disappeared, it would take at least eight minutes for Earth and all its earthlings to notice that the sun is gone and for our planet to shoot out of its orbit.

Unfortunately, this was in direct contradiction to Newton’s predictions. Newton’s definition of gravity said nothing about how long the effect of gravity took to manifest; it directly assumed the effect was always present. This meant that according to Newton, the effect of the sun disappearing would immediately be felt on all planets which would then instantaneously shoot out of their orbits.

Until then, only the effects of gravitational force were studied and defined, but Einstein felt that something crucial was missing from Newton’s definition, and that the answers lay in the true nature of gravity, which was hitherto un-pondered upon.

Ten years of research bore fruit in the form of what is probably Einstein’s greatest accomplishment, the General Theory of Relativity.

A brief description of Einstein’s version of gravity.

Einstein postulated that the three dimensional space that we can see and move about in and time are actually interlinked in a four dimensional fabric that makes up the universe. Gravity was simply an effect of objects having mass interacting with this fabric.

The best way to understand this rather abstract concept is to think of the four dimensional fabric of space-time as an ordinary stretched elastic sheet. A ball placed in it would form a depression around it, causing any nearby objects to ‘fall’ towards it. This is the gravitational force that we feel, that keeps us on the ground. The heavier the ball, the greater the depression and the greater the ‘gravitational force’.

What are gravitational waves, you ask?

Einstein’s theory of the nature of gravity was successful on two counts. It agreed with Newton’s definition that gravity depended on nothing but the mass of each object and the distance between them. But it also managed to explain how gravity was not an instantaneous phenomenon, and in doing so, brought forth a new concept, never thought of before, called gravitational waves.

Gravitational waves are what happens when something disturbs the fabric of space-time significantly. The analogy would be if two very heavy balls on the elastic sheet collided with enough force, the sheet would vibrate and ripples would spread out from the point of collision. The ripples represent gravitational waves.

Just as when a ripple passes through the elastic sheet, the fabric is stretched and compressed along perpendicular axis in succession, when a gravitational wave passes through matter, it stretches and compresses it. However, the degree of stretching very, very small, only one ten thousandth the size of a single proton, hence we don’t feel gravitational waves passing through us.

Einstein mathematically showed that the speed of these gravitational waves is equal to the speed of light, thus also putting a number on the speed at which the effect of gravity ‘travels’.

 So A Big Reflecting Thing is Built.

Einstein proposed his theory of the nature of gravity, but there was no experimental proof to support it, and in science, experimental proof is everything. Thus, scientists were looking to find ways of proving the theory experimentally.

The most obvious and conclusive evidence in favor of the theory would be the detection of gravitational waves, but how did one go about detecting something whose effect was so immeasurably small?

The answer lay in the Laser Interferometer.

A brief description of the working of the laser interferometer.

A Laser Interferometer works on the principle of interference of light waves. A laser beam is directed at a prism, which splits the beam into two mutually perpendicular beams, each traveling along identical paths ending in mirrors. The two beams reflect and return to the prism, where they interfere, producing light of a particular intensity, measured by a detector.

Under normal conditions, the two beams are perfectly out of phase when they interfere, and they completely negate each other giving zero intensity. However, if a gravitational wave were to go through the setup, it would stretch one arm and contract the other. Now the two paths are no longer identical and when the light beams interfere, they have some phase difference between them, giving non-zero intensity at the detector. As the gravitational wave passes, the stretching and contraction of each arm of the interferometer varies periodically, leading to a periodically varying intensity being seen at the detector.

The origins of LIGO, the instrument that detected gravity waves.

Since the effects of gravitational waves are so small, the interferometer used has to be incredibly precise.

 How is this done?

By making it big. Very big. And by using the best equipment in the world.

Each arm of the interferometer is 4 km long. The laser used is the steadiest source of light ever made. The mirrors are polished to a degree never attempted before and they are suspended by a system of wires and magnets that prevent any inadvertent vibration from interfering with the result. The whole setup, built by university researchers and students from all over the world, after years of toil, was called the Laser Interferometer Gravitational wave Observatory, or LIGO for short.

But there isn’t just one of them.

Two LIGO were built and were operational by 1997 on opposite ends of the continent of North America. This was to ensure another level of precision, by making sure both observatories recorded the same data for gravitational waves.

In spite of all this effort, for many years, LIGO found nothing.

2016: Eureka! The discovery of gravitational waves.

After years of finding nothing, it was decided in 2004 that LIGO just wasn’t sensitive enough to pick up the miniscule effects of the gravitational waves. It was closed for almost ten years while its sensors and equipment were revamped and improved. In late 2015, almost immediately after it reopened as Advanced LIGO, it began to pick up on gravitational waves, later shown to be caused by the collision and successive merging of two massive black holes 1.3 billion light years away.

Some scientists claim this to be the single greatest moment in the history of science yet, and with reason, for the discovery of gravitational waves opens new doors that we never even knew existed until now.

What the discovery of gravitational waves means for science.

The discovery of gravitational waves proves that Einstein’s theory about the fabric of space-time, called the space-time continuum, is true. This will provide a basis for other theories about cosmic phenomenon, and the cosmos itself to come up. But most importantly, gravitational waves can be used to observe events that could not be observed before due to inadequate technology.

What kind of events, do you ask? We can now observe the core of stars, for example. Until now, any radio waves could not get through the dense, hot mass of gasses around the star to reach the core, but by observing the gravitational waves coming from the core, the composition of the core and other such observations can be made. We could observe black holes in greater detail, or merging of galaxies and stars.

But the thing that have scientists most excited is that we can finally observe the Big Bang. The very creation of the space-time continuum sent large ripples through it in the form of gravitational waves, which we can observe now, giving us information about the creation of the universe. This is something that, arguably, the whole of the field of science has been building up to, to learn more about our creation.

With the help of the two established LIGO facilities and the new LIGO to be set up in India, researchers across the world hope to conquer new boundaries of knowledge and push the limits of science ever further, in this ultimate quest for understanding.


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