Gravitational Waves for Dummies.

March 06, 2016 Unknown 1 Comments


How do physicists celebrate I wonder. Maybe peek through the Hubble telescope and watch two comets collide and yell CHEERS? Maybe accelerate 2 protons in a hadron collider and yell MOZEL TOV when they collide, spilling out compositional matter? Or maybe just clink glasses of champagne like the rest of us boring folk. However the method, On February 11th 2016, celebrate they did, and the world joined them, when they announced the disturbance detected on September 14 2015 was in fact caused by gravitational waves proving their existence and giving credibility to Einstein’s prediction made a hundred years ago!




Now a lot of us joined in the celebration, clapped and smiled, but deep down wondered what the heck are these guys talking about? Gravitational waves? LIGO? I don’t get it! Let’s put the entire discovery into simple terms and get on track with these genius minds and be a part of this significant time in history.

The Universe: When Einstein was formulating the theory of ‘special relativity’, he found that the Universe was a fabric woven with space and time. He named it the ‘spacetime continuum’. Imagine the entire universe as a giant, stretched cloth, woven not with silk but with space and time. We now have our Universe, the cloth. Figure a and b give a better view.
  
         (Fig a: The Universe - A Stretched Cloth)                                                                      (Fig b: The Universe - Scientific rep.)
Gravity: Gravity was discovered by Sir Isaac Newton, and the geometry of Newtonian Gravity was already in place. Einstein, after publishing his theory of special relativity, wondered how gravity could be incorporated into his idea of the spacetime continuum (the stretched cloth). After many, many years of research and hypothesizing Einstein concluded that objects with mass, cause a dent in the fabric of spacetime. A body with mass causes a curve (depression) in the fabric of spacetime. If the mass is large enough, it can cause a curve large enough for other bodies to fall into and stay in orbit. This is Gravity. Remember our cloth? Imagine holding the cloth at the four ends, stretching it out, like how our universe is stretched. Now imagine a Bowling ball placed on the cloth. The weight of the ball will cause the cloth to sag (Fig c). This is what bodies with mass do to the spacetime continuum. They cause a depression. Now imagine a small golf ball thrown into this depression, sideways. It will revolve around the bowling ball, and the depression will not allow it to leave the outer vicinity. This is the force of gravity exerted by the bowling ball on the golf ball, causing it to stay in place. Now the smaller ball also has mass, but with respect to the bigger ball it is negligible. We now have Gravity.

(Fig c: Large ball depressing the cloth, smaller ball falling in its pull)                           (Fig d: Moon attracted by Earth's Gravitational pull)
Gravitational Waves: We now know how a body with mass induces Gravity. Einstein went on to predict, that when Objects with mass move, they cause ripples in the spacetime fabric. These ripples are disturbances, and they tend to stretch out and also compress the fabric of space. Imagine our bowling ball being pushed around on the stretched cloth. It will cause the cloth to distort, sending out ripples. These are the Gravitational waves. These waves are what Einstein predicted a hundred years ago. He went on to lament that they may never be detected or may not even exist.


                     (Fig e: Ripples caused by a stone are an example)                (Fig f: Distortion in space time by a large mass- the gravitational waves.)
That however didn't stop our brilliant physicists. They went ahead adamantly, searching for the waves and a hundred years after the prediction, discovered them.

Now the waves emitted are generally too small to be ever detected or even felt. When a giant star moves, it sends out gravitational waves which stretches out and compresses the universe. In essence, you and I have been stretched out and compressed many, many times, but we have never felt it because the effect is astronomically small.

For a wave large enough to be measured, the size of the bodies must be relatively huge. If two giant blackholes came near each other and got trapped in each other’s gravitational fields, they should spiral towards each other they should distort spacetime and generate gravitational waves moving away from each other, and these should be significantly more detectable than waves emitted by singular bodies. This is exactly what happened on September 14th 2015. But first, scientists built LIGO (Lie-go)

LIGO (Lie-go), Laser Interferometer Gravitational waves Observatory, was built to measure gravitational waves. It does so by detecting and measuring the distortions in spacetime caused by gravitational waves. Here's how it works:

LIGO is an L shaped detector, with both arms of the L exactly equal in length (Fig g). A laser beam is split and sent in two directions, within the arms. They travel precisely the same distance within the long vacuum tubes, are reflected off mirrors at the extremes and sent back to the starting point where they combine (Fig h). The light waves are made to exactly line up so that they cancel each other out on reaching back and nothing is read on the photodetector.
        
(Fig g: LIGO Hanford Observatory)
                                                                 (Fig h: How LIGO Works)
Now when a gravitational wave passes through Earth, it stretches Earth and everything in her in one direction and compresses in another direction. This changes the distance between LIGOs arms(image below) and in turn will cause the light waves to no longer line up, and now we will have a signal read in the detector. 
A signal read in the photodetector would mean something caused a disturbance in LIGOs arms. If there were no natural disturbances, for example earthquakes or other such causes, it would mean the disturbance was due to an invisible force, namely the gravitational waves.
Because the detector is extremely sensitive to even the most minute disturbances, 2 identical LIGOs were built on opposite sides of the United States, and for a signal detected to be considered as gravitational waves, both LIGOs must detect a reading of the same magnitude at the same time.



A: LIGO arms equal. B and C: LIGO arms                          Light Waves in Sync                                   Light waves out of  Sync                                             
distorted in length by gravitational waves

What happened on September 14th 2015 was a thing of beauty. Imagine two giant black holes, oblivious to the other, travelling through space. They come scarily close to each other and get trapped in each other’s gravitational pull. They spiral towards each other, spinning at great speeds, getting pulled towards the epicentre and end up combining, sending ripples through the universe. The universe is alive, it is a living organism bustling with activity.



These waves were detected by LIGO and after months of data analysis, on February 11th 2016, scientists announced the discovery they made was gravitational waves. It is and will be a monumental step forward in the understanding of our universe.

Just to give you a sense of it's implications, imagine the Radio Waves. Since their discovery, they have opened a whole new method of communication for us. Something that is invisible to the naked eye is the backbone of today's method of communication. Gravitational Waves, the same way, will open up a whole new possibility for mankind. They will give us new insight and deeper understanding of the universe and it's origins.

A round of applause to the never faltering persistence of humankind.



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