Relativity was called “Einstein's Symphony”. Such was its beauty and Mathematical consistencies. Einstein had proposed his Special Theory of Relativity in 1905 and further generalised it to the gravitational case in his General Theory of Relativity in 1915. Among many predictions, the bending of light due to gravitational fields was the most accessible at the time. This was confirmed on May 29, 1919, at the end of First World War by Sir Arthur Edington when he observed total solar eclipse by travelling to South Africa. This was a major triumph and set the base for further exploration in General Relativity and gave it the credibility. It predicted many phenomena like Black Holes, Gravitational Lensing etc. All of these were confirmed and one of the last unconfirmed major predictions was Gravitational Waves.
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| An artist's impression of Merging of Black Holes |
So what is a Gravitational Wave? In short, it is like any other ripple that you see in some like medium like water ripples or ripples in a tightly held cloth/fabric when you vibrate it. Just like these Gravitational Wave is a ripple in space-time fabric itself(in relatively we see that space and time as interrelated). Consider a charged particle. Every charged particle is associated with an electric field. Whenever this particle is accelerating it will emit electromagnetic radiation losing energy (if no external energy is supplied) and is governed by Maxwell's laws of electromagnetism. This phenomenon is what invalidated the Solar system model of atoms and led to Bohr's atomic model and pushed the Quantum Revolution. Now the essential point is that an accelerated charge emits radiation. In the same way, we consider every particle with mass having an associated gravitational field. This we know to be true even without Einstein's gravity just by Newtonian Gravitational laws. Now extending the above ideas we expect an accelerated mass to emit radiation which was theoretically confirmed by General relativity (GR). Now particle associated with Electromagnetic field when accelerated emit electromagnetic waves and by that logic, mass associated with the gravitational field will produce ........yes, you guessed it right- "Gravitational Waves". This was all predicted and proved way back around 1915-1916 when GR was formulated. But Einstein himself did not believe that this could be experimentally proved, why?....because even the most optimistic approximations gave the amplitude of these waves to be very small and way beyond the technology imaginable.
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| LIGO Observatories |
But why do scientists even bother trying to find these waves? Can't we just use theoretical predictions of already proven theory?. Obviously, it will help us give further proof of GR but that is not the sole reason to spend such amount of resources on it. These waves are pure and give information exactly as it is since they are almost completely transparent to any kind of external effects after their production and act as excellent "Cosmic Messengers" which is why they dubbed as "Einsteins Messengers". Biggest payoff Physicists expect is observing Gravitational Waves produced during the Inflation (or simply during Big Bang). Over and above that, simple answer a Physicist will give is curiosity and for a feeling of fulfilment. These were the bread crumbs nature left for us as the part of treasure hunt and it is the duty of the Pirate to find and unravel its mysteries.
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| An artist's impression of gravitational waves generated by binary neutron stars |
Since Einstein we have come a long way. We have discovered that Black holes do exist and over that we have "observed" black hole mergers, neutron star, pulsars and what not. Hulse and Taylor analysed the data from PSR B1913+16 pulsar (neutron star which emits radiation in regular intervals like pulses) in 1974. They observed the variation in time of pulses and decay orbit size. These results turned out to be exactly as predicted by GR and confirmed that they lose energy in the form of Gravitational Waves. This was the first discovery of Gravitational Waves either directly or indirectly. And this earned them the 1993 Nobel prize.
The first attempt at directly observing the Gravitational waves was done around 1960-1970's by a Physicist called Joseph Weber who is rightly called "Father of Gravitational Waves". But this was a very primitive device and as expected gave a null result. However, this set the tires rolling and created much interest in the field.
By the period of 1990's there was much hope and in 1994 LIGO construction began which went on till 2002. But even this was a very weak detector and enhanced LIGO project began in 2008. This was the game changer. They started observing black mergers. When two black holes merge a huge amount of energy is released in the form of Gravitational waves. These have enough energy to destroy all objects around them. But luckily or unluckily for us, these are so far from us that by the time they reach us, their energy can only stretch a 4km long rod by only 1/thousand th the size of the proton. This is what makes the detection monstrously hard. Laser interferometer technique in a 4km long tunnel is used to detect these waves. Finally, they were discovered on 14 September 2015 while observing two ~30 solar mass black holes merging about 1.3 billion light-years from Earth. This detection happened just two days after the newly enhanced detector started working.
The LIGO detectors have already discovered 3 gravitational by now. As always in science, new observation leads to some unknown phenomenon. Until now Physicists believed that not many heavy black holes (>20 solar masses) existed. But out of the three detections, two of them involved these heavy black holes. Hence already there is a scent of new paths. Most anticipated use of these is Gravitational Wave Astronomy just like EM wave astronomy or Neutrino astronomy. While EM wave/normal astronomy fairly good but they fail us at a very large distance while Neutrino astronomy has potential to go very close to the origin of the universe however not close enough. This is where Gravitational Wave Astronomy comes in as we can go very very close to Big Bang and observe what happened. Exciting times are ahead!!!
As much difficult as it may be, it is ALWAYS worth it-
" PHYSICS IS IMAGINATION IN A STRAIGHT JACKET "
- John Moffat
P.S- LIGO India called INDIGO has been planned (Yay!!!). A site in the Hingoli district in western India has been selected.
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