"We have detected gravitational waves. We did it," said David Reitze, executive director of the Laser Interferometer Gravitational-Wave Observatory (LIGO), at a press conference in Washington on 11 Feb 2015.
Physicists have now finally confirmed the detection of gravitational waves; 100 years after these waves were predicted by Albert Einstein in his General Theory and after a long quest of 50 years. Certainly, it is one of the biggest Astrophysical discoveries of the past century – more important than Higgs boson discovered in 2012. Einstein's soul must be happy with his last prediction in the General Theory being proven directly by an experiment. Now both of his theories – Special and General Theory – stand experimentally tested. There is absolutely no doubt that Einstein's theory of General Relativity is one of the greatest impressive intellectual achievements of all time, though he received noble prize for photoelectric effect in 1921. Einstein published this monumental work in the form of four papers each separated by a week during the month of Nov 1915, followed by a summary paper in March 1916.
The General Theory of Relativity has been quite fantastic in proving the precession of the orbit of Mercury around the sun, deflection of light from distant stars while grazing the edge of the sun (or any massive body) and gravitational red shift – the useful implication for the GPS system, the only pending prediction of General Relativity was to detect the gravitational waves. After publishing General Relativity in 1915, in an address in France, Einstein once said, "If my theory of relativity is proven successful, Germany will claim me as a German, and France will declare that I am a citizen of the world. Should my theory prove untrue, France will say that I am a German, and Germany will declare that I am a Jew." Einstein's greatest success came in 1919, when Arthur Eddington a British astronomer, led expeditions to set up two locations to experience a total eclipse of the Sun on 29 May 1919, proving that the light from distant stars while passing by the sun bends as predicted by General Theory of Relativity.
Even today, in the modern age, we are deeply indebted to the prediction- gravitational red shift of the General theory, which resulted in the development of the Global Positioning System called GPS-inbuilt in our cars and phones. In one of the papers Einstein shows that the passage of time also depends on the strength of the gravitational field. The clocks that experience stronger gravity tick more slowly than those in a weaker gravitational field. This prediction was first tested in 1971 when four very precise atomic clocks were flown around the world and compared with the clocks left stationary in the laboratory. The results reported different elapsed time and indeed in exact agreement with the predictions of General Relativity. In GPS system we compare both satellite and Earth-bound clocks, realizing that the fact that the clocks in satellites tick more quickly than their terrestrial cousins on ground. If General Relativity were not accounted for, the difference in the clocks would lead to the mess in the GPS system and it will tell you that you are in a wrong place.
Before Einstein, people thought time to be absolute, which means that one big clock measures the time for the entire Universe. Consequently one hour of time in India is same as one hour in USA and one hour on Mars. Special as well as General Relativity has shown that space and time are tied together. We live in 4 dimensional world (3 space and one time). When we move through one of the space dimensions we also travel, unwittingly, through time. Time is a manufactured quantity. It can be slowed down or speeded up by motion and also by gravity. The massive object not only warps the space around it but also slows down the interval of time. So it makes sense to link space and time together as one thing – space-time. Space-time is a sort of background in which everything happens. It can be curved, stretched and squeezed. The particles which enter this warped space have to follow this path and these objects feel like they are on a steep hill and roll inward towards it. Thus we see that, matter forms the shape of the space-time and the space-time tells the matter how to move.
The gravitational waves are ripples in the fabric of space-time just like ripples we create on the surface of water by throwing a stone in it. When space can bend and twist under the influence of matter because of gravity, then moving mass can set up vibrations of space, which are gravitational waves. Black holes are enigmatic objects in the space which cannot be seen even with the powerful telescopes. They are collapsed stars which have lost fuel and crumbled under their own gravity to infinitesimal points. They are bottomless gravitational pits; on its surface even light can't escape. Because of huge mass and small size the space around the black holes is highly curved and time move extremely slowly. Long ago, deep in the space, two massive black holes, one 36 solar masses and the other 29 solar masses (1solar mass is our sun) slowly drew together. They started spiraling even closer, until, about 1.3 billion years ago, they whirled about each other at half the speed of light and finally merged and formed a new black hole of 62 solar masses with the release of energy. The collision sent a shudder through the universe. It was a strange coincidence that five months ago, they washed past our Earth, for the first time and Physicists detected these waves.
In the Interferometer experiment with two L-shaped tubes, each 4 kms in length, the laser light going through each tube, bounces back and forth between the mirrors, comes together at the middle and line up in such a way that they cancel each other. When the gravitational waves are passing through the earth, they cause the lab space itself to stretch in one direction and get squeezed in perpendicular direction. The beams have now to travel different distances along the arms. The light takes a little longer to get down in one arm than the other and that messes up the perfect alignment at the middle, so one can see that the perfect cancellation has been lost. So researchers measuring those small changes would actually be measuring and observing gravitational waves. The relative lengths of the two arms of LIGO can be compared to within 1/10,000 the width of a proton which means enough sensitivity to see a passing gravitational wave as it stretches the arms by different amounts. The researchers sensed a weak wave that stretched space by one part in 1021, making the entire Earth expand and contract by 1/100,000 of a nanometer—about the width of an atomic nucleus.
The gravitational waves are extremely weak and thus difficult to sense and because of this it took a long time to detect them directly. In 1974, U.S. astronomers discovered a binary pulsar – a rapidly rotating neutron stars that emits regular radio signals. They demonstrated that they are very slowly spiraling toward each other –as they should, if they are radiating gravitational waves. This indirect detection of gravitational waves won scientists Nobel Physics Prize in 1993. Now after a series of experiments conducted on the Earth and employing a range of technologies, the search is complete. The gravitational wave signals were detected by Physicists at LIGO on September 14 last year and since then the researchers have been rigorously studying this signal to see if it could have been caused by anything else. But the overwhelming conclusion is that the blip was caused by gravitational waves. "Gravitational waves are akin to sound waves that travelled through space at the speed of light," said one researcher David Blair recently. "Up to now humanity has been deaf to the universe. Suddenly we know how to listen. The Universe has spoken and we have understood."
The discovery of gravitational waves is certainly a big milestone in Physics. It will open a new window on Universe. Till date the only source of information from the outer space was through electromagnetic waves, but now we can receive information by means of gravitational waves. The discovery will throw more light on the origin of the Universe and Big Bang. It has confirmed the existence of black holes. We can test other theories on our universe, learn about the formation and death of stars and observe the catastrophic events happened long back, millions of light years away, from remote corners of the world. Quantum Mechanics and General Relativity are two branches in Physics which deal with microscopic and macroscopic world respectively. They are both accurate and reliable but are at loggerheads from the time they were invented. The most important thing is to see how the gravitational waves can help to unite the two theories. It remains to be seen whether the discovery reveals some other mysteries of the universe. Only time will tell.
(Dr Muhammad Amin Malik is Associate Professor, Department of Physics, Amar Singh College, Srinagar. Feedback at: firstname.lastname@example.org)