Romancing the distant whisper

Romancing the distant whisper

Submitted by Rashmi on Sat, 2016-03-26 17:47 Physicists for the last few decades have been on a hot pursuit of detecting one of the weakest signals ever, tuning their giant instruments to sense the presence of waves emanating millions of light years away during certain astronomical events. These waves are so feeble that even the sound of a passing wind on a nearly windless day is a thunderous roar compared to it.Einstein predicted that massive collisions of celestial objects produce ripples in the space time fabric. He called these waves gravitational waves. These waves have a peculiar nature of shrinking and expanding space in two different directions alternatively. Now you would stop and ask me “Did you say space?” Yes! That is right, space! This prediction was a consequence of his celebrated theory of gravitation, known as the general theory of relativity. According to this theory both space and time are interwoven in one continuum. And the curvature of this continuum is what we perceive as gravitation. These curvatures on the space time continuum are created by any massive astronomical objects like planets or stars.Detecting these waves was thought to be almost impossible in the early days of its prediction. Eventually however some courageous experimentalists decided to give it a shot, a task so daunting that for a good hundred years after its prediction some of the best experimentalists in the world failed to detect them despite huge efforts and spending billions of dollars.To understand the fundamental difficulty in detecting these waves let us consider the following situation. Imagine on a fine morning your house expands to double its size. Well who would not like a bigger house? You step out to announce to your neighbors, the good news, only to discover to your dismay that their houses also have mysteriously grown bigger, and the streets stretched accordingly. You wonder whether it was you who shrunk or everything else expanded around you. Had you also expanded along with all other objects, including the furniture and the house, would you have noticed anything at all? Absolutely not! If everything expanded then even the rulers that one would use to measure the expansion would have themselves expanded making it impossible to tell the difference.How then would one detect such waves? If one had a special ruler which would not shrink or stretch one could measure these elastic changes. Indeed physicists have found such a ruler. It is not a ruler in a strict sense but something that does not get affected by the stretching: light! The fact that its speed is independent of external movements was discovered by Einstein himself! Thus if space itself expands light would take more time to travel a particular distance, thereby making it possible to measure the difference. The light could be used not as a ruler, but as a stopwatch.The gravitational waves stretch space asymmetrically. Meaning, if you consider the north- south and the east-west directions, when the waves shrink the space along the north- south direction they would expand it in the east-west direction.If we could send light in these two perpendicular directions and reflect them and measure the difference of their arrival at a point, we could tell whether there was stretching or not! Light being a wave interferes with another light wave just like waves created by two pebbles on a pond add or cancel each other when they meet. When they are in sync, they are enhanced, and, when out of sync they destroy each other.The change in path length of the light sent in two different directions and then combined produces a shift, as the extent of overlap is different for different path differences. This is what is technically called the fringe shift. There is indeed a very simple and beautiful device which achieves exactly this: Michelson’s interferometer developed in the 1880’s which was used to search for the existence of the elusive ether.However, there is another significant practical difficulty. Einstein’s calculations show that these gravitational waves are so weak that one would need to make these paths of light very large in order to observe any measurable difference. And the device that detects such differences has to be extremely sensitive! The changes that we talk about are unimaginably small. The relative change (change in length to original length) is so small that if the relative changes were to be for earth system (Diameter of the earth is about 12,700 km) then the expected change is about 1000 times thinner than the thickness of our hair.Researchers at LIGO, at the United States of America, bounce light off two mirrors, each weighing 40kg, kept at the end of two arms of an L Shaped arrangement( both the arms are of length of 4 kilometers) and receive them back on a detector to see whether these two paths are the same or not. This is a highly modified version of a Michelson’s interferometer. They use an extremely sensitive Michelson’s interferometer along with a number of devices which enhance the sensitivity of the measurements. The reason why the mirrors are made so heavy is that they do not move under the influence of random vibrations coming from various sources from earth which could completely destroy the possibility of detecting the weak gravitational wave signals!And the entire lengths of the two 4km arms are maintained under Ultra high vacuum conditions to reduce vibrations. To begin with, these two paths are made equal and then one would wait for a massive astronomical collision to occur. The collision sends ripples in the space time and when the ripples arrive they stretch these two paths in different ways resulting in a change in the path length which results in a fringe shift which can be detected.In the history of science these measurement are considered to be one of the most ambitious and the most criticized, for many believed that it is a futile venture. However on 14th of September 2015, LIGO and Virgo Collaborations measured the whisper of two merging black holes about 1.3 billion light years from Earth Proving beyond doubt Einstein's theory of the existence of the gravitational waves. It was published on February 11, 2016.  This has opened up a whole new area called Gravitational Astronomy which was non-existent till the discovery of the gravitational waves.Gravitational astronomy can be used to detect the presence of black holes and other astronomical bodies which are not accessible to conventional astronomical observations. These might eventually help us unravel the secrets of black holes, which hold the key to understanding the universe about which very little is known.  Footnote:Abbreviations: LIGO: Laser Interferometer Gravitational-Wave Observatory LIGO(The author is an Assistant Professor at Department of Sciences, Amrita School of Engineering, AMRITA VISHWA VIDYAPEETHAM (UNIVERSITY ), Ettimadai, Coimbatore )