Gravitational Waves, in simplest terms, are a ripple in space-time curvature. This implies that the ripple generated due to the collision of two black holes or any other massive body in the universe travels as a wave. The simplest implication of gravitational wave hitting earth is that when it passes by us, the distances will appear to oscillate i.e. the distance between two objects in the path of the gravitational wave will grow, then shrink and then grow again, and so on. These were predicted by Albert Einstein in 1916 as a result of his special theory of relativity. According to him, everything which has mass is a source of gravitational waves and the impact depends on the mass of the object or the intensity of events like the merging of black holes, the collision of stars etc.
Fig: Left: Ripple effect, Right: Merging of two black holes
Consider space-time curvature as a rubber sheet and on its surface, galaxies, stars, black holes, planets etc. are present. This causes the sheet to bend, like a bowling ball on a trampoline. The more the mass, the more that space-time curvature gets bent and distorted by gravity. It is because of the similar type of distortion caused by our Sun which leads to planets rotating around it. Since the sun is a very massive object, if you try to move in a straight line around it, you will find yourself moving in a circle. When heavy objects start accelerating in this space-time “Rubber sheet”, this produces gravitational waves. Since gravitational force is comparatively weaker than other forces of nature, therefore, gravitational waves show an effect when extremely massive objects move very fast creating ripples big enough so that we can detect. So, to detect the distortions caused by gravitational waves, time taken by light to travel back and forth between two points is taken as a scale. If this time period increases, that means the distance stretched and if it decreases the distance contracts.
Fig: Space-time distortion by Sun
LIGO, which stands for “Laser Interferometer Gravitational-wave Observatory” exploited the above-mentioned concept in its recent success in detecting gravitational waves. The team of researchers built two detectors, one in Livingston and other in Hanford. In LIGO laboratory, two 4 km long tunnels are built which are perpendicular to each other and have mirrors at their respective ends. A Laser produces a sharp ray of light which then hits the beam splitter which further divides the beam into two parts which are coherent. This light enters both the tunnels and travels till the end, get reflected by the mirror on the end of the tunnel and again meet at the beam splitter (Refer image).
Fig: LIGO setup
Since both the tunnels are of same lengths and the frequency of both the light beam is also same, they cancel out each other without producing any output on photo-detector. But when the frequencies do not match, it means that the lengths of tunnels are different hence producing an output on photo-detector. This is how LIGO team was able to achieve the first-ever detection of gravitational waves. When these waves pass through Earth, they distort the time-space and every physical entity on Earth, therefore changing the lengths of the tunnels and getting recorded on the photo detector.
Now the question arises why Gravitational waves are so important? This discovery has been hailed by everyone as an extremely significant achievement. Just like when Galileo pointed the telescope towards space for the first time, we were able to see things beyond the earth. Similarly, the discovery of these waves is equivalent to be able to hear after being deaf for a very long time. This is important because it might help in finding new dimension other than space and time, i.e. Gravity. It will give a brand new perspective to several phenomenon and aid in unveiling the answers to countless questions.