Can you imagine that there could be wrinkles in space and time caused by some natural phenomenon in the universe? A gravitational wave is a wrinkle caused by a stellar crash of neutron stars. A discovery that happened in September 2015 but has been predicted by Einstein more than a century ago. Einstein suggested in 1916 that his general theory of relativity pointed towards the occurrence of gravitational waves, which are caused when very massive objects in space spiral towards each other and distort the fabric of time and space. This space-time distortion then sends ripples across the entire cosmos. This article will define gravitational waves and look at the gravitational wave equation along with a few other essential characteristics of the gravitational wave theory.
To give a gravitational wave definition, first, we need to understand what happens in the cosmos. Many violent and forceful processes in the universe cause gravitational waves. Something special happens when two massive accelerating bodies like black holes, neutron stars, or planets orbit each other. Such movements can disrupt time and space, which would travel like ripples. Gravity waves would travel from the source like water in a pond would spiral out when a stone is thrown in it. The gravitational radiation then propagates in directions away from the source, carrying information about its origins. Gravity is a wave that also carries with it clues to the nature of gravity itself.
Cataclysmic events produce the strongest gravity waves. A few examples are black holes colliding, supernovae, huge stars that explode once they reach the end of their lifetime, and neutron star collisions. The other less strong waves are predicted to be created by the movements of those neutron stars that are not perfect spheres. The remaining gravitational radiation is believed to be remnants of the Big Bang.
Einstein was not entirely convinced by his own idea, though many scientists accepted his theory. For the next few decades, Einstein fiddled with his theory and published many papers that refuted his original idea.
The initial proof of the existence of gravitational waves came in 1974, 20 years after the death of Albert Einstein. In the Arecibo Radio Observatory (Puerto Rico), two astronomers discovered a binary pulsar. Pulsars look like a blinking star from afar, but they are planets that orbit the rapidly rotating neutron stars. These seemed to them exactly like the system that was predicted earlier to generate gravitational waves. The astronomers felt that this could be proof of Einstein's prediction years ago. Hence they started measuring how the orbit of these stars changed over time.
After 8 years, they found that these stars were getting closer to each other at a rate which the theory of general relativity aptly predicted if these were radiating gravitational waves. Since then, many astronomers have tried studying the pulsar radio-emissions and found similar results, but these results did not come through direct contact but either as a mathematical deduction or other indirect ways.
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Finally, on September 14, 2015, LIGO (Laser interferometer gravitational-wave observatory) found a breakthrough and proved the existence of gravitational waves, which were caused by the collision of 2 black holes that were 1.3 billion light-years ago but reached Earth only in 2015. This could be deemed as one of humanity’s biggest scientific achievements.
On October 3, 2017, the founders of LIGO Rai Weiss, Barry Barish, and Kip Thorne won the Nobel Prize in Physics for this discovery.
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A gravitational-wave observatory or detector detects the gravity waves. As a gravitational wave passes earth, it goes on squeezing and stretching the space. LIGO can detect this movement of space by gravitational waves. A LIGO observatory consists of two ultra-sensitive detectors. These identical L-shaped detectors are 4 kilometers long and situated in Washington and Louisiana. By employing lasers and mirrors, each of these detectors can catch tiny aberrations in spacetime, which the gravitational waves cause. If a gravitational wave crosses these arms, their length changes slightly. The laser bouncing back and forth between the two mirrors can track how far apart these mirrors are to an incredibly precise degree. A real signal will show up in both the detectors.
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Now another observatory, the European gravitational observatory’s detector, is also live, which has a similar design as LIGO. So, there are a total of three working observatories for gravitation waves in the world. This would help scientists precisely identify the source of the gravitational waves in space. Similar observatories are anticipated soon in Japan and India too.
Gravitational waves can not be seen; they are invisible.
They are very fast and travel at the speed of light, 186,000 miles per second.
The gravity waves stretch and squeeze everything on their way as they pass by.
The objects that cause gravitational waves are very far away from earth and sometimes might not reach the earth if they are very weak. That is why these waves are very difficult to detect.
Though destructive and violent events cause gravitational waves, they are much smaller when they reach the earth (thousands of billions of times).
The gravitational waves that reach the earth result from an event that occurred billions of light-years ago.
Q1. What is the Gravitational Wave Equation?
Ans 1: The wave equation that gives gravitational fields like electromagnetic waves is characterized by a wavelength λ and a frequency f. Gravitational waves travel at the speed of light, which is given by λ * f.
Gravitational waves are transverse waves, i.e., they result from shear stresses linked with shear strains. Two states determine the polarization of these waves, i.e., plus (+) and minus (-). Gravitational waves can be understood as a small disturbance in flat space, and the gravitational wave equation can be given as:
ds2 = gµνdxµdxν = (ηµν + hµν )dxµdxν
where ηµν -> the Minkowski metric for flat spacetime.
hµν -> the small perturbations (also called the wave metric).
Q2. How can LIGO be Sure that a Signal in the Data is Actually From a Gravitational Wave and not Other Disturbances?
Ans 2: LIGO can sift through the vibrational noise using a combination of different techniques.
It uses microphones, seismometers, gamma-ray detectors, and magnetometers to measure sources of all known noises like winds, ocean, earthquakes, farming activities, and even molecular vibrations in the LIGO mirror. It then filters out vibrations caused by such noises from its data.
Noises that are local to the detectors are ruled out by looking for similar, simultaneous signals from various detectors worldwide (LIGO, Virgo, GEO600). If more detectors feel the same vibration simultaneously, then it becomes certain that these are not local disturbances.
It constantly compares the signal that comes out of the interferometers with the gravitational wave patterns established by known phenomena that are derived from the general relativity equations.
It also compares the arrival time of a supposed gravity wave with an EM (electromagnetic) event detected by EM observatories.