Abstract
Einstein’s General Theory of Relativity predicted the existence of Gravitational Waves in 1916, marking the beginning of decades-long controversy over their existence. With further research, it was found that sources such as coalescing compact binary systems, stellar collapses and pulsars can be sources of Gravitational Waves. Highly sensitive and promising models of Gravitational Detectors were introduced and implemented which include LIGO in USA, TAMA 300 in Japan, VIRGO in Italy and GEO 600 in Germany. The first ever detection of Gravitational Waves was made on September 14, 2015 by the LIGO detectors in the USA. It detected the undulations in space-time induced by gravity waves generated by two colliding black holes 1.3 billion light-years apart.
Keywords
Gravitational Waves, Binary systems, Interferometric techniques, Michelson interferometer,
Binary Black hole Merger.
1. Introduction
The existence of gravitational waves, also described as ‘ripples' in space-time, was predicted by Albert Einstein in 1916, shortly after the definitive formulation of the field equations of General Relativity. Einstein’s mathematics showed that massive accelerating bodies can act as a source of gravitational waves producing waves of space-time. Gravitational waves have a very small amplitude, but they may be detected by highly sensitive detectors. Efforts for the detection of Gravitational waves began in 1960s from resonant mass detectors and a network of cryogenic resonant detectors. Use of interferometric detectors were suggested in 1970s which were more sensitive than other detectors. By the 2000s, initial detectors such as LIGO in the United States, TAMA 300 in Japan, VIRGO in Italy, and GEO 600 in Germany had been established. Observations were made by the combination of these detectors from 2002 through 2011. Advanced LIGO was the first of a far more sensitive network of advanced detectors to start taking measurements in 2015.
2. Observation of Gravitational waves
On 14th September 2015, LIGO Detectors in USA marked an end to the decades-long debate on the existence of gravitational waves by physically sensing gravitational waves from a binary black hole merger for the very first time. The detection of the signal GW150914 was exclusively made by LIGO detector since LIGO was only observing at that time. VIRGO was being upgraded and GEO 600 was operating but not in observational mode and was not sufficiently sensitive. The chirp signal lasted 0.2 seconds and rose in frequency and amplitude from 35 Hz to 250 Hz in around 8 cycles. The black holes' relative tangential (orbiting) velocity grew from 30% to 60% of the speed of light throughout the course of the 0.2-second observable signal.
Since the gravitational waves have a significantly small amplitude, its detection was a challenge due to the interference of various forms of noises including photon shot noise, displacement noise, seismic noise and thermal noise. Photon shot noise is reduced by using interferometric techniques, seismic noise is reduced by suspending test mass as the last stage of a quadruple pendulum, and thermal noise is reduced by employing low mechanical loss material in the test mass and suspensions. All components, excluding the laser source, are positioned on vibration isolation stages in ultra-high vacuum to reduce extra noise. There was no indication that it was an instrumental artefact after extensive analyses of instrumental and environmental disruptions. No abnormalities were detected by any of the environmental sensors. There were no long-range associated disruptions discovered. Space-time volume exceeds prior observations by an order of magnitude for binary black holes of similar mass. Hence it was confirmed that the gravitational waves detected by LIGO has its origin at a binary black hole merger.
2.1. LIGO Detectors
Advanced LIGO Detector (Laser Interferometer Gravitational-Wave Observatory) is a modified Michelson interferometer. Each arm has a resonant optical cavity, a partly transmissive power recycling mirror, and a partially transmissive signal recycling mirror in order to increased sensitivity. A photo detector, a 20W LASER input, and a beam splitter are also included. Each arm of the LIGO detector is 4 kilometres long. Space itself stretches in one direction while simultaneously compressing in a perpendicular direction as a result of gravitational waves. This causes one arm of the interferometer to get longer while the other gets shorter, and vice versa, as long as the wave is passing. The latest LIGO detectors are 3 to 5 times more sensitive to strain than the original LIGO detectors in their most sensitive range, 100–300 Hz; at lower frequencies, the increase is significantly larger, with more than ten times higher sensitivity below 60 Hz.
3. Conclusion
The LIGO detectors were the first to detect gravitational waves emitted by two stellar-mass black holes merging. The presence of binary stellar mass black hole systems is demonstrated by these findings. This marks the first observation of a binary black hole merger and the first direct detection of gravitational waves. The signals from neutron star and black hole interactions are opening up a new age in which we will get a better understanding of the universe. Along with LIGO, the LISA project which started as a joint effort between NASA and ESA will together define the future of astronomy. LISA would be the first gravitational wave detector to be sent into space which has a goal to use laser interferometry to directly measure gravitational waves. In the coming decades, with the help of far more advanced detectors and observatories, we’ll be able to observe and understand new unknown sources.
References
1. B.P. Abbott, Observation of Gravitational Waves from a Binary Black Hole Merger, PRL 116, 061102 (2016).
2. S. Rowan and J. Hough, THE DETECTION OF GRAVITATIONAL WAVES, Glasgow G12 8QQ.