Astronomy Paper on Gravitational Waves
Introduction and Background
Gravitational waves are postulated by Einstein’s theory of general relativity, which offers a new explanation of the phenomenon of gravity. According to the theory, space and time are considered a single entity, called spacetime. An important property of this physical entity is that its curvature is a function of mass and its proximity to a given point/region in spacetime. In general terms, the more massive and closer a mass is, the sharper the curvature at the region of interest, or at the boundary of the volume of the mass.
It follows that any motion of a given mass through spacetime, which actually involves a continuous relocation of the mass within spacetime, will result in an analogous distortion of the spacetime. These continuous disturbances in spacetime behave like waves, as might be observed in a ripple tank. On rare occasions, some masses in the universe – for example orbiting binary stars – accelerate in such a manner that their perturbation of spacetime generate waves that spread out at the speed of light. It is these vibrations of spacetime propagating at the speed of light that are termed gravitational waves (Harrison 231).
A passage of gravitational waves through spacetime causes an alteration of distances between bodies on its path in correspondence with it the gravitational wave’s frequency. The change in distance between any two bodies is a function of the distance of the two bodies from the source of the wave (Harrison 233).
A scientifically interesting property of gravitational waves is their ability to propagate regions of space that are opaque to electromagnetic waves, as they do not depend on the medium but spacetime itself. Their use as a medium of observation along with the electromagnetic waves, is speculated to unravel regions of the universe that are currently inaccessible.
Goals: The Search for Gravitational Waves
Until recently, gravitational waves were just a theoretical prediction of Einstein’s theory of general relativity. The existence of such waves had been a question of interest to physicists, especially considering the fact that the Einstein’s theory of relativity has withstood every experimental test it has been subjected to so far. This interest has been furthered by the knowledge that gravitational waves, unlike electromagnetic waves, are immune to impedance by materials in the medium. Therefore, the question of whether or not gravitational waves actually exist became significant to astronomy and physics as a whole.
An experiment was thus designed to test the existence of such waves. The experiment is named Large Interferometer Gravitational-Wave Observatory (or LIGO, in short). The main goal of LIGO, which is both an experiment and observatory, is twofold – (1) to detect gravitational waves from the cosmos, if at all they exist; and (2) to develop a new astronomical tool that relies on the gravitational waves for observation (Barish and Rainer 44).
The experiment was first conceived at Massachusetts Institute of Technology and, a year later, Caltech University. The two institutions later pooled together their resources to form a joint LIGO experiment. The project experienced operational difficulties, until its operations stabilized in 2002. Eight years since, gravitational waves remained elusive to the experiment. The failure to detect gravitational waves prompted a refinement of the experiment in terms of sensitivity and precision. The refinement was completed in the beginning of February 2015, and, in September of the same year, LIGO began a new phase of observation. This new phase of observation actually culminated in the detection of gravitational waves.
Observations Made by LIGO
In 2015, the LIGO experiment made an observation of gravitational waves using a telescope that is in many ways different to conventional telescopes. A mention of a telescope conjures up images of dishes pointing into the sky – mirrors, in the case of optical astronomy; or plane dishes with some sort of antennas, in the case of a radio astronomy. The LIGO telescope, however, departs from the conventional design. The primary element of the LIGO telescope is the interferometer.
The setup of the LIGO observatory consists of a laser beam generator, a beam splitter and two partially reflecting laser mirrors. The two reflectors are set up such that they lie on perpendicular planes (lines perpendicular to their surfaces intersect at right angles). The beam splitter is placed at the point of intersection of these two lines so that the beam splitter equally bisects the perpendicular angle between the two lines. The laser generator is then placed at a point where it is parallel to one reflector and perpendicular to the other reflector. The two reflectors are placed 4 Kilometers from the beam splitter (“LIGO’s” par.3).
The setup works on the principle of interference. The laser generator generates the laser beam in the direction of the beam splitter. The beam splitter splits the beam into two – it partially reflects the beam to the reflector that is parallel to the beam generator, and partially transmits the beam to the other reflector that is perpendicular to the beam generator. The use of partially reflecting mirrors is necessitated by the need for Fabry–Pérot cavities, which increase the path length of a laser beam. The two reflectors then reflect their incident beams, which are kept out of phase so that in the absence of external influence, the photodiode detector of their combination records a zero.
Observation and Data Collection
LIGO operates on the principle implying that a gravitational wave is a disturbance of spacetime, and, as a consequence, a resultant alteration of the length of either cavity, or the lengths of both cavities, can be detected. The change(s) in length(s) is a function of both the polarization of the incident gravitational wave and the gravitational wave’s source. The change in the length of the cavity results in a difference of phase between the incident light and the light that is already inside the cavity. A gravitation wave, therefore, causes periodical incoherence between the cavities and the beams, resulting in a signal that can be measured (“LIGO’s par. 8).
A gravitational wave – the postulated physical phenomenon whose existence is to be tested – is allowed to pass through either arm of the interferometer. (Actually, it is the experiment setup that is located at a place where a gravitation wave is likely to be detected). The gravitational wave alters the lengths of the cavity, resulting in a relative phase between the two reflected beams (as discussed earlier). The periodic change (according to the frequency of the gravitational wave) in the distances along the arms of the interferometer means that sometimes the beams come in phase, resulting in a resonance, which is detectable by the photodiode.
A gravitational wave passed through LIGO’s arm in September 2015. The gravitation wave is believed to have been produced by a spiraling and an eventual merging of two black holes in the cosmos, culminating in single black hole. The relative motion of the two black holes had resulted in the production of gravitational waves that are strong enough to reach the Earth.
The Detection and Announcement of Gravitational Waves
On the 14th of September 2015, it was announced that LIGO had successfully detected gravitational waves. Upon the detection of resonances by the photodiode detector, the data was subjected to a complex computer algorithm for the extraction and processing of the captured data. The analysis of the data thought to be associated with gravitational waves went on for months since the detection. It involved a repeated analysis and cross checking, all for assurance that the data had actually been a result of a gravitational wave effect, and not just some experimental noise. When the scientists had done countless repetitions of the analysis of the data and found the same results almost after every other analysis, they finally had the confidence that they had actually detected gravitational waves, and finally announced the discovery.
The detection of gravitational waves is believed to be confirmative of Einstein’s theory of general relativity, the theory that predicts the existence of the waves. In fact, until their detection, gravitational waves had been the missing affirmation of the general theory of relativity (Steele par. 8). The general theory of relativity (or the theory of relativity, its limiting case) and quantum mechanics form the two pillars of modern physics. The detection of gravitational waves, therefore, has an enormous impact not just on astronomy and astrophysics, but on our understanding of physics as well.
The detection of gravitational waves also paves the way for a new field of astronomy – gravitational waves astronomy. Traditionally, the optical and the radio spectrum of the electromagnetic spectrum have been used to observe the universe, but they have their limitations in the sense that they are deflected, refracted, scattered and diffracted by the medium through which they travel. They are affected by the medium through which they travel, resulting in the detection of distorted images/data. The main advantage of use of gravitational waves is that they are affected by nothing but mass on their path, which has a comparatively insignificant effect on the data accuracy.
Future works and Conclusion
Work on LIGO did not end with the detection of gravitational waves. The remaining data is still being analyzed, and the sensitivity of the LIGO apparatus is also being improved (Steele par. 17). Besides the continuation of the analysis of the remaining data and the continuing improvement of the sensitivity of the detectors, there are also other versions of LIGO around the world. In Italy, there is another experiment dedicated to the detection of gravitational waves. Their version of LIGO is called Virgo observatory. Another similar detector is being constructed in Japan, and there are plans for a fifth detector in India (Steele par. 19). The more the number of detectors there are, the better the precision necessary for the detection of other gravitational waves.
The exploitation of gravitational waves can serve as a window into the rest of the universe. Furthermore, gravitational waves are also speculated to be highly influential on the field of physics as a whole. One particular area is high-energy astrophysics, in particular gamma ray bursts, whose sources are believed to be the same as those that generate gravitational waves (Steele par. 29). In general, the importance of gravitational waves to physics and astronomy is undeniably significance. Continued detection and study of the waves will arguably have an invaluable impact on physical science.
Harrison, Edward R. Cosmology: The Science of the Universe. New York, NY u.a.: Cambridge
Univ. Press, 2000. Print
Steele, Jim. Gravitational Waves Detected. The University of Alabama in Huntsville (UAH),
11th Feb. 2016, http://www.uah.edu/news/research/gravitational-waves-detected-100-years-after-einsteins-prediction . Accessed 11 November 2016.
Barish, Burry C., and Rainer Weiss. “LIGO and the Detection of Gravitational Waves.” Physics Today 52 (1999): 44-50.
LIGO’s Interferometer. LIGO Caltech, n.d, https://www.ligo.caltech.edu/page/ligos-ifo . Accessed 11 November 2016.