The discovery of gravitational waves ushered in a new era of astronomy
Gravitational waves are ripples in the curvature of space-time produced by the universe’s most powerful catastrophic events. Albert Einstein predicted their existence in 1916, in his theory of general relativity. Einstein’s theory predicted that if there was an acceleration of the movements of huge objects such as black holes and neutron stars, it would cause a distortion of space-time, which would lead to the creation of waves that would become evident by deformations of space.
According to the theory of relativity, these deformations would spread with the speed of light and carry information about the cataclysmic event that caused them. In addition, they would also present a valuable source of information about gravity itself.
According to forecasts, the most intense gravitational waves are produced by the collisions of black holes, collapses of the cores of huge stars (supernovae), by the merging of neutron stars or white dwarfs, by rotations of stars that are not perfect spheres and also by the remains of the gravitational radiation released in the process of the creation of the universe. LIGO interferometer registered gravitational waves from the first process - collisions of two black holes.
Historically, the existence of gravitational waves was indirectly proven in 1974. This indirect prove was based on the observation of a binary pulsar - a system of two extremely large stars with a high density of matter that orbit each other. According to the theory of general relativity, such system should radiate gravitational waves. Astronomers observed the changes in the orbital period of this system and compared them with the predictions based on the theory of general relativity.
After a several years of observations, they came to the conclusion that the predictions of the theory about the orbital period and stars getting closer to each other are consistent with the experimental observations and since the theory required the existence of gravitational waves, their existence was indirectly proven. A few more similar observations were carried out in this field. Later, however, the experiments conducted, including LIGO, were carried out in order to “sense” the gravitational waves.
What is the complexity of such experiments? Even though gravitational waves are generated during highly energetic processes, on Earth, their manifestations are very weak (because they occur far and are caused by our existence). On Earth, they are manifested through the changes of the movement of space-time, or simply said through length changes, but these changes are a thousand times smaller than the dimensions of an atomic nucleus - below the current dimensions of elementary particles. The result of the LIGO experiment is based on the ability to measure such small changes in dimensions.
The LIGO interferometer is a very complex and complicated device, but its essential physical part is the Michelson interferometer that was constructed approximately 135 years ago. Its scheme can be seen in the figure. Both of these devices are similar in that:
they have the shape of an L
at the end of their arms, they have mirrors that reflect light in order to combine two rays, which results in interference
they measure the light intensity of the superposition of the two rays.
The difference between the classical Michelson interferometer and the LIGO interferometer is mainly that LIGO is substantially larger and more complex. LIGO is the largest and most sensitive interferometer in the world. The length of its arms is 4km. This length is important because the greater the distance the light from source P travels, the greater the sensitivity of the device is. Since 4km long arms are not sufficient enough to be able to measure the distance difference between two mirrors Z1 and Z2 at 10,000 times smaller than the size of a proton, other adjustments had to be made in this basic configuration. Adding the so-called Fabry-Perot resonators that are right behind a semi-permeable plate that divides the rays into the two arms solved this problem. Further, there are additional mirrors at the beginning of the resonator set so that the ray is reflected between them and the mirrors at the end of the resonator 280 times and so that the superposition takes place with the ray from the second arm.
This arrangement has two advantages: (1) The light stays in the interferometer for longer, (2) it leads to lengthening the path that the light travels in each arm to 1120km which again increases the sensitivity of the interferometer. Another effect that it causes is based on the fact that the magnification of focal length in optical telescopes, which is equivalent to lengthening the path that the rays travel in the interferometer, leads not only to an increase in the resolution but also to a decrease in visible vibrations. In binoculars, vibrations are unwanted, but while using this device we are measuring them since they are caused by gravitational waves.
Other improvements of the classical interferometer are needed because of the laser radiation intensity and the laser power generating signals. Of course it is also necessary to separate vibrations caused by things other than gravitational waves (thermal, seismic…) the way LIGO dealt with amplifying the output signal and removing vibrations also makes it a unique device. (The details of the description of these approaches are beyond the scope of this article, but they can be found on the LIGO experiment website that the author of this article also drew from).
Observation results are pictured below. Individual panels in the picture show the results of the measurements of the gravitational waves signals from two LIGO observatories in Livingstone, Louisiana and Hanford, Washington. As reported, the signal comes from the collision of two black holes with the mass of approximately 30 times the mass of our Sun located approximately 1,3 billions light years from us.
What is the significance of this discovery?
First and foremost, the discovery of gravitational waves confirmed the accuracy of the predictions of Einstein’s theory of general relativity. The discovery ushered in a new era of astronomy based on gravitational waves, which was added to the astronomy in the visible part of the electromagnetic spectrum, infrared, X-ray etc. We also got to research the curvatures in space-time, which made the possible predictions about multidimensional universes and parallel universes not to seem to be such a distant fiction. In the future, we will be able to examine catastrophic events in the universe using gravitational waves - this will present new opportunities for us to study objects that were once inaccessible. As one of the astrophysicists said: “This discovery substantially changed our understanding of the heavens.”
Author: prof. RNDr. Jozef Masarik, DrSc., Faculty of mathematics, physics and computer science, Comenius University in Bratislava
Photo: archive of the author; fmph.uniba.sk
Illustrational photo: www.pixabay.com
Edited by: Marta Bartošovičová NCP&T at SCSTI
Published by: ZVČ
Translated by: Dorota Jagnešáková
About the author:
Prof. RNDr. Jozef Masarik, DrSc. – a notable scientist and a nuclear physicist, studied nuclear physics at the Faculty of mathematics, physics and computer science (FMPCS) at CU in Bratislava. Since 1983 up until now, he's been working at the Department of Nuclear Physics and Biophysics as a Dean of FMPCS. He focuses on nuclear and sub nuclear physics and its application in the field of geophysical and space research. One of his most important results achieved in the scientific field is the development of a program that simulates interactions of cosmic rays and the nuclear reactions caused by them. He also contributed to the development of hafnium – tungsten chronometer, a contribution to the discovery of water on Mars and an interpretation of many important phenomena in the radiation and geomagnetic history of the Earth.
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