Gravitational waves were proposed by Henri Poincaré in 1905 and subsequently predicted in 1916 by Albert Einstein on the basis of his general theory of relativity. There are so many aspects of physics that are aesthetically pleasing. These aspects are not necessarily pleasing due to their visible or on the surface features, rather, they are aesthetically pleasing in their detail. Gravitational waves are certainly one such phenomenon. They have immense importance, and their impact in understanding the theories of physics is considerably high.
Gravitational waves explained
Gravitational waves are ripples in the fabric of space-time that are formed due to the acceleration of masses. These ripples propagate outwards from the source of mass. One must understand that distortions are created in the fabric of space-time by bodies of mass. To visualize this concept, think of this fabric as a piece of paper or a blanket, with people holding on to it from all sides. When an object of mass is placed on the paper or blanket, there is a visible dent or distortion of the shape of the paper or blanket at the position where the object was placed. Now when these bodies of mass are moved about, that is, they are provided acceleration, these distortions also move about in the fabric of space-time. These accelerated bodies lead to the formation of waves in space-time. These waves are the gravitational waves.
As you would imagine, larger bodies tend to create larger intensity waves. Theoretically, any movement of a body having mass can cause these ripples. A person walking on the pavement, in theory, also causes these ripples. However, these ripples caused by a walking person are very minuscule and insignificant.
So, what is space-time?
The universe was long thought to be consisting of the three dimensions of space only. But, Albert Einstein proved that the universe consisted of a fourth dimension, time. It would be impossible to move in space without moving in time. Similarly, it would also be impossible to move in time without moving in space. Space and time, therefore, have a very integral relationship. Einstein stated that there is a profound link between motion through space and passage through time. He hypothesized that time is relative. Objects in motion experience time slower than objects at rest.
The three dimensions of space and the dimension of time are viewed as the four-dimensional space-time. Hermann Minkowski provided a geometric interpretation that fused the three dimensions of space and the dimension of time to form the space-time continuum. This was called the Minkowski space.
In three dimensional space, the distance, D between any two points can be represented using the Pythagorean theorem as:
D2=(Δx)2 + (Δy)2 + (Δz)2
Δx represents the difference in the first dimension, Δy represents the difference in the second dimension and Δz represents the difference in the third dimension
The spacetime difference of two points given by (Δs)2 varying by time Δt would be given as:
(Δs)2=(Δct)2 – (Δx)2 + (Δy)2 + (Δz)2
c is a constant, representing the speed of light that enables conversion of units used to measure time to units used to measure space.
Gravitational pull and formation of waves
Every body that has mass tends to attract other bodies. Whether the mass is small or large, every body exerts a force on the other. This attraction is the gravitational pull. The greater the mass of the object, the larger its gravitational pull. The larger the distance of an object from another object, the lower its gravitational pull on it. Since every object, however large or small, tends to exert this pull on every other object, changes in gravity can provide insight into the behavior of these objects.
If the distance between two bodies is doubled, the force of attraction F between them will be:
Since the force of gravity acting between any two objects is inversely proportional to the square of the separation distance between the object's centers, Force F will be reduced by 1/2 x 1/2 = 1/4 times.
Consider the earlier example of the distortion caused by placing an object on paper or blanket, now, if we were to place a larger object, this would result in an even larger distortion. The larger object would cause a larger depression in the paper or blanket and hence, is said to have larger gravity. If the two objects were placed on the paper or blanket together, the larger object with the larger distortion would seem to be exerting a larger force of attraction towards the other object. If these objects moved, there would be ripples formed on the paper or blanket. This is similar to how gravitational waves are formed, the only difference being that the paper or blanket would be replaced by the fabric of space-time.
These gravitational waves cannot be felt easily. To detect these, you would require special equipment. These detectors are L shaped instruments with generally long arms.
Detection of gravitational waves
Gravitational waves were first witnessed in September 2015. Scientists observed the waves that were a result of two black holes colliding. These black holes were said to possess masses several times that of the sun. The black holes were attracted to each other due to the gravitational forces and slowly, over the course of several years, began to spiral into each other. One day, they finally merged. Before they merged, they let out gravitational waves that were felt on earth billions of years later in 2015.
This was picked up by a detector called Laser Interferometer Gravitational Wave Observatory (LIGO). This signal was very short lived and lasted only a fifth of a second. These wobbles in space-time picked up by the LIGO was thousands of times smaller than the nuclei of atoms. This is because the gravitational waves over the course of time gradually became weaker. The Laser Interferometers were configured in such a way that even these small ripples could be picked up.
LIGO consists of two gigantic laser interferometers located thousands of kilometers apart. Each detector consists of two 4km long steel vacuum tubes arranged in an ‘L’ shape. A special covering is provided to these tubes to ensure protection from the environment.
These tubes are the arms. The lengths of these arms are measured with lasers. If the lengths are changing, this could be due to compression and relaxation of arms due to gravitational waves. Studying these gravitational waves enables scientists to derive certain information about the objects that produced them. Information such as the mass and size of the orbit of the object that created the wave can be extracted from studying these gravitational waves. In the year 2017, The Nobel Prize in Physics was received by Rainer Weiss, Kip Thorne and Barry Barish for their role in the detection of gravitational waves.
Significance of gravitational waves
These gravitational waves help scientists gain information about the physical properties of the objects that created the waves. These gravitational waves provide a new way to observe the universe. A way that never existed previously.
The detection of the gravitational waves allows us to understand interactions in the universe in a completely new way. The waves detectable by LIGO are waves generated due to the collision of two black holes, exploding stars, or perhaps the birth of the Universe.
Before this form of understanding the universe was realized, most observations of the universe were made based on electromagnetic radiation. Something like the collision of black holes would have been impossible to have been picked up by electromagnetic radiation.
A major difference between gravitational waves and electromagnetic waves is the fact that gravitational waves interact very weakly with matter. Electromagnetic radiation, on the other hand, reacts strongly with matter and could face several alterations in its properties. Gravitational waves can travel through the universe virtually unimpeded.
The information, such as the mass and orbit of the object that caused the waves could be understood in a clearer manner. The information carried by the waves is free from any alterations or distortions that result from interaction with matter present in the universe.
The gravitational waves can also penetrate regions of space that electromagnetic radiation cannot. These properties have led to the creation of a new field of astronomy, called gravitational field astronomy. Gravitational field astronomy aims to study large entities in the universe and their interactions through unadulterated properties of gravitational waves.
Famous basketball player, John Wooden once said, “It’s the little things that are vital. Little things make big things happen”. In the case of gravitational waves, the little things are the ones that provide the knowledge of the larger things. Little observations made on the properties and complexities of the gravitational waves are what gives rise to the details pertaining to the larger bodies existing in the universe. There is no denying the fruitfulness of the existence of gravitational waves. One can even go so far as to say that gravitational waves have revolutionized physics. I can say without a cloud of uncertainty that gravitational waves will surely help us uncover more secrets of the universe in the future.