A better version of this article is — THEORY OF RELATIVITY IN BRIEF which is also a featured article on our blog. So, we recommend you to read that article instead of this, and, also that article describes RELATIVITY completely.
According to General Relativity, Gravity is not a force like other but is a consequence of curvature of space-time. The presence of mass and energy curves space-time that means near a mass, space contracts and time dilates and away from the mass, space is less contracted and time is less dilated. All the objects follow the shortest path, that is a straight path in 4-dimensional space-time. These paths are known as geodesics and space is pushing the objects to follow these geodesics (straight paths), but, the objects seem to move in curved paths in 3-dimensional space.
Space-time is a 4-dimensional structure, but we can’t actully even visualize a 4-dimensional space-time because we can’t see the dimension of time as our brain creates a 3-dimensional model of everything we see. So, we represent it with two dimensions, like a fabric:
And when matter and energy distort (or curve) this fabric, it is represented as:
We can also represent space-time with three-dimensions, and the curvature is represented as:
This fabric of space-time can be ‘disturbed’. These disturbances in the fabric of space-time are known as ‘Gravitational Waves’. A wave can be defined as a disturbance in a medium (solid, liquid or gas) or field or this fabric of space-time. The waves which disturb a medium [like water waves are disturbances in water, sound waves disturb any medium (solid liquid or gas)] are called mechanical waves. The waves which disturb a field, like light waves disturb electromagnetic field, are called electromagnetic waves. Similarly, these gravitational waves disturb space-time fabric. These waves can be represented as:
You can see how ‘ripples’ are created in this fabric as are created in water when you throw a pebble in it. I described in the post titled ‘Classical Physics in Brief’, that Newton’s gravity predicts that gravity is an instantaneous force which means that it travels faster than light. But this violates Einstein’s special theory of relativity, which predicts that anything can’t travel faster than light. The speed of light is a kind of ‘cosmic speed limit’. If sun disappears suddenly, Newton’s gravity predicts that all the planets, comets, asteroids will instantly, leave their orbit. But general relativity predicts that when sun will disappear, waves will be created in space-time fabric and will take some time to reach the planets and then they will leave the orbit, for example it will take 8 minutes for these waves to reach the Earth, it means that gravitational waves travel exactly at the cosmic speed limit, the speed of light. This catastrophe, of Sun’s disappearance is illustrated below:
About the Author – Madhur Sorout
Sorout is currently a last third year high school student, living in India. His main fascination lies in Physics mainly in the field of general theory of relativity and topics related to it like Big Bang, Black Holes and Evolution of Universe. He likes to make out the meaning of what he see in this universe. He loves to read books by Stephen Hawking, Neil deGrasse Tyson and other authors (and Physicists). He is an atheist and believes that Physics completely rejects the idea of a god. He likes to play cricket also and he wants to continue down the route of research in Theoretical Astrophysics.