The Theory of Relativity usually is composed of two interrelated theory by Albert Einstein- Special Relativity and General Relativity. This theory, in 1905, led to the beginning of a new age for theoretical physics. This theory showed a new way of visualising and understanding different phenomenon in the universe. The Special Theory of Relativity was published in “On the electrodynamics of moving bodies” by Albert Einstein in 1905, which is also called ‘the miracle year’ as Einstein published five consecutive papers in that year.

Special Theory of Relativity

Einstein devised Special Relativity in 1905. In Special Relativity, Einstein put forward some ideas which was very important for the development of physics. He showed that the speed of light within a vacuum is the same no matter at which speed an observer is travelling. As a result, he found that space and time were interwoven into a single continuum known as Space-Time.

Special Relativity is a theory of the structure of Space-time. It proved that time and space are not absolute. It is based on two postulates:

1. The laws of physics are the same for all the observers in any inertial frame of reference relative to on another, which is the principle of relativity.

2. The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the light source.

Relativistic mechanics copes with experiment better than classical mechanics. The 2nd postulate is the direct result of the Michelson-Morley experiment. In Michelson-Morley experiment, the non-existence of vacuum or the existence of the luminiferous aether(a medium permeating space which was supposed to be the carrier of light waves) was being proved. But the results found were negative. This led to the birth of postulate 2. In fact Einstein recognised acceleration instead of momentum unknowingly. Einstein mentioned that mass of a body increases with the increase in the motion or velocity of the body. Classical mechanics dealt with the rectilinear principles of motion. But Einstein imagined the space-time is curved and all motions are curved and attempted to establish the curvilinear principle. This theory has many consequences like – relativity of simultaneity, time dilation, length contraction, mass energy equivalence, etc. Time dilation and length contraction are two very important consequences of Special Relativity. Time dilation showed that time dilates and contracts either due the relative velocity between two bodies considered or due to fluctuation in gravitational potential difference in different space-time. On the other hand length contraction showed that a moving object’s length is less than its original length(measured in its own rest frame) in the direction of motion, it is also known as Lorentz contraction. This phenomenon is clearly noticeable when the speed of the object is near to the speed of light. In the principle of equivalence Einstein explained the gravitational and inertial masses are same. But this concept needed more elaboration as it could only explain mass with gravitation. Beyond the gravitational field body is unable to express mass. This led to the birth of the General Theory of Relativity. To understand Special Relativity one has to transfer from Galilean transformations of classical mechanics by the Lorentz transformations of relativistic mechanics.

General Theory of Relativity

Einstein spent 10 years trying to include the idea of acceleration in his theory and published the General Theory of Relativity in 1915. The development of the general relativity began with the Principle of Equivalence, in which the states of accelerated motion and being at rest in a gravitational field are physically identical. In his equations, he devised the fact that massive objects cause a distortion in space-time, which is nothing but gravity. This is same as setting a large body in the centre of a trampoline, causing it to dimple. The body would press down into the fabric, causing it to dimple. A marble rolled around the edge would spiral inward toward the body, pulled in much the same way that the gravity of a planet pulls at rocks in space. At first this theory would seem meaningless, but the greatest beauty of our galaxy that is the back holes confirmed the theory. Some of the consequences of the general relativity are –

1. Gravitational lensing: Gravitational field bend light. Light around a massive object such as a black hole is bent causing it to act as a lens for things that lie behind it. This method is usually used by astronomers to study stars and galaxies behind massive objects. Also light approaching the sun is curved due to the distortion in space-time.

2. Gravitational Redshift: The electromagnetic radiation of an object is stretched out slightly inside a gravitational field. This is a simple consequence of the principle of equivalence.

3. Gravitational Waves: These are ripples in space-time caused by some most violent and energetic processes in the universe. Einstein’s equations showed that massive accelerating objects such as black holes, neutron stars orbiting each other would disrupt space-time in such a way that waves of undulating spacetime would propagate in all directions away from the source. These cosmic ripples travel at the speed of light. This has been proven from the collision of two black holes and captured by LIGO.

4. Gravitational Time Dilation: Clocks run slower in deeper gravitational potential wells.

5. Expanding Universe: The universe is expanding and the far parts are moving away from us faster than the speed of light.