Special theory of relativity :

 Einstein's special theory of relativity is a physical theory of relativity based on the assumption that the speed of light in a vacuum is a constant and the assumption that the laws of physics are invariant in all inertial systems. Einstein's theory of relativity is the theory that space and time are relative concepts rather than absolute concepts. The special theory of relativity was based on two main postulates: first, that the speed of light is constant for all observers; and second is that observers moving at constant speeds should be subject to the same physical laws. Following this logic, it was theorized that time must change according to the speed of a moving object relative to the frame of reference of an observer. The theory states that both space and time are relative, which holds true in a special case where the gravitational field is absent. Relativity was the concept from where comes the Einstein's mass-energy equation, E=mc², which implied that matter and energy were equivalent and that a single particle of matter could be converted into a huge quantity of energy. However, since the special theory of relativity only held true in the absence of a gravitational field. The theory of special relativity is based upon two postulates namely: Principle of Relativity: The laws of physics are the same for all inertial reference frames. Principle of Constancy of the Speed of Light: Light always propagates through a vacuum at a definite velocity, c, which is independent of the state of motion of the emitting body. Special relativity yields to several consequences from applying Lorentz transformations at high velocities. They are: Time dilation, Length contraction, Velocity transformation, Relativistic velocity addition, Relativistic Doppler effect. Simultaneity & clock synchronization, Relativistic momentum, Relativistic kinetic energy, Relativistic mass, Relativistic total energy.