Time is an obvious physical quantity that can be felt in daily life but defining it remains a challenge even for the physicists of current times. A famous scientist once said ‘time is something that flows from past to present and to future’. The vagueness of the understanding of the concept of time can be interpreted in that single sentence. Does time remain constant for every one in the universe? For example does a person sitting on some distant galaxy experience time just like we do? The answers remain in Einstein’s theory of relativity.

Albert Einstein, once a patent clerk, published his theory of relativity (special theory of relativity) in 1905. The repercussions of the theory are counterintuitive to the point of disbelief. One of the postulates of the theory is that the speed of light (in vacuum) is constant irrespective of the velocity of the observer. This is the result of Michelson-Morley’s experiment. The second postulate states that the laws of physics remain same for any observer in inertial frames of reference.

Let’s discuss one of the important consequences of special theory of relativity; E=mc^{2}. E=mc^{2} states that mass is also a form of energy and a loss of mass from space is associated with release of some other form of energy and the amount of energy released is equal to corresponding mass times the square of the speed of light in vacuum (c^{2}). In a four vector space, a vector space with four dimensions, one of the vectors is Energy and the other three being momentum in x, y, z directions respectively. The four vector of a body at rest is given by (E, 0, 0, 0).

To see how this vector changes for a person moving at a velocity ‘v’ along X-axis, one can do something called as Lorenz transformation. The result of Lorenz transformation is (E, E.v/c^{2}, 0, 0). Thus the momentum now has a value of E.v/c^{2}. We have learnt during our Physics tuition class that momentum is the product of mass and velocity. And one can use both the premises to obtain E=mc^{2}. A conclusion that is drawn on entirely theoretical lines has serious implications. The concept of E=mc^{2} explains large amounts of energy released in an atomic explosion or a nuclear reactor. Another implication of special theory of relativity is time dilation. Time does not tick at the same rate for an observer moving at certain speed compared to that of one at rest. This results in something called as twin paradox. Lets discuss this in detail in Relativity series 5.