100 years ago, in 1915, Albert Einstein came up with his General theory of relativity. Einstein published special theory of relativity in 1905 which dealt with inertial systems or systems that are in non-accelerating uniform motion. General theory of relativity is in a way an expansion of his previous theory to moving object, both accelerating and non-accelerating. This article is in lieu of the centenary celebrations of Einstein’s General theory of relativity. General theory of relativity explains the phenomenon of black holes, complex paths taken by planets around their stars, gravitational lensing, gravitational time dilation, gravitational waves, gravitational-shift in the frequency of light, etc. Let’s discuss the basic principle on which Einstein based his theory and one of the consequences of such principle.

Assume that you are sitting in a car, accelerating at a rate of 9.8 m/s2 (yes, it’s the same as acceleration due to gravity which we often used during our JC Physics tuition classes). Now, every one of us knows that when a moving vehicle accelerates we are pushed against our seats by some kind of force and when it suddenly decelerates (when brakes are applied) the opposite happens. It is like there is a force pushing us towards the seat or pulling us out of the seat. Physicists call such forces as fictitious forces or pseudo-forces. It is the same force we experience while sitting in a merry-go-round where you feel like you are pulled away from the center of the merry-go-round.

Now coming back to our car that was accelerating at 9.8 m/s2, the person experiences the force even if he is moving in space under no influence of gravitational pull. The physical laws remain the same for this person and a person under the influence of Earth’s gravitational pull. This is called as equivalence principle. This forms the basis of Einstein’s general theory of relativity.

Gravitational shift in the frequency of radiation can be explained using the above stated principle. Let’s say a person is accelerating at certain rate with respect to a reference. When light of a particular frequency is shone on the person in a direction opposite to the direction of his motion, then the person observes the light to have shifted to higher frequency. This phenomenon is called as Doppler shift. Now since a person under the influence of gravity is equivalent to a person accelerating, when light is shone on a person on a moving object like Earth, then he notes the light to be of slightly higher frequency. This is experimentally observed and is called as gravitational shift in frequency. General theory of relativity says that massive objects distort the space-time fabric, just like a massive object placed on trampoline, and as a result induce gravitational time dilation and other effects.