In the last article, we have understood the concept of quantum physics, and how it explains the laws of the universe. In this article let us learn the theory of relativity which brought a massive revolution in modern physics.

Before that we need to know how and who came with this idea. Albert Einstein came up with the ideas of theory of relativity. One day when he was working as a patent clerk at Bern, finished his work and boarded a tramp car on his way home. He used to finish his work as soon possible so that he can contemplate the truths of the the universe. In his free time, he used to think of different experiments for the better understanding of the universe.

That day, he devised one experiment on that tramp car which changed modern physics forever. While travelling in the tramp car, Einstein saw a clock tower, and he is moving away from it. He imagined what would happen if the tramp car would move with the speed of light? The speed of light is 186,000 miles per second; he realised if he travelled at that speed, the clock’s hands would appear to be completely frozen. At the same time, he knew that back the clock tower the hands would tick along with normal pace. Time has slowed for Einstein.

This thought blew his mind; he concluded that the faster you move in space, the slower you move through time. His work was heavily influenced by two of the great physicists of all time. First, the “laws of motion by” “Sir Isaac Newton” and second by the “laws of electromagnetism” by “James Clerk Maxwell.” According to the laws of motion, velocities are always relative. For example, imagine a train travelling at 50 kilometres per hour. The speed of the train is 50 Kilometers per hour for someone at rest. It is only 30 kilometres per hour if another train is travelling with respect to this train and in same direction with a speed of 20 kilometers per hour.

This is true for the earth, sun, Milky Way galaxy and other galaxies. According to Maxwell, the speed of an electromagnetic wave(in this case light) is constant, regardless of the observer. If we take Newton’s laws into this, there is a clash, according to newton the velocity and speed are relative but the speed of light is constant regardless of the observer. Thus Einstein proposed his “Special Relativity theory” in 1905.

**Special Relativity Theory:** There are two postulates of special relativity.

1. The laws of physics take the same form in all inertial frames of reference.

2. As measured in any inertial frame of reference, light is always propagated in space with a definite velocity “c” that is independent of the state of motion of the emitting body. Or: the speed of light in free space has the same value “c” in all inertial frames of reference.

The first postulate says that the laws of physics are the same for the person who is in moving in constant velocity or at rest. If we apply any laws of physics the results will be the same. The second postulate is very explanatory.

**Mass–energy equivalence:** Mass-energy equivalence states that the total mass of a system may change, although the total energy and momentum remain constant. In simple words, mass can be converted into energy and visa-versa.

**E=mc ^{2}**

Where E = Energy.

M = Mass.

C = Speed of Light.

**Applications of special relativity:** There are many applications of some of the important ones are GPS (Global Positioning System), Cathode-ray tubes (Old Television screens), modern computer chips and most important of all, by the concept of “Energy – Mass equivalence” was essential for the development of “nuclear fission” and “nuclear fusion” which are used in all nuclear plants.

So we understood the concept of special relativity, now let us understand the concept of general relativity. Before that, we need to understand the back story of the same.

Again back to Einstein and his brilliant imagination. One day he was observing a window washer on a ladder from his office. He imagined what would happen if the washer fell, for most of us it is a horror story which will never end will. But Einstein thought it differently, he imagined himself in the place of the washer while falling. He didn’t imagine what would happen if he hits the ground but imagined what he would experience while falling.

He realised that gravity was the only force acting on him while he was falling and accelerating towards the ground. Without the wind resistance, he would be free in fall, which is no different from being weightless as the ground is not coming up. This is nothing, but he is falling in space where there is zero gravity. He wanted to connect gravity to his theory of relativity through acceleration.

Einstein imagined a room without windows and it has a weighing scale and checked his weight, it would be constant at any point on the earth. Now he imagined the room is in a spaceship, which is moving upward with a speed of 9.8 meters per second which the speed of acceleration due to gravity on earth. Now, what would happen if he stepped on the scale? The weight should be the same as the space ship is accelerating upwards with the same speed that of acceleration due to gravity but this time it is upwards.

Einstein thought about telling the difference between acceleration and gravity, now he imagined shooting a light or laser beam from one point of the room to the other while the spaceship is moving upwards. He saw that the light is bent while moving from one point to another. The size of the light would be lower compared to the projection point. This is because the room is moving upwards with a speed of 9.8 meters per second. The light beam would appear a bit curved downward.

However if you are on earth, you wont see this phenomena. The projection point and the projected points height would be the same. Einstein thought why is this different as it would the equivalence of gravity. The force is constant while it is upwards or downwards. He thought light must bend under the influence of a gravitational field, but how could this be possible as the light always takes the shortest path. May be the shortest point between two points is a curve?

Imagine earth which is spherical in shape, the surface if the earth is never a straight line, it is always a curve. So may be the gravitation causes the curvature of space somehow. He hypothesized may be the straight line is not the shortest path in space, with the presence of mass and energy space somehow becomes curved. This is again different from Newton’s hypothesis in which space and time are fixed. In Newton’s model, gravity acted within space and time.

Einstein’s theory was gravity emerges when there is an interaction between space and time. This is General Relativity in a nutshell.

**General Relativity: **The central idea of general relativity is that space and time are two aspects of spacetime. Spacetime is curved when there is matter, energy, and momentum resulting in what we perceive as gravity.

**Spacetime:** Space-time is a mathematical model that joins space and time into a single idea called a continuum.

**Applications of general relativity:** It can explain why planets revolve around in their orbits, why light bends at blackholes, correcting high precision clocks on satellites, gravitational lensing in astronomy, and a large variety of corrected astrophysics calculations.