Theory of Relativity : What is Theory of Relativity? There are 2 types of relativities. First is special and general relativities. Let’s understand both of them.

1]The Theory of Special Relativity

What would you see when you will travel at the speed of light[30,00,00,000 m/s]? This question was asked by Albert Einstein to his teacher when he was a school student. His teacher told him to stand outside of the class for this question. This question became the base for theory of relativity.

Firstly, we have to understand that we live in a 4 dimensional universe in which 3 dimensions are of space [length, breadth, and height] and 1 dimension is of time [past, present, and future]. Every object is moving towards its future. Every object can also move in any of 3 dimensions of space. These 4 dimensions together are called ‘space-time’.

Let’s consider a person at rest, a car moving with a speed of 50km/hr, a jet plane at a speed of 11000 km/hr, and speed of light. Now let’s try to draw a 2-D representation for these 4-d motions. We will draw a graph which will have space in x-axis and time in y-axis.

A person at rest:- The person is moving in time continuous. He is not moving in space. For this person , the line on graph will be parallel to time and perpendicular to space.

The car at 50 km/hr:- This car is moving in space-time. This car is slower in time than the person at rest.

The jet plane at 11000 km/hr:- This jet plane is even slower than the car in time

Speed of light:- The light is the fastest object ir n the universe. For light time is completely stopped. The line on graph for light is parallel to space and perpendicular to time. This is one of the reason that light is the fastest object in the universe.

From these graphs, we came to know that increase in speed results in slowing of time.

Now let’s answer the question I asked you in the start of the chapter. What we will see if we travel at the light speed. For us, the time will be stopped. In addition, when another person see you travelling with light speed, you will also be stopped for him.

Now let’s take other question. Are you at rest or in motion? Yes, you are at rest. Can you think my way. Firstly, forgot everything about motion. Now think of a bus passing from a road. When you observe the bus, it is in motion. Now just try to think what the passenger in the bus will say about your state of motion. He will say that you are in motion with respect to him. In fact, all trees, cars, etc. which he see moving backwards is in motion for him. Only on object which is moving at same speed and direction as of bus will be at rest for him.

Now think of a person who is observing your state of motion from space. What will he say? He would say that you are in motion! You are moving with Earth. Our Earth is moving around our Sun. So you are also moving around the Sun. These observation tell us that there is nothing called absolute rest or rest. It also tell that, motion is relative.

Science is based on proofes. We want to prove it. So we took two atomic clocks. one atomic clock was at Earth The other clock was set in a jet plane. The jet plane orbited our planet and came back at the starting point. The time in both atomic clocks was compared. The time in the clock on Earth was more than the time in the moving clock. The difference in time of both clocks was very less, but it was enough to prove theory of special relativity .

This can help in time travels. Travelling in past is hypothetical and impossible for us. Travelling in future is a thing which is possible for us in near future.

To travel travel in future, we would need energy to accelerate ourselves to 10-90% of the light speed. The faster we go, the slower the time will be. So we will be alive for thousands of years. We would have to go out o our solar system, and then return to Earth. When we will return to Earth, thousands of years will be passed and you will be in future.

Even though we can’t travel to past let’s take a look to the theory for travelling past. Remember the graphs we have drawn. Now let’s try to draw a graph for a hypothetical object named tacion which is even more faster than light.

tacion [faster than light]:- tacion is not real. It is faster than light. if it is faster than light, then it must travel backwards in time. If they were real, then we could send messages in our past.

you must be wondering ‘Why nobody has travelled to future?’ and ‘can we accelerate more than the speed of light?’. Unfortunately we do not have enough energy to accelerate at even 1% of speed of light! We can never accelerate at the speed of light. This is because to accelerate a massfull object to the speed of light, we need infinite amount of energy. This is impossible forever.

2] The Theory of General Relativity

What will you see near a black-hole? How a person near a black-hole look like? These questions and other questions make the understanding of the theory of general relativity.

Do you remember the 4-D space-time? Let’s modify it a little bit. Now think of a very-very-very-very-large sheet of cloth. This sheet has a size of our universe. Now think about all stars, planet, comets, celestial bodies, etc as balls on this sheet. these balls have mass. What will happen to the sheet on inserting these balls? Yes, the sheet will bend. More mass means more bending. Thus, the space-time fabric is flexible in nature.

This is what happens in our universe. The only difference is that The universe is 4-D and the sheet is 2-D. Now you would think ‘how time can bend?’ Time is attached to space. So time has to bend with our space. We see this bending in the form of slowing of time. Yes! Gravity can slow down the time.

This is the source for gravity. This bending in Space-Time is what we call gravity This bending is brought by an object having mass. Thus, gravity is formed due to mass.

Now just think of a small ball[planet] in our previous example. this planet is placed in the curve which is formed due to the bending. What will be the effect of this star on the planet. Let’s start by drawing figure in 2-D

This planet will start orbiting the star. Our Earth and other celestial bodies in our solar system orbit around our Sun due to this reason. The only difference is that, this model is 2-D and our universe is 4-D. In 2-D the planets would simply fall inside the star. But in 4-D universe, these planets and celestial bodies don’t fall directly into the star.

Do we want proof for gravity and slow time? Let’s start with slow time:-

We all know about artificial satellites. Here on Earth, there is more gravity than near the satellites. The time on Earth is slower than the time on the satellite. This is the reason that, we have to reset the time of satellites again and again. This proves that time is slower in more gravity and vice-versa

Remember the space-time fabric. A very massive object like a neutron star, a black hole, etc. bends the space-time fabric. When light [Which is massless] comes in the curve of this massive objects gravity, the light changes its path. The light which travels straight takes a curved path! This is because of the gravity of the massive object. The light is massless. So it interacts with gravity with the help of curves formed in space-time. This process of bending of light due to a massive object is called ‘gravitational lensing‘. It proves the existence of space-time fabric.

What will happen if we place black hole in our 2-D model of space-time? Let’s try to place on our 2-D space-time :-

Black holes are infinitely dense and is a very extreme object in the universe. A black hole has very strong gravity. It has a boundary of no return. This boundary is called ‘event horizon’. Even light with its speed of 30,00,00,000 m/s can’t escape, once entered the event horizon of the black hole.At event horizon, the curvature of sspace-time is very high that, time gets stopped. This means, at event horizon, the space time are broken.

What happened? The fabric was broken. A black hole made a hole in space-time fabric. In 4-D universe it curves space-time 360°. Black holes are the most extreme objects in the universe.It has its all mass in an infinitely small point. This point is called singularity. It is at the centre of the black hole and is infinitely dense. It has many mysteries in it. It need a whole chapter on black holes to explain them.

Remember a time travel we did with theory of special relativity. Let’s do it again using general relativity. But this time, travel in future is possible. Let’s travel in future.

As we know, time slows down near a massive object. We have to stay near a massive object. A little massive object like Jupiter will take you few years ahead. A very massive object like black hole will take you thousands of year ahead. Unfortunately, you will not be able to go again in the past.

3]Twin Paradox

i] Special Theory of Relativity style :-

Imagine there are 2 twin brothers. Both born on same day at only 5 minute difference. When these twins hit the age of 20, the elder brother was sent to a space tour at a speed of 50% of light. When he returned back home, his younger brother was 50 years old. He was still 20 years old. This happened because the elder brother has done a time travel in future using the theory of special relativity.

ii] General Theory of Relativity style.

Again, imagine the same twins. At the age of 20, younger brother was sent to live near a black hole. When he returned back home, his brother was 50 years old. He was still 20. This is because younger brother has time travelled to the future using the theory of general relativity.

4]Mythological evidences of Theory of Relativity

In Hindu mythology, there’s a story. The great king Muchukunda was invited in Devlok for a fight with evils and devils on the call of Lord Indra. A fierce fight took place between Gods and Evils. King Muchukunda showed his bravery, courage and power in the fight. The fight lasted for 1 year in Devlok. Finally Gods won the fight. Lord Indra told king Muchukunda that he would be blessed with anything he want.. He told that he want to return. Lord Indra told him that, 1 year in Devlok means 1 yug on Earth. The king’s family has been finished. The king was given the bless that he would sleep forever and if anyone tries to woke him up, the person would immediately burn into ash.

It is said that Lord Brahma live in Brahma-lok.It is said that 1 second in Brahma-lok is equal to Thousands of years on Earth.This mean, Brahma-lok is travelling very near to the speed of light or Brahma-lok is located near event horizon of a black hole.

Daily Use English Sentences for Students #Theory of Relativity