And which topic we are going to talk about today, is always being a subject of discussion so far. Most of the people can’t understand this concept after reading once, but today we are going to talk about this with a simplified explanation… If you want to understand then read it till end.

We are talking about , THE SPECIAL THEORY OF RELATIVITY. Which are originally proposed by the genius Albert Einstein in 26 September 1905 with the title of ” ON THE ELECTRODYNAMICS OF MOVING BODY”. And it is generally accepted and experimentally confirmed physical theory.

After coming this theory , the way of watching the universe has been changed completely.

If we want to understand this theoretical concept. Then we have to start it from starting point. Then let’s start…

Going further in the article , we have to take a look at the basic idea behind this theory , which is termed as the postulate of special theory of relativity. There are two postulate regarding to this theory. Which is as follows.

1). The laws of physics are invariant ( identical) in all inertial frame of reference.(ie. non accelerating frame of reference).

2). Second is , the speed of light in vaccum is same for all observers , regardless of the motion of the observers or light source.

In other word we say that any observable mass can’t travel with the speed of light or more than it, the speed of light is considered constant in all type of frame of reference.

These are two basic idea on which the special theory of relativity are proposed by the Albert Einstein.

This theory is widely accepted by the people and give rises the brand new concept like – time dilation , space contraction , mass energy equivalence , relativistic mass , constancy of speed of light, relative of simultaneity. Now we are going to take dive in the postulate of this theory, to understand this.

## EXPLAINATION OF THE FIRST POSTULATE

In the inertial frame of reference , in which any particle moves with the constant velocity. Then in this type of situation laws of physics are invariant, means it does not change. In inertial frame of reference Newton’s laws of motion are applied good.

Let’s take it differently,

Take a ball and drop it from a finite height towards the ground, in this event you can apply simply laws of motion to find speed , time interval , acceleration and displacement of the ball. Because this event has occurred in inertial frame of reference.

Now think for a plane, which is moving with a constant velocity with respect to its airport. If it moving with a constant velocity wrt it’s airport , then it is the example of inertial frame of reference, now take that same ball and drop it from a finite height inside the rocket , then you will find that laws of physics are also applicable here , same as on the ground. And here you can also find velocity , displacement, acceleration , time interval by applying Newton’s laws of motion.

Einstein shows that motion , speed and time all are relative.

Take a look and find out how is it relative.

### RELATIVITY OF SPEED

Have you wondered that when are you sitting in the moving train, then you must have noticed that when a train which is moving in opposite direction of your train passes your train very fastly, and we think that how slow our train is moving. And a another thing you also must have been noticed that , when two train are moving with same speed in same direction then we see that train are at rest and seems as they are not moving.

Here the concept of relativity of speed applied.

If there are two train having speed u m/s and v m/s and if they are moving in the opposite direction then the relative speed of train for a person , sitting in any train is given by =(u + v) m/s and this is the speed of the other train which is moving opposite to our train.

And if they are moving in same direction then relative speed is given as =(v – u) m/s and this is the speed of other train which is moving parallel in same direction of our train. This gives the feel of rest if the velocity of each train is same.

### RELATIVITY OF MOTION

Einstein shows that motion is relative phenomenon, let’s take a example to understand it.

Think of a train which is moving with u km/h , and you are outside the train. You are watching that your friend sat on the roof of the moving train. He has a ball in his hand, the velocity of the ball and your friend is same as the velocity of the train. Here your are watching the train so you are a observer, and from your reference train is moving in some direction. But do you know what your friend feel, when he sees at you , he saw that you are moving with same velocity as train in the opposite direction that of train, for your friend reference of seeing is changed, your friend velocity wrt train is zero, and u km/h wrt ground and you in the direction of motion of train. So he saw that you are moving in opposite direction that of the train. So we can say that nature of motion changes if reference point changes.

### RELATIVITY OF TIME

In the era of sir Isaac Newton , Newton strongly believe that time is a absolute, he always says that if 3 hrs for me then 3 hrs for every objects in the universe. But Albert Einstein did not agree with him , Einstein think that time is a relative quantity and it is also true so far. Einstein says that time is effected with the velocity, if anyone who is travelling with a very high speed, then time passes slowly for him, in comparison with person who is at rest.

In the verification of this statement , scientists performs various experiment, one of the experiment is –

Scientists take two atomic clocks , they synchronised their time reading, after synchronization one clock is sent for the tour around earth with high velocity, while another clock keep at rest in the laboratory.

After coming back from the tour, their time reading is noticed and scientists found the difference in time reading in both clock.

This shows that time is relative, and it is found that time also slows near the high masses objects such as black hole , neutron stars , etc. Great Pyramids of Giza also slows the time for little bit.

A interesting phenomenon with relatively of time is that , if you go for a journey by a probe which can travels at the speed of light, for one year, when you come back from your journey then you saw that your age is changed only by two years ( one year in going and one year is coming) but on the earth many years has been passed, this gives a new concept of TIME TRAVEL. How it happened, Is time travel possible? we will talk about it in another post.

Now you understand that time is a relative quantity. Look at the following formula.

T = D/S. If speed is very large then time become very small and vice versa.

Now let’s talk about the second postulate.

## EXPLAINATION OF THE SECOND POSTULATE

We have all read that second postulate says that speed of light is constant in all type of point of reference or constancy of speed of light.

For understanding this statement let’s take an example.

Consider a train which is moving with u km/h and a your friend is sitting in the roof of the train and he takes a ball in his hand.

And you are an observer and stands on the ground outside of the train. Train in moving with velocity u km/h and suddenly your throw the ball with v km/h in the direction of the train motion, then you saw that the velocity of ball is faster than the speed of the train by (u+v) km/h , but what happened if your friend throw the ball with the speed of light, and what will you see. Do you see the speed of the ball is (c+u) km/h , ( where c is the speed of light in km/h). You as an observer sees that the speed of ball is only the speed of the light, because nothing in the stellar space whose speed is greater than the speed of light, means speed of light is maximum constant speed ever found. (Speed of light is maximum as per our researches, calculation and discovery, may be some objects have speed larger then the speed of light, and yet to be discovered).

the concepts which is come into existence after coming the special theory of relativity are TIME DILATION, SPACE CONTRACTION, RELATIVISTIC MASS , SPACE TIME AND MASS ENERGY EQUIVALENCE. we are not going to talk about these topic today.

We will discuss these topic in detail in coming articles.

Now let’s talk about why special theory of relativity is special.

This theory is “special” because this theory is applicable only in special case, where the space time is flat, ie. The curvature of space time should be 2D , and if space time is 2D or flat then it is described by the energy – momentum tensors , where gravitational effect is negligible. This make this theory special.

But Einstein developed a new theory to describe the effect of gravity in space time and that theory is THE GENERAL THEORY OF RELATIVITY. which is published in 1915.

And we will discuss about this theory in coming articles.

So this is all about THE SPECIAL THEORY OF RELATIVITY. Also read General theory of relativity