# The Basic Idea of Relativity

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A better version of this article is — THEORY OF RELATIVITY IN BRIEF which is also a featured article on our blog. So, we recommend you to read that article instead of this, and, also that article describes RELATIVITY completely.

## INTRODUCTION

In this article, the ‘MEANING’ of Einstein’s Theory of Relativity will be discussed.

Relativity was developed by a famous German physicist, Albert Einstein. Some contributions of Hermann Minkowski, Hendrik Lorentz and Max Planck is also there.Theory of Relativity comes in two different ‘versions’ – ‘Special Theory of Relativity’ and ‘General Theory of Relativity’ or simply special relativity or general relativity. By name, special relativity seems to be more complex than general relativity, but actually, special relativity is much simpler than general relativity.Special relativity is called ‘special’ because it is applicable only on objects moving with a constant speed. This is a very rare or ‘special’ case because in our universe hardly anything moves with a constant speed, almost everything accelerates in our universe.General relativity is applicable on objects moving with a constant speed as well as accelerating objects. Acceleration is a common or ‘general’ case in our universe.

## 1. WHAT IS RELATIVITY?

Relativity means ‘to be relative’. It can be understood by an example. Suppose there is a very hot cup of tea. It is very hot, but, hotter from what? Now, suppose another cup of tea which is further hotter than the first one.So, which cup should be referred as hot?The right answer to the question is that the first cup is colder than second one or second is hotter than first one. This is what we call relativity.There isn’t anything fixed. No quantity (except speed of light) is fixed.So, more technically, relativity can be explained as any quantity measured by one observer may be different from that measured by another observer. Let’s understand it with an example.Suppose you are in a train and let the train is moving with 40 mph (miles per hour) of speed. When you will see a person sitting in the train in front of you, the speed of the person will seem to be zero for you but an observer outside the train, the person will be moving with a speed of 40 mph.So, this can be elaborated as, with respect to you, the person is in rest, but, with respect to an observer outside the train, the person is in motion. This is the basic principle on which the theory of relativity is based on. So, we can observe that the basic principle of relativity is –
Nothing is absolute in this universe, but everything is relative to each other. Only the speed of light is absolute, it doesn’t depend on velocity of observer.
So, RELATIVITY tells us that every motion should be considered relative.

## 2. SPECIAL THEORY OF RELATIVITY

As described above special relativity is called ‘special’ because it is applicable only on objects moving with a constant speed. This is a very rare or ‘special’ case because in our universe hardly anything moves with a constant speed, almost everything accelerates in our universe.It is actually the special theory of relativity which tells us that the size of a moving object contracts and that the time for a moving object slows down, and, also a moving object has its mass increased.The most FAMOUS conclusion of special relativity is that energy and mass are inter-convertible, and they are the same thing.As described above special relativity is based on the principle that everything is relative.The whole Special Relativity is based on two postulates:
1. 1. The laws of physics remain same for every object moving at a constant speed (or, is at rest).
2. 2. The velocity of light remains constant for all the observers no      matter what their velocity.

### 2.1 THE IDEA OF FOUR DIMENSIONAL SPACE-TIME

According to the second postulate of special relativity, velocity of light is independent of velocity of observers, this means velocity of light is absolute.
We know that space is not absolute.
It should be noted that space and distance are not dissimilar things because they are measured in a same FUNDAMENTAL unit, that is, METER. (There are also other units like CENTIMETER, KILOMETER, MILES etc. but the standard unit is METER)For example, when you look at the night sky filled with stars, the distance between them seems to be very small, but they are actually many light years away from each other! (One Light Year is the distance travelled by light in an year. One light year is equal to 9,461,000,000,000 kilometers.) So, this proves that distance (or space) is not absolute.
And we also know that,
velocity of light = distance travelled by light ÷ time taken
so, time taken = distance travelled by light ÷ velocity of light
⇒ time = non-absolute quantity ÷ absolute quantity
⇒ time = non – absolute quantity
So, special relativity concludes that TIME isn’t absolute and is dependent upon distance (or space) and space is also dependent upon time.In 1908, Hermann Minkowski (one of the former mathematics professors of Albert Einstein), using the above principle of non-absolute time, presented the idea of four-dimensional structure called space-time or Minkowski space.He suggested that the three dimensions of space and single dimension of time are fused in a single four-dimensional structure called space-time or Minkowski space.