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A law belonging to a field subject like physics can be expressed either in the form of a theory, or expressed mathematically. We call the statement and mathematical expression as law if it satisfies the principle of relativity.

The principle of relativity tells us that a law must be the same for all possible observers. By observer, we mean it is a system that is competent to verify the law. 

Therefore, for two different laws, there might be two different sets of possible observers.

The relativity theorem was brought up in 1905, by a German-born theoretical physicist named Albert Einstein, where he described relativity as one of the two pillars of modern physics.

It states that, ‘space and time are relative and all motion must be relative to the time of reference.

However, as explained above that the laws of physics are different for different sets of observers. This theory seems simple, however, seems hard to understand.

Relativity Theory

The theory of relativity is also known as Albert Einstein’s theory of relativity. 

Relativity is just a concept that states objects in motion are relative to each other.

For example, in a truck, two kids are playing catch the ball game and the truck is moving with a constant speed i.e., 50 mph and the kids are also moving with the speed of 50 mph even if they are standing still.

As kid A is passing on the ball to kid B at 10 mph while an observer outside the truck observed the speed of the ball by adding the speed of the ball and the speed of the truck, given by,

50 mph + 10 mph = 60 mph.

Do you think this boy is standing still?

Well, this boy is also constantly in motion with the rotation of the earth and the sun and the galaxy is in motion too.

Therefore, nothing is ever at absolute rest or in absolute motion.

Things just move relative to each other.

This theory is also called the classical theory of relativity.

Theory of Relativity Definition

The theory of relativity states that laws of physics are the same for all sets of observers.

 This theory states that:

  1. There is no absolute reference frame. An observer can measure velocity if the object or momentum is only in relation to other objects.

  2. The speed of light remains invariant whosoever measures it how fast an observer is moving or how fast he is measuring.

Relativity the special and general theory

Theory of relativity encompasses two theories named:

  1. General relativity

  2. Special relativity

When we refer to the theory of relativity, we actually refer to general theory.

Special relativity is the peculiar case of general relativity.

The combination of these two principles helps us explain many subjects varying from the motion of the planets to the effect of gravity on light due to the existence of black holes.

We will talk about these two in this article.

General Theory of Relativity 

General relativity theory was developed by Albert Einstein in the year 1915.

He predicted that the space-time around Earth would not only be distorted but also twisted by the planet’s rotation, and determined that the massive objects may cause a warp in space-time which he termed as gravity.

It states that being at rest in the gravitational field and accelerating are identical physically.

For example, a boy can see the free-fall of a ball in the same way on the Earth and while sitting in the rocket.

This is due to the fact that acceleration of the rocket is also equal to 9.8 ms-2.

Basically, this theory relates to Newtonian’s gravitational theory and special relativity.

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Special Theory of Relativity 

In 1905, a theoretical physicist, Albert Einstein discovered that the laws of physics are the same for all non-accelerating observers. However, the speed of light in a vacuum is independent of the motion of all observers.

This theory was named as the special theory of relativity and it introduced the new framework to the entire world of physics and proposed new concepts of space and time.

The special theory of relativity: Part 1: Time-dilation

Consider a beam of light being reflected between two mirrors.

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Light-reflecting between mirrors

Case1: Let’s consider case 1 when the mirror is at rest.

The time taken by the light to bounce between the two mirrors at a distance ‘d’ is say, 1 second.

Which gives us the speed of light ‘s.’

As we know speed = distance/time = d/1  


 s = d

Case 2: The mirrors in motion

Now, when the mirrors move at a certain speed, the clock travels at the same speed in the same direction.

Here, light has to travel an extra distance along with the mirrors from distance ‘d’ to distance ‘D’ with a speed of light ‘S’ in time ‘t = 1 second.’

So,  S = D/1 


 S = D  

Which states D > d

But in the 1800s, the Michelson-Morley experiment tried to detect the relative motion of the matter through ether and concluded that speed of light is constant stated as c.

So, in our scenario 

s = S = c                               

So, the time slows down when you move faster and faster during the speed of light.

This theory is known as time-dilation theory and it broke the traditional understanding of time as a constant entity and proved the fact that time is different for different observers.

FAQ (Frequently Asked Questions)

 1. Why is it Called Relativity?

The theory of relativity describes how measurements of space and time can differ for different sets of observers for each object moving relative to another object.

2. What is E = mc2 Called?

E = mc2 is an equation derived by a theoretical physicist named Albert Einstein in 1905. 

Through this equation, he explained the relationship between energy and mass.

Here, E represents units of energy, m represents units of mass, and c2 is the speed of light squared. 

He used c because he was specifically measuring the speed of light in relation to two systems in motion.

This theory explained how much energy can be released in stars and nuclear explosions.

3. Is  E = mc2 The Theory of Relativity?

Einstein’s equation, E = mc2states that energy and mass are interchangeable. It is a special theory of relativity that how space and time are linked to an object moving with a constant speed equivalent to the speed of light.

4. How Do We Use E = mc2 Today?

The equation, E = mc2 gave birth to an atom bomb.

We take advantage of this theory where E = mc2is used in PET scans and similar diagnostics used in hospitals.