**“SPECIAL THEORY OF RELATIVITY” By Gaurav Gawade**

**INTRODUCTION**

The special theory of relativity was derived by the popular physicist Albert Einstein and was published in 1905. It was undoubtedly a major breakthrough in the early 20th century in the field of science. The special theory of relativity brought a revolution in high-speed physics. It completely changed or rather developed our attitude towards the universe.

In this blog, we will not go deep into the mathematics behind the special theory of relativity; but we will discuss the impact and outcomes of the theory.

To know the basics, let us first read the postulates of the Special theory of relativity:

**The laws of physics work exactly the same in all inertial frames of reference.****The speed of light is constant.**

In 1915, Einstein published his “general relativity”, which was the next step in the special theory of relativity.

Now, the popular question arises is:

**What is the difference between the Special Theory of Relativity and General Relativity?**

Essentially, the core difference between the Special Theory of Relativity (or special relativity) and General Relativity is that Special Relativity is applicable only in the non-accelerated (inertial) frames of reference; whereas General Relativity tells us about both accelerated(non-inertial) and non-accelerated(inertial) frames of reference.

Further differences:

**E = mc²:**

The special theory of relativity gifted us with the famous equation E = mc², which talks about the direct relationship between energy and mass.

As we can see here, energy is nothing but the mass of the body multiplied by the square of the speed of light. This is the main meaning that this equation carries.

For simplification, if you weigh 50kg, then the amount of energy your physical body carry is 50×(3×10⁸) ² = 45×10¹⁷ J. That’s huge! But don’t worry, with our current technology, we cannot completely convert a mass into energy.

**Does speed have a limit?**

How fast can your superbike/supercar run? Max 482km/hr. How fast can a jet go? Max 7200 km/hr. But do you know how fast light travels? 299,792,458 meters/second. About 300 million meters in one second!

According to the special theory of relativity, the speed of light (in vacuum) is the limit of the speed. Nothing can ever exceed the speed of light. WHY? Well, special relativity tells us that if you try to move as fast as light, your weight will reach infinity. Infinite mass! And infinity is always impossible in the real world. Although, if somehow you could travel at a higher speed than that of light (hypothetical wormholes might help you to travel at a higher speed than that of light), then you can go back in time. But in the real world, it is next to impossible for a body or a particle with mass to travel faster than light.

**TIME DILATION:**

The special theory of relativity tells us that if we travel faster in space, then we move slower in time. This directly means if you leave the earth today with a spaceship that can travel about 95% of the speed of light, and when you will return to earth, suppose after 10 years, then around 60 years would have passed on the earth. So if you had a kid son when you left the earth, then he would be much older than you when you return. How fascinating is that! But here comes the popular paradox: Twin Paradox

**TWIN PARADOX:**

Suppose you have a twin named Sally. Today at the age of 25, Sally is a doctor and you are an astronaut. As usual, you have a space mission where you need to test the latest spaceship which can travel at 95% of the speed of light. Now is the time for you to leave the Earth and travel in space for 10 years. So, Sally came to say goodbye before you left with the spaceship. After successfully completing your space travel, you return to Earth. But you came to know that your twin sister, Sally, is too old to welcome you back.

Here comes the paradox! Before 5 years (before 60 years for Sally), you left the earth with near lightspeed. So, from your point of view, Sally or the earth is the one who is speeding with 95% of the lightspeed. This means that Sally shouldn’t have turned this old. But from Sally’s perspective, things are exactly the opposite. Which one of them is right and who is mistaken here? This is the TWIN PARADOX. The special theory of relativity was unable to solve this perplexing paradox.

**CONCLUSION:**

The twin paradox was one of the main limitations of the special theory of relativity. Many physicists (and even philosophers) tried to solve the twin paradox. But the knowledge of physics was incomplete at that time to solve the twin paradox. So, when did this paradox be solved? After a decade of publishing the special theory of relativity, the great Einstein proposed the general theory of relativity.

With the help of general relativity, the paradox was solved successfully. Because the astronaut accelerated while leaving the earth and also the point in space from which the astronaut turned back, these two cases cannot be treated with the special theory of relativity, making the astronaut’s perspective wrong.

Basically, general relativity solved many mysteries which were unsolved by the special theory of relativity. General relativity shed light on the knowledge that was untouched by the special theory of relativity. Also, we can derive the special theory of relativity from general relativity. General relativity is the bigger fish! Despite all this, without understanding the special theory of relativity it is not possible to grasp the concept of general relativity.