Einstein was a great physicist who developed numerous theories in the 20th century. He had formulated lot of theories but he was especially famous for the theory of relativity which brought about a massive change in this world.In 1905 he formulated his theory of relativity, which explained the relationship between space and time, the time dilation, and the twin paradox,length contradiction and so on. He called this, the special theory of relativity. In 1915 he developed his special theory of relativity named it as general theory of relativity, which explained the propagation of gravity and invalidated Newton's laws of gravitation recently.In this article we will look at the theory of relativity in detail and in ways that can be understood.Before we see about the theory of relativity we shall look on to the classical theory also known as Galilean theory

### Classical Theory:

By now we understand that physics is concerned with motion of objects,and the quantitative analysis of that motion,but this can be tricky business if you really think about the fact that everything is in constant motion.For example, When we say that a car is moving at 100 km/hr on the freeway we are ignoring the fact that the surface of the earth is rotating around the center at about 1700 km/hr at the equator.Earth is also moving around the Sun at around 30 km/s.The entire solar system moves around the galactic center at 20 km/s and the Milky Way galaxy is moving around through the universe at about 230 km/s .

So it's not completely wrong to say that the car is actually moving with this incredible speed as well,meaning that in a certain context,there's practically no difference between flying in a plane and sitting still in terms of absolute velocity.

But that's why we tend to discuss relative motion.We don't notice the movement of the earth and the Sun and the galaxy in our everyday lives.To us,the earth seems to sit still,and a few dozen km/hr may be insignificant in the grand scheme of the universe.So we often discuss motion that is relative to the surface of the earth.When we do this we are assigning something called an inertial reference frame.This is the object or person or location that we pretend is completely still and we access the motion of other objects relative to this inertial reference frame.For example,if we treat the earth as an inertial reference frame and we ignore its rotation and movement through space,it becomes meaningful to say that this car is moving directly north at 100 km/hr relative to the ground.But if we are inside the car moving with constant velocity it seems as though we are not moving but rather than the surroundings are rushing past us at 100 km/hr.In this case the car has become inertial frame that is standing still and everything is in motion around us.

It may seem strange to pretend that the car isn't moving, but remember it's just as incorrect to pretend that earth isn't moving,so it really is all relative.Assigning frames of reference and understanding what they mean for the motion of objects is a big part of physics,and although some aspects of this kind of thinking seem obvious,they were not always understood.Galileo was the first to rigorously describe relative motion after doing experiments.He used these experiments to prove that velocity measurements depend on which reference frame you adopt.These ideas were extended to describe the motion of the earth around the Sun instead of the other way around by proving that the earth could be moving even if we don't feel it.The portion of this work is known as Galilean relativity and these concepts will be built upon to enter very strange territory when we cover Einstein's Special theory of relativity.But for right now,we won't go flying through space,we have much more work to do on earth,which despite its rotation and motion is actually quite a good approximation of an inertial reference frame,one that will be used frequently throughout our study of physics.With each topic we go through,whenever we examine an object's velocity,just remember it's all relative.

### Special Theory Of Relativity:

In classical theory we have seen about the relative motion.When Einstein got his hands on Galilean relativity,he gave it a huge lift.The part that stayed the same was the importance of assigning initial reference frames,which we pretend are motionless while everything else is moving relative to that frame,which could be the earth,or a person,or a train.Each inertial reference frame has its own set of axes and a clock,whether real or imaginary,to measure time.

But Einstein wondered what would happen if you assigned an object moving very close to the speed of light as the inertial reference frame,or even a beam of light itself.What would happen then?As it turns out,some pretty strange things,

Special theory of relativity is comprised of just two postulates,which at first glance seem very simple,but we quickly realize that in order to them to be true we have to completely restructure our understanding of space and time.First let's recall that an inertial reference frame is one where no acceleration is taking place.It must have some constant velocity,including zero if at rest,and we typically approximate the earth as an inertial reference frame.The first postulate states that the laws of physics are the same in every inertial reference frame.Whether you are standing still on earth or travelling with constant velocity in a car or plane or spaceship,the same law of physics always apply.But the speed of light,represented by the letter c,is a law of physics.It is a constant that is used in numerous equations,so the second postulate states that the speed of light in a vacuum will be the same in every inertial reference frame.No matter what you do,it's the same.Experiments have verified this,because when comparing two objects emitting light,one stationary and one in motion,they always yield the same value for the speed of light.But how can this be possible?How is c always the same and why can't we catch up to it?

It's not just because we don't have the technology to go so fast,it's because the speed of light is a fundamental law of physics.It is the universal speed limit.The problem arises when we now have to try to account for these different reference frames.Let's say with an example,imagine a person on the ground and a person travelling in the slower car.They measure different speeds for the faster car.and this agrees with our everyday experience.But in order for the person on the ground and the person in the spaceship moving near the speed of light to measure the same speed of light,they must be experiencing time in different ways.This is the first incredible conclusion we can derive form special theory of relativity.Time is not some rigid detached parameter as Newton envisioned.It doesn't flow at an absolute rate.Time is relative.It flows at different rates for different observers.

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