Time is important part of our life. We know it never stops and always moves ahead.The arrow of time moves from present to future. But it's just basic understanding of time.

we frequently asks time from our friends or our family. They say,' It's 9o'clock or its 11o' clock' something like that,Yes or No?Which device they use to tell you the time?Clock,isn't it?But, Clocks are just the device which we use to calculate the flow of time. So,what the thing Time actually is?

I know somewhen this question would have came to your mind and you perhaps thought that,' what stupid I'm thinking'? But actually it is a very complicated question even for great Scientist ever lived.

Once Aristotle said,"Time is most unknown of all Unknown things".

That was nearly 2500 years ago and it still stands true today.

To understand time we have to took a dive in ocean of Physics. Histoey of time is very long but we started understanding time and its characteristics when we stuided about fastest thing in the Universe “ Light ”. The researches on its charcterstics and speed makes us compelled to consider the term Time.

In Mid 19th Century, Scottish Scientist James Clerk Maxwell told us that -

Light is a Electromagnetic waves which travels at constant speed in vaccum like space.

But, this phenomenon given by Maxwell created lot of problems in World of Physics.

These problems are later solved by legendary Albert Einstein. But, for Albert Einstein theory of classic Relativity states that Anybody in the Universe isn't in state of absolute rest or motion. Everybody moves relative to each other. Example- 

A truck is moving at speed of 50km/hr. Two boys are play with ball in that truck. If a boy throws a ball with velocity of 10km/hr for those two, balls moves with velocity 10km/hr but if someone see them playing in truck he/she will see the ball moving with more speed than 10km/hr. According to his/her perception speed of ball is 50km/hr + 10/hr =60km/hr.But, is this person is in absolute state of rest?No,because our whole earth is moving even our galaxy is also moving.

So, anything in the Universe isn't in the absolute state of rest as everything is moving.

This is Classical Relativity. 

In the example we saw that speed of ball adds upto the speed of the truck.Is everything speed is relative to each other? Does law of relativity applys to each and every body in Universe? Is there anything which remains constant at any situation ?

In this subject James Clerk Maxwell gave a big contribution. He gave four brilliant equations on Electicity-Magnetism. In this he also told us that light is nothing but a electromagnetic waves travelling through empty space whose speed is no matter what will remain constant for all observers.Example - speed of light of a moving car with speed of 100km/hr and stationary car will be always same.

So, Light is a Universal constant.

Does this thing is possible according the the theory of Classic Relativity ?

The speed of light is 186000 miles/s. But, According to the law of Classic relativity Speed of light is relative to what?

This statement of Maxwell raised lot of questions in 19th Century.

So, Albet Einstein accepted both statements which are 

Speed of light is constant in the Universe for all observers.

And, he also accepted that

Light is the fastest moving thing in entire Universe 

The both statements are contradictory but Albert Einstien accepted them and started his research.

And, In 1905 he published his Hypothesis which is one of the most famous theory of Albert Einstein. He called this theory “Theory of Special Relativity”.

In this theory he defined the exception of Relativity which is ‘Light ’ in a detailed way.

In this theory Albert Einstein told that in order for the speed of light to be constant at speed of 186000 miles/s . It is necessary to slow down of a element , And slowdowned  element is “Time”.

Now, lets take a example if a body is travelling at almost 90% of speed of light. Then for this body the speed of time will slowdown to maintain the speed of light which will be constant and consistant speed of light.The speed will still be 186000 miles/s and constant irrespective to the speed of that body.

It is a funfact that,the object will not experience any slowdown in time beacause it will experience the same laws of Physics as stationary Object !

This phenomenom is called “Time Dilation” by Albert Einstein.

So, Time Dilation is slowing down the passage of time for an object to maintain the consistency for speed of light.

According to Albert Einstein Space and Time are interconnected to each other. 

According to Einstein, Speed is inversely propotinal to Time. In simple words how faster you will be in space dimension that much slower you will be in time dimension.

If somehow you will reach speed of light then time will stop for you.

But, as we know the particles of light which are Photons , are actually massless.

In reality it is impossible to travel at that much speed because with mass the body will start to gain more mass with increase in acceleration.And, this increase in mass will decrease the acceleration.

It is claimed that if something will move faster than time then it can be possible to travel in past.

But,Many scientist claims that after the Big Bang when time comes into existence since then the nature of time is to move forward.There is no way to to go back.

We can assume this fact also in law of thermodynamics which says that Entropy increases with time.

Entropy states the transformation of a body from orderly state of existence to disordered state of existence with time.

It help us to understand that before big bang there should have less entropy than today. As universe is expanding since bigbang the Entropy is increasing.