Time..for general public it is the normal tick-tick-tick of the clock but for physicist it have been a great mystery from the very beginning of our understanding of nature.
Isaac Newton was the first one who gave a scientific description of time.
Well,this is not actually true,Rig Veda contain very beautiful description of time,however it also describes time as static and moving forward.But there is an another calculation in Indian ancient literature,describing the 1 day of Lord Brahma which shows that people were aware of time dilation...i will explain time dilation later.It all shows that ancient Indians have a deep knowledge than we have today about world,however most scientist don't believe the knowledge Vedas contain.
Newton also described time as something which cannot be changed,it flows like a river.You cannot change it's pace.It is like an arrow always moving forward i.e towards the future.
However,200 years later after Newton,an unknown physicist in 1905 drastically changed the all earlier accepted notions of time.A simple idea overturned the old concepts of time.Albert Einstein...with his Special theory of Relativity shows us that time is not static it changes with our movement in space and is also effected by the curvature of the space itself.Time is dynamic.Time ticks differently for different observers.
Time Dilation...in simple words,it says the faster you move the slower the time will ticks.At first it seems all logical that the faster i move the less time i will take to reach a place.But here what matters is how fast you are moving.
First consider this postulates of Einstein's theory-"Speed of light is constant for every observer"
That is if you are standing on a ground and you measure the speed of light you will measure it to be 671 million miles per hour and even if you measure the speed of light while moving in an air plane with speed 200 miles per hour then you will still measure it to be 671 million miles per hour..it will remain constant no matter you are standing or moving with respect to it.
Now,to understand time dilation,consider this hypothetical situation.Suppose you have a car which can move close to the speed of light and you are about to leave to see an old friend of yours.Before starting your journey you match your wrist watch with a grand clock tower,which is so big that it can be seen from far off places clearly.Now you start your journey..moving at 100 miles per hour things are as usual and normal.You are sitting in your car and the people are passing by you i.e you feel you are at rest and others are moving relative to you.The time(the ticks)of your wrist watch matches exactly well with the grand clock.
However in hurry to reach your friend quickly you accelerate to 400 million miles per hour.You again look at both clocks(your wrist watch and the grand clock) to make sure you are going by time. you will be amazed to see that the grand clock is ticking slower than your wrist watch,soon you will realize that it is not the clock only everything outside your car is slowing down.Peoples are walking slowly,talking slowly,eating slowly as if someone has pressed the slow motion button for outer world.This slowing down of time is known as Time Dilation.You will further realize that everyone outside the car will appear to be shrunken in width(this is known as length contraction),for you things outside car will appear to be exceptionally thin.However you perceive things in your car absolutely normal.
It is just the one side of the story.
For people outside the car you are moving with respect to them.So when they will observe your wristwatch(just assume that you can observe it at this great speed) they will find that your clock is ticking slowly compared to the grand clock and for them,you will appear to be moving slowly in your car.Also they will find that your car as well as you has shrunk in length.You will never feel it physically or mentally because both have also been effected.
The effects are relative..and both(you and people outside) are absolutely correct.It is all because....for you, inside the car,everything else is moving and you are at rest whereas for people outside car you are moving.
Now people may think that what if someone will move faster than light then...one should abandon the idea of going faster than light because relativity shows that faster you move slower the time ticks and heavier you become.That is if you accelerate your car more and more close to the speed of light, the car will become more and more heavier thereby decreasing you velocity.
Please don't think of the news of neutrinos going faster than light...it was found later that there was a flaw in their experiment which result in that result...however it may be possible that we reach that limit some day.
However their are theoretical particles,called tachyons,but they are theoretical there is no physical evidence of them yet.
10 years later Einstein published his General theory of relativity..a theory dealing with gravity.
Newton when discovered gravity doesn't himself know the true origin of gravity i.e what causes gravity.
Although,he was successful in developing the equations to calculate gravity acting between different objects and on different planets..the equation holds at every point in universe,making it an Universal Law.
This new theory,General Relativity tells us what gravity actually is.
Just like the Special theory of relativity,general theory of relativity revealed a more drastic behaviour of space and time,showing a relation between space and time..collectively known as space-time.
Space is the emptiness we see around us(not the space outside the earth) i.e. nothing but mind you this nothing isn't actually nothing it is among the major players who govern the universe.
Space is everywhere,everything exist in this space.Anything which you see as solid is actually 99% empty
(i.e contains space) because an atom which is the fundamental(not most fundamental)constituents of everything is itself 99% empty.Say for example Tajmahal,if you remove all the space from all the atoms making the Taj Mahal then it will shrink down to the size of a grain but weighing millions of ton.
The secret of gravity lies in this space.The presence of mass/energy warps the space-time around it.
To understand what it means see these images..the grids are representing the space.Since we see space as nothing..to illustrate the bending of this 'nothing' grid representation is used.
An another way of understanding,suppose you and your friend are holding a long rectangular rubber sheet stretched.Now i put a heavy metal ball on it.What will happen? The ball will warp the sheet around it,creating a depression around it.
The space behaves similar to that rubber sheet whenever mass(heavenly bodies) is present... they warp the space in the same manner the ball warp the rubber sheet.
Now whenever a heavenly body comes near this depression in the fabric of space..it follows the warp and start revolving around it.Here's pic and a video to illustrate this.
So it is due this warping of space that a planet or any other heavenly body revolves around the other.Therefore,the force of gravity is nothing but the curved (or warped) space.
The physical insight which Newton lacked about gravity is provided by Einstein.
Now,if you still find it hard to believe that how can space be warped or curved.. then let me know you it has been experimentally verified by NASA.
ON 20th April 2004 NASA launched GRAVITY PROBE B to detect whether the warping of space described by general relativity is true or not....and to their satisfaction the probe verified the warping of space...earth is warping the space around it.