Time

            Since time is a hot topic today that runs across many theories in physics such as Newton's classical physics ,Quantum Mechanics, Relativity, String theory etc, is the concept of time.
         Time is used to define the speed of a moving objects in a classical sense and measure the gap between the two events. In the classical world , time always moves forward, and they are absolute, that is time is same for everyone. We all age with time and it is a witness to our ageing.
      But in relativity physics time is not independent. Time is always related to space-time. Your time is determined based on where are you in space time. And time depends on your speed. Hence time ticks slow as you move faster. Finally when you reach the speed of light, time freezes. Assume that you have a time machine that could take you on a  space ride at the speed of light for 20 years and bring back home. When you meet your friends they would have  aged 20 years while you would still be as young you were when you left 20years ago.
           As we exist in a particular space (sweet home) , we also exist at a particular time( right now). Space as we all know, is a spacial three dimensional entity. It has three dimensions, height, width and depth. And what  Einstein deduced was that, time is the fourth dimension that needs to be added to the three dimensions of space. As an example, of your friend wants to know your address, you would normally tell him with three bits of information such as 8th Street, 47th avenue and 1st floor. (These three bits of information specifics three spacial dimensions). Einstein added the fourth bit of information as time, say 5pm. Thus here space and time are combined into a single entity called space-time.
              How much time the earth takes to orbit around the sun depends on how much the space time is curved in it's path of orbit. However if it would have orbited around a black hole, the Sense of time world be very different. The sharp bending of space-time makes earths orbit steeper and making it move faster, resulting in time ticking slower.
               Some theory say that we can go back in time and change past events from the history that could in turn affect future events. But we are here in the present only due to that past events that occurred. If past events would have been different you would not be here in the present. So for every cause there is an adverse effect. And also anything you do in the present events will lead to change in future events. There is a cause first and and later comes it's effects. These are all retrocausality, for they are philosophical paradoxes and thought experiments which are not yet proven, these all possibilities come from Einsteins theories.
          But what if time stands still? We know that we can stop the flow of time only if we travel at the speed of light. For light itself, time does not pass. Our sense of time and it's speed of flow must have come to us through a long evolutionary process. Our body clock is synchronized to the motions of sun, Moon and stars.
           According to science and some theories, time and space got created from an event called the Big Bang, some 13.7 billion years ago. Time has never ceased to exist since then. Time has been flowing in the forward direction for us ever since. Our universe has been expanding ever since the big bang and this expansion  will continue for as long time to come. But as soon as it reaches it's maximum size it will end through process called Big crunch, where time will end.