Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future. Usually we divide time into three:-
Time takes two distinct forms: the calendar, a mathematical tool for organizing intervals of time, and the clock, a physical mechanism that counts the passage of time. In day-to-day life, the clock is consulted for periods less than a day whereas the calendar is consulted for periods longer than a day. Increasingly, personal electronic devices display both calendars and clocks simultaneously.
One of the most interesting concepts of time is time-travel. Time travel is the concept of moving backwards or forwards to different points in time and space; and it is different from the normal “flow” of time to an observer. Time travel has been a plot device in fiction since the 19th century. Travelling backwards in time has never been verified, presents many theoretical problems, and may be impossible. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is known as a time machine.
Cause and Effect- Time Paradox
A central problem with time travel to the past is the concept of cause and effect; should an effect precede its cause, it would give rise to the possibility of a temporal paradox. Some interpretations of time travel resolve this by accepting the possibility of travel between branch points, parallel realities, or universes.
Another solution to the problem of causality-based temporal paradoxes is that such paradoxes cannot arise simply because they have not arisen. As illustrated in numerous works of fiction, free will either ceases to exist in the past or the outcomes of such decisions are predetermined. As such, it would not be possible to enact the grandfather paradox because it is a historical fact that your grandfather was not killed before his child (your parent) was conceived.