Everyone, even the youngest child, knows what time is because he has experienced it. Unfortunately, time is a concept that is very difficult to explain or define. Someone once defined time as "God's way of keeping everything from happening all at once." If time didn't exist, there would be no beginning or end, and the only "time" we could perceive would be "always." Time is an experience that goes on inside people's heads but it is also a phenomenon that takes place in the physical universe.
In the physical universe, time is usually perceived as the interval between one event and another event. The passage of time is used to measure the duration from a designated start time to an ending time. Time, according to Einstein's special theory of relativity, is considered a fourth dimension after the three spatial dimensions of length, width, and height. .
Time seems to move like an arrow from some point in the past to some point in the future, such as from the time of the Big Bang when the Universe was created to some unknown future. Meanwhile, at present, galaxies continue to fly away from each other as the Universe keeps expanding and growing. The age of the universe is considered the most important frame of reference for physicists.
There are two schools of thought regarding time. One is that time is absolute and fixed in the universe and that it occurs even without the presence of an observer. This view prevails for Newtonian physics as well as for the realm of quantum mechanics. The other view, which is part of Einstein's theory of relativity, is that time is different for each observer; that time slows down for an observer moving at a high enough speed, such as that close to the speed of light. The Einsteinian view also considers that within a black hole with infinite mass and infinite gravity, time would not exist. It is very difficult to reconcile these two views or to say which one is correct.