Physicists define time as the progression of events from the past to the present into the future. Basically, if a system is unchanging, it is timeless. Time can be considered to be the fourth dimension of reality, used to describe events in three-dimensional space. It is not something we can see, touch, or taste, but we can measure its passage.
The second law of thermodynamics states that the universe cannot return to exactly the same state in which it was at an earlier point. Time, put simply, cannot move backward.
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