According to Sir Albert Einstien, there is no such thing as absolute motion or absolute rest. Objects move relative to each other. For example, at a busy crossroad where hundreds of cars are passing by at the same time and everything/everyone around you is in one kind of motion, but you stand still at one particular spot, are you really not at all in motion? No. The earth is constantly in motion. So even if it seems through our naked eye that we are not moving at all, according to science we all are constantly in motion. Therefore, nothing is ever really in absolute rest or absolute motion. This theory in known as Classical Relativity.
For the most of us time slows down when we are upset or anxiuous, there are times when you feel like 10 minutes seem to be hours. For example when we wait for someone outside the airport , the time of the wait usually feels longer than usual. The same thing happens vice versa. When we are busy doing something or too engrossed in our work we feel time passes by so fast. This is called Time Dilation.
Now let’s understand the concept of Time Travel.Time slows down for objects close to the speed of light, It stops for objects travelling at the speed of light and Time travels backwards for objects which are faster than the speed of light. Till date humans have not created any object faster than the speed of light and because of this we cannot travel back in time.
To understand this concept we have to understand that time is not a fixed unit for everyone.