What is time?
Classical mechanics takes time as something fundamental in the universe, although this is obviously just for the sake of calculation. Classical mechanics would never query what time itself is, and possibly no branch of physics is able to answer this either. It's almost like asking 'what is mass?'. This is a question for philosophy more than anything else; it takes a scientist to handle a concept, but a philosopher to define it from its first principle. Einstein showed that our perception of time is relative, with his famous thought experiment of the lightning bolts and the train. However, the fact that there is a sequence of events to perceive in the first place (even though another person may perceive the same events in a different order) shows that there is something fundamental about time. Otherwise there would not be any 'sequence' to perceive in the first place. Perception is limited, because a human being only perceives the events that reach him in a specific