The question posed is a very general one. I want to understand the nature of time exactly in this thread. Time seems to have 3 commonly ways of looking at First is in special relativity we have world lines evolving in a space time continuum with the Lorenz transformation the space time metric. The second is in quantum mechanics there seems to be a world clock that ticks at a constant rate. The phase of each energy eigenstate evolves as (e^iEt/h) In quantum field theory also time is a unitary evolution. However the Lagrangian is taken to be Lorentz invariant. And the third where the direction of time is given by the direction of entropy increase of the system. And a also relating to the time symmetry of the underlying laws. The purpose of the thread is I hope to get clarity over the various things related to time.