I know the topic of time has been brought up on multiple threads, and they are interesting. But I would like to ask the question a bit differently than I've seen it asked. When I took Physics 1A the instructor basically said that Einstein showed that time was an independent variable, then showed how that impacted a lot of equations. Over the years, I've enjoyed reading the various laymans physics books like Elegant Universe. In these books I've seen several different definitions of time. One suggested that time is a physical dimension of space/time, just like the normal 3 physical dimensions. It said that we are traveling through the time dimension at the speed of light, and that whenever we accelerate we are just "redirecting" velocity from the time vector into a spatial vector. It also suggested that matter has a "shape" in the time dimension. It also went on to suggest, based on Einsteins work, that the past, present and future all exist simultaneously and that for some unknown reason we just "experience" the forward arrow of time. This had something to do with relative acceleration causing bodies to experience a "plane" through the time/space continuum like a sideways slice in a loaf of bread. Others have said that while the passage of time may be relativistically tied to space, that when people refer to it as a "dimension" they simply mean in the mathematical sense (as in an independent variable). Anyway, I'm sure I've butchered these theories and made myself sound completely ignorant. What I'm curious about is the latest thinking into the physicality of time and if we have any idea why relativistic velocities have an impact on how an object experiences time.