So much physics, so little time
One of the concepts which appears mysterious to students first learning relativity is the notion of "spacetime". And indeed, spacetime is the most important and fundamental concept in relativity. Unfortunately, many treatments of relativity present spacetime as something new in relativity that does not exist in the Newtonian mechanics learned in high school. This is unfortunate because space and time certainly exist in Newtonian mechanics and there is, in fact, a "spacetime" in Newtonian mechanics just as there is in relativity. What changes between the two theories is the relationship between space and time and the corresponding geometry. Understanding relativity will be much easier if we first understand the concept of spacetime in Newtonian mechanics, which is the focus of this set. We start with time.
Question 1: What is a valid definition of time from a physical process perspective? By physical process, we mean that time has to be defined by some real, physical process that happens in the universe.