I noticed that the relations in mechanics can be seen like: 1) assume conservation of angular momentum 2) from this you can mathematically *derive* that energy as defined by dE=v dp is conserved (if all forces are inverse square) Up to here we haven't specified whether we are dealing with classical or relativistic mechanics, i.e. we do not know the function p(v). Now instead of writing E=mc^2 you could *equivalently* write p dt=E ds from which all of relativity follows. Now I am wondering if p dt has a physical meaning. I vaguely recall seeing something in a path integral formulation. What do you think?