Mass-energy equivalence is fundamental in relativity, but it seems like energy and momentum are also different aspects of the same thing. They've each got very important conservation laws. In SR, the space coordinates of the four-momentum give the momentum while the time coordinate gives energy. In QM, -ih d/dx is the momentum operator, and ih d/dt is the energy operator. P=h/wavelength and E=h/period. It seems that what momentum is to space, (mass-) energy is to time. What is behind this symmetry? Is there a mass-energy-momentum equivalence principal? Also, I notice that p=mv classically, where v is in units of distance/time. It almost looks like v is a conversion from time units to space units, where mass-energy is the time unit and momentum is the space unit. Is this a valid way of looking at it?