Hi all, My Physics tutor's reasoning for no well-defined energies existing in nature for a system in time was that if a system had a well defined energy in time, such that |psi> = |E0> then, evidently, the energy cannot vary. And, his logic goes on to say that if an energy cannot vary, you cannot measure the energy of the system accurately as that requires changing its energy - measuring a difference. Has anyone a better answer? His isn't very satisfying. Surely I could measure the energy through knowing the mass and Einstein's relation, if it were at rest in my frame? Much appreciated!