In the case of electrons, you are mixing this with potential energy. Indeed the latter is negative (Coulombic potential). However, electrons obey the positive square root relation for energy.
marlon
Thanks a lot. My problem is really quite basic, I´m afraid. Are you saying something like...
In the transition from "basic" quantum theory to quantum field theory, both in the derivation of the Gordon-Klein and of the Dirac-equations, there is a big problem with solutions with negative energies. Can anyone explain me why this is being considered a big problem in the first place (it...