Relativistic Terms in Quantum Mechanics

The fact that an electron has an intrinsic spin comes out of a relativistic formulation of quantum mechanics. Even though the Schrodinger equation does not predict it, wave functions that are antisymmetric and have two electrons per orbital are used for nonreiativistic calculations. This is necessary in order to obtain results that are in any way reasonable. 

Also arising from relativistic quantum mechanics is the fact that there should be both negative and positive energy states. One of these corresponds to electron energies and the other corresponds to the electron antiparticle, the positron. 

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