Relativistic Quantum Mechanics Lecture 7 Books Recommended: Lectures on Quantum Field Theory by Ashok Das Advanced Quantum Mechanics by Schwabl Relativistic Quantum Mechanics by Greiner Quantum Field Theory by Mark Srednicki http://www.quantumfieldtheory.info/

Spin of Dirac Particle We define 4-dim matrix which is generalization Of Pauli matrices This is related to

As and thus Note that

Also

With above, free Dirac Hamiltonian gives

Orbital angular momentum is We calculate

To find spin angular momentum, we note that Thus

Total angular momentum Defining spin angular momentum In particular