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To quote Weinberg Vol1, Pg 14 :
So to speak, Dirac equation alone cannot determine g-factor uniquely, but quantum field theory can? How?
And immediately he said:(iii) One of the great successes of the Dirac theory was its correct
prediction of the magnetic moment of the electron. This was particularly
striking, as the magnetic moment (1.1.8) is twice as large as would be
expected for the orbital motion of a charged point particle with angular
momentum [tex]\frac{\hbar }{2}[/tex]; this factor of 2 had remained mysterious until Dirac's theory. However, there is really nothing in Dirac's line of argument that
leads unequivocally to this particular value for the magnetic moment. At
the point where we brought electric and magnetic fields into the wave
equation (1.1.23), we could just as well have added a 'Pauli term'
[tex]\kappa {\alpha _4}[{\gamma ^\mu },{\gamma ^\nu }]\psi {F_{\mu \nu }}[/tex]
with arbitrary coefficient к. (Here F_uv is the usual electromagnetic field
strength tensor) This term could be obtained by first adding a term to the free-field equations proportional to [tex][{\gamma ^\mu },{\gamma ^\nu }]{\partial _\mu }{\partial _\nu }\psi [/tex]which of course equals zero, and then making
the substitutions (1.1.22) as before. A more modern approach would be
simply to remark that the term (1.1.32) is consistent with all accepted
invariance principles, including Lorentz invariance and gauge invariance,
and so there is no reason why such a term should not be included in the
field equations. (See Section 12.3.) This term would give an additional
contribution proportional to к to the magnetic moment of the electron, so
apart from the possible demand for a purely formal simplicity, there was
no reason to expect any particular value for the magnetic moment of the
electron in Dirac's theory.
As we shall see in this book, these problems were all eventually to be
solved (or at least clarified) through the development of quantum field
theory.
So to speak, Dirac equation alone cannot determine g-factor uniquely, but quantum field theory can? How?