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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?