I'm currently reading Sakurai's 'Modern Quantum Mechanics' (Revised Edition) and at page 76 he introduces a spin half hamiltonian(adsbygoogle = window.adsbygoogle || []).push({});

[tex] H = - (\frac{e}{mc}) \vec S \cdot \vec B.[/tex]

But what is c doing in this hamiltonian? Clasically the energy of a magnetic moment in a magnetic field is

[tex] E = - \vec \mu \cdot \vec B = - \frac{e}{2m} \vec S \cdot \vec B[/tex]

and as far as i know what one does to go to QM is to introduce the g-factor. What am I missing here?

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# What is the speed of light doing in the spin half hamiltonian?

**Physics Forums | Science Articles, Homework Help, Discussion**