# Magnetic moment is an intrinsic property of elementary particles?

1. Jan 11, 2008

### cks

My question is stated in the title. I wonder is it that scientists are not able to explain the origin of magnetic moment of elementary particles and say that it's an intrinsic property just like mass.

2. Jan 11, 2008

### cks

I mean, magnetic moment of a current loop is the integration of current over vector dA. but the magnetic moment of elementary particle can only be measured by experiment, but cannot found by calculation?

3. Jan 11, 2008

### vanesch

Staff Emeritus
The magnetic moment of elementary fermions such as electrons is calculable from first principles ; however, the magnetic moment of composed hadrons is much harder to calculate from first principles as it is a complicated bound QCD state.

Edit:

for an elementary fermion, the magnetic moment is given by:

g e hbar / (2 m0) x spin

where g is a calculable number in QED, which equals 2 in tree approximation, and which takes on higher-order calculations when taking into account loop diagrams.

Last edited: Jan 11, 2008
4. Jan 12, 2008

### cks

Thank you very much.

5. Jan 17, 2008

### danime

In fact it is not a calculation from first principles.
The intrinsic angular momentum of the electron pops from the Dirac Equation. The relation of this to the magnetic momentum is another thing. If you argue that any spinning charge have magnetic momentum I must to ask you the size of the charge, which we now is zero. The connection is an experimental fact. The QED calculations starts from the fact there are such connection.