# Fine dipole moment - numbers
⏎
⏎
## Fine dipole moment
⏎
$\alpha_{QED} = \frac{e^2}{\hbar c} = \frac{d^2}{a^2 \hbar c$
⏎
this is the definition of fine dipole moment depending on the length $a$ between charges $e$.
The lengtha can be taken either as https://en.wikipedia.org/wiki/Bohr_radius or https://en.wikipedia.org/wiki/Classical_electron_radius
⏎
## imagine that there are no free charges, but only dipole interactions
⏎
- let us compare with fine coupling in low-T glasses
⏎
$\alpha_{QSNS} = P \frac{\gamma^2}{\rho c_t^2$
⏎
where $P=\frac1}{erg*cm^3}$, $\gamma=erg$, and $\rho c^2= erg/cm^3$
⏎
# Parents
⏎
* Fine structure constant⏎