# Fine dipole moment - numbers


## Fine dipole moment

$\alpha_{QED} = \frac{e^2}{\hbar c} = \frac{dd_{QED}^2}{aa_B^2 \hbar c}$

this is the definition of fine dipole moment depending on the length $a$ between charges $e$.
The lengtha can be taken either as $a_B$ https://en.wikipedia.org/wiki/Bohr_radius or https://en.wikipedia.org/wiki/Classical_electron_radius

The dimensionality of dipole squared is $d^2=erg*cm^3$⏎
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## 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 $P=\frac{1}{erg*cm^3}$, $\gamma=erg$, and $\rho c^2= erg/cm^3$

To write down explicit dipole moment, we need to introduce length $\xi$
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$\frac{\hbar c}{\xi} = \frac{\gamma^2}{\rho c_t^2} \frac{1}{\xi^3}$, therefore $\xi=\gamma \sqrt{\frac{1}{\hbar \rho c_t^3} }$
$\xi \sim 50A \sim 10* a_B$
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Therefore $d^2_{QSNS} = P \gamma^2 \xi^6$
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## compare $d^2_{QSNS}$ and $d^2_{QED}$
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- this ratio $d^2_{QSNS}/d^2_{QED}$ implicitly depends on the ratio $a_B/\xi$
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# Parents

* Fine structure constant