PX156 - H1 - the electromagnetic force

boson: photon ( $γ$ )
- rest mass: $0$
- electric charge: $0$
- only couples electrically charged particles
conserves all quantum numbers
infinite range:

F_{E M} \sim \frac{α_{E M}}{r^{2}}

where, $α_{E M} =$ coupling constant, 'intrinsic strength of the force'; $r =$ distance

α_{E M} = \frac{q_{1} q_{2}}{4 π ϵ_{0}}