No Dispersion

The same B field interactions between photons travelling down a beam is attractive.

By this alone, a light beam does not disperse.  From a previous post "τo The New Guy On The Block", it was proposed that hot bodies repel each other.

$F_T=\cfrac{T_aT_b}{4\pi\tau_or^2}$

where  $T_a$ and $T_b$ are the temperatures of the two bodies involved.  This is the repulsive force that is responsible for the dispersion of light.

Taken together, this would suggest that the spread of a light beam in free space is dependent on temperature.  At higher temperature a beam disperse wider than when the ambient is at low temperature.