Gauge transformations as Lorentz-Boosted rotations

The E(2)-like little group for massless particles is obtained from the O(3)-like little group for massive particles in the infinite-momentum/zero-mass limit. It is shown therefore that rotations around the axes perpendicular to the direction of the momentum become gauge transformations in this limit. The limting procedure is shown to be identical to that of group contraction through which O(3) becomes E(2). (orig.)