  In gratings travelling at nearly the velocity of light a symmetry breaking
transition is observed between free-flowing fluid-like Bloch waves observed at
lower grating velocities and, at luminal velocities, condensed, localised
states of light captured in each period of the grating and locked to its
velocity. We introduce a new technique for calculating in this regime and use
it to study the transition in detail shedding light on the critical exponents,
and the periodic oscillations in transmitted intensity seen in the
pre-transition regime.
