  We show that weakly incoherent optical beams propagating in a Kerr medium
exhibit a universal algebraic coherence after a short propagation time,
mimicking the quasi-long-range order of ultracold quantum Bose gases in two
dimensions. If two plates are inserted in the medium, this optical
quasi-condensate gives rise to a long-range Casimir-like force, attractive at
large distances and repulsive at short distances.
