Fluctuation induced forces (FIF) are ubiquitous [1], and are caused by the confinement of long-range correlated fluctuations. Their most celebrated and initial version, also known as Casimir forces, were first predicted and measured using perfectly conducting plates immersed in the QED vacuum [2].

Here, we consider intensity fluctuations of classical light propagating through a scattering medium. In the multiple scattering regime, the average light intensity behaves diffusively. Underlying mesoscopic coherent effects give rise to spatially long-ranged fluctuations [3]. The resulting FIF are described here using an effective Langevin approach which properly incorporates the coherent mesoscopic corrections. Their magnitude depends on the dimensionless conductance g. This Langevin description bears a similarity with corresponding FIF recently identified in nonequilibrium systems [4], resulting from long-ranged density fluctuations around the steady state density profile.

References
[1] M. Kardar, R. Golestanian. Rev. Mod. Phys., 71, 1999.
[2] H. B. G. Casimir. Proc. Kon. Ned. Akad. Wet., B51:793–795, 1948.
[3] E. Akkermans, G. Montambaux. Mesoscopic physics of electrons and photons. Cambridge University Press, 2007.
[4] A. Aminov, Y. Kafri, M. Kardar. Phys. Rev. Lett., 114, 2015.