Equilibrium Pressure & Fluctuation-Induced Force
Particles: Pressure is a function of state.
Force per unit area of the wall is a bulk property;
independent of the shape of the container and potentials at the wall
independent of the dynamics of the particles (momentum integrations)
For a dilute gas: 
(non-universal corrections)s
Radiation: Photon density is temperature dependent:
Stefan-Boltzmann law of black-body pressure:
Casimir Force: Virtual photons (QED fluctuations) at low temperatures: 
"On
the attraction between two perfectly conducting plates," H.B.G.
Casimir, Proc. K. Ned. Akad. Wet. 51,
793 (1948)
Quantum fluctuations
of electromagnetic field in the vacuum between leads to an attractive force between perfect mirrors
"The
Theory of Molecular Attractive Forces Between Solids," E.M.
Lifshitz, Soviet Physics 2,
73 (1956),
Generalizes the result by considering fluctuating current sources in the bodies, and to finite temperatures.
"Pressure" =>"Interaction"; Force per unit area of the wall is
Attractive
Strongly dependent on the shape of the container
Since 1995 confirmed in myriad experiments.
Particles: There are corresponding quantum fluctuations at low temperatures, but more relevant
classical interactions emerge when inter-particle forces lead to large correlation lengths 
.
(Long-ranged, as opposed to short-range "depletion" forces.)
Finite-size behavior of free energy confined between plates takes a universal form at criticality
M.E. Fisher and P.-G. de
Gennes, C. R. Acad. Sci. Ser. B 287, 207 (1978)
Experimental confirmation for colloidal particles in critical oil/water mixture.
Hertlein,
Helden, Gambassi, Dietrich & Bechinger, Nature 451, 172
(2008)
Long-range correlations lead to long-range fluctuation-induced forces. (Universal?)
Goldstone mode fluctuations following continuous symmetry breaking are long-range correlated
H. Li . Kardar, Phys.
Rev. Lett. 67, 3275 (1991)
Experimental confirmation in wetting of superfluid films
R. Garcia and M.H.W. Chan, Phys. Rev. Lett. 83, 1187 (1999)
