  In dense, static, polydisperse granular media under isotropic pressure, the
probability density and the correlations of particle-wall contact forces are
studied. Furthermore, the probability density functions of the populations of
pressures measured with different sized circular pressure cells is examined.
The questions answered are: (i) What is the number of contacts that has to be
considered so that the measured pressure lies within a certain error margin
from its expectation value? (ii) What is the statistics of the pressure
probability density as function of the size of the pressure cell? Astonishing
non-random correlations between contact forces are evidenced, which range at
least 10 to 15 particle diameter. Finally, an experiment is proposed to tackle
and better understand this issue.
