  In the late 1980s, Sam Edwards proposed a possible statistical-mechanical
framework to describe the properties of disordered granular materials. A key
assumption underlying the theory was that all jammed packings are equally
likely. In the intervening years it has never been possible to test this bold
hypothesis directly. Here we present simulations that provide direct evidence
that at the unjamming point, all packings of soft repulsive particles are
equally likely, even though generically, jammed packings are not. Typically,
jammed granular systems are observed precisely at the unjamming point since
grains are not very compressible. Our results therefore support Edwards'
original conjecture. We also present evidence that at unjamming the
configurational entropy of the system is maximal.
