The ubiquitous clumpy state of the ISM raises a fundamental and open problem
of physics, which is the correct statistical treatment of systems dominated by
long range interactions. A simple solvable hierarchical model is presented
which explains why systems dominated by gravity prefer to adopt a fractal
dimension around 2 or less, like the cold ISM and large scale structures. This
has direct relation with the general transparency, or blackness, of the
Universe.Comment: 6 pages, LaTeX2e, crckapb macro, no figure, uuencoded compressed tar
file. To be published in the proceeedings of the "Dust-Morphology"
conference, Johannesburg, 22-26 January, 1996, D. Block (ed.), (Kluwer
Dordrecht
