The zero-body problem:
29 October 2010 at 18:58

“It might be noted here, for the benefit of those interested in exact solutions, that there is an alternative formulation of the many-body problem, i.e., how many bodies are required before we have a problem? G.E. Brown points out that this can be answered by a look at history. In eighteenth-century Newtonian mechanics, the three-body problem was insoluble. With the birth of general relativity around 1910 and quantum electrodynamics in 1930, the two- and one-body problems became insoluble. And within modern quantum field theory, the problem of zero bodies (vacuum) is insoluble. So, if we are out after exact solutions, no bodies at all is already too many!” — Richard D. Mattuck, A Guide to Feynman Diagrams in the Many-Body Problem

http://science.slashdot.org/comments.pl?sid=1845884&cid=34068552