  This article was written for the Logic in Computer Science column in the
February 2015 issue of the Bulletin of the European Association for Theoretical
Computer Science. The intended audience is general computer science audience.
  The uncertainty principle asserts a limit to the precision with which
position x and momentum p of a particle can be known simultaneously. You may
know the probability distributions of x and p individually but the joint
distribution makes no physical sense. Yet Wigner exhibited such a joint
distribution f(x,p). There was, however, a little trouble with it: some of its
values were negative. Nevertheless Wigner's discovery attracted attention and
found applications. There are other joint distribution, all with negative
values, which produce the correct marginal distributions of x and p. But only
Wigner's distribution produces the correct marginal distributions for all
linear combinations of position and momentum. We offer a simple proof of the
uniqueness and discuss related issues.
