  It was asked by J.Birman, Williams, and L.Rudolph whether nontrivial Lorentz
knots have always positive signature. Lorentz knots are examples of positive
braids (in our convention they have all crossings negative so they are negative
links). It was shown by L.Rudolph that positive braids have positive signature
(if they represent nontrivial links). K.Murasugi has shown that nontrivial,
alternating, positive links have negative signature. Here we prove in general
the old folklore conjecture that nontrivial positive links have negative
signature.
