Do quasars have cosmologically long lifetimes

Turner and Tyson have independently suggested that the apparent evolution of quasars may be an artifact caused by (unseen) gravitational lenses; some of the problems inherent in the usual picture of space density evolution are thereby avoided. We discuss how these problems may be similarly avoided without invoking any such gravitational effects: apparent (and unreal) density evolution follows as an immediate consequence if quasar lifetimes (the only free parameter in our model) are of the order of 3 x 109 years. If the lifetimes are indeed this long, quasars may occur much less frequently than previously thought but, at the same time, the local density of quasars may have been grossly underestimated