Perseus Books's Frontiers in Physics series has, since 1961, made it possible for
leading physicists to communicate in coherent fashion their views of recent devel-
opments in the most exciting and active fields of physics—without having to
devote the time and energy required to prepare a formal review or monograph.
Indeed, throughout its nearly forty-year existence, the series has emphasized
informality in both style and content, as well as pedagogical clarity. Over time, it
was expected that these informal accounts would be replaced by more formal
counterparts—textbooks or monographs—as the cutting-edge topics they treated
gradually became integrated into the body of physics knowledge and reader
interest dwindled. However, this has not proven to be the case for a number of the
volumes in the series: Many works have remained in-print on an on-demand
basis, while others have such intrinsic value that the physics community has
urged us to extend their life span.