Very few people have contributed as much to twentieth-century physics as
Julian Schwinger. It is therefore appropriate to offer a retrospective
of his work on the occasion of his sixtieth birthday (February 12,
1978). We hope, in offering this selection of his papers, to bring to
light ideas and results that may have been partly overlooked at the time
of the original publication. Schwinger has published prodigiously on a
great variety of subjects, as is evident from the comprehensive list of
publications arranged in chronological order which appears on p. xiii.
Needless to say, only a small subset could be included in the present
modest volume. In the selection, great weight was assigned to papers
that seem to be less widely known or appreciated than they deserve. Many
important papers are therefore omitted. (Examples: Paper [64] 'On
Gauge Invariance and Vacuum Polarization' and Paper [69] 'On Angular
Momentum', both of which have been reprinted elsewhere. ) The collection
is a personal one, having been chosen by Schwinger himself, and is
therefore of particular interest. It would probably not be interesting
to offer an analysis, by the editors, of Schwinger's contributions to
physics. However, we are very pleased to be able to include Schwinger's
own informal and very personal comments about each article that appears
in this volume. These comments indicate his reasons for choosing these
particular articles and, in many cases, provide a capsule synopsis of
what he considers most valuable.