  The world appears to be well described by gauge theories; why? I suggest that
gauge is more than mathematical redundancy. Gauge-dependent quantities can not
be predicted, but there is a sense in which they can be measured. They describe
"handles" though which systems couple: they represent real relational
structures to which the experimentalist has access in measurement by supplying
one of the relata in the measurement procedure itself. This observation leads
to a physical interpretation for the ubiquity of gauge: it is a consequence of
a relational structure of physical quantities.
