In his paper, “Grand Unified Gauge Theories and the Number of Elementary Particles,“ Robert Weingard suggests what he calls the Extended Redhead's Principle for elementary particles: “Two particles for which there are conceivable circumstances in which one can be ‘rotated’ or reoriented into the other are the same particles”. The philosophical soundness of such a principle is questionable.

Writing the history of elementary particle physics has all the problems common to writing the history of any other subject “in the making”. There is, however, an additional characteristic, unique to this branch of physics. The development of particle physics, unlike the situation in other branches of physics, reveals a continuously changing picture of what its object of investigation is, of what, in other words, the things we call particles are and how elementary they should be considered. The history of (...) elementary particle physics is, in a way, the history of the continuous reinterpretation of both the ontological and methodological status of the notion of elementarity. Hence, examining the history of elementary particle physics is also an attempt to explicate this changing collective consciousness of the scientific community about the elementarity of particles. In studying, therefore, (practically any aspect of) the history of elementary particle physics, one has to be sensitive about a number of philosophical, and primarily methodological issues which have acquired an added significance due to the relatively recent and mainly theoretical developments. (shrink)