© David Kaplan.
Rotations in physical space are analogous to rotations in "particle space." One can represent the changing of a particle from one type to another as a rotating arrow. The picture on the left shows an arrow that is rotated from an eastward direction to something between east and north. The picture on the right shows an arrow which is rotated from the A direction to something between A and B. The direction represents what the particle type is, and thus allows a description in which the particle is neither A nor B, but a combination of the two. If the theory has a symmetry with respect to particle type, it means that physics experiments do not depend on which direction the arrow is pointing—which particle, or particle combination, is in the experiment. (Unit: 2)