Recently I tried once moreto reflect your thought experiment but I'm not pretty sure if I completely got how it explains the locality philosophy by Yang and Mills, especially how does it interplay with concept of "gauge" they introduced. Could you check if I now understand your explanation correctly? (I think in my last comment I confused a lot of things)
So, say as before we model our universe as certain manifold ##M## and wonder how "our physics practiced on Earth" (say arround spatial point ##m_E \in M ## on the manifold) of eg pions differs from physics of pions "practiced" by physicists on "another planet" around point ##m_P \in M## far away from ##m_E##.
As you said assume we on Earth choose the convention to write everything in terms of eigenstates of ##\hat{t}_3 ## and on another planet the choose the convention to work with eigenstates of ##\hat{t}_1 ##.
One should expect that "the physics on both places" is the same. To keep it precise: Do you mean "the physics should be same" phrase in the naive sense that the Lagrangian describing the physics on Earth ##\mathcal{L}_E## should differ from the Lagrangian ##\mathcal{L}_P## with which they work on the other planet only by certain symmetry transformation ##\psi(x) \to \widetilde{\psi}:=e^{i \theta(x)} \phi ##, right so far? Then, so far, nothing new.But then you write
Also all experiments physicists can do are in principle pretty "local", i.e., what we observe with our experiments is influenced by the circumstances not too far away. So it's plausible to demand that the symmetries should also be local, i.e., the choice of which iso-spin component I use to define the charge states of the pion within this theory may vary locally.
and I'm not completely sure what you mean there. Do you mean it in the sense that you regard within this thought experiment the procedure of taking "an explicit choice of which iso-component our model locally use" as a symmetry transformation itself? Right?
If yes, then, what do you the precisely mean by
the choice of which iso-spin component I use to define the charge states of the pion within this theory may vary locally.
in mathematical terms? Do you mean by this that the message of locality philosophy by Yang and Mills is that every such "family of choices varying from point to point" ##(\hat{t}_{s(m)})_{m \in M}## aka symmetry transformation on the underlying manifold which iso-spin component is used by physicists at point ##m \in M## to model the pion physics vary "continuously"? In other words, that gauge transformations must be "continuous" on underlying space as mathematical function, so no "jumps" etc?
And then in order to legitimize the naive locality, we continue the reasoning by, say on the Earth - ie at point ##m_E ## - we change with time our choice of which iso-component our pion model use, ie ##s(m_E)=s(m_E)(T)## is time dependent. Then the physicists on the other planet at ##m_P## percive our permanent procedure to vary with time the iso-component we are working with only after "some time has past", ie as only after some time has passed the ##t_s(m_P)## can be affected by "our wheeling and dealing on Earth".
Is that what they mean by "locality"?
Or do I mising the pun you intended to emphasise in your thought experiment?