Role of $SU(2)$ group in isospin and in the weak interaction I know that the $SU(2)$ group describes internal symmetries such as isospin and the weak interaction. But isospin and weak interactions are quite different, so more precise what is the role of $SU(2)$ in the two cases?
 A: Saying that $\textrm{SU}(2)$ describes internal symmetries as the isospin is somewhat incorrect. Rather, it is the gauge group describing the weak interaction.
In quantum field theory the equations of motion for the fields and the particles involved therein are described by means of a Lagrangian which is supposed to be invariant under some gauge group of transformations. According to how you choose the gauge group you have different couplings among field-field interactions, field-particle, particle-particle and so on and so forth. For an initial description you can have a look at the related wikipedia article.
Electromagnetism is fully described by taking a Lagrangian that is invariant under the action of $G=\textrm{U}(1)$; likewise the weak interaction is described along the same line by taking $G=\textrm{SU}(2)$ and the dimensionality of the gauge group gives back the number of different force carriers in each case: for instance one (the photon) for the electromagnetic field and three (the weak $W_{\pm},Z$ bosons) for the weak one. When exploiting the calculations you will find that, similarly to the electric charge, there is an associated conserved quantity under the action of $G=\textrm{SU}(2)$ which we refer to as isospin, but that is not an inner consequence of the gauge group, it is an additional consequence of the Noether theorem and is apart from the description of the weak interactions as gauge theories themselves.
A: I think it's fair to describe both $\textrm{SU}(2)$ symmetries as "internal symmetries" (as opposed to spacetime symmetries). However, there's an important distinction between the case of $\textrm{SU}(2)$ as used in isospin and the weak interaction: in the case of isospin, $\textrm{SU}(2)$ is a global symmetry, while with the weak interaction $\textrm{SU}(2)$ is a local or gauge symmetry. At some level, this seems like a small distinction, since it just specifies whether the group transformation is the same at every point in spacetime (global) or can vary at different spacetime points $x_\mu$ (local/gauge). However this has profound effects on what quantities are conserved and what interactions are allowed.
