  It is demonstrated that there is a dynamic isospin breaking effect in the
near threshold $\gamma^{*} N\to \pi N$ reaction due to the mass difference of
the up and down quarks, which also causes isospin breaking in the $\pi N$
system. The photopion reaction is affected through final state $\pi N$
interactions (formally implemented by unitarity and time reversal invariance).
It is also demonstrated that the near threshold $\gamma \vec{N} \to \pi N$
reaction is a practical reaction to measure isospin breaking in the $\pi N$
system, which was first predicted by Weinberg about 20 years ago but has never
been experimentally tested.
