  In this paper, for the first time a method is proposed to compute
electromagnetic effects in hadronic processes using lattice simulations. The
method can be applied, for example, to the leptonic and semileptonic decays of
light or heavy pseudoscalar mesons. For these quantities the presence of
infrared divergences in intermediate stages of the calculation makes the
procedure much more complicated than is the case for the hadronic spectrum, for
which calculations already exist. In order to compute the physical widths,
diagrams with virtual photons must be combined with those corresponding to the
emission of real photons. Only in this way do the infrared divergences cancel
as first understood by Bloch and Nordsieck in 1937. We present a detailed
analysis of the method for the leptonic decays of a pseudoscalar meson. The
implementation of our method, although challenging, is within reach of the
present lattice technology.
