  We discuss the implications of causality on a primordial magnetic field. We
show that the residual field on large scales is much more suppressed than
usually assumed and that a helical component is even more reduced. Due to this
strong suppression, even maximal primordial fields generated at the electroweak
phase transition can just marginally seed the fields in clusters, but they
cannot leave any detectable imprint on the cosmic microwave background.
