  In the representation theory of split reductive algebraic groups, it is well
known that every Weyl module with minuscule highest weight is irreducible over
every field. Also, the adjoint representation of $E_8$ is also irreducible over
every field. In this paper, we prove a converse to these statements, as
conjectured by Gross: if a Weyl module is irreducible over every field, it must
be either one of these, or trivially constructed from one of these.
