One problem is that we actually already have a direct energy to momentum generating "EM" drive. It is called a laser; light possesses momentum (in some sense,

One problem is that we actually already have a direct energy to momentum generating "EM" drive. It is called a laser; light possesses momentum (in some sense, it is nothing but momentum) and emitting light asymmetrically produces thrust. The "EMDrive" claims to be orders of magnitude more efficient than laser propulsion (as well as not incinerating anything in it's wake), which is why it was both interesting and probably impossible.