If a person gets a phd studying controls theory, shouldn't they have the skills (as a researcher) to publish electrical engineering research one year, economics the next year, and biology the next? Since someone that studies "systems" generally works in high abstractions, I don't see why the subject at hand really matters. Sure, some systems will require far too much background knowledge to be practical, but I don't see why many topics would require a vast background. So, if a person studied heavy on controls theory in an Electrical Engineering graduate program, what would keep them from suddenly conducting economics research or something in a field completely unrelated to their thesis used to obtain the phd? Is it unrealistic for someone in academia to jump around between fields constantly?