How hard is it to switch fields in science? For example, say you do your Ph.D. in an area of theoretical physics and are very successful, but cannot find further funding at some point (can't land a postdoc in your chosen field, no professorship offers, denied tenure, etc.). How difficult would it be to change to a field where there is more funding available? Can you fairly easily switch to another area of physics? What about an area outside of physics? Bioengineering, systems biology, electrical engineering, chemistry? I hear conflicting stories on how difficult this is. Is there any hard data?