Can people in western countries do graduation and post graduation in two completely different fields? It might be possible here in India too, I haven't researched on it, but in one movie I have seen the guy is studying mathematics in MIT but his future plan was to study medicine at Harvard. I kept wondering how could he do that.  Here at India at 11th class we have to usually choose either Mathematics or Biology and then our eligibility changes accordingly.
How unusual is getting a Masters degree in a slightly different field than Bachelors? Would it be out of the ordinary to get a Bachelors in Mechanical Engineering and then go for a Masters in something like Civil or Structural Engineering?  I don't know a lot about how transitioning to a Masters program works.. Do people usually continue on in the same field, or is branching out common?
Introduction to string theory I am in the last year of MSc. and would like to read string theory. I have the Zwiebach Book, but along with it what other advanced book can be followed, which can be a complimentary to Zwiebach. I would like a more mathematically rigorous book or lecture notes along with Zwiebach.   Specifically, mention whether the book discusses string theory   Rigorously? Intuitively?   What's the scope of the book? Does it cover the advanced materials, e.g. Matrix string theory, F-theory, string field theory, etc. Maybe even String Phenomenology?