I am a physics undergrad, and plan to pursue a PhD in mathematical physics (string theory?). I have heard from a lot of people, who have personally seen the research scenario at universities both in the US and Europe, that it is much easier to get a PhD from a European university, that it takes about 3-4 years in a good university in Europe, while more than 5 years in American universities. Another thing I have been told is that in Europe you get your PhD after 4 years atmost by default, even if you have not done any original research. Is it true?