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?
Which country in the EU is that one supposed to be? In Portugal to enter a PhD program you either need to have a Masters or an exceptional Bachelors with high marks and letters of recommendation from senior researchers (only accepted in cases where applicants already have a publication record). A PhD usually takes around 5 years to complete. Exceptional people may do it in 4 years and quite a lot of students take 6 years or more to get one.
You do not get a PhD without a solid publication record. The requirements vary by institution and may include the number of publications and the impact factor of the publications.
If you are not motivated to spend a considerable amount of time studying that subject you should not bother getting the PhD. Also like others have said the career prospects and financial gains are not worth it either during the PhD or for the first 5 years after the PhD. Even then your position is not ensured. A leading academic researcher with that kind of income will be someone who is good at bringing research funding. Those kinds of people would manage just as well or better financially in the private sector.