I'm about to finish my master's degree in physics (theoretical physics). I'm applying for several professors in Europe. I want to do my PhD in another field than my master's thesis, but both fields are theoretical and to some extend related. But, I didn't take some relevant courses on this topic in my undergraduate studies, instead I took courses which were related to my master's thesis. My question is the following:
How much details should I know about the research field of the potential professor?
I don't understand his papers quite well, although I'm passionate about the topic and have many ideas or maybe fantasy.