How familiar should I be with research topics of a potential PhD supervisor? 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.
You should at a minimum make an attempt to understand some of the papers written by a professor you want to work with, but the professor shouldn't expect you to understand every aspect of their work. It's also fine to go into your meetings with some questions about a paper, but do make sure you've made some effort to understand by reading other related papers, etc.