I am doing a Masters in theoretical physics. It's been half a year since I've begun my Masters. Recently, I find myself intrigued by machine learning and would like to do a PhD in machine learning after I finish my Masters in theoretical physics. I am rather afraid to break it to my supervisor that I plan to move into machine learning after my Masters. I am also afraid how he might react if he finds out that I am taking courses in the Computer Science department. I plan to work with machine learning profs to get two recommendation letters, but I guess I must have a third letter from my physics supervisor. I am afraid that he will not write me a good letter for PhD admission into machine learning. What should I do?