like many others I'm here to ask for advice about switching groups. I'm a third year PhD in physics. I have been working in my current lab for about a year. Prior to that, I worked on another project with a different professor for about 2 years. I want to leave my current group because I realize this area is not my passion. Good thing is there's a professor from math department who wants to work with me on a project, and I absolutely want to work with him. But I'm not sure how to tell my current advisor that I want leave.
I must say that my current advisor is an excellent advisor in all aspects. He is supportive (both academically and financially); he is absolutely smart and has deep understanding of the field. He cares about his students' overall progress and growth. In addition, the group dynamics is great. I have zero things to complain about. The only thing making me want to leave is that I realize I have absolutely no interest in this area.
His financial support (RA) to me has one more month before it expires, and he is willing to extend it for the summer. But I do not want to stay for another two months, since it does not fit my academic development and I don't think it's ethical to take his money and not willing to work productively. I don't know how to tell him my decision. I definitely don't want to be ungrateful or disrespectful, because I very much appreciate his support in the past year.
Any suggestion would be greatly appreciated. Thank you very much for the help.