I worked for a particle physics experiment last summer. I have now just graduated and am supposed to continue working on that same project this Monday. I've determined that what I'm really interested in is medical physics. I just received an offer from a professor in medical physics to do a summer job with him, that would most likely lead to an MSc. with him. This is my only opportunity to do an MSc. in the field that I want. However, doing this means that I have to turn down my other supervisor on Monday, when he is under the impression that I'm working for him for the summer. I feel horrible about doing this, especially on the first day that I'm supposed to work, but I really feel that I just can't pass up this opportunity. I fear that my other supervisor will be very angry. Is there any way I can break the news to him lightly?