I recently graduated from my Master's program in theoretical physics. Throughout the program I did most of my coursework in high energy physics and did my thesis in the same. However, I also did courses on condensed matter physics. Right now I am considering to apply for PhD positions. But I am faced with intense fear of missing out as in, I have been interested in many areas of theoretical physics along with interdisciplinary areas such as biological physics, complex systems and modelling (did few projects while I was doing my Master's along with few colleagues/friends) and application of physics to areas like economics. My problem is that if I were to choose the hardcore theoretical physics side (which I will enjoy, no doubt) then I would be just a spectator on the research that goes on in the interdisciplinary side (I would like to be an active contributor). And vice-versa if I were to specialize my PhD in interdisciplinary fields such as complex systems, physics of biological systems etc. Has anyone gone through such a phase? If so how would you take care of this irrational fear, if I can say that?