Yes, it is possible to pursue a Ph.D. in theoretical physics without having a Master's degree. While it is more common for students to complete a Master's degree before entering a Ph.D. program, some universities may admit exceptional students directly into a Ph.D. program after completing their undergraduate studies.
Admission requirements and policies vary among institutions, so it's important to research and contact specific universities or departments to inquire about their policies regarding direct Ph.D. admission. In such cases, the selection process is typically highly competitive, and applicants are evaluated based on their academic performance, research experience, recommendation letters, and other relevant factors.
It's worth noting that while not having a Master's degree may not be a strict requirement, having a strong foundation in physics and relevant coursework is typically expected for admission into a theoretical physics Ph.D. program. Students without a Master's degree may need to demonstrate their proficiency in theoretical physics through their undergraduate coursework, research experience, or standardized exams (such as the Physics GRE).
Overall, while it is less common to pursue a Ph.D. in theoretical physics without a Master's degree, it is not impossible, and exceptional students with a strong background in physics may be considered for direct admission into Ph.D. programs.