I assume by the not knowing anyone, it's a confidence thing? I don't think you should be too worried about socializing with people on your course... it's physics at Cambridge, I doubt there'll be too many jocks around. 
In fact, I'd be willing to bet that there are a fair few people on your course who are feeling the exact same way, about not knowing anybody.
I should know - I'm doing Computer Science, I know the kinds of people you get on those courses, lots of shy people, hate leaving their rooms. Computer Science students actually started networking through Facebook - set up a group, people posting questions and answers, organizing socials, posting links that they think others might be interested in (like techy videos)... it basically provided a means for the shy people to communicate, make friends, form groups, and now some study together, etc, etc... find out if there is a similar group for your course, if not... make one.
