Popular Science References in Statement of Purpose for Ph.D I am currently in the process of writing my SoP for applications to PhD in Physics programs starting Fall 2018 (in the US schools). I am definitely writing about the content that I have studied rigorously as well as about the research projects that I have been involved in and so on. But I also want to involve something about a paragraph (not a specifically small one) about some of the colloquial talks and articles that I have read during my undergraduate years that have played a significant role in shaping my interests inside theoretical physics. I have been quite a YouTube buff and have watched a lot of semi-popular level talks and have been quite motivated by them in my interests. These talks have been very useful in making me aware of the current frontiers of research as well as in narrowing down my interests for good. Of course, these are not the only factors but these are among the non-negligible factors. So, I wanted to ask whether mentioning this part would be considered good or it would give my statement a non-serious and a childish tone. Or worse, might it look like I am confusing real physics with what is shown in the popular level talks? 

Along with anyone interested or knowledgeable about this kind of a situation, those with a first-hand experience of looking at someone’s statements with such references or those with a first-hand experience of writing such a kind of statement are particularly requested to comment/respond.
From my experience, telling the tale why you became so fascinated with one particular topic is to be kept to a minimum. It does not add any weight or edge to your SoP, other people will have their more or less interesting story. You can use maybe one or two sentences and I don't see any need to have a "not a specifically small" paragraph on why you became fascinated with a topic. It might sound harsh, but they will not care. You have to use the limited space in your SoP for other things. But that's another question/answer.

In short, I would avoid citing pop science to a very bare minimum, especially in a field as rigorous as physics. Unless you use this reference to show something you have achieved. For example, I read pop science X which inspired me to run experiment Y which has produced Z.