How to decide when it's worth investing the time to learn something new Background

I am 3 months into a 3 year theoretical physics PhD in the UK. Since I am fairly early on in my research career, there are many new ideas and techniques for me to learn, as well as the need for me to work directly on my research questions.

Either through reading papers or attending seminars, I often come across new ideas which seem interesting and conceivably applicable to my research. However, I find it difficult to decide whether to further study those ideas to more deeply understand them.

On the one hand, my impression is that it is important to deeply understand ideas to be able to apply them to one's own research. On the other, attaining good understanding usually requires a significant investment of time, and this is time taken away from working more directly on the details of my current research. In some sense, this could be seen as balancing short- vs long-term investments of time and mental energy.

My question is about how one should decide whether to invest the time learning new ideas now, or wait until they come up more directly in later research. Of course, the difficulty with research is that one is often not sure if or when a new idea will come in handy.

Question

What thought process do you go through to decide whether to spend time now in learning a new idea?
First of all, good luck on your PhD! 

The way I was explained it, the point of a PhD is to become super good at something. To do so, a shallow understanding of lots of things is oftentimes less useful than a deep understanding of one thing. This is just practicality: you need to graduate in X years with a PhD certificate. I would say 80/20 ratio in favor of deeper understanding of a topic vs reading something new, at least. 

I would attend seminars anyway to keep abreast of what’s going on, and to learn how to speak about one’s work.