Is it normal that what I studied in my bachelor's and master's is rarely useful to my PhD? Short version: Do you expect your knowledge in previous degrees to play a big part in your PhD?



Just to give you a bit of my background, I studied maths in my previous degrees and I am also doing maths in my PhD (trying to solve some kind of discrete optimisation problem). All in the UK, by the way.

Now some of you probably know that it's usually hard (NP-hard, if you like) to solve discrete optimisation problems. I got to learn some of them in my previous degrees and found them really fun and enjoyable, probably because I could understand how to solve them and why such and such methods work. (Well I guess it's obvious that they would only teach me "easy" problems at that stage.)

I know that I am expected to learn new things during a PhD, but since I started my PhD, I have been feeling that I rarely get to use what I studied before to tackle my research. It's like I started from zero again. Anything I learned before doesn't seem to be useful and I had to learn new techniques from scratch. All this made me wonder that I probably chose a wrong topic because I have almost zero knowledge to do this topic. 

Have you ever had a similar feeling? Is it just another normal PhD life? I'm just no longer sure if I made the right decision to do this particular topic (or even to do a PhD).

Edit: I tried to think about this again and realised that I shouldn't have asked "how normal is it?", but rather "has anyone else experienced this and how did you deal with it?"
It's quite normal for your bachelor's and master's studies to have little, if any, influence on your PhD research, particularly in the sciences and engineering. I think this is especially true in engineering, as the contents of engineering curricula tend to be more "conservative" than the research ongoing in most departments. 

Moreover, though, one of the skills you need to learn to survive as a research professional is the ability to learn a brand new topic to which you have not previously been exposed. This is because it's quite likely that you'll move into another research area multiple times during your professional career, and the only way to figure out what makes for good research in the field is to become an expert in it. So studying something brand new in your PhD is not only not a problem, it represents a great opportunity for your future career.