How hard is it to change a (sub-)field right after starting a CS PhD? I am a senior undergrad in computer systems field (specifically, computer networks) and applied for CS PhD this year.
My question is:
If I find a new interest in a different field before starting a PhD, is it possible to change from the field that I applied for to the different field as I start my PhD?
I can see many problems that would prevent me from changing my field. For example, I would have almost no depth in the field yet compared to other students who came for that field, but I would have to somehow find a way to find an advisor in the field who can advise me.
And I just wonder if you think any problem you see would make it almost impossible to change, so I would just have to get research experience again in the field and then reapply for CS PhD for the field
To give more about my context, I am now a 4th year undergrad and enjoying research projects I have participated in, but I really don't think those (computer networks) are what I can do or enjoy for at least next 5 years. I've been always thinking of exploring as many fields as possible and changing my field (which can be a huge change like from system to theory) after going to PhD. I think I already knew such a huge change would be almost impossible, but maybe I just pushed myself to continue my research because I felt like it was quite irresponsible to quit in the middle of the projects that professors worked with me on (I just... don't know...).
I feel very lost now as I realized I applied for PhD without knowing anything about it. If needed, I am even willing to explore new fields for several months, find a research assistant position in a new field, and prepare for grad school application again.
Thanks for reading this long post.
If this is for the US, then there is plenty of time after acceptance to a doctoral program to choose a specialty, assuming you start with just a BS/BA degree. The early couple of years are course rich to get you prepared for qualifying exams. This gives you a chance to see advanced material in a number of specialties and also get acquainted with several faculty members.
If you take advantage of that, and establish a few relationships, then, by the time comps/quals roll around you will be in a good position to choose some narrow specialty and work with an advisor to find a suitable problem.
Other places the system and expectations might be quite different, but if you are now an undergraduate it is probably too early to choose. I studied mathematics and started grad school with an interest in both Analysis and Topology. I might have jumped either way, but wound up in Analysis. But both of those are still pretty broad. Both have several subfields.
An advantage of choosing (rather) late is that you get a chance to evaluate the faculty for their likely support for your studies. I started with a poor advisor, but finished with a really great one.