How do I choose my research area? Choosing your research area

Relevant and adding to that. 

I am planning to pursue a PhD (currently at masters) and have many research interests. Some of my interests are more attainable and my knowledge in the domain is good enough to make contributions and publish papers and get a PhD faster than usual. However I feel that those research areas have small impact as compared to other and makes me not want to dedicate a significant amount of time to something I know will probably impact a very small slice of the pie.

Other research interests I don't know as much about, or what I know is from a class settings. Some of those interests I didn't enjoy working on in the past as part of my courses but I am still considering a PhD on them. Mostly because they can have a much larger scientific impact.

My main motivation for doing a PhD is the scientific impact that my research can have and how I can help solve problems that can have some sort of application.

Research Interests Include:

AI, Reinforcement Learning, Machine Learning with specific applications in Software Engineering and/or Social Networks, Robotics, Motion

So here are my questions:


Is it acceptable to apply for a PhD program that I know not as much about but am willing to learn? 
How would it reflect on a PhD program application diverse research interest topics with more than a handful faculty? 
What should be my main motivation behind choosing a topic?
What questions should I be asking to narrow down my interests?
If you have a relatively open set of research interests, I would focus much more on finding an advisor who you believe you will get along well with on a personal level. "Success" (which is admittedly an ambiguous term) in a PhD often matters much less on what you are studying specifically and much more on how engaged and dedicated you are to the research process as a whole. It is very difficult to be engaged in research on a topic you love, but with an advisor you despise. 

I would not be too worried about getting trapped in a certain research field long term. You doctoral work is a relatively short portion of your research career as a whole. I have already changed my research focus twice in my career (MS was one topic, PhD was another, full-time employment is yet another). And I still have three plus decades of research to go (so my research career is still young). I did what work my advisor was interested in, put my own flavor into it, then changed my focus as necessary in  my employment.   

As for your research making an impact, understand that very, very, very few doctoral researchers make a ground breaking impact as a PhD student. Most PhD researchers go their whole career in fact and only impact "a very small slice of the pie." That is how research is modernly. You can make an impact in any of the research fields you listed. Don't sell yourself short of course, but also be aware that most people with PhDs do not in and of themselves change the world.