Choosing an unfamiliar PhD topic? Is it okay to apply to a PhD without knowing precisely what you want to achieve? I heard advisors have projects, so would it be okay to state that you are interested in one of those projects without necessarily being an expert in it? 

If so, how much knowledge do you need anyway? and in the statement of purpose letter, how do you state your interest in a specific project efficaciously giving that you are not fully aware of it?

Now you may ask, why would someone be interested in working on a project if he is not fully aware of it, right? But the thing is, some research topics are claimed to be HOT, at least that's the word on the web, some claim that certain topics have great potential in the foreseeable future. So, who does not want to work on something desirable with a future!
What you describe is often the common case for a PhD. But, in a letter and also in real you should have some related interdisciplinary or methodological background. Starting as a complete novice a PhD is very risky. An expert you have to become during your PhD, no one is one at the beginning, this would be a pointless odyssey. But a background is necessary, otherwise everybody could start a PhD without educational requirements like a bachelor in natural sciences/engineering  when looking for a STEM PhD.