The PhD is awarded to the candidate who can come up with some original and innovative understanding in the knowledge domain of that degree, like physics, English lit, economics, etc.
To come up with original and innovative understanding in any field requires that the candidate has a thorough and comprehensive skill set in that field. That takes hours of study and experimentation or original work. And that study begins in kindergarten and builds up over the next twenty or so years of formal training. When you graduate from HS, you'll have finished just 12 of those 20 years.
So to reach the PhD, at a minimum you have eight more years of formal education. During that period you'll get the MS in physics or related. That typically takes two to three years. Then, if your grades are good enough, you get into a PhD program which includes about two years of classwork followed by a weekend of comprehensive exams, written or oral depending on the school.
If you pass your comprehensives, you'll be cleared to demonstrate and defend your original and innovative contribution to the knowledge base of your field by researching, experimenting, and writing a peer reviewed dissertation. You will typically have a professor in your chosen field as your sponsor and lead for your dissertation committee of three. The committee guides and reviews your work.
The dissertation can take from two to six more years or so to finish. Mine took six years because I was still working full time; so I had to work on the dissertation only part time.
Half the class was eliminated by the first PhD seminar, which wasn't even in the domain of knowledge. It was a "weeder" course about how to write a dissertation. It was called a weeder class because it was designed to weed out students who had little chance in making it to the PhD.
It was given first because the school found that most of the students who took all the technical courses and got to the dissertation stage were unable to properly write a dissertation. So they failed to get their PhD after spending large sums of money and time with the course seminars. So the school opined that if the students were going to fail the dissertation, it was more fair to the students that they do it in the beginning of the course rather than later after spending big bucks.
As to how hard...my PhD class started with thirty students; each had their MS degree. Out of that thirty, eight of us actually earned the PhD. How hard is it to get the PhD...damn hard.