What is the General Theory of Relativity?
One of the best sources if you’re really interested in this stuff is Einstein himself, not his papers, but his book on relativity (1916 I think), written for a general audience. He’s a very clear, intelligible writer, and the math is down to the bare minimum (and what there is you can skip if necessary). Essentially, he sets forth the growth of the ideas, which anyone can follow. Personally, I find the man himself much more understandable than most of the people who have set out to simplify him.
Einstein’s Special Theory of Relativity came first. Special here meaning “special case”. The “special case” was that it dealt the motion of objects as seen by an observer moving at constant speed and constant direction, and without rotation. The equations are independent of observer … provided the observer was moving at a constant speed and constant direction. In a rotating or accelerating frame of reference, “fictitious forces”, e.g. the centrifugal force, have to be added to the equations. The Special Theory was unsatisfactory as it required observers to be able to tell whether they were rotating or accelerating … but rotating or accelerating with respect to what? The Special Theory had just dispensed with the idea of “absolute rest” against which you could measure this. Einstein showed using Special Relativity, that acceleration, including gravity, appears to bend space-time. Objects then follow paths that to them are always straight ahead, “geodesics”, but can result in curved
