I started this journey in my late fifties because I decided I wanted, finally, to come to grips with the classic depiction of how mass affects spacetime.
You know the one. A trampoline, for want of a better descriptor, with a heavy ball in the middle and a much smaller ball circling it, rolling round and round because of the curvature in the surface.
I asked, and I duly received. The small ball was not being pulled into a circle. It was travelling in a straight line. To the outside observer the path looked like a circle, but that was the observer's problem, not the ball's: the heavy mass had curved spacetime, and the ball was going as straight as spacetime allowed. The straightest possible line through curved spacetime is called a geodesic. The circle is what a geodesic looks like when you flatten away the dimension in which it is straight.
Then I did something that turned out to matter more than the physics. I decided to update the firmware my brain was running, so that I could perceive my everyday reality with this as the ground truth. The "pull of gravity" did not, in fact, exist. What I feel standing on the floor is the floor pushing me up, off the straight line I would otherwise be following. Weight is the sensation of being deflected. The only time I travel straight is when I fall, which is why falling feels like nothing.
That was one correction: a curve that turned out to be a line. Once the firmware would accept one it would accept others, each closer to home than the last. The body that turned out to be a worldline. The moving "now" that turned out to be a slice. The self that turned out to be a rendering, and the thing watching the rendering that turned out not to be a thing. The manual you are holding is the sequence of those corrections, in the order I met them.
What this is, is the result of an unfolding conversation, with an AI as the teacher and me as the curious student. I asked the questions in the order they occurred to me, and I have kept them in the book, set apart, because the question is the part that was mine. The answers built The Framework.