AP Feed I have always been fascinated by gravity, mainly because I never understood it. Richard Feynman, who gave us the heuristic diagrams of quantum interactions, famously observed that “Nobody understands quantum mechanics”; the same is true of gravity. Like everyone, I know it as a tugging force, dragging you back as you climb a hill, pulling you forward as you descend—a force needing to be fought, to struggle against going up or coming down. Seeing images of astronauts floating in their space capsules was a reconciling factor; at least we were grounded, a relief not to find oneself in a condition of permanent levitation. Yet it remains a mystery defying resolution and comprehension.