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General relativity: “gravity” is the result of space and time curving in the presence of mass. One of the fundamental tenets is that gravity and acceleration are interchangeable and it follows that (locally) to a freefalling observer, space looks flat. That’s why you feel weightless when you’re moving under the influence of only gravity, even though Newtonian mechanics claims that you’re being tugged on constantly by a force.
People often say that Newtonian mechanics might be wrong, but it’s right enough to put a man on the moon. While that’s true, I dislike that it doesn’t really give you a sensible picture of what’s going on in terms of the nature of gravity. A person standing on Earth may think he’s not going anywhere, but when he tosses a ball it moves in the same way he’d expect if his reference frame was accelerating. Meanwhile, a freefalling observer sees a thrown ball obey Newtonian mechanics in the absence of gravity even though Newton would claim that gravity is making the freefalling observer accelerate. One of Einstein’s great insights was that a freefalling observer is the one who can truly claim to be unaccelerated and, consequently, in zero gravity.