When a plane accelerates, we feel the back of the seat pushing us to keep up with the plane, but parachutists in free-fall feel no force at all as gravity accelerates them to the earth, so:
“It’s not the fall that kills you; it’s the sudden stop at the end.” Douglas Adams
But if there is no force, isn’t that being at rest? Einstein’s insight was that free-fall acceleration equates to being at rest, so gravity isn’t a force at all but the earth curving space and time around it, which he called “the happiest thought of my life!”
Gravity is then indistinguishable from acceleration, so passengers in a rocket accelerating at 1g feel an effect like gravity on earth and can sit down and have a cup of tea just the same. But if gravity equates to acceleration, isn’t acceleration caused by particles?
Einstein instead proposed that the earth’s gravity warps the time and space around it, which replaced Newton’s inexplicable force-at-a-distance by an equally inexplicable distortion of space and time. The idea that objects act on a fixed stage then gave way to the idea that the stage could change! A particle moving in a straight-line curves under gravity because its space and time change, not because a force influences it.
This also explains Galileo’s finding that but for friction, all masses fall at the same speed. A heavy object has more inertia, so it should be harder to move, but if gravity is equally greater, the effects cancel. A ton of lead hits the ground at the same time as a feather because according to relativity, gravity varies with mass as inertia does. It was a brilliant solution, but it left the standard model with a force that none of its particles could explain.
If gravity causes changes in space and time, the standard model claim that gravitons cause gravity denies general relativity, just as its claim that particles move on fixed paths denies quantum theory. Why then does physics accept a model that contradicts its two best theories?