When a plane accelerates, the back of the seat pushes passengers to keep up with the plane but parachutists in free-fall accelerate without feeling a force at all. A parachutist jumping from a plane in a free-fall feels no force as gravity accelerates them to the earth so as Douglas Adams said:

“It’s not the fall that kills you; it’s the sudden stop at the end.”

But how can matter go faster and faster when nothing is pushing it? Einstein’s insight was to link the force of gravity to a free-fall acceleration that equates to being at rest. His conclusion that gravity isn’t a force at all but due to the earth curving space and time around it was he said “the happiest thought of my life!” He deduced that the force of gravity is equivalent to an acceleration, so people in a rocket accelerating at 1g feel a force pulling them down exactly like gravity on earth. Inside the rocket, they can sit down and have a cup of tea just as they do on earth thanks to gravity. Gravity is indistinguishable from an acceleration except that no particles “push” matter to make it happen.

Instead of particles causing the acceleration of gravity, Einstein deduced that the earth warps time and space around it. He replaced Newton’s inexplicable force-at-a-distance gravity by a gravity that inexplicably distorts space and time. For Newton, space was the stage on which objects acted in common time but for Einstein, matter changed the space and time that define movement. If the earth distorts space and time, particles following straight paths curve as if under the influence of a force. Einstein’s gravity redefined what it means to move in a straight line.

Centuries earlier, Galileo discovered that but for friction, all masses fall at the same speed because gravity and inertia both increase equally with mass. A heavier object has more gravity pull but it also has more inertia, so the effects cancel. Einstein added that it is so because gravity is an acceleration that varies with mass just as inertia does. It was a brilliant solution but it left the standard model with a force that none of its particles could explain.