When a plane accelerates, the seat back pushes passengers to keep up with the plane but a parachutist in the initial free-fall accelerates without feeling any force at all. A parachutist who jumps from a plane feels no force in a free-fall even though gravity is accelerating them to the earth. As Douglas Adams said:
“It’s not the fall that kills you; it’s the sudden stop at the end.”
Einstein’s insight was to equate the force of acceleration to the force of gravity. He called realizing that accelerating while falling from a building is like being at rest “the happiest thought of my life“! He concluded that the force of gravity is equivalent to an acceleration, so people in a rocket accelerating at 1g will 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 gravity lets them do on earth. Gravity is indistinguishable from an acceleration except that no matter is being “pushed” to make it happen.
Instead of concluding that a particle caused the acceleration, Einstein deduced that the earth warped the time and space around it. He replaced Newton’s inexplicable force-at-a-distance gravity by general relativity gravity, that lets the earth distort space and time. For Newton, space was the fixed stage on which objects acted in a common time but for Einstein, matter changed the space and time that define movement. The earth distorts space and time so particles following straight paths now curve as if under the influence of a force. Einstein’s gravity works by redefining what it means to move in a straight line.
Centuries earlier, Galileo showed that, but for friction, all masses fall at the same speed since gravity and inertia both increase equally with mass. Einstein added that this is so because gravity is a constant 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.