Se non ci fossi io a segnalarvelo...
Einstein had an epiphany when he was still a patent examiner. What if someone fell from a roof? During the time of the fall, the person would feel no gravity, and so in that frame of reference, gravity would not exist. He described this as the happiest thought of his life. Einstein began to think of gravity as not really a force but rather the absence of a force. When we are sitting in a chair, it's not that gravity pushes us downward; it's that the Earth by way of the chair pushes us upward. A frame of reference that is falling freely, like a satellite in orbit, is the logical analogue of a frame of reference that is just moving along at a constant velocity in space.
Gravity is, therefore, a fictitious force. It is like the centrifugal force that we feel outward on a carousel. The real force is the centripetal force of the floor pushing our feet inward. The idea of the centrifugal force comes from ignoring the fact that the carousel is rotating, but it's really just inertia.
