free fall

Free fall is motion under gravity alone, with nothing else pushing or holding you. A skydiver before the parachute opens, an apple dropping from a branch, and astronauts orbiting the Earth are all in free fall. The surprising feature is what it feels like: weightless. Inside a falling elevator you would float, a ball released from your hand would hang beside you, and a glass of water tipped over would not pour — because everything falls together at the same rate.

This is where the equivalence principle bites. If gravity pulled different objects differently, you could feel it inside the falling box. But it does not: every object accelerates downward in step, so relative to the box gravity simply vanishes. The free-falling room is, locally, just like a room drifting in deep space far from any star. That is why orbiting astronauts feel no gravity even though Earth's pull on them is nearly as strong as it is on the ground — they and their spacecraft are falling together, endlessly missing the Earth as they circle it.

In general relativity this changes the meaning of 'natural' motion. In everyday physics we say an undisturbed object moves in a straight line at constant speed, and gravity is a force that bends it away from that path. Einstein turns this inside out: free fall IS the undisturbed, force-free state of motion. A freely falling object travels along the straightest possible path through curved spacetime, called a geodesic. What we feel as the force of gravity pressing us into a chair is really the floor pushing us OFF our natural free-fall path.