Gravity and acceleration are equivalent

"Einstein first recognized that the force we feel upon a uniform acceleration and the force we feel when under the control of gravity are one and the same. In the jargon of physics, gravity and a constant acceleration are “equivalent.” There is no difference between being pulled down on the Earth by gravity or being pulled backward in an accelerating car. To arrive at this conclusion, Einstein imagined a windowless room far out in space, magically accelerating upward, moving faster and faster with each passing second. Anyone in that room would find their feet pressed against the floor. In fact, without windows to serve as a check, you couldn’t be sure you were in space. From the feel of your weight, you could as easily be standing quietly in a room on Earth. Both the magical, accelerating space elevator and the Earth, with its gravitational field keeping you in place, are equivalent systems. Einstein reasoned that the fact that the laws of physics predict exactly the same behavior for objects in the accelerating room and in Earth’s gravitational hold means that gravity and acceleration are, in some fashion, the same thing."