A space shuttle orbiting the Earth is in a free-fall motion, bound by the Earth's gravitational force. However, an astronaut inside the space shuttle feels weightlessness, as if no force is acting upon him. Suppose the space shuttle accelerates upwards at 9.8 m/s2; the floor of the space shuttle room will move up towards the floating astronaut. However, to the astronaut, it appears as if he is accelerating downwards towards the space shuttle floor at 9.8 m/s2. For him, the acceleration creates a pull in the space shuttle, similar to gravity on the Earth's surface. Sitting inside the closed space shuttle room, the astronaut cannot distinguish whether he is on the Earth or accelerating in space. Einstein, therefore, stated that there is no difference between a uniform gravitational field and a uniform acceleration in the absence of gravity. This is known as the Principle of Equivalence. He concluded that gravity is not a force between two masses but emerges from the interaction of matter and space.