Transcript: Einstein's general theory of relativity is based on his assertion that there's no way to distinguish between acceleration due to gravity and acceleration due to any other force. Consider the following situation: you're in a sealed and closed elevator with no way to see outside. In one situation the elevator is stationary on the Earth's surface, and you feel your normal gravity which would cause an acceleration of 9.8 meters per second per second if you dropped an object in the elevator. In the other situation the elevator is free floating in space but is being accelerated at 9.8 meters per second per second. Standing in the elevator you would feel your normal weight, yet the situations are entirely different. In an analogous situation imagine an elevator falling in free fall on the Earth's surface. You would be weightless within the elevator. Gravity is operating the same on the elevator as on your body, but there's no way to distinguish this situation from the situation of being in the elevator in free space weightless. No gravity is operating in the second situation.