Watch a visualization of the thought experiment that Albert Einstein used to conclude that gravity and acceleration are the same phenomenon, in this video from NOVA: Inside Einstein’s Mind. To work out a complex idea that would later feature his theory of general relativity, Einstein carried out an experiment in his mind. He envisioned a man in a box. Einstein realized that there was no way this man could tell whether he was sitting in a gravitational field or being accelerated. Because of this, these two situations were equal. By extension, Einstein concluded that gravity and acceleration are the same thing.