““Einstein recalled realizing that a person falling from the roof of a building would feel no effects of gravity as he fell. He called this "the most fortunate thought of my life," and he made it into a principle, which he called the principle of equivalence. It says that the effects of acceleration are indistinguishable from the effects of gravity. So Einstein succeeded in unifying all kinds of motion. Uniform motion is indistinguishable from rest. And acceleration is no different from being at rest but with a gravitational field turned on.””
