Enlarge / From left to right, a time lapse of a Bose-Einstein condensate forming. (credit: NASA/JPL) At the heart of Einstein’s theory of gravity (general relativity) is the equivalence principle. The equivalence principle says that there is no difference between being stationary and subject to gravity tugging you and accelerating in a vehicle that's free of gravitational pull. In practice, this means that there is no difference between inertial mass (the mass a rocket works on) and gravitational mass (the mass the Earth tugs on).