Einstein’s General Relativity, the curvature of space means that falling into a gravitational field makes you gain energy, and climbing out of a gravitational field makes you lose energy. The full and general relationship, then, for any moving object, isn’t just E = mc2, but that E2 = m2c4 + p2c2. (Where p is momentum.) Only by generalizing things to include energy, momentum, and gravity can we truly describe the Universe.