General relativity extends Newton's theory of gravitation, by taking into account a local causal structure described by coordinate-invariant light cones. This proposal predicts some novel features around stars. Ultimately, it predicts black holes as fundamental objects and gravitational radiation.

It was Einstein's great insight to consider Lorentz invariance of Maxwell's equations as a property of spacetime. All physical laws hereby are subject to one and the same causal structure. To incorporate gravitation, he posed a local equivalence between gravitation and acceleration. This introduces the concept of freely falling observers in the limit of zero acceleration and described by geodesic motion.

The accelerated motion of the proverbial Newton's apple freely falling in the gravitation field is fundamental to gravitation. The weight of the apple when hanging on the tree or in Newton's hand is exactly equal to the body force when accelerated by hand at the same acceleration as that imparted by the gravitational field in free-fall. The mass of the apple as measured by its “weight” is unique whether gravitational or inertial.

Rapidly moving objects show kinematic effects in accord with special relativity. These effects may be attributed to the associated kinetic energies. In the Newtonian limit, the gravitational field may be described in terms of a potential energy. Kinetic energy and potential energy are interchangeable subject to conservation of total energy.