Gravitational effects cannot propagate with infinite speed. This is obvious both from the lack of Lorentz invariance of infinite speed and from the causality violations that are associated with signal speeds in excess of the speed of light. Since the speed of light is the only Lorentz-invariant speed, we expect that gravitational effects propagate in the form of waves at the speed of light.

As a concrete example, consider an apple that hangs on a tree. At some time, the stem of the apple breaks and the apple falls to the ground, which means there is a sudden change in the terrestrial mass distribution. The gravitational field surrounding the Earth must then adapt itself to this new mass distribution. The change in the field will not occur simultaneously throughout the universe – at any given point of space the change will be delayed by a time equal to the time needed for a light signal to travel from the Earth to that point. Hence the disturbance in the gravitational field propagates outward at the speed of light. Such a propagating disturbance is a gravitational wave.