The theory of Observational Relativity (OR), as a new theory in physics, has generalized and unified Newtonian mechanics and Einstein theory of relativity in the same theoretical system under the same axiom system, becoming what Hawking called Complete Theory. OR serial report 1 has clarified the logical consistency and theoretical validity of OR, as well as, the empirical basis and experimental evidence of OR. The theory of OR has had significant discoveries: as reported in OR Serial Report 1, the speed of light is not really invariant, spacetime is not really curved; as reported in OR Serial Report 2, the rest mass of photons is not really zero. Now, OR Serial Report 3 reports to readers: Einstein mistakenly predicted gravitational waves; LIGO mistakenly detected gravitational waves. In 1916, Einstein derived a wave equation from his gravitational-field equation of general relativity, in which the wave function has two characteristic physical quantities: the Newtonian gravitational potential ; the speed of light c. Accordingly, Einstein decided that the wave in his wave equation must represent gravitational waves, and the speed of gravitational waves must be equivalent to the speed of light. From the GOR gravitational-field equation under the general observation agent OA(u) (u>0), the Gravitational theory of OR (GOR) has also derived a wave equation with two characteristic physical quantities: the Newtonian gravitational potential X; the information-wave speed u of OA(u), which suggests that the wave in the GOR wave equation is the information wave of OA(u), and the Newtonian gravitational potential X is the information about gravitational spacetime loaded or carried by the information wave of OA(u). The theory of OR is a theory of the general observational agent OA(u), in which Einstein’s theory of relativity is only a special case, a theory of optical observation with the optical observation agent OA(c) (u=c). It is thus clear that the wave in Einstein’s wave equation does not represent gravitational waves, but the information wave of the optical agent OA(c), that is, light or an electromagnetic wave transmitting observed information at the speed of light. So, what did LIGO detect?