by starting from the mathematically demonstrated quantum fluid dynamic reformulation of Newton's law of universal gravitation [1], it is shown how the principle of equivalence can be likewise reformulated according to fluid dynamics. From this, special and general relativity are incorporated in fluid quantum gravity, which also explains GR's curved spacetime. Moreover, if superfluid quantum space (SQS) replaces quantum vacuum, light propagates through it. Indeed, many clues and equations indicate how photons could be superfluid phonons of space's quanta, without contradicting the Michelson-Morley experiment. In this way, the unification of gravity and Einstein's relativity with QFT would be completed, without resorting to gravitons or string theory. It seems only necessary to reformulate the equations of relativity according to the fluid equivalence principle, whose theoretical framework simplifies relativity, e.g. showing that there is only a primary relativistic effect, the increase of mass, while time dilation and length contraction are respectively second and third-order effects. A minor correction in the classical formula of weight would be likewise necessary. Indications are given for experimental verification.