As Einstein was seeking a relativistic theory of gravity, he thought that special relativity should be upgraded to general relativity thus promoting the Minkowski space to curved pseudo-Riemannian (Lorentzian) one. Does this mean that special relativity as a theory never discussed gravity from any perspective?
[Physics] Special Relativity and Gravity
general-relativitygravityrelativityspecial-relativity
Related Solutions
Yes, special relativity is a special case of general relativity. General relativity reduces to special relativity, in the special case of a flat spacetime. I.e., general relativity reduces to special relativity, in the special case of gravity being negligible, for example in space far from any objects, or when considering a small enough piece of space in freefall that gravity is unimportant to the problem.
Like special relativity, general relativity also assumes that the speed of light is universal. However, when spacetime is curved, the universality of the speed of light can only be applied locally, within regions of spacetime that are small enough that the effects of gravity aren't important within the region.
This issue is discussed somewhat in Steven Weinstein's essay "Naïve Quantum Gravity", published in Physics Meets Philosophy at the Planck Scale: Contemporary Theories in Quantum Gravity (eds. Callender & Huggett, 2001). He begins by noting that
[a]n alternative way to conceive of gravity would of course be to follow the lead of other theories, and regard the gravitational field as simply a distribution of properties (the field strengths) in flat spacetime.* What ultimately makes this unattractive is that the distinctive properties of this spacetime would be completely unobservable, because all matter and fields gravitate. In particular, light rays would not lie on the "light cone" in a flat spacetime, once one incorporated the influence of gravity. It was ultimately the unobservability of the initial structure of Minkowski spacetime that led Einstein to eliminate it from his theory of gravitation and embrace the geometric approach.
So it's perhaps more elegant to view GR as a theory of curved spacetime; but could we get away with thinking about it as a weird non-linear field theory on Minkowski spacetime anyhow? It's certainly possible in some circumstances; famously, Steven Weinberg's 1972 book on general relativity tries to eschew geometric thinking as much as possible, viewing GR as a field theory that has the Equivalence Principle as a fundamental principle and showing how object such as the metric and curvature tensors can be thought of as arising from this principle. It seems to me that this is not far off from the OP's idea in the second paragraph that "any set of assertions compatible with the prediction of GR as we understand them today (again, when expressed in a GR-free and purely physical language including background independence) necessarily leads to GR and the identification of gravity with spacetime", though Weinberg (in 1972) might have disputed the last part.
However, Weinstein notes that there are still a few problems with this approach:
First, the "invisibility" of the flat spacetime means that there is no privileged way to decompose a given curved spacetime into a flat background and a curved perturbation about that background. Though this non-uniqueness is not particularly problematic for the classical theory, it is quite problematic for the quantum theory, because different ways of decomposing the geometry (and thus retrieving a flat background geometry) yield different quantum theories. Second, not all topologies admit a flat metric, and therefore space times formulated on such apologies do not admit a decomposition into flat metric and curved perturbation. Third, it is not clear a priori that, in seeking to make a decomposition in the background and perturbations about the background, that the background should be flat. For example, why not use a background of constant curvature?
* Weinstein includes a footnote here which refers the reader to an "interesting philosophical analysis of this line of thinking" in Reichenbach's The Philosophy of Space and Time (Eng. translation), (1958 [1927]).
Best Answer
It all hinges on the luminiferous aether which was prevalent in the 19th century theories:
The Michelson Morley experiment was crucial in discovering that there does not exist a luminiferous aether.
To start with the Lorenz transformations were discovered/invented to make consistent Maxwells equations with the existence of a luminiferous ether, i.e. an inertial framework against which everything else would be moving with classical mechanics equations of motion.
Here is the history of Lorenz transformations, the lynch pin of special relativity.
The "out of the box" thinking of Einstein comes when he applied the Lorenz transformations to particles, not light. It took some time to confirm it , and the real validation comes from nuclear physics and the huge number of particle physics experiments which can only be interpreted by assuming a four dimensional space time.
As you see from the above precis gravity does not enter into the special relativity validation.,