If we assume that spacetime doesn't exist, but is merely a space, then there isn't anything to distinguish spacetime from the rest of non-existence beyond it. Therefore, spacetime would have to be ontologically real. When you construct physics as a formal system, you get answers to these types of questions. I would read the abstract and section 1.1 here: Preprint THE FRAMEWORK OF EVERYTHING (FOE)

Given that all ponderable masses are made of the only carriers of charge and mass in the universe, the charged and massive elementary point-like behaving elementary particles of which all atoms are made, whose accumulations make up all large masses in existence, and that GR does not recognize the existence of charges that interact as a function of 1/d^2, it seems doubtful to me that GR may have any ontolotical meaning.

I would say that a pure general theory of relativity (as suggested by Einstein in 1950) should be ontologically sound, but that Einstein's own 1916 attempt isn't.

In the context of "pure GR", the principle of equivalence of inertia and gravitation ("PoE") requires all inertial masses to have associated gravitational masses, and therefore gravitational fields. The physical existence of every real mass is expressed by the existence of its curvature/distortion "footprint" in the shape of the spacetime metric.

So putre GR distinguishes between purely hypothetical masses (with no footprint) and real masses (which do). It is the interactions of the real masses that give us the real laws of physics. Any laws derived specifically for the "unreal" masses are not physics, because they only describe the behaviours of things that physically do not and cannot exist, and which are defined geometrically (though the lack of a footprint) as not existing.

Additionally, the relative motion of particles is "real", because velocity-dependent gravitomagnetic distortions cause the gravity-wells of real particles to have the bottom of their throats tilted to point in the same directions as their worldlines.

So a pair of real objects approaching each other have converging worldlines, and their gravity-well throats are similarly tilted to point somewhat towards each other. If they mutually recede, the throats point somewhat away from each other, as do the worldlines. The tilt of the gravity-wells fade away with the distances of the sources. The "distance-dependent tilt" of a massed particle's gravity-well is the moving particle's gravitomagnetic field.

The point of mentioning this second, velocity-dependent curvature effect is that it results in the state of motion of the body also being physically written into the metric as curvature. The metric "knows" the location, mass, and state of motion of every particle of matter in a region, it acts both as the computing memory that holds the data to be computed with, and the data-processing engine that convolves the data to predict what happens next.

So motion is also a physically real thing. If we take a snapshot of one moment in space, the frozen-in distortions have enough information to tell us what happens next, even though nothing in the snapshot is "moving". The fact that motion effects have a kind of holographic duality that allows them to be identified and calculated, even in a static spatial description that has the time axis/axes suppressed, solves Zero's paradox against motion.

So far so good.

----

Where Einstein's 1916 general theory screws up, ontologically, is in its adoption of "SR physics" as a foundation. Instead of calculating the correct laws of motion that should apply in the new context of GR, from scratch Einstein "cut a corner" and simply cut-and-pasted the SR equations into GR1916, arguing that SR was a limiting case of GR, and that SR physics was therefore a subset of GR physics.

Not physically, it isn't.

Special relativity assumes that the presence and relative motion of objects and observers has zero effect on the metric. That violates GR's "PoE".

So within the context of a general theory, SR describes only the hypothetical actions of purely-hypothetical objects that demonstrably have no real physical existence, because they don't leave footprints in the metric. The objects and observers of special relativity have different behaviours and different associated geometries and properties to the objects and observer-masses of general relativity.

Einstein logically identifies the objects and observers of SR with the objects and observers of GR, but these are two different sets of entities, that share the same names, but not the same properties. Similarly, "SR spacetime" is not "GR spacetime", they have different properties, and merely share the same name, "spacetime".

An "SR object" is not the same as a "physical GR object", an "SR observer" is not the same as a "physical GR observer-mass", and SR's absolute Minkowski spacetime (in which the acceleration of a mass is absolute, and never causes distortions) is a different thing to the interactive dynamic spacetime of a general theory (in which an accelerated mass always causes gravitomagnetic curvature).

In a general theory of relativity, there is no such thing as SR physics, because there are no such things "in reality" as SR objects, or SR observers, or SR spacetime. The two descriptions might appear at first sight to "touch" for the special case of empty space ... but the condition of flat empty space can only be achieved in a GR context when there is no matter present.

SR at best is a "null solution" of GR, that doesn't apply to real matter, or to real observers (or to real space). The rules that do apply to the real things are then necessarily different, because the geometrical behaviours of the real things are different.

In fact, it's actually worse than this: SR does not even "touch" GR as a limiting special case where the number of objects is zero. Relativity requires a minimum of two real masses (or two observer-masses) to compare observations, so GR's actual limiting case happens when the number of real masses present, n, is n=2 . Special relativity can only apply when the number of real masses with associated gravitational fields is n=0.

The values zero and two are not contiguous. There is a range between them, where SR has already stopped applying (because of the curvature associated with a single mass, or a single nearby (partial) mass), but GR's relativistic physics (n>=2) has not yet kicked in. So it's not the case that SR lies on the "logical boundary" of GR, at the limits of what's included in GR, on the edge of the boundary of the GR set in a Venn diagram ...

No, it lies definitely outside the territory of GR, with a clear gap separating the two.

And this is why the 1916 theory, philosophically speaking, is a dysfunctional piece of junk.

----

The reason most people didn't see the problem was that theoretical physicists nowadays are trained in "mathematical physics", and to see problems in terms of logical tokens and symbols that can then be manipulated using symbolic logic, and all the mathematical machinery of modern general relativity. They are trained to think linguistically, or symbolically, rather than geometrically.

So when Einstein used the same "tokens", and "words" to refer to different sets of entities under the two theories, as if they were the same entities, he broke the system. It's as if he mislabelled two sets of white plastic pipes that he encountered in different parts of a building site with the same names, not knowing that one was for water and the other set for gas. Or identifying one bare wire that you pull on to open a window with another wire that carries mains electricity. Yes, they are both nominally sets of "pipes" and "wires", but they must not be cross-connected, or the results will not be happy.

People with a math physical background felt that it was perfectly normal to identify these identically-named entities with each other, and use these supposedly common entities to stitch the two theories together. It was like merging two databases together by their common named fields, without realising that the fields with identical names referred to different and incompatible things, with incompatible properties.

Preprint Einstein's SR limit, and why it doesn't work

Some people say that it is a phenomenological theory. It simply models behavior through geometry .

It describes how things behave, not what they fundamentally are.

Both yes and no. Ontologically, general relativity provides a robust geometric framework to describe gravitation and curvature, but it doesn't, by itself, account for emergence. Much of what we attribute to general relativity today - expansion, quantum interaction, cosmological structure - has been grafted onto it out of necessity, not inherent sufficiency.

This suggests there's a missing piece - something deeper and more foundational that underlies emergence itself. Until that piece is observed or measured, it may remain dismissed or invisible to current theory. But it may not be an addition at all; it may be something modern physics has simply overlooked.

Panagiotis Michael Mougoyannis, Einstein's 1916 system certainly attempts to "model behavior through geometry", and to "describe how things behave".

But it fails. Because its two main components, the "special relativity" foundation, and the GPoR-based chunk of theory added on top, geometrically contradict each other.

Special relativity only has the correct equations of motion and Doppler equations if matter never drags light. But the Fizeau experiment shows that moving groups of particles do drag light, so the Earth's atmosphere and the Sun's must drag light, and every active star must drag light, along with every galaxy and dustcloud. Gravitational theory also says that moving horizons must totally drag light, and then we have other known effects like the slingshot effect, and the dragging of a rotating planet or star.

Light-dragging changes the momentum and energy of light-signals as a function of relative velocity, so any body that drags light would seem not to be correctly described by "undragged" special relativity. An additional effect unmodelled by SR that that changes a signal's energy as a function of velocity will alter the Doppler relationships away from the SR set.

And the relativity principle says that all moving masses must obey exactly the SAME Doppler relationships. So if a moving black hole must have a non-SR Doppler formula, then every other piece of matter in the universe must share the same non-SR Doppler formula.

The architecture of the SR-based 1916 system is honestly a piece of junk that Einstein never got to work, and could never fix. That's why he disavowed it in 1950.