Einstein’s general theory of relativity.  By showing that spatiomaterialism can explain the truth of Einstein’s special theory of relativity (STR), I have answered the first part of the Einsteinian reservation about using spatiomaterialism as the foundation for demonstrating ontologically necessary truths. In this section, I will answer the second part. Einstein’s general theory of relativity (GTR) also makes it appear that this is not a spatiomaterial world, and I will show how its truth can also be explained by spatiomaterialism. . 

The way Einstein’s general theory of relativity explains gravitation does not, at first, seem compatible with spatiomaterialism. The foundation of the general theory is spacetime, for gravitation is explained as a “curvature” in spacetime, and since substantivalism about spacetime is incompatible with substantivalism about space, it seems out of the question that what the general theory refers to as “curved spacetime” could turn out to be an aspect of space and matter as substances enduring through time. (For a very accessible account of Einstein's general theory of relativity, see Clifford M. Will's Was Einstein Right?)

It is, however, possible for spatiomaterialism to explain why Einstein’s general theory of relativity is true. The key is what spacetime turns out to be in the ontological explanation of the truth of the special theory of relativity, for that makes it possible to explain curved spacetime as well. Curved spacetime is also an aspect of space and matter, even though as substances that endure through time, space and matter exist only at the present moment.

Though I go on in the next section to suggest an ontological explanation of quantum mechanics and, in the following section, take up some basic issues in cosmology, this explanation of the Einstein’s general theory of relativity pays off the second mortgage that we took out in order to use spatiomaterialism as the foundation for our philosophical argument. (See Necessary Truths.) Quantum mechanics is not so crucial to this project, because there is continuing disagreement about its ontological implications and some of the possibilities are compatible with spatiomaterialism.

We have already seen how the existence of consciousness can be explained in a spatiomaterial world (though the unity of consciousness will not be explained until I take up the mammalian brain in the sixth stage of evolution), and I have yet to take up the nature of goodness and holiness. But one of those four mortgages will be repaid when we see that Einsteinian physics provides not reason for denying that this is a spatiomaterial world.

In fact, spatiomaterialism might welcome the challenge of explaining Einstein’s general theory of relativity, because that means it does not have to defend Newton’s theory of gravitation. Newton’s theory is prima facie less hospitable to spatiomaterialism than general relativity. If a force did act immediately at a distance, it would contradict the principle of local action, implying that spatiomaterialism is false.

Newton’s theory describes an attractive force by which every material object acts immediately on every other material object, including those at a distance. Newton introduced it, in effect, as the best efficient-cause explanation of Kepler’s laws of planetary orbits, and it was confirmed by the deduction of many surprising, quantitatively precise predictions of measurements, becoming the model for the empirical method in physics. Despite its predictive success, Newton’s law of gravitation had nothing to say about how such forces are exerted on objects at a distance, except that they act instantaneously at a distance.

Action at a distance was puzzling to classical physicists, since it did not fit well with their intuitive understanding of nature as composed of space and matter in time. Even Newton was uncomfortable with the notion, and he refused to make any hypotheses about how gravitation worked in his Principia.^[1] But action at a distance could not be rejected for being incompatible with spatiomaterialism, for that would require using space as an ontological cause, and Newtonian physics did not recognize the validity of ontological arguments. Still, when Einstein proposed an explanation of gravitation that implied that gravitational forces propagate at a finite velocity, even physicists were relieved at not having to believe in action at a distance. And it did remove what would otherwise be an insuperable objection to spatiomaterialism. 

Einstein’s general theory of relativity was, however, another highly mathematical hypothesis, which predicted many quantitatively precise measurements, and since it implies that gravitational acceleration is caused by a curvature of spacetime, a realist interpretation of Einstein’s theory seems to imply that spacetime is a substance. But if the real nature of what exists in addition to mass and energy is spacetime, that is, a four-dimensional entity in which time is one of the dimensions along with space, then existence is not in time and “real change” is not ontologically possible. Thus, general relativity solved one ontological problem, but only by introducing another. The challenge is, therefore, to explain how curved spacetime can be understood as an aspect of a world constituted by space and matter as substances that exist only at the present moment.

Curved spacetime. Having discovered STR by assuming the local equivalence of all inertial frames, Einstein sought to use the same approach in explaining acceleration due to gravity, that is, by including reference frames that were being accelerated by gravitation. Thus, the main assumption of his general theory of relativity is the equivalence of inertial frames to reference frames falling freely in gravitational fields.

What Einstein himself called the “principle of equivalence” assumes that nothing can be detected within any reference frame (that is, locally) that would distinguish a reference frame in inertial motion from one in free fall.

Or, to put Einstein’s equivalence principle the opposite way, a reference frame at rest in a gravitational field is indistinguishable from one being accelerated by a force; the push that we ordinarily call the “force” of gravity is actually the force of the earth accelerating us upward from what is equivalent to inertial motion.

This further equivalence can be only local, however, because free-falling frames are obviously different in how they are related to the rest of the world, or globally. Though inertial frames simply continue in motion indefinitely, free-falling reference frames eventually collide with the center of gravity, because gravitational fields are imposed by matter concentrated at certain locations. Thus, what makes the general theory of relativity general is that it includes both inertial and free-falling reference frames, and Einstein’s highly mathematical description of how they fit together as parts of a single world is a theory of acceleration due to gravity.

Einstein’s strategy in GTR paralleled that of his special theory. In STR, Einstein used his principle of relativity (implying the equivalence of all inertial frames) to derive a mathematical description of how they must be related globally (the Lorentz transformation equations). In his general theory, Einstein started with the assumption that reference frames in free fall are locally equivalent to inertial frames, and using the four-dimensional, spacetime mathematics from special relativity, he derived equations describing how all reference frames, inertial and free-falling frames, are related to one another. In both theories, the equivalence of reference frames means that the laws of physics hold the same way on each of them. That means that there is a mathematical transformation of explanations of events given on any one reference frame into explanations given on the other in which the laws of physics have the same form. In special relativity, only a Lorentz transformation was required, making them Lorentz covariant. But in general relativity, it is a more general transformation, which includes both inertial frames and free-fall frames, called “general covariance”. How objects change their motion depends on centers of mass in their neighborhoods, and using general covariance as a constraint, Einstein was able to deduce equations that describe what classical physics attributed to a force of gravity.

Einstein’s general theory of relativity describes a spacetime world in which the accumulation of matter (both mass and energy) causes a “curvature” in the surrounding spacetime. This curvature explains the acceleration that Newtonian physics attributed to a force of gravitation, because it determines, in turn, the inertial path for any matter located there. (Such an inertial path though curved spacetime is called a “geodesic”).

GTR also predicts various new phenomena, including the bending (and slowing down) of light rays passing through gravitational fields, the precession of the perihelion of Mercury, and a gravitational red shift. These predictions all differ from classical physics, and since GTR entails the possibility of black holes, including rotating black holes, it has become the foundation of cosmology. Except for the precession of Mercury’s perihelion, these phenomena were not even expected before Einstein’s argument, much less explained, and so the confirmation of these predictions justified accepting the general theory by the empirical method of physics.

Realism about the general theory of relativity, like realism about the special theory, makes it hard to avoid thinking of spacetime as a substance on a par with what it contains. The curvature of space­time is supposed to cause the acceleration of mater that is ordinarily attributed to gravity, and it would be hard to explain how a property of spacetime can have such an effect on what it contains, if spacetime did not exist independently of matter.

GTR is, like STR, a highly mathematical theory. Gravitation is described by the Einstein field equations, which relate the distribution of mass and non-gravitational energy to the curvature of spacetime. Currently, GTR is usually interpreted in terms of differential geometry. Spacetime is postulated as a four-dimensional continuous manifold of points (M), and there are two kinds of (tensor) equations defined everywhere on the manifold. The metric-field tensor (g) defines the metric (and geometric) relations among points in spacetime, and the stress-energy tensor (T) represents the distribution of matter (mass and energy) in spacetime (and its effects).

Jointly, M, g, and T are called a “model” of GTR, and even for a world with a particular distribution of mass and energy, there are infinitely many different, yet empirically equivalent models. They all predict the same gravitational phenomena, but each model involves a different coordinate system, for each is based on a different local inertial reference frames at its location in spacetime, that is, adapted to material objects with different free-fall trajectories.^[2] Their empirical equivalence is an assumption that Einstein used to derive his field equations, and it is one of the meanings sometimes given to “general relativity”. On this geometrical approach, GTR also seems to imply substantivalism about spacetime, because the four-dimensional manifold of points (M) must be postulated in order to define the metric-field tensor (g) and stress-energy tensor (T).^[3]

The challenge that GTR poses for spatiomaterialism is that it implies that what exists is spacetime, rather than space and matter existing as substances in time. In a spacetime ontology, time is another dimension of what exists on a par with the spatial dimensions (except for a change in sign and the velocity of light as a scaling factor). Its implications about time were used in Spatiomaterialism: Time to show that spatiomaterialism is a better ontological explanation of nature than spacetime ontology (or “spatiotemporalism”). Substantivalism about spacetime makes it impossible to explain “real change”, because if what exists is a four-dimensional entity, and time is part of its structure, then nothing can be coming into existence or going out of existence as time passes.

As we saw in Spatiomaterialism, there is no way for spacetime substantivalism to avoid refutation by the fact that our experience of change itself take place through time and we are parts of nature, except by postulating an additional, subjective substance, for whom spacetime and the events it contains have the appearance of real change. Not only does the addition of such a subjective substance make spacetime ontology more complex, but it also poses the problem of relating eternal and enduring substances as parts of the same world, a problem that Plato never solved. And even if it could be solved, this modification would be ad hoc, for it would explain nothing but the appearance that change takes place through time. There is, therefore, no question that spatiomaterialism is a better ontology, if it is possible.

In order to show that spatiomaterialism is possible is to show that it can explain why GTR appears to be true, and that means explaining all the relevant phenomena on the assumption that nothing exists but space and matter enduring through time. This is to describe a model or solution of the Einstein field equations that differs from the prevailing geometrical interpretation because, instead of postulating a four-dimensional manifold and defining geometrical objects on it, spatiomaterialism postulates space and matter as substances enduring through time. Nothing exists in a spatiomaterial world but what exists at present, and thus, the interaction of space and matter must somehow have an aspect that explains what Einsteinians are referring to when they talk about “curved spacetime” and that aspect must explain all the phenomena predicted by the general theory.

We can tell that not in principle impossible for a world of substances that exist only at the present moment to explain the truth of GTR, because even on the received geometrical interpretation, there is a standard of simultaneity implicit in each model’s assignment of space and time coordinates to every event in the universe. All the spacetime events with the same temporal coordinates that we now have in some model for our universe (a certain “simultaneity hypersurface” in curved spacetime) could be all that actually exists at the present moment, and their spatial coordinates could be referring to parts of a three dimensional Euclidean substance. Of course, this could be true of only one model, for although every model assigns some coordinates to us now, different models entail different standards of simultaneity, and if different models were ontologically equivalent, the substances constituting the world would have to include spacetime.

Moreover, in order to hold, in effect, that one of all possible models represents absolute space and time, spatiomaterialism would have to show that there is a law of gravitation that explains not only the approximate truth of the Newtonian theory in it, but also all the new phenomena predicted by GTR. We can also tell that such a law is not in principle impossible, because GTR itself implies that the relevant events in that model are all related in a regular way. Still, the regularity would have to be described without referring to space­time or spacetime curvature, that is, explained as constituted by (Euclidean) space and matter enduring through time. And there would be problem about the regularity, only if its description turned out to be very complex.

Finally, since spatiomaterialism would take reality to be equivalent to what exists in a single model of GTR according to the received geometrical approach, we should also expect the spatiomaterialist law of gravitation to explain why different models are observationally equivalent, that is, to explain “general relativity”, in the sense that enabled Einstein to derive his mathematical representation of gravitation.

This is a tall order, but it is possible, as I will show here by giving an ontological explanation of why Einstein’s GTR is true. It is an intuitively intelligible explanation, rather than a mathematical explanation, because what is required to explain the truth of any theory ontologically is showing that there are aspects of the substances postulated by the ontology that correspond to the theory. That requires a qualitative argument, which identifies the kinds of regularities and how they are related according to the theory, and then shows that they can all correspond to aspects of the same world. To be sure, the aspects of the substances pointed out must be quantitatively adequate as well. But that is rather trivial, once the qualitative argument has shown what the parameters are, how they are related to one another, and the signs and order of magnitude of their quantities, because substances can be postulated as having whatever quantitative aspects are required to make the measurements come out correctly. Thus, I will leave it as a challenge to those who would disprove spatiomaterialism to show that the aspects identified here cannot all be quantitatively accurate.