What is the Composition of Spacetime?

[ZirkMan]

Does it have a sense to ask what is Spacetime made of?

Because we know:

Spacetime can be bent, which results in gravity and gravitational lensing effects etc. If it was made of "nothing" it could'be bent, right?

Spacetime "flows" into a black hole and at the event horizon the speed of its flow is faster than c. Again, to speak about flow of "nothing" wouldn't make sense.

So Spacetime is clearly "something" it has an objective and observable existence and it's probably meaningful to ask what it is made of.

Do we have any clue or this question doesn't make sense?

 

[HallsofIvy]

Does it have a sense to ask what is Spacetime made of?

Because we know:

Spacetime can be bent, which results in gravity and gravitational lensing effects etc. If it was made of "nothing" it could'be bent, right?

No, not right.

Spacetime "flows" into a black hole and at the event horizon the speed of its flow is faster than c. Again, to speak about flow of "nothing" wouldn't make sense.

Why not?

So Spacetime is clearly "something" it has an objective and observable existence and it's probably meaningful to ask what it is made of.

Do we have any clue or this question doesn't make sense?

You are making assumptions about what is "possible". Perhaps the problem is that you do not understand what "bent" and "flow" mean here.

 

[Vanadium 50]

The question doesn't make sense. You are ascribing properties to spacetime that it doesn't have ("Spacetime "flows" into a black hole"), and also arguing based on popularizations "Spacetime can be bent, which results in gravity and gravitational lensing effects etc. If it was made of "nothing" it could'be bent, right?")

 

[ZirkMan]

Great, then I would love to understand the correct meaning of "bent" and "flow".

1. Bent - when I look at picture of any gravitational lens like http://en.wikipedia.org/wiki/File:Gravitational_lens-full.jpg I can see, well a lens of deformed space. If there was not a physical bent of trajectories of light how else could you explain the phenomena?

2. Flow - you hit the nail. I have just watched a BBC Horizon epizode on Black holes and Prof. Max Tegmark explains the event horizon exactly as I have described - standing by a waterfall he says that even when you swim as fast as you can the water (meaning the Spacetime) will flow faster than you can swin. So this is not true explanation?

 

[espen180]

Are "bent" and "curved" synonymous here?

1. The trajectories of objects (geodesics in space-time) are the straightest possible lines in space-time. If space-time is flat, these are straight lines in space. Otherwise, they may be curved in space. Objects are simply flowing along the "gradient" of space-time, and their path is given by the geodesic equation.

2. If you watched it on BBC it was probably dumbed down for the general audience, which generally does not have sufficient command of tensor analysis to understand the technical explanation. The reason is simply that going in a straight line in space-time implies a spatial acceleration towards masses, as explained by Einstain's equation.

 

[Dale]

Spacetime possesses geometrical properties, distances, curvature, etc. (In fact, in a purely relational theory spacetime would be the geometric relationship between different material objects) It does not seem to possesses material properties, composition, velocity, etc. I would think that the question "what is spacetime made of" is asking for material properties.

 

[ZirkMan]

Spacetime possesses geometrical properties, distances, curvature, etc. (In fact, in a purely relational theory spacetime would be the geometric relationship between different material objects) It does not seem to possesses material properties, composition, velocity, etc. I would think that the question "what is spacetime made of" is asking for material properties.

The paradox I'm struggling with is how there can be something with geometrical properties, yet without any material properties of some sort (these do not have to be material in the classical sense)?

I also heard that empty space has some virtual particles in it and that its total energy even when its "empty" doesn't have to be zero (one possible explanation of Dark energy). Could this virtual field of particles/energy be a base of the Spacetime and give rise to its geometrical properties?

 

[Dale]

The paradox I'm struggling with is how there can be something with geometrical properties, yet without any material properties of some sort (these do not have to be material in the classical sense)?

That seems like an odd connection to make. Do you expect that something with material properties must have personal properties like preferences or feelings? Or, perhaps a more related question would be, do you expect that even fundamental (material) particles must have a (geometrical) size?

 

[Temporal]

The part about the waterfall must be a misunderstanding of what the BBC show was trying to explain. Perhaps they were referring to in-falling matter or maybe they were just using an analogy so as not to confuse the viewer with concepts such as severely distorted spacetime. In any case, spacetime is distorted (curved) around the black hole, but doesn't "flow" into the gravity well.

I think, however, that the second part of ZirkMan's question is perfectly valid. I don't think there's much question as to the distortion of spacetime by matter. I wouldn't refer to this concept as a "popularization" since it's a well accepted theory among the scientific community. Since matter appears to distort spacetime, this would imply that there is some interaction between matter (energy) and what we refer to as spacetime. If interaction exists, then there must be something there for the matter to interact with.

I haven't delved into this to any great extent, but I do remember reading about some concept in quantum mechanics that was called "Quantum Foam" where the foam represented subatomic spacetime turbulence.

 

[Dale]

What is a circle or a triangle made of?

 

[George Jones]

2. Flow - you hit the nail. I have just watched a BBC Horizon epizode on Black holes and Prof. Max Tegmark explains the event horizon exactly as I have described - standing by a waterfall he says that even when you swim as fast as you can the water (meaning the Spacetime) will flow faster than you can swin. So this is not true explanation?

I think Tegmark based his analogy on the more quantitative American Journal of Physics paper

http://arxiv.org/abs/gr-qc/0411060.

 

[Dale]

AFAIK, the river model works for Schwarzschild and Kerr black holes, but not in general spacetimes like pp-waves or the FLRW metric. It seems to be a limited but defensible model specifically developed for popularizations.

 

[ZirkMan]

That seems like an odd connection to make. Do you expect that something with material properties must have personal properties like preferences or feelings? Or, perhaps a more related question would be, do you expect that even fundamental (material) particles must have a (geometrical) size?

I don't think my expectations are my problem (I try not to have any) but my imagination.

It's really hard to look at a picture of a gravitational lensing effect and not see these multiple flares of the same object as a result of some invisible but still clearly defined lens like object around a cluster of galaxies and when light from the distant source comes closer to it it splits and follows its curved shape creating these multiple images or Einstein rings.

But I really like your relational theory definition. It says that you can get these effects simply out of pure geometrical relations. I can imagine that, makes more sense now, thanks.

 

[Dale]

I am glad it helped. The distinction between geometrical and material properties is the idea which has made the most sense to me so far.

 

[ZirkMan]

What is a circle or a triangle made of?

I would like to answer this question and test the new geometrical insight to find out more about the nature of Spacetime.

Circle is a result of two different points and a rule that moves one of the points on a flat 2D plane so that their distance doesn't change.

Triangle is a result of straight connection of 3 different points on a flat 2D plane.

Well, this could be one of valid definitions. But how about this one in the same style:

Spacetime is a result of straight connections between two or more points in 4D (x,y,z,t) and a rule that states that more mass/energy each point has the more the connecting line will be curved in 3D (x,y,z) but will remain straight in 4D (x,y,z,t) (the last part I'm not sure).

 

[Mentz114]

Spacetime is a result of straight connections between two or more points in 4D (x,y,z,t) and a rule that states that more mass/energy each point has the more the connecting line will be curved in 3D (x,y,z) but will remain straight in 4D (x,y,z,t) (the last part I'm not sure).

That is an oversimplification. Between two spatial locations there are an infinite number of 4D paths, whether the spacetime is flat or not. Like road racers, who all follow (roughly) the same 3D path but have different 4D trajectories. From experience it seems that a straight line is the path followed by light, which has the distinction of winning every race. I think a spacetime might well be described by all the possible paths between masses.

But spacetime has to remain an abstraction. You can't touch or feel space nor time like you can matter and energy.

 

[ZirkMan]

But spacetime has to remain an abstraction. You can't touch or feel space nor time like you can matter and energy.

I would be fine with this answer if only that abstraction remained abstract and left me alone. But now it pulls me down every time I try to jump up. Can you tell me how?

 

[Dale]

I would be fine with this answer if only that abstraction remained abstract and left me alone. But now it pulls me down every time I try to jump up.

I like this reply.

All theoretical constructs are mathematical abstractions so Mentz114 is correct in one sense, but since they are also subject to experimental validation I am fundamentally uncomfortable with statements like Mentz114's. I think that space and time are on equal scientific footing to matter and energy: they are measurable quantities in an experimentally validated physical theory.

 

[aeon.rs]

The clue obviously can be found within the theory of special relativity. The theory concerns about the fundamentals: the space, time, energy, and matter. The special relativity has unified those fundamentals into two distinct entities: the spacetime as the unification of space and time and the energy substance from which matter is derived (E=mc²).

The spacetime and energy are not two separate entities as we think. The spacetime is not like a sort of container and energy something that fills the container. On the contrary, they are inextricable just like water substance and its spherical form in a drop of water. The spacetime is merely the geometrical quality of energy. In plain English, energy is noun and spacetime adjective.

The spacetime does not have existence on its own. Energy is the one and the only independent reality in nature. It is primordial; neither can be destroyed nor created, omnipresent; permeates throughout every part of an object or a place. Everything else is derived from it. The concept of empty spacetime loses its meaning. This is the deepest meaning of the energy conservation, the most primitive law of nature.

 

[pallidin]

Nice thread! Just posting to keep track of it.

 

[aeon.rs]

Nice thread! Just posting to keep track of it.

Thank you. If you are interested, may I suggest you to follow my postings in Re: E2 - p2c2 = m2c4 - meaning of symbols (#12,14,18,19,21) which are related / elaborate further this thread?

 

[ZirkMan]

The special relativity has unified those fundamentals into two distinct entities: the spacetime as the unification of space and time and the energy substance from which matter is derived (E=mc²).

You are right, this is how far the Special theory could go in regards to Spacetime and Energy. But as far as I can see it doesn't say anything about unity of Spacetime and Energy. It only explains how measurements of space and time change for observers with different relative velocities in inercial frames so that the condition of c being a constant in all those frames is maintained.

In plain English, energy is noun and spacetime adjective.

Then what is a black hole? A noun transformed by its adjective? But transformed to which of the two?

 

[aeon.rs]

You are right, this is how far the Special theory could go in regards to Spacetime and Energy. But as far as I can see it doesn't say anything about unity of Spacetime and Energy. It only explains how measurements of space and time change for observers with different relative velocities in inercial frames so that the condition of c being a constant in all those frames is maintained.

Then what is a black hole? A noun transformed by its adjective? But transformed to which of the two?

Did not the advisor talk about the spacetime possessing geometrical properties and not material properties, and about geometry (circle, triangle) to exemplify the nature of spacetime? The spacetime is merely a geometrical quality … but of what? The special relativity gives us a clue. The spacetime is the structural quality of nothing but energy. The spacetime is an adjective and energy is noun, and not vice-versa.

You are right, the special relativity theory stops there keeping the spacetime and energy remain separated. However, during the latest years of his life, Einstein was still thinking on the nature of spacetime. He asserted that the spacetime has no separate existence, independently of the actual objects of physical reality (although what he meant by the physical reality in this context was the field). Albert Einstein:” Relativity, The Special and The General Theory”, Appendix V, Crown Publishers, Inc., New York (1952).

 

[ZirkMan]

Did not the advisor talk about the spacetime possessing geometrical properties and not material properties, and about geometry (circle, triangle) to exemplify the nature of spacetime? The spacetime is merely a geometrical quality … but of what? The special relativity gives us a clue. The spacetime is the structural quality of nothing but energy.

Maybe I'm really without imagination but I don't get this "spacetime is a geometrical quality of energy". Geometry in what dimension is meant by that? Does the need for existence of spacetime somehow arise from whether Energy has a shape of triangle/circle (in 2D) or a sphere/cube (in 3D)? What type of geometry and dimensions are we speaking about precisely?

Let's see how Spacetime arises from Energy in the framework of the Special theory of relativity. As already discussed when relative velocities of Energy differ their relative spatial (3D) and temporal geometry is different compared to state when the difference in their relative velocities is zero.

This change in measures of spatial and temporal coordinates that goes with change of relative velocities is called the spacetime because space coordinates change in equal measure as temporal. But why to call it "geometry" when it's more like a "transformation rule"? Maybe the general theory gives a clue to this?

You are right, the special relativity theory stops there keeping the spacetime and energy remain separated. However, during the latest years of his life, Einstein was still thinking on the nature of spacetime. He asserted that the spacetime has no separate existence, independently of the actual objects of physical reality (although what he meant by the physical reality in this context was the field).

Well maybe separated, maybe it's really just the spacetime geometry of Energy that changes for different observers. In that sense spacetime would be just a rule of the transformation and wouldn't indeed exist without existence of that to what it applies to i.e. Energy.

 

[Hoku]

Here's what works for me (although I'm just a layman, and I know some people disagree with me):

As people have said, there is no *single* geometry for spacetime. Every point within spacetime has an infinite number of possible geometries that pass through it. So there is no *absolute* geometry that "spacetime" has. There is only the description of how matter moves through it. The path that matter follows can be predicted using geometries as a tool, but there is no "fixed" geometry, except where the energy of matter "fixes it" while relating to the energy of other masses.

In other words, IMO, spacetime curvature is used to predict the location of matter in 4D and should not be taken too literally.

 

[aeon.rs]

Maybe I'm really without imagination but I don't get this "spacetime is a geometrical quality of energy". Geometry in what dimension is meant by that? Does the need for existence of spacetime somehow arise from whether Energy has a shape of triangle/circle (in 2D) or a sphere/cube (in 3D)? What type of geometry and dimensions are we speaking about precisely?

Let's see how Spacetime arises from Energy in the framework of the Special theory of relativity. As already discussed when relative velocities of Energy differ their relative spatial (3D) and temporal geometry is different compared to state when the difference in their relative velocities is zero.

This change in measures of spatial and temporal coordinates that goes with change of relative velocities is called the spacetime because space coordinates change in equal measure as temporal. But why to call it "geometry" when it's more like a "transformation rule"? Maybe the general theory gives a clue to this?

Well maybe separated, maybe it's really just the spacetime geometry of Energy that changes for different observers. In that sense spacetime would be just a rule of the transformation and wouldn't indeed exist without existence of that to what it applies to i.e. Energy.

To simplify things, as we discussed previously the spacetime and energy are not two separate entities as what we think. The spacetime is not like a sort of container nor does the energy something that fill it. As for your imagination I gave a metaphor that the energy and spacetime are inextricable like water substance and its spherical form in a drop of water. You can imagine that at the cosmic scale we have a 4-dimensional gigantic “spherical” drop of water, the water being the energy and 4-dimensional spherical form being the spacetime.

 

[Hoku]

To simplify things, as we discussed previously the spacetime and energy are not two separate entities as what we think. The spacetime is not like a sort of container nor does the energy something that fill it. As for your imagination I gave a metaphor that the energy and spacetime are inextricable like water substance and its spherical form in a drop of water. You can imagine that at the cosmic scale we have a 4-dimensional gigantic “spherical” drop of water, the water being the energy and 4-dimensional spherical form being the spacetime.

What are your thoughts about this quote on the Stanford Gravity Probe B website?

http://einstein.stanford.edu/SPACETIME/spacetime2.html

In 1918, Einstein described Mach's principle as a philosophical pillar of general relativity, along with the physical principle of equivalence and the mathematical pillar of general covariance. This characterization is now widely regarded as wishful thinking. Einstein was undoubtedly inspired by Mach's relational views, and he hoped that his new theory of gravitation would "secure the relativization of inertia" by binding spacetime so tightly to matter that one could not exist without the other. In fact, however, the equations of general relativity are perfectly consistent with spacetimes that contain no matter at all. [...] The bare existence of such solutions in Einstein's theory shows that it cannot be Machian in the strict sense; matter and spacetime remain logically independent. [...]

[...]Space and time do act on matter, by guiding the way it moves. And matter does act back on spacetime, by producing the curvature that we feel as gravity. Beyond that, matter can act on spacetime in a manner that is very much in the spirit of Mach's principle. Calculations by Hans Thirring (1888-1979), Josef Lense (1890-1985) and others have shown that a large rotating mass will "drag" an observer's inertial reference frame around with it. This is the phenomenon of frame-dragging, whose existence Gravity Probe B is designed to detect. The same calculations suggest that, if the entire contents of the universe were to rotate, our local inertial frame would undergo "perfect dragging" — that is, we would not notice it, because we would be rotating too! In that sense, general relativity is indeed nearly as relational as Mach might have wished. [...] Within the context of Einstein's universe, however, the majority view is perhaps best summed up as follows: Spacetime behaves relationally but exists absolutely.

 

[aeon.rs]

What are your thoughts about this quote on the Stanford Gravity Probe B website?

http://einstein.stanford.edu/SPACETIME/spacetime2.html

To respond your standpoint I oblige to refer again to Einstein’s views in his book [Albert Einstein: “Relativity. The Special and the General Theory”, Appendix V, Crown Publishers, Inc., New York (1952)].

The concept of spacetime as an independent reality is inherent in the special relativity theory. Recall the Minkowski famous statement: “Henceforth, space by itself and time by itself are doomed into mere shadows and only the union of the two remains as an independent reality”.

I quote Einstein’s views in that book regarding to this issue:

[...] “Thus, as in classical mechanics, space is here also an independent component in the representation of physical reality. If we imagine matter and field to be removed, inertial-space or, more accurately, this space together with the associated time remains behind. The four-dimensional structure (Minkowski-space) is thought of as being the carrier of matter and of the field. […]

This rigid four-dimensional space of the special theory of relativity is to some extend a four-dimensional analog of H.A.Lorentz, rigid three-dimensional aether. For this theory, also the following statement is valid: The description of physical states postulates space as being initially given and as existing independently”. (p. 150-151)

Einstein, however, continued his views with regard to the concept of spacetime in the general relativity theory:

“We are now in a position to see how far the transition to the general theory of relativity modifies the concept of space. In accordance with classical mechanics and according to the special theory of relativity, space (spacetime) has an existent independent of matter or field. In order to be able to describe at all that which fills up space and is dependent on the coordinates, spacetime or the inertial system with its metrical properties must be thought of at once as existing, for otherwise the description of “that which fills up space” would have no meaning. On the basis of the general theory of relativity, on the other hand, space as oppose to “what fills space”, which is dependent on the coordinates, has no separate existence. Thus a pure gravitational field might have been described in term of the g_ik (as functions of the coordinates), by solution of the gravitational equations. If we imagine the gravitational field, i.e. the functions g_ik, to be removed, there does not remain a space of the type (I), but absolutely nothing, and also no “topological space”. For the functions g_ik describe not only the field, but at the same time also the topological and metrical structural properties of the manifolds. […] There is no such thing as an empty space, i.e. a space without field. Spacetime does not claim existence on its own, but only as a structural quality of the field”. (p. 154-155)

 

[Hoku]

To respond your standpoint I oblige to refer again to Einstein’s views in his book [Albert Einstein: “Relativity. The Special and the General Theory”, Appendix V, Crown Publishers, Inc., New York (1952)].

Understood. The website author acknowledges these points of view when he says, "Einstein [...] hoped that his new theory of gravitation would "secure the relativization of inertia" by binding spacetime so tightly to matter that one could not exist without the other."

I know that many people maintain Einstein's point of view. I'm just saying that not everyone does, and these other people have good reasons not to. I'm just trying to offer balance on an issue that is not universally agreed upon. But I respect your position on the matter and your knowledge in general. Hopefully this doesn't put you off from helping with one of my current or future questions.

 

[peteb]

This is an interesting thread, but I think the question is not answered simply by referring to mathematics or physically observed phenomena nor by the equations of relativity directly. These are questions of metaphysics.

First of all, this really delves into metaphysical issues beyond the physical reality of spacetime. Just for a start, there are more than one kind of spacetime, so one must clarify which concept of spacetime is under discussion. There is Minkowski (relativistic) spacetime, Newtonian spacetime, Galilean spacetime, neo-Newtonian affine spacetime (in which space itself does not even exist, only the events in spacetime have form), among others. As well, the issue is just as complex and metaphysical for the concept of time.

The various concepts of spacetime relate, among other aspects, to the concept of space itself as having a physical form, or simply as being a relational framework for real physical entities and properties. IOW space is analyzed as either a physical entity or as simply a collection of properties of other physical things existing in the real physical universe. This is a philosophical debate involving the very undecided issue of the substantivalist versus the relativist views of physical reality.

It also involves the issue of time as reality: does time exist as a physical entity or is it simply a property of the real physical universe revealed by the various ways we observe the universe (especially by the various ways we can view spacetime)? Most modern physicists would likely assert that time does not even exist, because that is what relativity theory shows.

The question relates to philosophical and scientific issues both. Relativity itself involves what is **observed** about spacetime, what we can measure, not what spacetime is as a fundamental physical entity. Relativity tells us what we can observe and measure, not why we measure what we do.

May I suggest three very interesting books on this subject to you:

1) "Relativity and the Nature of Spacetime (The Frontiers Collection)"
by Vesselin Petkov
Publication Springer (2009), Edition: 2nd ed., Hardcover, 316 pages
Publication date 2009
ISBN 3642019528 / 9783642019524

and even more to the issue:

2)"Time and Space"
Author Barry Dainton
Publication McGill-Queen's University Press (2002), Paperback
Publication date 2002
ISBN 0773523065 / 9780773523067

3) "Nothingness: The Science of Empty Space
Author Henning Genz
Publication Perseus Books Group (2001), Edition: 1st, Paperback, 352 pages
Publication date 2001
ISBN 0738206105 / 9780738206103


These are rather advanced technical books but they really treat this topic in depth, far above the "popularization" level"; but I must advise you that the material presented is not easy to grasp in many aspects. The second book, especially, discusses the various forms of space as related to spacetime, and focuses on the physical analysis of the concept of absolute spacetime. It also presents the mataphysics of time as another aspect of this issue.

The third book listed delves very deeply into the physics of space itself, and would help with understanding what "space" really is.

Just my contribution to this topic here. I am no expert but I have been reading very extensively on these ideas for quite some time. I have not seen any real answers to the questions posed here, most are still open issues of the philosophy of physics.

Pete B

 

[ZirkMan]

Thank you Pete for your contribution. As OP let me summarize what I think was and was not answered in this thread so far.

Since this is a question of nature of spacetime from theory of relativity's point of view we learned that only the relational point of view makes sense because the theory refuses any absolute frames of reference. If you don't know what a Relational theory is see http://en.wikipedia.org/wiki/Relational_theory

In the relational framework there are at least two types of spacetime to deal with in the theory of relativity. First is the Minkowski spacetime of the Special theory of relativity.

The nature of the Minkowski spacetime can be quite easily demonstrated to be just a relative geometrical property of energy dependend only on relative velocities of observers in inercial frames of reference. This spacetime doesn't exert any kind of "force" on energy. It is just its geometrical aspect in dimensions of space and time measurements for different observers.

Second type would be the "Stationary spacetime" of the General theory of relativity. Nature of this spacetime lies in the heart of my original question because as a result of its existence we feel gravity which has a direct consequence of how energy behaves for all observers (as stated by the strong equivalence principle).

What is the relation of this spacetime of the General theory to energy? Is it again purely geometrical or is it something else? I don't know and I would love to hear an opinion of somebody more versed in the General theory.

 

[peteb]

The reason I posted what I did was because it appeared to me that nobody was directly responding to the questions you asked. Like I stated, I am no expert here, but I think you are confusing issues regarding spacetime with questions that delve into the nature of space and time itself.

Actually, your statement "Since this is a question of nature of spacetime from theory of relativity's point of view we learned that only the relational point of view makes sense because the theory refuses any absolute frames of reference" is not quite correct. For one, accelerated motion, that is, motion in non-inertial frames in spacetime, is absolute in relativity theory; it is only motion in inertial frames that is not absolute.

Spacetime itself is not a "thing", it is a way of viewing,analyzing, or just perceiving phenomena in the unified four-dimensional continuum or framework of space and time. Whether space itself is truly perceived via a relativistic or substantival approach is as I said still an open question, there are many issues with the relativist view of space that cannot be answered by relativity.

Remember that, as I said, relativity theory does not concern or deal with the intrinsic nature of space (or spacetime) itself, it deals with the entities and phenomena **in** space that we observe and measure. For example, relativity does not deal with the nature of light per se, the universal limitation on the speed of light and so on; it simply accepts that, for whatever reason, nature manifests light as the universal "speed limit" in the physical world, and relativity then deals with the effects that aspect of light has on the motion of entities and other phenomena in the physical world, the limitations and particulars of what we can observe and measure in phenomena that are affected by the nature of light.

Similar limitations apply to all the other aspects of relativity theory, especially regarding the nature of the time interval and such things as the Lorentz contraction of length and the relativistic perception of the time interval in its dependence on the framework of observation and measurement.

Special relativity is a kinematic theory of motion, it details how we perceive and measure the motion of physical entities in the universe. General relativity is a geometric theory that describes the properties of space and the mass in that space, among other things. But again, what GR describes is how we perceive and measure the phenomena presented by mass and motion in space, it does not tell us **why** the universe presents itself that way nor does GR describe the intrinsic nature of such phenomena.

Point is, the nature of spacetime is not limited to just the relativistic implications offered by the theory of relativity. Thus, asking "what is spacetime made of" will not be answered solely by examining the relativistic aspects on phenomena we observe in that spacetime. It is far more complicated than that, and is as yet an open question. The answers you seek will not provide all the information involved in analyzing the issue.

Again, I suggest reading the books I mentioned. They are not just popularized dumbed-down presentations, they are instead university textbooks, on both an undergraduate and graduate-level basis, that will help you sort out the complex aspects involved. But the study of this is very "deep", and in the end you will find that there is no real answer to your question. MOst complex of all are the issues dealing with the intrinsic nature of space itself, these become metaphysical questions involving just what we can expect to know about space and so on, and as discussed in the books, you will find that not only are the answers unknown, often it is the questions themselves that are either incomprehensible or else of a nature that cannot be explained using physics alone.

My view, anyway...

Pete B

 

[Hoku]

[...]only the relational point of view makes sense because the theory refuses any absolute frames of reference.

These "points of view" are points of measurement. They are measuring spacetime. In other word, bodies are required to measure spacetime and no single body can exist as an absolute frame of reference for this. But that doesn't mean spacetime doesn't exist without them, it only means it can't be measured without them.

 

[IRobot]

According to Einstein, Space-time is made of the gravitational field.

 

[ZirkMan]

According to Einstein, Space-time is made of the gravitational field.

According to Einstein gravitational field doesn't exist. Gravity is an accelerated frame of reference and that is a big difference.