What is the Composition of Spacetime?

[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.

 

[Phrak]

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).

I read this to mean that objects have not separate existence, independent of spacetime. The task is not to attribute material-like properties to spacetime but to discover how objects are elements of spacetime properties.

 

[ThomasEdison]

This is a really fun thread to read.


I assume that for time: time only happens where an action takes place. All actions that take place succumb to the 2nd Law.

The way I see it is: if there is no object to experience the 2nd law of thermodynamics then there is no time movement.
So along the same lines wouldn't spacetime need energy/matter within it to define itself as well?

 

[espen180]

This is a really fun thread to read. I assume that for time: time only happens where an action takes place. All actions that take place succumb to the 2nd Law.

The way I see it is: if there is no object to experience the 2nd law of thermodynamics then there is no time movement.
So along the same lines wouldn't spacetime need energy/matter within it to define itself as well?

Not really. GR doesn't know about the laws of thermodynamics. That's why the existence of time-reverse black holes (white holes) are predicted by GR.

There is also a vacuum solution (no matter/energy) which is SR.

 

[ThomasEdison]

Not really. GR doesn't know about the laws of thermodynamics. That's why the existence of time-reverse black holes (white holes) are predicted by GR.

There is also a vacuum solution (no matter/energy) which is SR.

I thought the idea of Hawking Radiation placed thermodynamics back into fold because if true it would mean Black Holes decay: decay says to me... 2nd law.

If they are time-reverse wouldn't they accrete instead of decay?

 

[espen180]

If they are time-reverse wouldn't they accrete instead of decay?

A white hole is just like a black hole, except instead of nothing escaping, nothing can enter it. Their existence is refuted by means of entropy arguments and the question "Just what would they throw out?".

 

[kochanskij]

Space-time is definitely a manifold different from "nothingness". It has a certain number of dimensions (4), it has properties that allow electric and magnetic fields to exist, it has a certain geometry (that is determined by matter and energy in the area), it has a certain topology. Although it is not a material object, it must be some kind of entity.

Asking what space-time is made of is a valid question that new quantum gravity theories will be trying to answer. There must be more primitive entities whose large-scale approximation is our familiar space-time. I don't have any idea what these primitive entities could be. I think Loop Quantum Gravity comes closest to describing them. It says that space-time (area, volume, time) is an approximation developed from interactions within a spin network.

 

[ZirkMan]

Space-time is definitely a manifold different from "nothingness". It has a certain number of dimensions (4)

No, those dimensions are properties that you measure for different matter/energy objects. You never measure dimensions of the spacetime itself.

it has properties that allow electric and magnetic fields to exist

Not necessarily. It was Einstein who showed that light can propagate in "space" without any aether. Light propagates as a progression of changes of electric fields to magnetic and vice versa. The rate of this change of fields is always constant for all observers and has a vector of direction. The finite velocity of c is a result of it.

it has a certain geometry (that is determined by matter and energy in the area), it has a certain topology.

Spacetime and its geometry and topology is again a result of relation of energy/matter distributed in the "space". It is a consequence of energy/matter being separated by what we call space. This separation would not have any meaning if it didn't take some "time" for different energy/matter to influence each other. The spacetime arises as a consequence of this delay in influence. But because we can measure time of those influences and therefore infer the notion of space doesn't mean the spacetime has any kind of objective existence other than in "the space" of measurments.

Although it is not a material object, it must be some kind of entity.

See above.

Asking what space-time is made of is a valid question that new quantum gravity theories will be trying to answer. There must be more primitive entities whose large-scale approximation is our familiar space-time. I don't have any idea what these primitive entities could be. I think Loop Quantum Gravity comes closest to describing them. It says that space-time (area, volume, time) is an approximation developed from interactions within a spin network.

The fact that there can be some non-zero energy even in what we call "empty space" doesn't mean that the spacetime of general relativity (which includes the spacetime of the special theory as a special case) has a real objective existence tied to existence of energy which is so far the only thing that we can say has objective existence. Everything else has only a virtual existence in our heads as it seems is the case with the spacetime. At least, this is what this thread explained to me.

 

[espen180]

Aren't all notions inferred in physics simply tools for approximating the bahaviour of nature? They make no claims as to what exactly is going on, only what happens as a result of it. The only theory I ever saw which makes claims about what is and isn't is String Theory.