What is the Composition of Spacetime?

[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.