Is space an entity, a relationship or a concetual framework?

[PeterDonis]

The predictions of general relativity follow from the equivalence principle, special relativity and some other assumptions.

Do you mean you have a theory using a different mathematical framework from GR, that reproduces all of its predictions? If so, please give a reference.

Or do you just mean that you prefer to interpret the same math of GR that everyone uses, differently from the usual curved spacetime interpretation? If so, that's a question of philosophy, not physics.

 

[Chalnoth]

The cosmological principle is a statement that the universe appears the same to all observers, you can't infer the homogeneity of the universe from one sole vantage point without additional assumptions. Sure special relativity doesn't hold in the FLRW metric, which assumes homogeneity.

The question was whether the cosmological principle implies a linear distance/recession velocity relationship. It does.

This does require defining a specific reference frame (given by a time slicing of the universe). But the point is that if you have homogeneity and isotropy (the cosmological principle), then it's always possible to define a reference frame where the first derivative of the proper distance between any two galaxies at the same time (given by this time slicing) is a linear function of that proper distance.

As for other possible expansion histories that would be consistent with observations, well, so far no such models have been shown to be accurate. It's particularly difficult to fit any non-homogeneous model with measures of large scale structure such as baryon acoustic oscillation measurements.

My point was precisely that the mathematics of general relativity are not the only way to arrive at predictions consistent with observations. While the much simpler explanation will not be equivalent to general relativity, what we are concerned about is whether it fits all the experimental evidence, not just one specific situation. And it is not necessary to invoke a 4-dimensional curved manifold to explain the evidence, as was done in this paper for the classical tests of general relativity: http://journals.aps.org/pr/abstract/10.1103/PhysRev.92.1557 (the same approach could be generalized to explain all the evidence, it could be formulated in a mathematically simpler way, other approaches could be used...the point is the facts of experience do not compel us to accept a 4-dimensional curved expanding manifold as a physical necessity)

Not the only way, sure. Technically, there has to be a different way because General Relativity cannot be entirely correct (due to its prediction of singularities). But there is no simpler way that will fit with all of the evidence.

As for that paper, I no longer have access to an institutional subscription. But I highly doubt that it would be in any way easier to grasp than General Relativity (and it's probably either wrong or reduces to General Relativity in the appropriate limit).

Gravitational waves may be measurable, but their existence does not imply the existence of space being a physically curved expanding thing. Just like the bending of light does not imply space is physically curved unless you postulate in the first place that light travels in straight lines, and just like the precession of gyroscopes in orbit does not imply space is physically curved. What they do show is that what we observe can be interpreted in a mathematical framework where a mathematical space is curved.

Except that it fits your own definition of space-time being a physical entity.

 

[yogi]

Since the original question is more of a survey - I will weigh in with my own interpretation of space as a functional entity. My background is engineering - I have BS, MS and JD degrees. Einstein notably changed his position on space as an ether during the years spent developing GR, he believed space to be conditioned by matter - and while he had the wrong view of the universe as static, the equations admit dynamic solutions as shown by de Sitter, Friedmann and others. A mental image of dynamic space as motion is not necessary... anymore than an image of static space as bent by matter. Both concepts are mathematically useful and both have been used to derive physical results.

 

[Gigel]

Space may be a superfluid Bose-Einstein condensate with very little interaction with fields and particles (except maybe the gravitational field) - just an opinion I found in a few articles.

 

[jack476]

No, a "space" is just a set of objects where there is some concept of distance, direction, and position. Vector spaces, point sets, the xy-plane, your computer's RAM (in a more abstract sense), the search space of a search engine, the solution space of a differential equation...etc.

So I'm going to go with "conceptual framework". Quantities like distance and position are not necessarily inherent to the universe itself, they are ideas that were invented by humans to quantify phenomena in the universe in order to make it amenable to human understanding. It might well be that the universe itself is made out of some identifiable material, but calling the universe a space in which we can quantify distance and direction is entirely a human invention.

 

[Feeble Wonk]

"Entity" ,
"relationship" or
"Framework".

Leibniz would say "relationship".