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This question has nothing to do with the expansion of space or the speed of the expansion. I do understand that space expansion is not constrained by the speed of light limit, but I am not asking about expansion.

I am not asking about the fabric of spacetime, what it is made of, or how it stretches.

I have read this question:

If the universe is expanding faster than the speed of light can this relegate light to past time-frames?

where Allure says:

No physical thing can travel faster than light, but the universe can still expand faster than light because it's not physical.

What is the Fabric of Spacetime?

The Fabric of Space-time?

I would like to disagree.

The physical Universe is defined as all of space and time[a] (collectively referred to as spacetime) and their contents.[10]

https://en.wikipedia.org/wiki/Universe

I do believe that spacetime itself is a physical thing, and all the events it is made out of are. Vacuum, as we define it, itself is physical too.

There are a few reasons why I believe spacetime itself is a physical thing:

1. Vacuum can produce particle antiparticle pairs. If spacetime itself was not physical, then how would it be able to produce particle antiparticle pairs?

2. Spacetime itself can bend (and expand too), how could it if it was not a physical thing?

3. Spacetime itself includes all events too.

4. It includes all the fields, for all forces, particles, and particles are just excitations of these fields, that are an intrinsic property of spacetime itself. How could spacetime not be physical if it includes the fields too (that manifest as excitations in matter and energy itself)?

Question:

  1. Is spacetime physical?
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    $\begingroup$ What’s your definition of physical? $\endgroup$ – G. Smith Nov 11 '19 at 23:06
  • $\begingroup$ General relativity makes accurate predictions about the shape of the universe. It clearly captures something that must be true about the universe. But, GR is only a model. Think of the parable of the blind persons examining an elephant: Each perceives something that is arguably true about the elephant, but each forms a very different picture in their mind of what an elephant is like. $\endgroup$ – Solomon Slow Nov 11 '19 at 23:49
  • $\begingroup$ why the downvote? $\endgroup$ – Árpád Szendrei Nov 12 '19 at 0:32
  • $\begingroup$ @G.Smith what I mean is, it is not just theoretical, not just a model like the definition here: "In physics, spacetime is any mathematical model" on wiki. I mean it does have physical manifestations, like excitation of fields, bending, expanding. $\endgroup$ – Árpád Szendrei Nov 12 '19 at 0:37
  • $\begingroup$ Well, then, by your definition it is obviously physical. I also consider it physical, because it has dynamics, transports energy, etc. It is not merely an arena in which other things have dynamics. I suspect that Allure had a different definition. “Physical” can mean different things to different physicists. Don’t worry about the ambiguity of nontechnical terms. (P.S. I am not the downvoter.) $\endgroup$ – G. Smith Nov 12 '19 at 1:33
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Your question presupposes that there is an authoritative meaning of the word 'physical'. There is not; people will use the word loosely without being sufficiently clear about its significance. For all the reasons you mention, you are entitled to consider spacetime as being 'physical'.

Your disagreement with the author who said that space was not physical is a classic example of an argument at cross purposes. The word physical can be taken to mean concrete rather than spiritual or abstract, and in that sense one could be justified in saying that physical things do not move faster than light. But the word also means relating to the laws of physics, and in that sense spacetime clearly is physical. The argument has arisen because two people have used the same word in connection with two legitimately different meanings of it, without realising they have done so.

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We do not know everything about the nature of spacetime today, because we are not able to quantize it, that means that today it is impossible to define spacetime in accordance with quantum mechanics.

However, we may define spacetime as coordinates for the observation of the universe.

For this topic, it is important to recall how curved spacetime has been developed:

  • In 1905, Einstein published SR.

  • In 1908, Minkowski provided a mathematical structure to SR, with worldlines in four-dimensional coordinates. He made the assumption (!) of a continuous spacetime manifold: "In order to leave nowhere a gaping void, we imagine to ourselves that something perceptible is existent at all places and at every moment." This assumption was not part of Einstein's SR, and in the beginning, Einstein was even surprised about Minkowski's interpretation of his theory.

  • Based on Minkowski, the idea of a fourdimensional spacetime manifold was adopted.

  • In the following, Einstein worked on general relativity. He became aware that for the description of general relativity he needed a new mathematical model, and he developed the semi-Riemannian manifold of curved spacetime. That means, that at the state of our current knowledge, spacetime in the form of a semi-Riemannian manifold is one possibility for the description of the ideas of general relativity and of the experimental evidence for general relativity, not more and not less.

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